future communications infrastructure - technology ... · 2.5.3 link budget ... future...

EUROPEAN ORGANISATION FOR THE SAFETY OF AIR NAVIGATION

EUROCONTROL

EUROPEAN AIR TRAFFIC MANAGEMENT PROGRAMME

Future Communications Infrastructure - Technology

Investigations Description of AMACS

Edition Number : 1.0 Edition Date : 02/07/07 Status : Issue Intended for : ACP-CG

Future Communications Infrastructure - Technology Investigations Description of AMACS

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DOCUMENT CHARACTERISTICS TITLE


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Edition Date: 02/07/07 Abstract

Description of AMACS using the Technology Assessment template.

Keywords future communication technologies FCI evaluation Datalink

COCR Terrestrial systems Satellite systems aeronautical commu-nications

shortlist Requirements spectrum AMSS/AM(R)S

STATUS, AUDIENCE AND ACCESSIBILITY Status Intended for Accessible via Working Draft General Public Intranet Draft EATMP Stakeholders Extranet Proposed Issue Restricted Audience Internet Released Issue

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DOCUMENT APPROVAL The following table identifies all management authorities who have successively approved the present issue of this document.

AUTHORITY NAME AND SIGNATURE DATE


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DOCUMENT CHANGE RECORD The following table records the complete history of the successive editions of the present document.

EDITION NUMBER

EDITION DATE

INFOCENTRE REFERENCE REASON FOR CHANGE PAGES

AFFECTED


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CONTENTS

1. L-BAND DATALINK TECHNICAL DESCRIPTION.......................................... 1

1.1 Overview................................................................................................................................. 1 1.1.1 Key facts .............................................................................................................................. 1

1.2 AMACS Functional Architecture .......................................................................................... 2 1.2.1 General ................................................................................................................................ 2 1.2.2 AMACS Network Architecture ............................................................................................. 3

1.3 Services Provided & Key Features ...................................................................................... 4 1.3.1 Data exchange services ...................................................................................................... 4 1.3.2 Supplemental services ........................................................................................................ 5 1.3.3 Timing concept .................................................................................................................... 5

1.4 Air Interface Description: PHY, MAC, Data-link & Network Sub-layers ........................... 5 1.4.1 Physical & MAC Sub-layers................................................................................................. 5 1.4.2 Data-link Sub-layer ............................................................................................................ 14 1.4.3 Network Sub-layer ............................................................................................................. 16 1.4.4 Services offered by the network layer: .............................................................................. 17

1.5 Standards ............................................................................................................................. 24

1.6 Technology Readiness Level (TRL)................................................................................... 24

2. APPLICATION OF TECHNOLOGY TO ATM................................................. 25

2.1 Concept of operation: cellular deployment ...................................................................... 25 2.1.1 Introduction ........................................................................................................................ 25 2.1.2 Representative C/I derivation and Cellular deployment .................................................... 26

2.2 Applicable Frequency Band and electromagnetic compatibility.................................... 28

2.3 Airspace Application........................................................................................................... 29

2.4 ATM services supported..................................................................................................... 29

2.5 Proposed Architecture for Technology System............................................................... 30 2.5.1 Avionics ............................................................................................................................. 30 2.5.2 Range ................................................................................................................................ 30 2.5.3 Link Budget........................................................................................................................ 30

2.6 Performance Assurance ..................................................................................................... 31

3. STATUS OF THE TECHNOLOGY ................................................................. 32

3.1 Summary .............................................................................................................................. 32

3.2 Status.................................................................................................................................... 32

ANNEX A : MODULATION OPTIONS..................................................................... 33


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A.1 Introduction.......................................................................................................................... 33

A.2 Modulation design choice .................................................................................................. 35

A.3 Practical modem implementation considerations ........................................................... 38

ANNEX B : ERROR CORRECTION CODES........................................................... 39

B.1 Introduction.......................................................................................................................... 39

B.2 Inner code properties.......................................................................................................... 40

B.3 Outer code properties ......................................................................................................... 41

B.4 Interleaving properties........................................................................................................ 42

ANNEX C : ESTIMATION OF A LINK BUDGET ..................................................... 44

ANNEX D : MESSAGE STRUCTURES................................................................... 45

D.1 Cell insertion message: CELL_INS.................................................................................... 46

D.2 Cell insertion reply message: GS_ALLOC........................................................................ 46

D.3 Cell exit message: CELL_EXIT........................................................................................... 47

D.4 Cell exit reply message: EXIT_ACK................................................................................... 47

D.5 Block reservation message: GS_BLOCK.......................................................................... 48

D.6 Framing message: GS_FRAME.......................................................................................... 48

D.7 GS data uplink message: GS_DATA ................................................................................. 49

D.8 CoS1 keep-alive message: KEEP_ALIVE.......................................................................... 49

D.9 CoS1 data downlink message: DATA_COS1.................................................................... 50

D.10 ACK/CTS message to all aircraft: CTS_ACK_ALL ....................................................... 50

D.11 GS ACK uplink message: GS_ACK ............................................................................... 51

D.12 CoS2 downlink message: DATA_COS2 ........................................................................ 51

D.13 A/C ACK downlink message: AC_ACK ......................................................................... 52

D.14 CoS2 random access short data message: DATA_RA_COS2_SHORT ..................... 52

D.15 CoS2 random access RTS message: RTS_COS2 ........................................................ 53

D.16 CoS2 random access CTS message: CTS_COS2 ........................................................ 53

D.17 CoS2 random access long data message: DATA_RA_COS2_LONG......................... 54

ANNEX E : SYSTEM OPERATIONS ....................................................................... 55


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E.1 A/C cell insertion ................................................................................................................. 55

E.2 A/C has data to send ........................................................................................................... 55

E.3 A/C has no data to send...................................................................................................... 57

E.4 CoS2 random access .......................................................................................................... 57

E.5 A/C-initiated cell exit ........................................................................................................... 59

E.6 GS request for A/C cell exit ................................................................................................ 60

E.7 GS has data to send ............................................................................................................ 62

E.8 GS framing message........................................................................................................... 63

E.9 GS changes the section sizes in the frame ...................................................................... 63

REFERENCES:........................................................................................................ 65


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1. L-BAND DATALINK TECHNICAL DESCRIPTION

This document contains a description of the L-Band Datalink system called AMACS (All-purpose Multi-channel Aviation Communication System). This document relies on a physical layer of the same family as the UAT standards and builds on the two E-TDMA studies per-formed by Sofréavia for the DSNA. Some elements have already been described in the ITT Phase 1 report [27] (Technology Assessment for the Future Aeronautical Communication System). See also the report on future communications prepared for LFV by Helios Technol-ogy [31].

1.1 Overview

AMACS is a multipurpose communication system, with cellular narrowband (50-400 kHz), operating in the 960-975 MHz frequency allocation designed for flexible deployment. Its key design drivers are flexibility, scalability and robustness. E-TDMA and XDL4 concepts have been merged to provide an adapted technical solution for the data-link communications needs of 2020+. The AMACS concept is intended to provide a data-only service with significant requirements for QoS for air/ground point to point, air/air point to point and broadcast modes. Its flexible slot structure is adaptable to meet local requirements. It can support different channel bandwidths and bit rates to cope with the various operational needs and traffic densities foreseen for Europe in the future. Its robust physical layer is based on the GSM/UAT modulation types associated with strong data coding, for achieving the highest QoS in terms of latency. A multi-level QoS system is proposed to permit use of channel resources according to the QoS level required. Specific channel slots are reserved for high QoS transmissions. The effi-cient handling of QoS is based on the TDMA structured MAC layer and gives a guaranteed transmission delay. These communications can support ATN or IPv6 networks. Common Signalling Channels (CSC), similar to those employed by VDL Mode 4, are pro-posed to maintain QoS levels during intervals of network degradation. Examples are the warm- and cold-start features if a ground station (or stations) should go off-line for any rea-son. CSCs would serve to broadcast new ground station frequencies to alert aircraft mobiles of the new channels to which they should tune. The AMACS frame length is designed for fast delivery of time-critical messages and has been set at 2 seconds, but could be adapted to a lower duration if necessary.

1.1.1 Key facts

The AMACS concept is based on several fundamental performance requirements. These include:

1) A very robust physical layer using the already-validated modulation family (CPFSK) used by GSM or UAT.

2) Contributions to data integrity and certification goals through careful, fast, error detection and correction mechanisms.


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3) Use of modular error correction where a unique FEC code is used for headers and for short and long slot data.

4) A high-integrity deterministic MAC sublayer employing deterministic slot schedul-ing and potentially Statistical Self-Synchronization (S3) and deterministic slot scheduling for remote area applications (without ground stations).

5) For ranging functions, fairly imprecise positioning performance may be adequate. 6) High throughput using low overhead for headers, FEC, and transmitter ramping.

AMACS is designed to simultaneously handle up to 175 aircraft per cell in high-density air-space. It has an efficient air-initiated cell handover mechanism, which uses aircraft knowl-edge of cell locations and characteristics – through on-board databases, Electronic Flight Bags (EFB) or a Common Signalling Channel (CSC). Its initial deployment will be in the lower L-band for new ATM point-to-point services requiring a high QoS, thus giving support to SESAR or NEXTGEN future concept. Broadcast services will be provided in a segregated channel if the spectrum availability in the lower L-band is sufficient. Air-to-air data communication is also provided in other segregated channels. It is expected that AOC data communications can be achieved if the necessary extra spec-trum is available.

1.2 AMACS Functional Architecture

1.2.1 General

A depiction of the AMACS functional architecture is shown in Figure 1.1.

Figure 1.1 – Functional Architecture of AMACS

The aircraft's communications system is linked to the ground station for the cell in which it is located. This ground station will provide all of the communication services required, such as



time synchronization. The aircraft will only need to initiate contact with other ground stations before it leaves the current cell. The information about cell parameters, ground station locations and frequencies necessary for the aircraft to initiate and maintain contact may be available through an on-board data-base (updated before the start of the flight), the EFB or CSC. Existing GSM/UAT radio technology will be used. Therefore new hardware radio develop-ment will be limited.

1.2.2 AMACS Network Architecture

The expected network architecture in support of AMACS is depicted in Figure 1.2.

Figure 1.2 – AMACS network architecture

The basic concept is that the ground AMACS infrastructure comprises a number of AMACS Ground Radio Stations, which are organized into clusters. Typically, the Ground Radio Sta-tions in a cluster will be geographically adjacent, or may have overlapping areas of coverage (using different frequencies). Each Ground Radio Station in a cluster will be connected to some redundant concentrator, the Ground Network Interface (GNI), which interfaces it to the transport network via an ATN Air/Ground Router or to other types of router (e.g. an IPv6 Router). The Air/Ground Routers supporting each cluster will themselves be interconnected by a ground transport network, which will also support Ground/Ground Routers for interconnection with end-users.

GNI

GNI

ATN A/G Router

ATN A/G Router

IPv6 Router

IPv6 Router

Cluster 1 ATN G/G Router

IP Router

ATN Applications

TCP/IP Applications

WAN

GNI

ATN A/G Router

ATN A/G Router

IPv6 Router

IPv6 Router

Cluster 2

Cluster 3

ATN G/G Router

IP Router

ATN Applications

TCP/IP Applications

WAN

GNI

GNI



From this description, the ATN A/G Routers and the IPv6 Routers are ground-based users of the AMACS sub-network service and the airborne ATN and IP routers are mobile users of the AMACS sub-network service.

1.3 Services Provided & Key Features

1.3.1 Data exchange services

The AMACS system provides services for reliable data transfer, ensuring delivery on a per- frame basis: the provision of an acknowledged connectionless service is expected to be suf-ficient in the context of ATN or TCP/IP communications where end-to-end connection will be ensured at transport level. AMACS has a strong robustness at physical layer level to ensure both the highest QoS in terms of latency and predictive behaviour in a typical distorted propagation channel. The communication service which is provided by AMACS addresses unicast destinations as well as multicast diffusion. AMACS will support the following specific communication types:

air-to-ground point-to-point; ground-to-air point-to-point; air-to-air point-to-point; ground-to-air broadcast; air-to-air broadcast.

The services that could be supported by AMACS are shown in Figure 1.3.

Ground network

Ground stationbroadcast

Autonomous mobilebroadcast

Mobilebroadcast

A/G point-to-pointcommunicationservices

Air-to-air point-to-point

Figure 1.3 – Possible uses of AMACS



AMACS is designed to be flexible and configurable, for use for h point-to-point and broadcast communications. The aircraft can use AMACS to communicate with each other as well as with the ground station (using the appropriate channels), and the ground station can selec-tively communicate with individual or all aircraft. Separate channels are used for point-to-point and broadcast communications.

1.3.2 Supplemental services

The AMACS high-level supplemental services include:

User/radio authentication; Data Compression: The AMACS data suite will include capabilities for both IP or

ATN header and user data compression; Mobility management: The AMACS data suite will handle the mobility of the air-

craft in order to maintain communications while they move, using handover algo-rithms;

QoS management: The AMACS data suite will handle different classes of traffic (CoS1 and CoS2), integrity, priority and a guaranteed minimum QoS.

1.3.3 Timing concept

UTC is the common time reference in AMACS, in support of the TDMA structure. A hierarchy of timing schemes will be defined in order to guarantee data exchange capability in situations where the the quality of time from available time sources is degraded. These schemes in-clude range measurement functions that also can be used to support security functions in the lower communication layers. The timing accuracy needs varies depending on the actual type of communication. Air-to-air communications are in general more demanding in this respect than air-to-ground communi-cations. The timing scheme for AMACS will be based upon the timing scheme developed for VDL Mode 41. The results of previous studies conducted to investigate the timing require-ments for VDL Mode 4 will be taken into account [26, 32].

