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WiMAX Capac i t y Whi te Paper


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Not i ce

The information in this manual is subject to change without notice. All statements,

information and recommendations in this manual are believed to be accurate, but are

presented without warranty of any kind, expressed or implied. Users must take full

responsibility for their use of any products.

No part of this publication may be reproduced or transmitted in any form or by any

means, electronic or mechanical, including photocopying and recording, or by any

information storage or retrieval system, without prior written consent from SR

Telecom Inc.

SR TELECOM, SYMMETRY, SYMMETRYONEand SYMMETRYMXare trademarks

of SR Telecom Inc. All rights reserved 2006. All other trademarks are property of

their owners. Information subject to change without notice.

2006, SR Telecom Inc.

All rights reserved. August 2006

Printed in Canada

WHITE PAPER 033-100743-001,ISSUE 1 2


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Table o f Content s

1 ABSTRACT................................................................................................... 62 PROTOCOL MODEL.................................................................................... 8

2.1 DATA/CONTROL PLANE..............................................................................92.2 MANAGEMENT PLANE ................................................................................9

3 PHYSICAL LAYER..................................................................................... 103.1 CHANNEL SIZE........................................................................................103.2 OFDM...................................................................................................113.3 FRAME AND SYMBOL SIZE........................................................................143.4 PREAMBLES............................................................................................14

3.4.1 Synchronization.........................................................................143.4.2 Ranging .....................................................................................153.4.3 Midambles .................................................................................16

3.5 SUB-CHANNELS.......................................................................................163.6 MINIMUM ALLOCATION UNIT .....................................................................173.6.1 Downlink....................................................................................17

3.6.2 Uplink ........................................................................................183.7 ADAPTIVE MODULATION/CODING AND POWER CONTROL .............................193.8 DIVERSITY..............................................................................................20

3.8.1 Space-Time Coding (STC) ........................................................203.8.2 MIMO.........................................................................................20

3.9 ADAPTIVE ANTENNA SYSTEM (AAS) .........................................................214 MEDIA ACCESS CONTROL LAYER ......................................................... 22

4.1 MAC HEADER ........................................................................................224.1.1 Sub headers ..............................................................................244.2 CHANNEL USAGE MAPS...........................................................................264.2.1 Frame Control Header...............................................................264.2.2 Downlink Map............................................................................264.2.3 Uplink Map.................................................................................284.2.4 Downlink and Uplink Channel Descriptors.................................28

4.3 BANDWIDTH REQUESTS ...........................................................................294.3.1 Contention .................................................................................294.3.2 Polling........................................................................................314.3.3 Piggyback..................................................................................32

5 CHANNEL BANDWIDTH CALCULATION................................................. 336 QOS ............................................................................................................ 36

6.1 CONSTANT BIT RATE SERVICES................................................................366.2 VARIABLE BIT RATE SERVICES .................................................................376.3 BEST EFFORT SERVICES..........................................................................386.4 SHARING NON-GUARANTEED BANDWIDTH .................................................386.5 OVER-SUBSCRIPTION..............................................................................39

7 CAPACITY SCENARIOS............................................................................ 42



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7.1 BROADBAND DATA-ONLY.........................................................................427.2 MIXED VOICE AND BROADBAND ................................................................43

8 SUMMARY.................................................................................................. 45



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List of F igures

FIGURE 1 - IEEE 802.16 PROTOCOL REFERENCE MODEL .......................................... 8FIGURE 2 - MAC PDU FORMATS ........................................................................... 22FIGURE 3 - GENERIC MAC HEADER FORMAT ........................................................... 23

FIGURE 4 - DOWNLINK BANDWIDTH CALCULATION SEQUENCE.................................... 33FIGURE 5 - UPLINK BANDWIDTH CALCULATION SEQUENCE ........................................ 34FIGURE 6 - WIMAX CHANNEL BANDWIDTH SPREADSHEET MODEL............................. 35FIGURE 7 AGGREGATE CHANNEL BANDWIDTH PARTITIONING .................................. 39FIGURE 8 - ALLOCATING NETWORK BANDWIDTH ....................................................... 41

1



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Abs t rac tThis white paper discusses factors influencing system capacity in IEEE 802.16

networks. IEEE 802.16, commonly known by its industry forum moniker of WiMAX, is

a wireless protocol intended for establishing metropolitan area networks that supply

broadband data and voice services.

The present standards document, IEEE 802.16-2004 as amended by IEEE 802.16-2005, is more accurately regarded as a family of distinct physical layer standards

sharing a common media access layer. This family of physical layers includes

separate standards for single-carrier, and two distinct orthogonal multi-carrier

physical layers. Historically the OFDM physical layer was the primary focus of the

initial effort to develop a protocol suitable for non-line-of-sight (NLOS) stationary

radio station operation