1.4 Air Interface Description: PHY, MAC, Data-link & Network Sub-layers

1.4.1 Physical & MAC Sub-layers

1.4.1.1 Introduction The aim is to re-use where appropriate the physical layer specifications of the UAT/GSM sys-tems, thus affording considerable advantage in the costs associated with the development, standardization and fielding of the technology. The derivation of the necessary physical layer properties is explained below. The system performance is designed to provide a required Residual Message Error Rate (MER) of 10-7 on the basis of a Physical Bit Error Rate of 10-3.

1 ICAO Manual [15] Part II clause 1.2.3 and ICAO Manual Part I clause 2.4.3.



The target net data rate is about 500 kbps in order to accommodate the most demanding communications load requirement in high density airspace (ref. COCR v2, [25]). These are summarized in Table 1.1 and Table 1.2 for addressed and broadcast services in high density airspace. Airport Surface TMA Enroute Oceanic ATS 40 40 30 20 AOC 200 4 60 30 Combined 200 40 80 40

Table 1.1 – COCR addressed communications load (kbps) for combined uplink and downlink in High Density volumes

Airport Surface TMA Enroute Oceanic C&P SURV - - 73 3.2 ITP SURV - 36 73 3.2 M&S SURV - 36 73 - SURV 219 91 249 7.7 TIS-B 18 272 249 -

Table 1.2 – COCR broadcast communications load (kbps) in post 2020

The figures in Table 1.2 are for individual services – the aggregate loading figure is close to 500 kbps. Previous work on E-TDMA has led to the definition of framing and error correcting codes. Much of this remains applicable to the design requirements of the AMACS system.

1.4.1.2 Modulation

1.4.1.2.1 Introduction Although AMACS makes use of UAT and VDL Mode 4 characteristics, it is not essential for AMACS to use the same modulation schemes. A balance has to be struck between the bit rate, the Bit-Error Rate (BER), the Signal-to-Noise Ration (SNR), the bandwidth and power. A description of possible modulation schemes and an analysis of the proposals which could be used for AMACS is presented in Annex A.

1.4.1.2.2 Modulation design choice Given the fundamental principles of GMSK modulation shown in Annex A, three proposals were drawn up taking into consideration the desirable characteristics, design goals, and the spectral environment that define the theatre of operations for AMACS.



In light of the observations, the known spectral constraints in the L-band, and cost advan-tages from reusing mass-market standards, the GMSK modulation proposal represents the best design compromise and is the design choice for the AMACS physical specifications. Chosen proposal :

• GMSK : h = 0.5 & BT = 0.3 • Gross bit rate : ~ 540 kbps • Channel bandwidth : 400 kHz • Expected C/I : ~ 9dB

The use of concatenated error coding is considered in Annex B, to make the objective of a C/I of 9 dB in co-channel interference attainable.

1.4.1.3 Introduction: Point-to-Point Air Interface The AMACS system makes use of a specific channel for point-to-point communications. This channel is designed to allow stations (air and ground) to have a minimum number of exclu-sive bits per slot for regular or high-QoS transmissions, with more bits available on request. The broadcast air interface description is given in Section 1.4.1.4.

1.4.1.3.1 Impact of the physical layer features on airborne co-site issues One of the key problems for a future communication component operating in the L-band (960 -1215 MHz) is co-siting with other radio transmitters that operate in the same frequency band. Even if a frequency separation is implemented, providing some decoupling with other spurious emissions, the robustness of the new communication link will be drastically affected by the proximity of other pulse transmitters on the same aircraft. Therefore the best solution will be to take advantage of Pulse Blanking Techniques that have been used in many cases to reduce the effect of strong interference (that is, the case on board aircraft due to very small system isolation). Such a pulse blanking mechanism has been defined in the UAT standards and has a com-mon bus interconnecting the avionics elements that could benefit from the information pro-vided (pulse blanking signal whenever a transmitter is on). Taking into account that two of the major interferers for the new communication system will be short pulsing transmitters (DME and SSR/Mode S), the duration of the jamming pulses will be equivalent to or lower than the AMACS bit duration: the impact of the interference will be therefore limited to a few bits in the frame for which data coding (presented in Section 1.4.2.2) will be the appropriate answer to mitigate the impact of the interference on the frame error rate. On the other hand the impact of AMACS onboard implementation on DME or SSR/Mode S will be limited by providing a frequency separation between the AMACS channel and the first DME receiving channel (i.e. 978 MHz) and by taking into account the small duty cycle of AMACS (0.15% in average on the basis of a 3ms usable slot duration).



1.4.1.3.2 Power control option In order to reduce the level of interference for point-to-point, a power control option is possi-ble. This can be done rather easily, using a small capacity in the signalling channels (af-forded by the high capacity offered by the GMSK modulation option). It requires the ground base station to perform a continuous measurement of the received signals from each aircraft and then return this information to each aircraft. On reception of this information, the aircraft terminal uses it to feed a power control algorithm, which is an adaptive algorithm that con-verges to the optimum power, i.e. the power which is required for normal operations and ac-ceptable BER. Advantage can be taken from algorithms developed for GSM.

1.4.1.3.3 Access and frame structure For point-to-point channels, AMACS will use the MAC layer principles developed for E-TDMA. It will have deterministic organization and a deterministic access to the medium. AMACS will have a frame repeating every 2 seconds, with specific 'uplink' and 'downlink' sec-tions. The frames are presented in Figure 1.4.

frame (N-1)AMACS cycle

frame (N)AMACS cycle

frame (N+1)AMACS cycle







Figure 1.4 – AMACS Frames

A frame consists of multiple slots and a slot consists of one burst. A frame is composed of successive time slots that each consists of:

• transmitter ramp-up, • synchronization interval, flags and addresses, • the data burst, • FEC/CRC code bits, • transmitter ramp-down, • propagation guard time.

This slot structure is depicted in Figure 1.5.



Guard tim

e depending on

cell size

individual slot structure

Synch signallingand data CRC

decay

total slot duration 4ms

next slotFEC/Ram

p-up

active slot duration

Figure 1.5 – AMACS slot structure

If a ground station or an aircraft is using several slots for one transmission, then the transmit-ted ramp-up time and synchronization interval will only be present at the start of the initial slot, and the transmitter ramp-down time and propagation guard time will only be present at the end of the last slot (the number and position of the FEC/CRC code bits will be dependent on the size of the transmission). This means that the size of the combined signalling and data bursts will be larger than the sum of signalling and data burst sections from separate, individual slots (the extra bit transmission capacity is of the order of 540 bits). This is shown in Figure 1.6.

signallingand data

ramp-up

merged slot structure

synch signallingand data

FEC/CRC

total slot duration

nextslot

FEC,CRCand

decay

guard timedepending

on cell size

total slot duration

signallingand data

ramp-up

merged slot structure

synch signallingand data

FEC/CRC

total slot duration

nextslot

FEC,CRCand

decay

guard timedepending

on cell size

total slot duration

Figure 1.6 – AMACS merged slot structure

Figure 1.7 shows a different view of the slot structure, showing the actual number of bits allo-cated to each section. It is based on a frame size of 2 seconds, a data rate of 540 kbps and a slot size of 4ms.



Ramp down

4 ms

Ramp up n

Flag 1 octet

Reservation header 3 octets (if required)

User data 148 octets

FEC / CRC 47 octets

Guard time 0·9 ms

m

Flag 1 octet

Addresses plus flags 4·5 octets (typical)

Ramp down

4 ms

Ramp up n

Flag 1 octet

Reservation header 3 octets (if required)

User data 148 octets

FEC / CRC 47 octets

Guard time 0·9 ms

m

Flag 1 octet

Addresses plus flags 4·5 octets (typical)

Figure 1.7 – AMACS slot structure sizes

The combined ramp-up and ramp-down time (m+n) is less than 0·1 ms. Slot characteristics:

• Active slot length: 4 ms – (ramp + guard times) = 3 ms • Bits per slot: Active slot length × Bit rate = 1,620 bits • Bits for FEC/CRC: ~30% of bits per slot = 376 bits • Remainder: Bits per slot – CRC = 1,244 bits = 155·5 octets • ISO flags + reservation header = 3 octets • Addresses plus administrative flags (typical) = 4·5 octets • User data space = 148 octets

The use of the uplink sections in the frame is configurable (dynamically) by the ground sta-tion. These sections are ground-reserved areas for uplinks and ground-directed signalling. The two downlink sections are separated for different Classes of Service (CoS). The first one (CoS1) is intended for a high QoS and each aircraft is allocated one exclusive downlink slot in CoS1 for high QoS messages. More downlink slots are available on request in the lower QoS section (CoS2). The slots are allocated based on QoS requirements, and may be based on the application or may be functionally grouped. Figure 1.8 illustrates this concept.



Shared section

Uplink section

Uplink section

Frame

CoS1 CoS2UP2UP1

Exclusive primaryslots for short, high QoS messages or

RTS messages

Shared slots, reserved or random access:

used for any messages

Second uplink for ACKs, CTS,

reservations

Reserved slots for uplink messages

Downlink section

Framingmessage

Cell insertion

Start of UTCsecond

Figure 1.8 – AMACS Frame Structure

1.4.1.3.4 Frame section usage

1.4.1.3.4.1 Introduction In Figure 1.8, the CoS levels indicate service delivery levels. For the highest level, a dedi-cated time slot is reserved in CoS1 for each aircraft in the cell, and transit times and mini-mum throughput rates are guaranteed. The use of deterministic slot assignments is important for QoS performance. For the lower-level CoS2 time slots, the time guarantees are smaller since these slots are potentially shared among many aircraft and time guarantees are meas-ured statistically. Specific uplink slots are reserved in each frame for the ground station framing message and for cell insertion messages. It is expected that the messages required for hand-off proce-dures will normally be exchanged in the UP1 and CoS1 sections, thereby taking place within one frame. The section lengths are not fixed and can be optimized by the ground station. The ground station will broadcast a framing message to all aircraft within the cell to indicate section length changes. Only the slot size and the overall frame size are fixed. If the ground station intends to change the frame section sizes, the aircraft will be notified a long time in advance (typically up to a few minutes).



Proposed messages required for AMACS are provided in Annex D. These structures indicate the different message fields and the number of bits required for each message field. All ex-cept the longest messages will fit into the single slot shown in Figure 1.7. System operation exchanges are provided in Annex E, indicating how the aircraft and the ground station interact using the proposed messages.

1.4.1.3.4.2 Message identifiers and acknowledgements The 'message identifier' fields are used in addition to the message type fields so that stations can be certain of which of their transmissions have been acknowledged. (For example, a long data message could be transmitted in several parts, and if one part is not correctly re-ceived it would be inefficient to have to retransmit the whole message) The identifier acts as a rolling sequence number, which can have values from 1 to 64. This range is acceptable because it is not necessary for every message to have a unique ID, merely for the messages from a station to be distinguishable within a period of time. The receiving station will include the message identifier in its acknowledgements so that they show which of the individual messages have been received; the message type field on its own would not be sufficient.

1.4.1.3.4.3 Insertion mechanism for an aircraft entering the cell An aircraft entering the cell will know (from on-board information) the frequency of the ground station in the new cell. It will listen on this frequency for the framing message transmitted by the ground station, which contains information about the slot structure, and will then announce its presence to the ground station by transmitting a message in one of the dedicated cell insertion slots. The ground station will reply in UP1 in the following frame, telling the aircraft the position of its allocated high-QoS slot in the CoS1 section. It will also give the aircraft a local address, used in the cell for identification instead of the longer 27-bit ICAO address. It is expected that the aircraft will be able to transmit in its allocated CoS1 slot very soon after reaching the new cell (as a framing message is transmitted by the ground station every 2 seconds).

1.4.1.3.4.4 Reservation mechanism for downlink In order to reserve a slot or a series of slots in the CoS2 section of the frame, the aircraft in-cludes a reservation request for CoS2 slots in its CoS1 slot transmission. These CoS2 slots are likely to be required when the aircraft has a large amount of data to transmit which can-not all be fitted into the CoS1 slot. A reservation flag (RTS) is set in the CoS1 slot transmission by the requesting aircraft and notice is implicitly provided to all members of the channel that future timeslots are requested by that aircraft. A reservation echo (CTS) is transmitted by the ground station, acknowledging and granting the request for time slots within the pool of secondary slots available in CoS2.



An aircraft may also transmit a slot reservation request (RTS) in CoS2 if it has further infor-mation to send to the ground station in the same frame. The ground station will reply in the first available slot, indicating whether or not any slots are available for the aircraft to use. If slots are available, the ground station will transmit a CTS identifying the available slots. If none are free then the aircraft will have to transmit in the next frame. In order to prevent conflicting transmissions, all aircraft will listen to all CTS transmissions to record in their reservation tables which slots have been reserved.

1.4.1.3.4.5 Uplink transmissions The ground station has two blocks in each frame, UP1 and UP2, which are reserved for up-link transmissions. These are used by the ground station for normal data transmissions to aircraft, acknowledgements to downlink messages, CTS messages and cell insertion and exit exchanges. The slots in the uplink section are concatenated and do not require separate ramp-up and ramp-down nor guard-time in between messages. Consequently the number of bits available for data transmission within each slot is greater. If the ground station requires more slots for uplink transmissions, it will examine the reserva-tion table for CoS2 and then broadcast a block reservation message to all aircraft. This mes-sage will indicate the start and number of slots of the reservation, which will only apply to the current frame. No aircraft will transmit in this block.

1.4.1.3.4.6 Hand-off mechanism for an aircraft leaving the cell There are two possible means of hand-off: controlled and uncontrolled.

1.4.1.3.4.6.1 Controlled hand-off Controlled hand-offs can be air-initiated or ground-requested air-initiated. An aircraft will know, from on-board information, when it is nearing the edge of the current cell. At an appropriate time, it will transmit a "cell exit" message in its dedicated CoS1 slot. The ground station will reply, in a UP2 slot, to confirm that the aircraft is leaving the cell and to indicate the number of a reserved CoS2 slot. As the "cell exit" message is being transmitted, the aircraft will also be searching for the ground station of the next cell, to ensure continuity of communications. When the aircraft receives the "exit confirmation" message, it will send a normal ACK mes-sage in this CoS2 slot, indicating that it is the "exit confirmation" message which is being ac-knowledged (this will be the last message that the aircraft sends to the ground station). When the ground station receives the ACK message from the aircraft, it will de-allocate the aircraft's CoS1 slot and will consider the link to be terminated.



The ground-requested procedure is very similar – the ground station will know, from the loca-tion information in ADS-B transmissions, when an aircraft is nearing the edge of a cell. If the ground station determines that a hand-off is appropriate, it will transmit a "cell exit" message to the aircraft. The aircraft, on receiving it, will attempt communication with the ground station in the next cell. If this is successful and the aircraft is allocated a CoS1 slot in the next cell, it will reply, in its current CoS1 slot, with an "exit confirmation" message. Then the current ground station will de-allocate the aircraft's CoS1 slot and will consider the link to be termi-nated. Otherwise the aircraft will maintain its communication with the current ground station. The hand-off from the current ground station will not be completed before the aircraft has made contact with the next cell.

1.4.1.3.4.6.2 Uncontrolled hand-off If communication between the aircraft and the ground station is lost, for more than a pre-determined time period, then both the aircraft and the ground station will consider their link to be terminated.

1. The ground station will de-allocate the aircraft's CoS1 slot as it will assume that it is no longer required by the aircraft.

2. The aircraft will determine the appropriate (new) ground station to contact and will begin the "cell insertion" procedure on the new frequency.

For this process to occur correctly, the appropriate value for the time-out period must be chosen (as it may be affected by local factors).

1.4.1.4 Broadcast Air Interface The AMACS system makes use of a specific channel for broadcast communications. The AMACS broadcast channel has the same MAC structure as VDL Mode 4, modified for a sin-gle channel and for the AMACS frame structure. One superframe is 60 seconds long and contains 15,000 slots, with a corresponding slot length of 4 ms. There is an increase in the allowable basic message size, making AMACS more convenient for ADS-B. Each superframe starts with a short ground-quarantine section. The remaining slots are available to all stations (air and ground) by random access, using modified VDL Mode 4 res-ervation protocols. Most VDL Mode 4 broadcast protocols will be used, but no point-to-point transmissions will be permitted on the broadcast channel. Therefore some modifications will be required.

1.4.2 Data-link Sub-layer

1.4.2.1 Segmentation and de-segmentation



The data-link sub-layer will handle the segmentation of user data queued for transmission by higher layers into appropriate blocks for the MAC layer and the de-segmentation (re-assembly) of received blocks from the MAC layer into a single user data packet for the upper layer.

1.4.2.1.1 Transmission The overall size of the message will already be known. The user data will need segmentation into blocks if the message size exceeds the maximum for one slot, which is 145 octets. Each additional block can contain up to 208 octets of data. These blocks shall carry se-quencing markers to indicate both the total number of blocks and each block's place in the sequence.

1.4.2.1.2 Reception On reception of user data blocks from the MAC layer, the data-link sub-layer will know (from the sequencing numbers) how many blocks to expect. The re-assembly of the blocks will be done by using the sequencing numbers; the sub-layer shall know that the first block contains 145 octets of the user data but that the other blocks may contain up to 208 octets.

1.4.2.2 Error correcting scheme and interleaving A key factor of AMACS is provision of deterministic access to the radio channel to cover the stringent latency requirements of future data-link services. In order to achieve this goal it is not only necessary to provide the proper mechanism at the MAC layer to ensure determinis-tic access to each frame to any aircraft logged in a cell but also to ensure that the probability of message rejection due to data corruption is kept very low. This is only achievable through the use of specific data coding ensuring a high level of error corrections: this data coding will be therefore dependent on the QoS associated to the data (e.g. the useful data throughput will be lower for high QoS due to the level of associated data coding. Different error correcting schemes are used in data communications : cyclic block codes, convolutional codes, turbo code and low density parity check code. The focus has been on cyclic bloc codes. Annex B provides the reasoning behind the choice of the Reed-Solomon (RS) and Bose, Chaudhuri and Hocquenghem (BCH) codes.



1.4.3 Network Sub-layer

Figure 1.9 and Figure 1.10 present the end-to-end connectivity architecture involving the AMACS subnetwork.

Figure 1.9 – Protocols stack model for ATN

Figure 1.10 – Protocols stack model for TCP/IP

These figures show the main associations:

1) Association between end users: In the context of both ATN and TCP/IP communications, end-to-end connections are provided at the Transport level.

TCP/IP Stack

IP Mobile AMACS SNDCF

AMACS datalink

AMACS Physical AMACS Physical

GNI

WAN AMACS datalink

WAN WAN WAN

IP Ground SNDCF

IP Mobile AMACS-SNDCF

IP ground SNDCF

TCP/IP Stack

1

43

AMACSGround Station

Air-Ground router Ground router Airborne router

2

ATN Stack

ATN AMACS SNDCF

AMACS datalink

AMACS Physical AMACS Physical

GNI

WAN AMACS datalink

WAN WAN WAN

ATN Ground SNDCF

ATN AMACS SNDCF

ATN ground SNDCF

ATN Stack

1

43

AMACSGround Station

Air-Ground router Ground router Airborne router

2



2) Association between AMACS network service users: In the context of both ATN and TCP/IP communications, channels are established between an air-ground router and an airborne router. These channels are established in reference to the QoS. Compression mechanisms will be set up (e.g. Deflate).

3) Association between the GNI and the Air-Ground router: The GNI will report aircraft connectivity to the Air-Ground router. This will be performed through use of Join and Leave messages. This interface will permit to handle the AMACS QoS management as defined in 1.4.4.3.

4) Transfer of data between an airborne AMACS system and a GNI though an AMACS Ground Radio Station, using the AMACS medium access protocol.

1.4.4 Services offered by the network layer:

1.4.4.1 Point-to-point air-ground Data transfer In the Airborne and Air/Ground routers, the data communication service is based on a dedi-cated AMACS SNDCF (which could be derived from the ATN Frame Mode SNDCF). The fol-lowing data transfer services are provided:

• SN-Unitdata.request • SN-Unitdata.Indication

Two mobility management information events will be raised by the datalink layer and pro-vided to the airborne router and the Air/Ground router:

• Join event, • Leave event.

These two events will be used as defined in Section 1.4.4.2. Following a join event, packets may be up-linked to the aircraft from the Air/Ground router or down-linked from the aircraft to the Air/Ground router respectively, using the SN-UnitData.Request. The Unidata.Request will support the following parameters:

• Source address, • Destination address, • Data to be sent, • QoS category, • Class of Service, • Priority, • Integrity.



The Source and Destination addresses will uniquely identify the Air/Ground router or the air-craft on the AMACS network. The ICAO 27-bit address will be used for the aircraft. The Air/Ground router will have a unique address coded over 3 octets. The maximum data size of each SN-Unitdata.request will be 145 octets. The values allowed for QoS category, Class of Service, Priority and Integrity parameters are defined in Section 1.4.4.3. For uplink data transmission, the information associated with the Unidata.Request will be sent though a reliable transport network to the GNI from which a join event has been re-ceived for the destination address aircraft. The GNI will then handle the received parameters to send the requested information to the destination aircraft, using the required QoS. Han-dling of the requested QoS parameters is described in Section 1.4.4.3. The data part of the PDU sent uplink by the GNI will consist of:

• The local identification of the Air/Ground router; • The user data part.

The local identification of the Air/Ground router corresponds to a local identifier (from the GNI point of view) that permits unique identification of the Air/Ground router among all the Air/Ground routers connected to the GNI. This identifier has been allocated locally by the GNI and was provided in the Join event triggered by the aircraft. It will permit identification of the data flows exchanged between the Air/Ground router and the airborne router. The GNI will use the received Source address information to find the local identification value to be used. The Destination address will be directly mapped to the corresponding AMACS datalink address. The GNI will implement message queuing mechanisms in order to temporarily store uplink transmission requests until they have been fully completed, i.e. sent and acknowledged at the AMACS MAC layer. In the case of failure to transmit the requested information, the GNI will react according to the requested QoS:

• If a high QoS has been requested (e.g. Guaranteed QoS requested with Deterministic CoS), it will report the transmission error to the A/G router, and the corresponding data packet will be discarded.

• If a medium QoS has been requested (e.g. Deterministic CoS), re-transmission will

be attempted as soon as possible, taking into account potential concurrent requests with higher priority requests arriving in the meantime. The maximum permitted num-ber of retries will be configurable. In case of failure after this number of retries, a transmission error will be reported to the A/G router, and the corresponding data packet will be discarded.

• If a low QoS has been requested (e.g. Concurrent CoS), re-transmission will be at-

tempted as soon as possible, taking into account potential concurrent requests with higher priority requests arriving in the meantime. The maximum permitted number of retries will be configurable. In case of failure after this number of retries, the data packet will be discarded.



When an aircraft receives an uplink PDU, the following information will be provided to the user through the SN-Unitdata.Indication service:

• Source address, • Destination address, • User data part.

The Destination address will correspond to the sub-network address of the airborne router. It will be locally inserted. The Source address will correspond to the sub-network address of the Air/Ground router. It will be translated from the local identification received in the PDU. For downlink data transmission, the aircraft will send the data part to the GNI through the AMACS Ground Radio Station of the current cell within which it has been inserted. The transmitted PDU sent downlink will consist of:

• The local identification of the Air/Ground router • The user data part.

The local identification of the Air/Ground router will be selected, according to the requested Destination address, from the list of available connectivity with Air/Ground routers (notified locally through Join events). Handling of requested QoS parameters will be similar to the GNI behaviour for uplink trans-missions, although queuing mechanisms have to deal with a unique sender. When a GNI receives a downlink PDU, it will identify the Air/Ground router to which it will forward the information through the reliable ground transport network. The GNI will translate the received local identification value to the real sub-network address of the Air/Ground router. Based on the datalink interface, it will also identify the sub-network address of the Source aircraft. The Air/Ground router will then provide the following information to the user through the SN-Unitdata.Indication service:

• Source address, • Destination address, • User data part.

The Destination address will correspond to the sub-network address of the Air/Ground router. The Source address will correspond to the sub-network address of the airborne router. As the GNI may be inter-connected with several Air/Ground routers, through several AMACS Ground Radio Stations, it will be able to handle concurrent transmission requests coming from and going to different network entities.

1.4.4.2 Mobility management This section presents the mechanisms that are used for mobility management, taking into account the above AMACS architecture and protocols stack model.



The aircraft AMACS Radio will use geometry information to identify the AMACS cell to which it should request insertion. The monitoring of the associated frequency and the description of the AMACS frame for this cell will allow the AMACS Radio to retrieve information regarding the presence of a ground station and its address, which will be encoded in one slot. When an aircraft first joins a 'user group' (i.e. an AMACS cell), this will result in a Join event being sent to the network user for both the ground A/G router and the airborne router. On the ground side, this will notify the service user that the identified aircraft has joined a 'user group' via the identified Ground Network Interface (GNI). On the air side, this will notify the service user of the GNI address as well as the supported A/G router(s). As the aircraft continues on its flight, a hand-off may take place to another Ground Station. When this occurs, it is signalled to both the airborne and ground users by another Join event. This Join event will identify the new GNI that both air and ground users must now use to communicate. On the airborne side, the Join event will include the following information:

• the address of the AMACS Ground Radio Station, • the sub-network address of the connected Air/Ground router, • Local identification of the connected Air/Ground router.

The sub-network address of the connected Air/Ground router will permit unique identification of the Air/Ground router on the AMACS network. This value will be coded in three octets. The local identifier of the connected Air/Ground router will permit unique identification of the Air/Ground router from the GNI point of view (local ground reference). This value will be coded in one octet. It is noted that the Sub-network address of the GNI will consist of the initial part of the ad-dress of the AMACS Ground Radio Station through which the aircraft has been inserted. This will be ensured when deploying AMACS Ground Radio Stations: all such systems connected to the same GNI will have a common address prefix value that will permit unique identifica-tion of the connected GNI on the AMACS network. When a GNI is connected to several Air/Ground routers, the airborne system will receive one Join event per connected Air/Ground router. On the ground side, the Join event will include the following information:

• Sub-network address of the GNI, • ICAO 27-bit address of the aircraft.

If an aircraft moves from one GNI to another GNI which are both connected to the same Air/Ground router, the second Join event will be identified as a hand-off event. Seen from the points of view of the Airborne router and the Air/Ground routers, there will be two possible simultaneous paths for communication between them. The two paths will be distinguished by the different GNI addresses.



Insertion into the new cell will occur before the aircraft leaves the previous cell, permitting the air-ground connectivity to be maintained throughout the flight. When the aircraft leaves a cell, a Leave event will be triggered on both the airborne and the Air/Ground routers. A hand-off between Ground Radio Stations in the same cluster (i.e. connected to the same GNI) will only have a minor impact on both air and ground. On the ground side, the aircraft will be inserted into the next AMACS Ground Radio Station cell and will leave the cell of the previous AMACS Ground Station. The GNI should thus not trigger a Join or Leave event, as routing information from the ATN or IP Air/Ground router's point of view is not impacted (the sub-network address of the GNI and the ICAO 27-bit address of the aircraft have not changed). On the airborne side, the AMACS Radio should recognize that it is still connected to the same GNI (same initial prefix of the address of the AMACS Ground Station Radio) in order to avoid triggering the Leave and Join events. The GNI will handle routing tables in order to identify the list of aircrafts connected and the identification of the AMACS Ground Radio Stations through which each is connected. When two routes exists to reach the same aircraft (handoff situation), the latest one established shall always be preferred.

1.4.4.3 Quality of Service (QoS) management

1.4.4.3.1 AMACS QoS management The AMACS system will permit handling of QoS based on four parameters:

• QoS category, • Priority, • Class of Service (CoS), • Integrity.

QoS category: this flag will indicate whether the QoS must be provided on a best-effort or a guaranteed basis. From the point of view of the user (pilot or controller), the provision of data link services on a best-effort basis may not be satisfactory. For example, in the case of a trajectory negotiation, it would be better for the user to know that the data link network cannot deliver the requested service in time rather than trying to negotiate another route in a situation which may no longer be optimal. Although QoS shall be handled from an end-to-end viewpoint, the air-ground link is a potential bottleneck. Priority information will be used to distinguish the relative importance of the exchanged data with respect to gaining access to communications resources and to maintaining the re-quested QoS. The priority of different message categories has been specified by ICAO in terms of the ATN priority.



When AMACS has multiple messages with different ATN priority, but the same AMACS transmit priority, queued to send then it shall take account of the ATN priority in deciding which messages to send first. Table 1.3 presents the priority mapping between the types of message category and the ATN network priority.

Message category ATN Priority

Network/systems management 14

Distress communications 13

Urgent communications 12

High priority flight safety messages 11

Normal priority flight safety messages 10

Meteorological communications 9

Flight regularity communications 8

Aeronautical information service messages 7

Network/systems administration 6

Aeronautical administrative messages 5

Unassigned 4

Urgent priority administrative and UN charter communications 3

High priority administrative and state/government communica-tions

2

Normal priority administrative 1

Low priority administrative 0

Table 1.3 – Mapping between message category, ATN priority, and AMACS priority classifi-cation

AMACS will provide two types of Class of Service (CoS):

Deterministic transmission (CoS1) The Deterministic transmission CoS will be used by applications that require a very high level of reliability for the transmission of short messages. Each aircraft will have a dedicated communication channel reserved for sending data at this CoS. This chan-nel shall always be available and maintained by the datalink services. In case of fail-ure to maintain such service, the user will be notified immediately.

Concurrent transmission (CoS2) The Concurrent transmission CoS will permit transmission of longer messages, but without a guaranteed delivery time for transmission. Transmission of data at this CoS will be done in a concurrent way between all aircraft in the same cell. All data trans-mitted using this CoS will be characterized by a priority value.

This Class of Service parameter will need to be understood in the context of the requested QoS category. The following table presents the impact of each of them where AMACS will handle a data transmission.



Best Effort QoS requested Guaranteed QoS requested Deterministic (CoS1)

In the case where there are no guaran-teed QoS requests for the current CoS1 slot, then if the transmission fits in a CoS1 slot, transmit the data in the current CoS1 slot. If the transmission is too long for CoS1, then request a slot in CoS2. In case of reception failure (no ACK), then re-transmit the data in CoS2 slots (later in the same frame).

For the highest priority guaranteed QoS request for the current CoS1 slot, then if the transmission fits in a CoS1 slot, transmit the data in the current CoS1 slot. If the transmission is too long for CoS1, then request a slot in CoS2. In case of reception failure (no ACK), then re-transmit the data in CoS2 slots (later in the same frame).

Concurrent (CoS2)

In the case where there are guaran-teed QoS requests for the current CoS1 slot, then transmit the data in CoS2 slots.

In the case where there are guaranteed higher priority QoS requests for the cur-rent CoS1 slot, then transmit the data in CoS2 slots in order of priority.

Table 1.4 – Impact of Class of Service on transmission

The integrity parameter will offer the highest level of integrity over the AMACS communica-tion channel. Based on the high level of integrity of the underlying channel, this service will consist of adding a simple CRC as part of the data message exchanged over the AMACS link. This CRC will permit the lowest residual bit error rate to be achieved. It should be noted that independently of the level of service to be provided, whether it is on a best-effort or guaranteed basis, mechanisms should be implemented to monitor the traffic load and usage in each AMACS cell. These mechanisms will allow a better anticipation of the capacity situation, permitting an early reconfiguration of the AMACS cell before capacity problems become a real issue.

1.4.4.3.2 End-to-end QoS management End-to-end communication will involve heterogeneous networks, including mainly an air-ground AMACS link and a ground transport network. Management of QoS on the AMACS link has been addressed in Section 1.4.4.3.1. In order to be able to provide end-to-end QoS management between the airborne system and the ground controller system, two alterna-tives are envisaged:

• Implementation of QoS management mechanisms on the ground network infrastruc-ture. Solutions based on IP based infrastructure, using IntServ or DiffServ model, are envisaged,

• Implementation of QoS management mechanisms at the transport level. This trans-

port protocol shall be designed to be used over a network layer that provides best-effort service differentiation (called EDS – Equivalent Differentiated Services). This solution has the advantage of providing this information directly to end users in order to decide whether the communication infrastructure is capable of providing the ex-pected QoS.



1.5 Standards

The AMACS Physical Layer uses features and characteristics of GSM and UAT, for which international standards are available ([15], [16], [18], [19], [20]). The AMACS MAC and Data Link layers use features and protocols that have been standardized in VDL Mode 4. Thus while international standards are not yet in place for AMACS, the system is already well specified and the development of the appropriate AMACS standards will be facilitated by the availability of the existing material.

1.6 Technology Readiness Level (TRL)

The system is effectively a collection of COTS components which are themselves extensively understood and deployed in the commercial marketplace. Standards for these COTS com-ponents are available and have been validated in an aeronautical context. A mention of these components is appropriate here:

• Physical layer (modulation) – UAT and GSM • MAC layer (access protocols) – VDL4

These components have been tested, validated and demonstrated in relevant environments. The choice of components for AMACS is taking advantage of real-life experience gained from actual use of the UAT, GSM and VDL4 systems. Therefore the AMACS system is expected to meet TRL Level 5.



2. APPLICATION OF TECHNOLOGY TO ATM

This chapter describes the application of the AMACS system to aeronautical communica-tions, which provides the basis for subsequent evaluation. This concept-of-use description involves: • Concept of operation: description of how the technology is able to operate in the ATM

environment. • Applicable Frequency Band: the band or bands that are appropriate for the implemen-

tation of AMACS for aeronautical communications. • Applicable Airspace: the airspace in which AMACS can practically provide aeronautical

communications. • Services Used and Performance required: the AMACS services that are best applica-

ble to aeronautical communications and meet the required levels of performance. • Architecture Integration: description of how the AMACS architecture integrates into the

architecture for aeronautical communications.

2.1 Concept of operation: cellular deployment

2.1.1 Introduction

The obvious characteristic of this system which is different from VDL subnetworks is that AMACS requires the aircraft receiver to have an a priori knowledge of ground station posi-tions. In today’s VHF ATC and AOC systems, the ground station is implicitly identified by the aircraft through use of a “pre-loaded” channel map by sector. In other words, the system knows a priori which channel to tune to by virtue of knowledge of the sector being traversed. The AMACS system’s use of cellular concepts is somewhat different. With regards to the identified operational scenarios, the AMACS system shall provide a spa-tial segmentation to take into account the typical message characteristics and requirements for different operational environment. Each cell shall be geographically distinct and will have its own dedicated ground station, which shall use a non-conflicting frequency (as described below). The cell partition will be built on the horizontal geographical plan, but will include vertical segregation as well. Each aircraft shall have knowledge of the parameters of all the cells including the relevant ground station frequencies. A cellular scheme will provide the adequate configuration to the airspace controlled by ATC. The size of the cell should (and could) be modulated according to the traffic. As a first as-sessment, three operational environments should be distinguished:

• En-Route Low Density cells, with a range of about the optical range 250NM (for lower airspace of the same type, smaller cells could be used taking into account the line of sight coverage limitations)



• En-Route High Density cells, with a maximal range of about 100 NM • TMA cells, with a range of about 50NM modulated by the size of the airport

The cells are tailored to operations, their sizes depending on:

• air traffic density • deployed applications • en-route, TMA, airport

Figure 2.1 presents an example of such cellular deployment over the ECAC area.

Figure 2.1 – AMACS cell deployment across ECAC

2.1.2 Representative C/I derivation and Cellular deployment

Application on a 12 frequencies pattern For further refinement of the AMACS system definition, the derivation of the C/I is a very im-portant task. This is not a trivial exercise. As a first approach the C/I can be estimated from

En-route ECAC Periphery 110-NM Radius Cell

Major TMA

En-route ECAC Core Area 55-NM Radius Cell



the C/N on a channel with AWGN. The degradation caused by a man-made signal, with the same characteristics (modulation, frequency, bandwidth, etc.) on the signal of interest, is smaller than the degradation caused by additive white Gaussian noise. This is because most of the time, the man-made signal is bound (the amplitude is limited, from a receiver point of view). This is not the case for a Gaussian random signal which corrupts the wanted signal. Some measurements on different systems have corroborated this hypothesis. The margin was about 3 dB, for PSK systems. In order to derive an estimation of C/I from first principles, a cell planning based upon the re-use scheme presented below is considered.

Figure 2.2 – 12 frequencies pattern for cell planning

With such a frequency plan, the worst-case interference scenario is an air-air interference, which is represented below in Figure 2.3. The following paragraphs present an evaluation of the C/I in the co-channel interference case. A cell radius R in the presented pattern is assumed. The distance from the interfered plane to its ground station is the wanted path dw = R. The distance from the interfered plane to its interferer on the same channel is the interfered path di = 4R. The following propagation model is assumed :

A(d) = (constant) + a.10 log(d) where the (constant) term stands for the contribution of frequency and other constant pa-rameters to the attenuation, and a is the exponent applied to the distance. Here, a is as-sumed to take the value of 2 (free space model) and could range from 3.5 to 4 in ground cel-lular network. Given these elements, and assuming an omni-directional receiving antenna, if the transmit-ted powers are kept the same, the C/I at the receiver is simply a function of distance:

C/I = A (di) – A (dw) = a.10 log(4R/R)

C/I = a.6 dB, and a ≥ 2



thus C/I ≥ 12 dB

The co-channel C/I ratio at the interfered terminal is always better than 12 dB with this pat-tern and these assumptions. However considering the GMSK modulation choice, the GSM FEC rate 260/456 = 0·57 and a very light interleaving, 9 dB C/I is considered sufficient. Furthermore, it is assumed here that the transmit power of the wanted signal (base station) is the same as the interfering signal (from an aircraft). For these reasons, at least the same performance is expected for the AMACS system.

dw=R

di=4R

dw=R

di=4R

dw=R

di=4R

dw=R

di=4R

Figure 2.3 – Co-channel interference in a 12 channels re-use pattern

2.2 Applicable Frequency Band and electromagnetic compatibility

AMACS systems shall be deployed in the lower L-band (960-975 MHz) which already has an Aeronautical Radio-Navigation allocation. The use of this band is subject to WRC approval of co-prime allocation to AM(R)S. Additionally, a new channelization scheme will have to be provided in the band, to accom-modate the AMACS system’s use of channels ranging from 50 kHz to 400 kHz. One of the key problems for a future communication component that is intended operate in the L-band (960-1115 MHz) is co-siting with other radio transmitters that also operate in the L-band. Even if a frequency separation is implemented, to provide some decoupling with other spurious emissions, the robustness of the new communication link will be drastically affected by the proximity of other transmitters on the same aircraft.



Therefore the solution is to take advantage of Pulse Blanking Techniques that have been used in many other cases to reduce the effect of strong interference (which is the case on board aircraft due to the very small system isolation). Taking into account that two of the major interferers for the new communication system will be short pulsing transmitters (DME and SSR/Mode S), the duty cycle of the jamming pulses will be lower than that of the AMACS bit duration: the impact of the interference will therefore be limited to a few bits in the frame for which data coding will be the appropriate answer to mitigate the impact of the interference on the frame error rate. Such a pulse blanking mechanism has been defined in the UAT standards and will have a common bus interconnecting the avionics elements that could benefit from the information provided (pulse blanking signal whenever a transmitter is on).

2.3 Airspace Application

The use of AMACS systems will provide A/G communications in continental airspace (Core Area as well as periphery), which includes En-route (ENR) and Terminal (TMA) areas. We believe that the surface (APT) area should be covered by another terrestrial-based system (such as WiMAX 802.16); oceanic and polar (ORP) communications should be supported by a satellite-based system.

2.4 ATM services supported

AMACS is designed to support distinct modes of operation:

The ground-supported mode where the aircraft fly within the range of ground datalink stations (these stations may be interconnected via ground links or not),

The autonomous mode where the aircraft fly without any ground datalink infrastruc-ture to support them.

Hence AMACS is designed to support all existing and foreseen types of datalink application:

Air-ground and ground-air point-to-point communications (as required today by AOC and also by emerging ATS applications such as COTRAC, ADS and CPDLC),

Air-air, air-ground and ground-air multicast (i.e. locally broadcast) communications (as proposed for ADS-B, FIS-B and TIS-B),

Air-air point-to-point communications (as envisaged for supporting autonomous sepa-ration assurance applications).



2.5 Proposed Architecture for Technology System

2.5.1 Avionics

Figure 2.4 provides a notional view of the avionics required for a AMACS implementation of ADS-B and AOC and ATS functions.

Figure 2.4 – Possible Avionics for AMACS

The air-to-air point-to-point connectivity is covered by the ADS-B function. For the air-to-air communication mode (generally supporting surveillance functions such as ASAS and ADS-B), two additional receivers are required in the avionics. While the primary receiver is tuned to the channel associated with the cell within which the aircraft is currently located, the additional receivers are tuned to downstream, adjacent cells to get any neces-sary signalling information associated with them.

2.5.2 Range

The expected range of a typical ground station is 150 NM. The size of each cell is the area of a hexagon with a radius of 150 NM.

2.5.3 Link Budget

See Annex C for an estimation of the Link Budget for AMACS.



2.6 Performance Assurance

Since even within the ATN framework the AMACS system will be in competition with other services, especially from commercially operated telecommunication systems, our design ef-forts have been focused at supporting very high performance data link services that will re-main unattractive to general purpose telecommunication operators. Both the perspective of a doubling or more of air traffic densities in the most developed coun-tries in the next twenty years and the progressive emergence of the "autonomous aircraft" operational concept will require that a future data link service simultaneously provides a very high integrity, a very high availability, an extremely short fault detection and recovery delay and a short and highly predictable transfer time. It cannot be assumed that the 95% maximum value of the transit time, which is the most usual metric today, will be a sufficiently rigorous specification in the future. Sooner or later, a datalink system supposed to address long-term needs yet unable to guarantee a 99% maxi-mum transit time for certain categories of message is bound to become a problem rather than a solution. A high degree of confidence in the provided Quality of Service (QoS) will be required: critical in-flight data communication services for applications such as the ASAS or CPDLC in dense areas will be difficult to certify unless the whole system is designed ab initio with QoS verifi-cation in mind. Since relaxing the performance constraints is not really an option, the only cost-effective ap-proach is to incorporate the demonstrability of performance into the very design of the AMACS system, on the one hand, and to avoid to translating the watchword of CNS/ATM integration into a multiplication of common failure modes, on the other hand. AMACS shall meet the COCR requirements for security, continuity, availability and integrity. For details of integrity see Section 1.4.4.3.



3. STATUS OF THE TECHNOLOGY

3.1 Summary

The AMACS technology solution has been developed from a baseline of the existing UAT/GSM and VDL Mode 4 systems. AMACS has a robust physical layer, with appropriate UAT and GSM specifications which are re-used where appropriate, affording considerable advantage in the costs associated with the development, standardization and fielding of the technology. The GMSK modulation scheme is proven and meets the AMACS requirements. The high-performance MAC layer is based on the existing E-TDMA MAC layer concept. The frame structure is devised to meet the high-QoS transmission requirements of AMACS. Existing VDL Mode 4 broadcast and reservation protocols are used. The broadcast experi-ence from VDL Mode 4 is used to take advantage of known operating practices.

3.2 Status

The design of AMACS is finalized at the Physical and MAC layer levels, with complete defini-tions of the frame, slot and message structures. The error correction coding definition is completed. The Channel structure, cellular deployment and network architecture are speci-fied All of the AMACS message types have been defined and the definition of services has been provided. The protocols and system operations are defined for both point-to-point and broad-cast communication



ANNEX A : MODULATION OPTIONS

A.1 Introduction

UAT is a recent aeronautical digital link technology operating in the lower AM(R)S band which is considered as the starting point for designing an optimal physical layer for the AMACS system. UAT uses the CPFSK modulation scheme, with a modulation index of h = 0.715 and a data rate of 1Mbps. UAT is designed to provide a reliable broadcast service in the L-band environment. The ser-vice usage profile is characterized by the fast transfer of short and periodic bursts of data, primarily in support of the ADS-B function. While attaining a high data rate for a safety-critical communications service is desirable, the physical layer needs to be designed around a sta-ble and robust scheme to provide the most reliable transport medium possible. In order to serve this goal, an adaptation of the UAT physical specification is considered to address the main attributes and known weaknesses for a communications link. The main design goals considered when addressing the suitability of modulation options are:

• A low Bit-Error Rate (BER) at a low Signal-to-Noise Ratio (SNR). • Good performance in multipath and fading environments. • Occupation of least possible bandwidth. • Introduce least amount of power in the RF environment. • Low sensitivity to timing jitter (good decision thresholds). • Easy and cost effective to implement.

Of these, a major design criterion is spectrum efficiency. This is especially true in the tar-geted L-band, thus the design of the spectrum mask of the waveform is chosen to reduce the extent of secondary lobes. In order to achieve this, a pre-filtered variation of CPFSK is con-sidered. The MSK (Minimum Shift Keying) family is a type of modulation offering a robust means of transmitting data in wireless systems where the data rate is relatively low com-pared to the channel bandwidth. MSK is a special form of FSK (Frequency Shift Keying). In MSK, the differential frequencies used to represent the data symbols are orthogonal, mak-ing reception particularly straightforward. The term 'minimum' results from the fact that from the FSK viewpoint, the two frequencies used are at the minimum allowable separation whilst maintaining orthogonality. The GMSK (Gaussian Minimum Shift Keying) variant is specifically being considered both for its technical qualities and because it is popular in the commercial mass market and is used with different characteristics in GSM, DECT and TETRAPOL. Hence it offers significant ad-vantage in cost reductions for the development and manufacture of avionics and ground-based equipment.



IF to RFIQ signalsgeneration IQ modulation

Gaussianfilter

Figure A.1 – Gaussian filtered modulation

Figure A.1 shows the block diagram of a GMSK modulator. The Gaussian filter used in GMSK is generally specified by its BT product, where B is the 3 dB bandwidth of the Gaus-sian filter and T is the symbol duration. Note that this is in contrast with the CPFSK specified for UAT where the filtering is applied after the generation of the I and Q component signals. For the Gaussian filter used in GSM, BT = 0.3. The filter is specified by its impulse response given by :

, with and BT = 0.3 for GSM. Pre-filtering in FSK can be considered as an extension of the concept of “continuous phase” modulation. A basic FSK shows abrupt phase transition from one symbol to another, which cause wider spectrum occupation. Continuous Phase FSK addresses this problem by provid-ing smoother transitions from one symbol to another. The concept of pre-filtered FSK is based upon the same considerations. With a CPFSK, the phase transitions are continuous,

but can still be abrupt: the frequency transition is not continuous. Pre-filtered Gaussian CPFSK specifically targets this problem. All the transitions are much smoother, as shown in Figure A.2. It represents the phase tree, i.e. the possible values of the phase throughout time, during transmission. The straight lines show the phase transitions for a CPFSK, while the curved lines show the transition for a GMSK.

These transitions result in a transmitted spectrum for which the secondary lobes are signifi-cantly reduced (Figure A.3). This is a major advantage in a cellular system where frequency reuse is mandatory. GMSK has high spectral efficiency, but it needs a higher power level than the more traditional schemes to reliably communicate the same amount of data. A first-order estimation of the transmitter power likely required by an AMACS transponder is pro-vided in Annex C, although this will largely depend on the deployment scenario (service vol-ume size etc.).

Figure A.2 – Gaussian filtered modulation



Figure A.3 – Spectral density for MSK vs GMSK

A.2 Modulation design choice

Given the fundamental principles of GMSK modulation shown above, three proposals have been drawn up taking into consideration the desirable characteristics, design goals, and the spectral environment that define the theatre of operations for AMACS. First proposal (GSM baseline):

• GMSK : h = 0.5 & BT = 0.3 • Gross bit rate : 270 kbps • Channel bandwidth : 200 kHz • C/I = 9 dB

Note - The above characteristics are those specified for the GSM system, including convolu-tional coding (see Annex B). It is expected that better performance may be attained. Other alternatives are considered in order to explore any trade-offs that may exist between system capacity and spectral efficiency, using different channelizations. Second proposal :

• GMSK : h = 0.5 & BT = 0.3 • Gross bit rate : ~ 540 kbps • Channel bandwidth : 400 kHz • Expected C/I expected to be in the region of 9dB2

2 Simulations will be required in order to confirm and substantiate this estimation.



Concatenated coding is considered in Annex B. It is expected to be more efficient than the coding used in GSM, hence the objective of a C/I of 9 dB in co-channel interference is attain-able. In former study efforts, a CPFSK solution with a modulation index h of 0.715 had been found to have suitable characteristics. Hence a third proposal is considered using this value, bear-ing in mind that, with all other parameters kept constant, the gross bit rate over a constant channel bandwidth is reduced as the modulation index increases. Third proposal :

• GFSK : h = 0.715 & BT = 0.3 • Gross bit rate : ~ 378 kbps • Channel bandwidth : 400 kHz

In order to illustrate the influence of the modulation index, h, on the performances of CPFSK, the following discussion presents the comparison of three modulations: BPSK (used as ref-erence), and binary CPFSK with h = 0·715 and MSK with h = 0·5. The results presented in Figure A.4 are exact for non-filtered modulations. It is noted that for filtered modulations, a significant performance enhancement is expected.

For all these modulations, in the presence of AWGN, the probability of error (BER) can be expressed as follows :

21

21

0

bNE

erfcP be

−×=

with b=1 for BPSK, and b=sinc(2h) for any binary CPFSK. They are represented against the Signal-to-Noise Ratio (SNR) in Figure A.4, where the blue curve represents BPSK, the green curve represents MSK, and red curve represents CPFSK. As can be seen in the figure, the degradation in terms of power, with respect to the BPSK modulation, is 3 dB for h = 0·5 and 2.2 dB for h = 0·715. Hence, using CPFSK (h = 0·715) instead of MSK (h = 0·5) provides a saving of 0.8 dB in the power link budget for a given BER, but costs about 30% in terms of bit rate, if the channel bandwidth remains constant.

Figure A.4 – Comparison of BPSK, MSK (h=0·5) and CPFSK (h=0·715)



The design of GMSK modems is the subject of several optimization studies, especially be-cause of its role in the GSM standard. The impact of variation of the BT product is of primary interest since it affects both the system performance and its efficiency. The impact of varia-tion of BT can be viewed in the time-domain in the form of the well-known “eye” diagram. Figure A.5 illustrates the ideal "eye" diagrams for the phase signals (I and Q) for GMSK in a standard GSM configuration, and for GFSK.

Figure A.5 – Eye diagrams for GMSK (left) and GFSK (right)

It is readily noted that the "eyes" for the I and Q counterparts are rather open for GFSK when compared to GMSK. This explains how the higher BT product for GFSK lowers the level of intersymbol interference at the expense of lower spectral efficiency. The I/Q "eye" diagrams suffer sever degradation if the modulation index, h, varies even slightly from 0·5. This makes GMSK susceptible to slight variations in h, and is the most prominent disadvantage identified for GMSK. GFSK is rather more tolerant to such variations making the hardware simpler and cheaper to implement. This said, the experience from the development of GSM products could be drawn on to build reliable GMSK hardware at a reasonable cost. It is considered unlikely that the mild power saving afforded by the GFSK option will provide any significant improvement of the RF footprint. Moreover, increasing the system bandwidth in the GFSK option in order to provide a higher data rate may prove unfeasible from the spectrum planning viewpoint. Although this is not a primary design criterion, it renders the GFSK option less desirable, and the GMSK option more sustainable, for demanding future services. Hence the third proposal is ruled out. A characterization of the typical application message profiles applicable to the Phase 2 (post-2020) period led to a design target of approximately 140 octets of user data per slot. Based on the effective 3 millisecond AMACS slot payload, the data rate required at the physical layer works out to be about 540 kbps. The lower data rates offered by the first and third pro-posals appear not to serve the data link’s mission for the intended services.



Therefore the second proposal is chosen.

A.3 Practical modem implementation considerations

In principle the design of a GMSK modulator is straightforward. However in practice there are practical limitations to the properties of the hardware components, such as the synthesizer, IF filter and power amplifier. This holds especially true for the synthesizer. Data patters consisting in consecutive ones or zeros have a spectral response extending down to very low frequencies. Most available frequency synthesizers will not respond to such low-frequency signals. A common and well-understood implementation method to countering such synthesizer shortcomings is quadrature modulation (see Figure A.6). In quadrature modulation, the Gaussian-filtered data is separated into in-phase (I) and quad-rature phase(Q) components. The modulated RF signal is then created by mixing the I and Q components up to the frequency of the RF carrier where they are summed together.

Figure A.6 – Quadrature modulation technique



ANNEX B : ERROR CORRECTION CODES

B.1 Introduction

One of the most powerful tools in modern mobile communications is error correction. This is considered first since it provides a baseline performance on which to select the attributes of the physical layer. Different error correcting schemes are available in transmission. Common examples of these are cyclic block codes, convolutional codes, turbo code and low density parity check code. Previous studies have shown that the complexity required to implement turbo code or low density parity check code was not worth the performance in our context. The focus has been essentially on BCH (Bose, Chaudhuri and Hocquenghem) and RS (Reed-Solomon) codes, which are cyclic bloc codes. The use of BCH and RS codes will be maintained for the defined AMACS frames and mes-sages structures. However in the case of messages which demand the highest quality of service, additional coding is considered. In this latter case, it is proposed to use a concate-nated code (see Figure B.1), based upon convolutional coding and RS coding. The RS code is called the outer code, and the convolutional code is called the inner code.

RS coding Interleaving Convolutivecoding

Figure B.1 – Principle of concatenated coding

This powerful coding scheme has been successfully implemented in other transmission standards, such as Digital Video Broadcast (DVB) operating in very perturbed radio chan-nels. Indeed, in DVB, outer code is a RS(188,204,8) code, which means the code words are made of bytes (where 1 byte = 8 bits), and 188-byte packets are encoded into 204-byte code words. For AMACS, it is proposed to use the RS(31,27,5) code as an outer code, which has been elected during the previous works on E-TDMA. The inner code will be the convolutional code used in DVB, as represented below. It is used in a puncturing scheme, which allows a trade-off between performances and code rate. Puncturing is a technique used to form a m/n rate code from a basic rate code (1/2). It is attained by deleting some bits in the encoder output. Available schemes for puncturing leads to the following code rates: 1/2, 2/3, 3/4, 4/5, 5/6 and 6/7.



G2 = 133(8)

D D D D D D

G1 = 171(8)

input bit

output 2

output 1

bk-4bk-1 bk-2 bk-3bk bk-5 bk-6

G2 = 133(8)

D D D D D D

G1 = 171(8)

input bit

output 2

output 1

bk-4bk-1 bk-2 bk-3bk bk-5 bk-6

Figure B.2 – 133,171 convolutional coding scheme

For illustration, the concatenated coding scheme is evaluated on the basis of a gross bit rate of 432 kbps after punctured convolutional coding with a rate 2/3. The overall coding rate is about 0.58. The following sections present an evaluation of the expected performance down the chain of such a coding scheme. The performance of convolutional coding is considered first, followed by a discussion of the interleaving scheme used “in between” the two codes. Finally, the overall performance is derived after considering the contribution of the outer RS code and overall concatenated coding.

B.2 Inner code properties

Convolutional codes are usually described using two parameters: the code rate and the con-straint length. The code rate, K/N, is expressed as a ratio of the number of bits into the con-volutional encoder (K) to the number of channel symbols output by the convolutional encoder (N) in a given encoder cycle. The constraint length parameter, K, denotes the "length" of the convolutional encoder, i.e. how many bits are available to feed the sequential register that produces the output symbols. For the inner code, AMACS will use a (133,171) punctured code (see Figure B.2) with a constraint length of 7. This requires a Viterbi algorithm of 64 states for decoding. This makes it a rather complex scheme, and carries a delay penalty – however it is necessary to provide the degree of reliability and determinism targeted by AMACS. In order to allow a simplified explanation of the scheme’s performance, a simpler coding scheme is considered at first instance (see Figure B.3). No puncturing is assumed, as its constraint length of 3 leads to lower performance than the (133,171) code, with constraint length of 7. In order to compensate for the lack of puncturing, a code rate of 1/2 (vs. rate 2/3), is considered.

G2 = 5(8)

D D

G1 = 7(8)

bk-1 bk-2bk

c2

c1

Figure B.3 – 5,7 convolutional coding scheme



In the case of a BPSK modu-lation (which is comparable to binary CPFSK in terms of BER performances, with a degradation of 2.2 to 3dB), the coded and non-coded BER performances can be derived easily 3. They are presented in Figure B.4. It shows that a BER of 10-3 (resp. 10-2) after demodulation (without coding) can be improved into a BER of 10-5 (resp. 5 × 10-3) with this coding scheme. The blue curve represents the uncoded results, while the green one represents the coded results.

B.3 Outer code properties

For the outer code, AMACS will use a “block code” technique called Reed-Solomon coding. The Reed-Solomon (RS) codes have high channel efficiency and are very versatile. Basically the RS scheme adds redundant parity symbols to the data to enable error correction. The data (and parity symbols) are partitioned into separate blocks which are processed as a sin-gle unit by the decoders and encoders. The number of parity symbols in each block is deter-mined by the level of correction required. A RS code is described as (N,K), where N is the block length in symbols (or codewords) and K is the message length in symbols. Another pa-rameter of significance is t, the maximum correctable symbols per codeword. The code used in AMACS is RS(31,27,5) where N=31, K=5 and t=2. The following expressions give the output probabilities of errors for a RS code, knowing its symbol error rate at the input, PM. A complete discussion may be found in 4. The probability of error for codeword at the output of the decoder is bound by

∑+=

−−≤N

ti

iNM

iM

Niec PPCP

1)1(

3 For a complete demonstration see section 8.5 of [29] A. Glavieux et M. Joindot, « Communication Numériques », MASSON 1996. 4 See [30] J. G. Proakis and Massoud Salehi, “Digital communications”.

Figure B.4 – Performance of convolution encoder with (5,7)



The probability of symbol error at the output of the decoder is given by

∑+=

−−=N

ti

iNM

iM

Nies PPiC

NP

1)1(1

The BER at the output of the decoder is given by

esk

k

e PP12

2 1

−=

−

The probability of error on a symbol at the input of the decoder is given by

kM pP )1(1 −−=

where p is the BER after Viterbi (inner code) decoding. These expressions provide an estimation of the performances over a memory-less channel corrupted by AWGN. It is noted that the expected BER without coding is 10-3, thus convolu-tional decoding should lead to 10-5 at the input of the RS decoder.

BER at the input of RS de-coder

Bound for BER at the output of RS decoder

Bound for Codeword Error Rate at the output of RS de-

coder 5.0 × 10-3 2.1 × 10-3 4.0 × 10-2 1.0 × 10-3 2.5 × 10-5 5.0 × 10-4 1.0 × 10-4 2.7 × 10-8 6.0 × 10-7 1.0 × 10-5 2.8 × 10-11 5.6 × 10-10

Table B.1 – BER results

The results shown in Table B.1 show that we can expect a good BER of 2.8 × 10-11, which is quasi error-free, for a BER after demodulation of 10-3. This is due to the strong correcting power of a concatenated code, where the inner code is ideally suited to removing isolated errors (but not bursts of errors). Following this, the interleaving plus RS code corrects bursts of errors with great efficiency. This has already been fielded with DVB-T and found to work well, with the same type of coding scheme : a BER of 4 × 10-3 after Viterbi decoding leads to 10-11 after RS decoding.. A complete assessment of performance is dependant on the modulation scheme and the characteristics of the transmission channel. Moreover, the effects of interleaving have not been accounted for in this discussion. For these reasons, the results presented above are only qualitative estimations. More precise quantification of the expected performance will require complementary works and software simulations of the transmission chain.

B.4 Interleaving properties

Interleaving is a powerful tool in increasing the effectiveness of block codes such as RS by effectively spreading out long burst noises. It does so by splitting bursts of errors into smaller bursts which can be handled by the error correcting code, with respect to its correcting



power. In the concatenated scheme used by AMACS, the interleaving stage comes in be-tween the inner and outer coding. A system’s noise environment can cause errors in a given transmission. These errors could be random or burst errors. In random errors, bit errors are independent of each other and are fairly well spaced. In burst errors, the bit errors are grouped up sequentially over several bits and occur sequentially in batches. Burst errors can be caused by fading in the communica-tions channel, a typical characteristic of aeronautical channels. Interleaver schemes come in various forms, including block interleavers, diagonal interleav-ers and random interleavers. The latter is more suited to turbo codes and is therefore not considered. Block interleaver : The block interleaver is the most commonly-used interleaver in commu-nications systems. It writes row-wise from top to bottom and left to right and reads out col-umn-wise from left to right and top to bottom. Diagonal interleaver : Diagonal interleaver writes column-wise from top to bottom and left to right and reads out diagonally from left to right and top to bottom. i 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Interleaver(i) 0 6 12 1 7 13 2 8 14 3 9 10 4 5 11

Table B.2 – Equivalent index position for original matrix index in diagonal interleaver

Given the above, two specific types of interleavers applicable to AMACS are block interleav-ing and diagonal interleaving. These will require testing with the use of the coding and GMSK modulation scheme which is defined for AMACS. Further evaluation should be performed by running software simulations on a simplified transmission chain (no synchronization process, framing), with the appropriate transmission channel model, which could be the Ricean chan-nel.



ANNEX C : ESTIMATION OF A LINK BUDGET

This annex intends to present some elements regarding link budget calculation. It is a rough approximation, but provides some ideas regarding the system requirements in terms of transmitted power and receiving threshold. Section 1.4.1.1 shows that a BER of 10-3 after demodulation is enough for operations. This BER, for a MSK, corresponds to a SNR value of 10 dB. The results will be different for a pre-filtered GMSK, but we start with this approximation. We assume our system uses a 400 kHz channel bandwidth : its Noise floor is :

N = FkTB = 6 -174 dBm + 20 log(400kHz) = -112 dBm. Assuming the C/N at the receiver is close to the SNR (Eb/N0) when the spectral efficiency is close to one, the operational receiving threshold is :

S = N + C/N = -102 dBm. For an aircraft in high altitude, and large cells (~110 NM), the free space propagation model is relevant. Considering antennae with 0 dB gain :

S = EIRP - 34 -20 log (970 MHz) -20 log (110NM) This leads to :

EIRP = -102 + 32 + 60 + 46 = 36 dBm An EIRP of 4W could be enough to set up an operational transmission.



ANNEX D : MESSAGE STRUCTURES

This section shows diagrams for all of the message types, showing the fields and the num-bers of bits required for each one. The following assumptions have been made:

• All messages shall start with the ISO/IEC 13239 flag • Single-bit flags are used for ACK, RTS and CTS • ICAO 27-bit addresses are only used when necessary • Aircraft local addresses shall be 9-bit addresses

° 0 0000 0000 is not permitted ° 1 1111 1111 means ‘broadcast’ to all aircraft

• Message type is followed by the destination address – the station will analyse the source address and the message type to see if the message is expected

6 bits are used for message identifiers to allow space for future codes. '00 0000' is not used.

Message type Binary code Message type Binary code CoS1 Downlink 00 0001 Uplink 00 1000 CoS1 Keep-alive 00 0010 Block reservation 00 1001 CoS2 Downlink 00 0011 Framing message 00 1010 CoS2 RA short 00 0100 CTS 00 1011 CoS2 RA long 00 0101 ACK 00 1100 CoS2 RA RTS 00 0110 ACK/CTS ALL 00 1101 Cell exit 00 0111 Cell insertion 00 1110

Some codes are used in more than one message – for example, the "cell insertion" code (00 1110) is used for both the aircraft's cell insertion message and the ground station's slot allo-cation reply message. The priority field follows the ATN numbering scheme and can have values from 0 to 14.

Message categories Q1

Network/systems management 14

Distress communications 13

Urgent communications 12

High priority flight safety messages 11

Normal priority flight safety messages 10

Meteorological communications 9

Flight regularity communications 8

Aeronautical information service messages 7

Network/systems administration 6

Aeronautical administrative messages 5



Message categories Q1

Unassigned 4

Urgent priority administrative and UN charter communications 3

High priority administrative and state/government communications 2

Normal priority administrative 1

Low priority administrative 0

Note.— This duplicates the ATN network priority numbering scheme. Q1 = 15 is reserved for future use.

D.1 Cell insertion message: CELL_INS

A/C Tx

ISO flag 8 Binary 0111 1110 Version number 2 Binary 00 Address length flag 1 Binary 1 for 27-bit ICAO address A/C ICAO address 27 Message type 6 Binary 00 1110 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS ICAO address 27 Destination ground station Authentication (32) Size not fixed

109 bits

A/C will listen for framing message to identify the cell insertion slots. GS reply to cell insertion message will be transmitted in the next UP1.

D.2 Cell insertion reply message: GS_ALLOC

GS Tx UP1

ISO flag 8 Binary 0111 1110 Version number 2 Binary 00 Address length flag 1 Binary 1 GS ICAO address 27 Message type 6 Binary 00 1110 INS message identifier 6 1 to 64 (00 0001 to 11 1111) A/C ICAO address 27 A/C local address 9 510 possible addresses (all 0’s and all 1’s invalid)GS local address 7 126 possible addresses (all 0’s and all 1’s invalid)CoS1 slot number 9 Measured from start of frame, range 100 - 459.

102 bits

GS will transmit reply to A/C in UP1 after reception of cell insertion message. Allocated CoS1 slot will be exclusive to the aircraft until it leaves the cell.



D.3 Cell exit message: CELL_EXIT

A/C Tx or GS Tx CoS1 or UP1

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 1 ICAO address 27 Transmitting A/C or GS Message type 6 Binary 00 0111 Message identifier 6 1 to 64 (00 0001 to 11 1111) ICAO address 27 Current ground station or destination A/C Authentication (32) Size not fixed

109 bits

Transmitted by the A/C in CoS1, by the GS in UP1. The GS reply to the CELL_EXIT message will be transmitted in UP2. The A/C reply to the CELL_EXIT message will be transmitted in CoS1.

D.4 Cell exit reply message: EXIT_ACK

GS Tx or A/C Tx UP2 or CoS1

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 1 ICAO address 27 Transmitting GS or A/C Message type 6 Binary 00 0111 Exit message identifier 6 1 to 64 (00 0001 to 11 1111) ICAO address 27 Destination A/C or GS ACK flag 1

ACK slot number 9 CoS2 slot for the A/C ACK, range 350 - 499. Set to 0 for an A/C reply to a GS CELL_EXIT

87 bits

Transmitted by the GS in UP2, by the A/C in CoS1.



D.5 Block reservation message: GS_BLOCK

GS Tx UP1, UP2

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 GS local address 7 Message type 6 Binary 00 1001 Message identifier 6 1 to 64 (00 0001 to 11 1111) A/C local address 9 Binary 1 1111 1111 to indicate broadcast Block length (slots) 7 Max value is 128 slots. Length 0 is invalid. Block start 9 Measured from start of frame, range 350 - 499.

55 bits

Adapted from VDL4 block reservation protocol. Used to get ground quarantine block in CoS2 section. Cell insertion slots are shown in the framing message so don’t need reserving.

D.6 Framing message: GS_FRAME

GS Tx UP2

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 1

GS ICAO address 27 Necessary for aircraft entering the cell to register this message

Message type 6 Binary 00 1010 Message identifier 6 1 to 64 (00 0001 to 11 1111) A/C local address 9 Binary 1 1111 1111 to indicate broadcast UTC time and date 28 Used for synchronization and time stamping Active UP2 length 8 Current number of slots, 20 - 200 Active CoS2 length 8 Current number of slots, 20 - 200 Insertion slots 4 Current number of slots at CoS2 start, 5 - 16

New frame number 8 Frame which will change to new section sizes, 0 if section sizes are not changing.

New UP2 length 8 New number of slots in UP2, 20 - 200 New CoS2 length 8 New number of slots in CoS2, 20 - 200 New insertion slots 4 New number of slots at CoS2 start, 5 - 16

135 bits

The last 3 fields are ignored by A/C if the 'New frame number' field is set to 0.



D.7 GS data uplink message: GS_DATA

GS Tx

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 GS local address 7 Message type 6 Binary 00 1000 Message identifier 6 1 to 64 (00 0001 to 11 1111) A/C local address 9 ACK slot number 9 Range 350 - 499 Data length (octets) 11 Range 1 - 2,048 octets

Data x

Max 1,177 bits to fit into 1 slot (147 octets) Each additional slot provides extra 208 octets Max 16,153 bits to fit into 9 slots (2,019 octets) Max 16,384 bits to fit into 10 slots (2,048 octets)

59 + x bits

D.8 CoS1 keep-alive message: KEEP_ALIVE

A/C Tx CoS1

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0010 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7

39 bits

Transmitted by the A/C when it has no data to send.



D.9 CoS1 data downlink message: DATA_COS1

A/C Tx ACK, RTS, data

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0001 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7 ACK flag 1 Reply to UP1 uplink ACK of message type 6 Message type acknowledged (all 0's if unknown)

ACK of message ID 6 Message identifier of the message being ACKed (all 0’s if unknown)

RTS flag 1 Request for CoS2 slot(s) Priority (0 to 14) 4 Unassigned if RTS flag is 0 Reservation length 5 Number of slots required (1 - 20) Data length (octets) 8 Range 1 - 146 Data x Max 1166 bits to fit into 1 slot (145 octets)

70 + x bits

Reservation length field is set to 0 if the RTS flag is 0.

D.10 ACK/CTS message to all aircraft: CTS_ACK_ALL

GS Tx UP2

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 GS local address 7 Message type 6 Binary 00 1101 A/C local address 9 CTS flag 1 Binary 1 indicates CTS RTS message identifier 6 1 to 64 (00 0001 to 11 1111) Priority (0 to 14) 4 Unassigned if RTS flag is 0 Reserved slot number 9 Binary 0 0000 0000 if no CTS, 350 - 499 Reservation length 5 Max value 20 slots, 0 if no CTS ACK flag 1 Binary 0 for NACK, binary 1 for ACK ACK of message type 6 Message type acknowledged (all 0’s if unknown)ACK of message ID 6 ID of message being ACKed (all 0’s if unknown)

24+47n bits

The highlighted box is repeated according to the number of aircraft (n).



D.11 GS ACK uplink message: GS_ACK

GS Tx CoS2, UP1

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 GS local address 7 Message type 6 Binary 00 1100 A/C local address 9 ACK flag 1 Reply to CoS2 downlink ACK of message type 6 Message type acknowledged (all 0’s if unknown) ACK of message ID 6 ID of message being ACKed (all 0’s if unknown)

46 bits

NACK to a transmitted message will trigger a re-send by the A/C. A/C will ignore ACK/NACK for an unrecognized message (type or ID).

D.12 CoS2 downlink message: DATA_COS2

A/C Tx ACK, data

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0011 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7 ACK flag 1 Reply to UP2 uplink ACK of message type 6 Message type acknowledged (all 0’s if unknown)ACK of message ID 6 ID of message being ACKed (all 0’s if unknown) Data length (octets) 11 Range 1 - 2,048

Data x


63 + x bits



D.13 A/C ACK downlink message: AC_ACK

A/C Tx CoS2

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 1100 GS local address 7 ACK flag 1 Reply to UP2/CoS2 uplink ACK of message type 6 Message type acknowledged (all 0’s if unknown)ACK of message ID 6 ID of message being ACKed (all 0’s if unknown)

43 bits

NACK to a transmitted message will trigger a re-send by the GS. The GS will ignore ACK/NACK for an unrecognized message.

D.14 CoS2 random access short data message: DATA_RA_COS2_SHORT

A/C Tx Data

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0100 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7 ACK slot number 9 CoS2 slot for the GS ACK, range 350 - 499 Data x Max 1,188 bits to fit into 1 slot (148 octets)

48 + x bits



D.15 CoS2 random access RTS message: RTS_COS2

A/C Tx RTS

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0110 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7 RTS flag 1 Binary 1 indicates RTS. Priority (0 to 14) 4 Unassigned if RTS flag is 0. Reservation length 5 Number of slots required (1 - 24)

49 bits

Reservation length field is set to 0 if the RTS flag is 0 If the RTS flag set to 0, the ground station will ignore the message.

D.16 CoS2 random access CTS message: CTS_COS2

GS Tx CTS uplink

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 GS local address 7 Message type 6 Binary 00 1011 Message identifier 6 1 to 64 (00 0001 to 11 1111) A/C local address 9 CTS flag 1 Binary 1 indicates CTS Priority (0 to 14) 4 Unassigned if CTS flag is 0 Reserved slot number 9 Binary 0 0000 0000 if no CTS, range 350 – 499 Reservation length 5 Max value 24 slots, 0 if no CTS

58 bits

CTS flag set to 0 will trigger A/C to re-send its RTS, or transmit in the next frame.



D.17 CoS2 random access long data message: DATA_RA_COS2_LONG

A/C Tx Data

ISO flag 8 Version number 2 Binary 00 Address length flag 1 Binary 0 A/C local address 9 Message type 6 Binary 00 0101 Message identifier 6 1 to 64 (00 0001 to 11 1111) GS local address 7 Data length (octets) 11 Range 1 - 2,048. Length 0 is invalid.

Data x


50 + x bits



ANNEX E : SYSTEM OPERATIONS

The following text indicates the possible sequences of events.

DOWNLINK Each A/C has an allocated CoS1 slot.

E.1 A/C cell insertion

• The aircraft uses its knowledge of the area to identify the frequency of the

ground station whose cell it is about to enter

• The aircraft listens on this frequency for a minimum of 2 seconds to hear the framing message (GS_FRAME) transmitted by the ground station

° This framing message contains the ICAO address of the ground sta-tion, the current sizes of sections within the frame and the UTC time and date

• When the aircraft has heard the GS_FRAME message, it transmits a

CELL_INS message in the dedicated cell insertion slots ° The message contains the ICAO address of the aircraft

• If the GS correctly receives the CELL_INS message, then it transmits a

GS_ALLOC message to the aircraft at the start of the next frame ° This contains the new local 9-bit address for the aircraft, the local 7-bit

address of the ground station and the location of the exclusive high-QoS CoS1 slot for the aircraft

° If the aircraft does not receive the GS_ALLOC message correctly then

it will re-transmit the CELL_INS message in the dedicated cell insertion slots

• If the GS does not receive the CELL_INS message correctly, then it will do

nothing and will wait for the aircraft to re-transmit

E.2 A/C has data to send

Data size is ≤145 octets

• If the data size is ≤145 octets the A/C transmits a DATA_COS1 message with RTS flag set to 0

• If the CoS1 transmission is correctly received by the GS –

° GS sets ACK flag to 1 in combined CTS_ACK_ALL message in UP2 and indicates which message is being acknowledged*



• If the A/C does not receive the ACK from the GS – ° It will re-transmit the data in CoS2 in the DATA_RA_COS2_SHORT

message, using random access to find a slot ° This message will have the repeat flag set to 1 and the repeat number

field set to 001

• If the CoS1 transmission is not correctly received by the GS – ° GS sets ACK flag to 0 in combined CTS_ACK_ALL message in UP2

and indicates which message is being NACKed*

° A/C then re-transmits the data in CoS2 using random access and the DATA_RA_COS2_SHORT message, with the repeat flag set to 1 and the repeat number field set to 001, also allocating a different CoS2 slot for the GS ACK

– If the DATA_RA_COS2_SHORT message is correctly received by the GS it will send a GS_ACK message in the allocated CoS2 slot, with the ACK flag set to 1, indicating which message is being acknowledged*

– If the DATA_RA_COS2_SHORT message is not correctly

received by the GS it will send a GS_ACK message in the allocated CoS2 slot, with the ACK flag set to 0, indicating which message is being NACKed*

° If the A/C does not receive an ACK for the DATA_RA_COS2_SHORT

message, or receives a GS_ACK message with the ACK flag* set to 0, then it will try to re-send the data in the next frame, with the repeat flag set to 1 and incrementing the repeat number field as appropriate

Data size is >145 octets

• If the data size is >145 octets, the A/C transmits a DATA_COS1 message with RTS flag set to 1

° This requests reservation of a specified number of slots in CoS2

• If the DATA_COS1 transmission is correctly received by the GS, the GS responds in the combined CTS_ACK_ALL message in UP2, setting the ACK flag to 1 (indicating which message is being acknowledged*), the CTS flag to 1 and indicating the allocated slots

° If the CTS in the CTS_ACK_ALL message is correctly received by the A/C then it will then transmit a DATA_COS2 message in the reserved slots

° If the CTS in the CTS_ACK_ALL message is not correctly received by

the A/C then it will then transmit an RTS in CoS2 using random access and the RTS_COS2 message, requesting a specified number of slots in CoS2 (see 3 below). (In the RTS_COS2 message the repeat flag will be set to 0)



• If the A/C does not receive an ACK from the GS in the CTS_ACK_ALL message –

° It will re-transmit the RTS in CoS2 using random access and the RTS_COS2 message. This requests reservation of a specified number of slots in CoS2 (see 3 below). (In the RTS_COS2 message the repeat flag will be set to 0)

• If the DATA_COS1 transmission is not correctly received by the GS –

° GS sets ACK flag to 0 in combined CTS_ACK_ALL message in UP2 and indicates which message is being NACKed*

° A/C then re-transmits the RTS in CoS2 using random access and the

RTS_COS2 message (see 3 below) (In the RTS_COS2 message the repeat flag will be set to 0)

E.3 A/C has no data to send

• If the A/C has no data to send and wishes to maintain the link with the GS, it

transmits a KEEP_ALIVE message

• If the CoS1 transmission is correctly received by the GS – ° GS sets ACK flag to 1 in combined CTS_ACK_ALL message in UP2

and indicates which message is being acknowledged*

• If the A/C does not receive the ACK from the GS – ° It will send a KEEP_ALIVE message in its allocated CoS1 slot in the

next frame

• If the CoS1 transmission is not correctly received by the GS – ° GS sets ACK flag to 0 in combined CTS_ACK_ALL message in UP2

and indicates which message is being NACKed*

° A/C then sends a KEEP_ALIVE message in its allocated CoS1 slot in the next frame

E.4 CoS2 random access

Data size is ≤148 octets

• If the data size is ≤148 octets, the A/C examines its reservation table to find the first free slot in CoS2

• It then transmits a DATA_RA_COS2_SHORT message –

° This has the repeat flag set to 0 unless this is a retransmission of CoS1 data

° It also contains the number of a free CoS2 for the GS to send an ac-knowledgement



• If the DATA_RA_COS2_SHORT message is correctly received by the GS –

° The GS will send a GS_ACK message in the allocated slot, indicating the message which is being acknowledged*, with the ACK flag set to 1 and the repeat flag set to 0

• If the DATA_RA_COS2_SHORT message isn't correctly received by the GS –

° The GS will send a GS_ACK message in the allocated slot, indicating the message which is being acknowledged*, with the ACK flag set to 0 and the repeat flag set to 0

• If the A/C does not receive a GS_ACK from the GS, or receives a GS_ACK

with the ACK flag* set to 0 – ° The A/C will re-examine its reservation table to see if there are any

more free slots in CoS2 in this frame. ° If a free slot is found (together with another free slot for a GS reply)

– It will re-transmit the data in CoS2 in the DATA_RA_COS2_SHORT message

– This message will have the repeat flag set to 1 and the repeat number field set to 010

– It also has the number of a free CoS2 for the GS to send an acknowledgement

° If a free slot is not found, then the A/C will re-send the data in the next

frame, with the repeat flag set to 1 and the repeat number incremented as appropriate

Data size is >148 octets

• If the data size is >148 octets, the A/C examines its reservation table to find the first free slot in CoS2

• It then transmits an RTS_COS2 message –

° This requests reservation of a specified number of slots in CoS2 ° The repeat flag will be set to 0

• If the RTS_COS2 transmission is correctly received by the GS –

° GS examines its reservation table to find the free slots in CoS2 ° It will then send a CTS_COS2 in the next free slot

– If the GS found a block with the requested number of free slots, the CTS_COS2 message will have the CTS flag set to 1 and will indicate the start of the reserved block

– If the GS did not find a block with the requested number of free

slots, the CTS_COS2 message will have the CTS flag set to 0

° If the CTS_COS2 is correctly received by the A/C and the CTS flag is set to 1, then the A/C will transmit the data in the allocated slots in a DATA_RA_COS2_LONG message

– This contains a slot number for the GS to send an ACK



• If the DATA_RA_COS2_LONG message is received

correctly by the GS, it will send a GS_ACK message in the allocated slot with the ACK flag set to 1, indicating which message is being acknowledged*

• If the DATA_RA_COS2_LONG message is not received

correctly by the GS, it will send a GS_ACK message in the allocated slot with the ACK flag set to 0, indicating which message is being NACKed*

– If the A/C does not receive an ACK for the

DATA_RA_COS2_LONG message, or receives a GS_ACK message with the ACK flag* set to 0, then it will try to re-send the data in the next frame, with the repeat flag set to 1 and the repeat number incremented as appropriate

° If the CTS_COS2 is correctly received by the A/C and the CTS flag is

set to 0, or the CTS_COS2 is not correctly received by the A/C – – the A/C re-examines its reservation table to find the next free

slot in CoS2 – if a free CoS2 slot is found:

• the A/C will re-transmit the RTS_COS2 message, with the repeat flag set to 1 and the repeat number field set to 001;

• see above for the steps which shall follow.

– if a free CoS2 slot is not found: • the A/C will try to re-send the data in the next frame,

with the repeat flag set to 1 and the repeat number in-cremented as appropriate

• If the RTS_COS2 transmission is not correctly received by the GS –

° The GS will send a GS_ACK message in the next free slot, with the ACK flag set to 0, indicating which message is being NACKed*

° A/C will then try to re-send the data in the next frame, with the repeat

flag set to 1 and the repeat number incremented as appropriate

E.5 A/C-initiated cell exit

• The A/C is about to leave the cell of the GS –

° It transmits a CELL_EXIT message in its dedicated CoS1 slot

• If the GS correctly receives the CELL_EXIT message – ° It transmits an EXIT_ACK message in UP2, with the ACK flag set to 1,

indicating an ACK slot in CoS2 for the A/C to confirm

° If the A/C correctly receives the EXIT_ACK message –



– the A/C will transmit an AC_ACK in the allocated CoS2 slot, with the ACK flag* set to 1, the ACK of message ID field set to CELL_EXIT and the repeat number field set to 000

° If the A/C does not (correctly) receive the EXIT_ACK message and

can still contact the GS – – the A/C will transmit an AC_ACK in the allocated CoS2 slot,

with the ACK flag* set to 0, the ACK of message ID field set to CELL_EXIT and the repeat number field set to 000

– the A/C will then attempt to re-transmit the CELL_EXIT mes-

sage in the next frame.

° If the A/C correctly receives the EXIT_ACK message and is out-of-range of the GS –

– the A/C will do nothing, and its dedicated CoS1 slot at the old GS will time-out.

° If the GS receives an AC_ACK message from the A/C with the ACK

flag* set to 1 – – the A/C's dedicated CoS1 slot will be de-allocated and the

hand-off will be complete.

° If the GS does not receive an AC_ACK, or receives an AC_ACK with the ACK flag* set to 0 –

– the GS will do nothing. If the A/C is still in range then the CELL_EXIT will be re-transmitted in the next frame, otherwise the link will time-out.

• If the GS does not correctly receive the CELL_EXIT message –

° the GS will do nothing. If the A/C is still in range then the CELL_EXIT will be re-transmitted in the next frame, otherwise the link will time-out.

E.6 GS request for A/C cell exit

• The A/C is about to leave the cell of the GS and the GS wishes to trigger a

hand-off procedure – ° The GS transmits a CELL_EXIT message in a UP1 slot

• If the A/C correctly receives the CELL_EXIT message, then –

° The A/C will determine the frequency of the new GS to contact, in the next cell

° It will attempt to hear the new GS on this frequency

° If the A/C receives a framing message from the new GS –

– It will commence the cell insertion procedure (as in E.5)

– If this is successful –



• The A/C will transmit an EXIT_ACK message in its dedicated CoS1 slot, with the ACK flag set to 1 and the ACK slot number field set to 0

• If the GS receives the EXIT_ACK message correctly, it

will reply with a GS_ACK message with the ACK flag set to 1 and the message ID field set to EXIT_ACK. It will then de-allocate the A/C's CoS1 slot and will con-sider the link to be terminated.

– If the cell insertion procedure with the new GS is not successful

then – • The A/C will transmit an EXIT_ACK message in its

dedicated CoS1 slot, with the ACK flag set to 0 and the ACK slot number field set to 0

– If the GS receives an EXIT_ACK message with the ACK flag*

set to 0, or does not (correctly) receive an EXIT_ACK at all – • it will re-transmit the CELL_EXIT message in the next

frame, unless the A/C transmits a CELL_EXIT mes-sage beforehand

° If the A/C cannot successfully communicate with the new GS on the

new frequency – – it will transmit an EXIT_ACK message in its dedicated CoS1

slot, with the ACK flag set to 0 and the ACK slot number field set to 0

° If the GS receives an EXIT_ACK message with the ACK flag* set to 0,

or does not (correctly) receive an EXIT_ACK at all – – it will re-transmit the CELL_EXIT message in the next frame,

unless the A/C transmits a CELL_EXIT message beforehand

• If the A/C does not correctly receive the CELL_EXIT message then – ° The A/C will transmit an EXIT_ACK message in its dedicated CoS1

slot, with the ACK flag set to 0 and the ACK slot number field set to 0

• If the GS receives an EXIT_ACK message with the ACK flag* set to 0, or does not (correctly) receive an EXIT_ACK at all –

° it will re-transmit the CELL_EXIT message in the next frame, unless the A/C transmits a CELL_EXIT message beforehand

NOTE the re-transmission of CELL_EXIT messages by the GS will only continue while the communications link is still open. If the link times-out then the GS will consider the A/C to have left the cell and will de-allocate the A/C's CoS1 slot.



UPLINK Each GS has dedicated uplink sections in the frame.

E.7 GS has data to send

Data size is ≤2,048 octets, if transmitted in UP1

• If the data size is ≤2,048 octets the GS transmits a GS_DATA message to the A/C with the ACK slot number field set to 0 0000 0000, the repeat flag set to 0 and the repeat number field set to 000

• If the UP1 transmission is correctly received by the A/C –

° A/C sets ACK flag to 1 in the DATA_COS1 message and indicates which message is being acknowledged*

• If the UP1 transmission is not correctly received by the A/C –

° A/C sets ACK flag to 0 in the DATA_COS1 message and indicates which message is being NACKed*

• If the GS does not correctly receive an acknowledgement for the GS_DATA in

the DATA_COS1 message, or receives a DATA_COS1 message with the ACK flag* set to 0 –

° the GS will re-transmit the GS_DATA message in UP2, with the repeat flag set to 1 and the repeat number set to 001

Data size is ≤2,048 octets, if transmitted in UP2

• If the data size is ≤2,048 octets the GS transmits a GS_DATA message to the A/C with the repeat flag set to 0 and the repeat number field set to 000, indi-cating the slot in CoS2 for the A/C to send an ACK

• If the UP2 transmission is correctly received by the A/C –

° A/C sends an AC_ACK message in the allocated slot in CoS2, with the ACK flag set to 1, indicating which message is being acknowledged*

• If the UP2 transmission is not correctly received by the A/C –

° A/C sends an AC_ACK message in the allocated slot in CoS2, with the ACK flag set to 0, indicating which message is being NACKed*

• If the GS does not receive an AC_ACK for the GS_DATA message, or re-

ceives an AC_ACK message with the ACK flag* set to 0 – ° if there is sufficient space in the remaining part of UP2 in the current

frame – – the GS will re-send the GS_DATA message, with the repeat

flag set to 1, the repeat number set to 001 and a new ACK slot allocation in UP2 for the A/C



° otherwise the GS will re-transmit the GS_DATA message in the UP1 section of the next frame, with the repeat flag set to 1 and the repeat number field incremented as appropriate

Data size is >2,048 octets

• If the data size is >2,048 octets, the GS will split the data into several GS_DATA messages (each one of which will contain up to 2,048 octets of data)

• These GS_DATA messages will then be transmitted separately

• The method for this is implementation-dependent and is outside the scope of

this document

E.8 GS framing message

Each GS has dedicated slot for the framing message.

• The GS wants to indicate the sizes of the sections within the frame – ° the GS_FRAME message is transmitted at the beginning of UP2 –

– the UCT time and date field is set to the current time and date (of transmission)

– the current sizes of the UP2, CoS2 and insertion sections are indicated

– the new frame number field is set to 0,

• The A/C receives a framing message from the GS with the new frame number field set to 0 –

° the A/C ensures that its on-board clock is synchronized with the GS and that it is using the same section sizes as the GS

° the A/C notes that the new frame number field is set to 0 and ignores

the rest of the message

E.9 GS changes the section sizes in the frame

Each GS has dedicated slot for the framing message.

• The GS wants to alter the sizes of the sections within the frame – ° the GS_FRAME message is transmitted at the beginning of UP2 –

– the UCT time and date field is set to the current time and date – the current sizes of the UP2, CoS2 and insertion sections are

indicated – the new frame number field is set to the number of the future

slot that will contain the new section sizes (at least 2) – the new section sizes are indicated



° this message will be re-transmitted in the next frame with the new frame number field decremented

• The A/C receives a framing message from the GS with the new frame number

field greater than 1 – ° the A/C ensures that its on-board clock is synchronized with the GS

• The A/C receives a framing message from the GS with the new frame number

field set to 1 – ° the A/C ensures that its on-board clock is synchronized with the GS

° the GS records the new frame sizes and ensures that it is using them

at the start of the next frame

NOTE once the GS has started to transmit GS_FRAME messages with a non-zero new frame number, the values of the current and new section size fields must not be altered before the new frame number reaches 0

*Note on acknowledgement messages An acknowledgement flag is followed by a field indicating the message type. If a station (GS or A/C) is sending an ACK/NACK message then the message type field should be set to the message type which is being acknowledged. If this is not known, for any reason, then the message type field must be set to "0 0000 0000". If a station (GS or A/C) is sending an ACK/NACK message then the message identifier field should be set to the identifier of the message which is being acknowledged. If this is not known, for any reason, then the message identifier field must be set to "00 0000". If an ACK or NACK is received by a station (GS or A/C) and the message type field does not match the message which was just sent, then it will be ignored. The station will then wait for another ACK/NACK message. If one is not received, this will be treated as a NACK. If an ACK or NACK is received by a station (GS or A/C) and the message type field is correct but the message identifier field does not match the identifier number of the message which was just sent, then it will be ignored. The station will then wait for another ACK/NACK mes-sage. If one is not received, this will be treated as a NACK. If an ACK is received (by GS or A/C) with the message type field set to "0 0000 0000", then it will be treated as a NACK for the message which was just sent. This shall be regardless of the value of the message identifier field.



References: Etude LDL (STNA3, 1997):

[1] D1.1 – Tranche 1: Eléments de base du système ETDMA – Lot 1: Protocole d’accès au système.

[2] D1.2 – Tranche 1: Eléments de base du système ETDMA – Lot 2: Protocole de ges-tion des liaisons logiques air-sol.

[3] D1.3 – Tranche 1: Eléments de base du système ETDMA – Lot 3: Modèle cellulaire. [4] D2.1 – Tranche 2: Architecture Réseau du système ETDMA – Lot 1: Gestion du

transfert du mobile. [5] D2.2 – Tranche 2: Architecture Réseau du système ETDMA – Lot 2: Service de cir-

cuit virtuel commuté. [6] D2.3 – Tranche 2: Architecture Réseau du système ETDMA – Lot 3: Interface ATN.

Etude ASTOR

[7] MACONDO project (Eurocontrol, 2002):

[8] D1 – Operating Concept of the Mobile Aviation Communication Infrastructure Supporting ATM beyond 2015 – ATM Context (Ed. 1, April 2002).

[9] D2 – Operating Concept of the Mobile Aviation Communication Infrastructure Supporting ATM beyond 2015 – Operating Concept for the Future Mobile Communication Infrastructure (Ed. 1, July 2002).

[10] D3 – Operating Concept of the Mobile Aviation Communication Infrastructure Supporting ATM beyond 2015 – Synthesis Report (Ed.1, August 2002).

MICADO project (STNA, 2001-2004):

[11] 8CA-MOE-NI-FP-01-006 - Identification et Quantification des Flux ATN Induits par les Applications Air/Sol (V0R3, May 2001).

ICAO Annex 10 – ATN SARPs (Standards and Recommended Practices) – Edition 3

[12] Sub-Volume II — Air-Ground Applications. [13] Sub-Volume IV — Upper Layer Communications Service (ULCS). [14] Sub-Volume V — Internet Communications Service (ICS).

[15] ICAO Doc 9816 (AN/448), Manual on VHF Digital Link (VDL) Mode 4, 1st ed., 2004.

EUROCAE/RTCA:

[16] ED-108A — MOPS for the Very High Frequency (VHF) Digital Link (VDL) Mode 4 Aircraft Transceiver

[17] ED-110A — Interoperability Requirements Standard for ATN Baseline 1. RTCA:

[18] DO-282A — Minimum Operational Performance Standards for Universal Access Transceiver (UAT) Automatic Dependent Surveillance – Broadcast

ETSI:

[19] EN 301 842 — Electromagnetic compatibility and Radio spectrum Matters (ERM);VHF air-ground Digital Link (VDL) Mode 4 radio equipment;



Technical characteristics and methods of measurement for ground-based equipment.

[20] EN 302 842 — Electromagnetic compatibility and Radio spectrum Matters (ERM);VHF air-ground and air-air Digital Link (VDL) Mode 4 radio equipment; Technical characteristics and methods of measurement for aeronautical mobile (airborne) equipment.

Institut National de Recherche en Informatique et en Automatique (INRIA) Research Reports

[21] End-to-end delay constrained protocol over the EDS service differentiation, N° 5030, December 2003.

[22] Flexible Management and Dynamic Control of Network Quality of Service in Grid environments: the QoSinus approach, N° 5083, January 2004.

[23] TCP adaptation to EDS, N° 5121, February 2004. ATNP Working Papers:

[24] Proposed Guidance Material for the Frame Mode SNDCF. Prepared and presented by Tony Whyman, ICAO ATNP/WG2/WP/598; Berlin – August 2000.

Eurocontrol:

[25] "Communications Operating Concept and Requirements (COCR) for the Future Radio System", version 2.0, EUROCONTROL/FAA Future Communications Study Operational Concepts and Requirements Team, May 2007.

[26] "Study on the options for time synchronisation in the VDL Mode 4 datalink system", version 1.2, Eurocontrol, January 2002.

NASA:

[27] NASA Technology Assessment for the Future Aeronautical Communication System, ITT Phase 1 report, May 2005.

[28] NASA support for the Future Communications Study, Phase II End of Task Briefing – Technology Evaluation Results, NASA/ITT, 21 June 2006.

Additional references:

[29] « Communication Numériques », A. Glavieux et M. Joindot, first edition, MASSON 1996.

[30] “Digital communications”, J. G. Proakis and Massoud Salehi, fifth edition, McGraw-

Hill, 2007. ISBN 0-0729-5716-6.

[31] "Feasibility of an Enhanced Service Orientated Aviation Communication System", Helios Technology Ltd, 2007.

[32] "VDL Mode 4 Timing Study", Helios Technology Ltd, 1999.

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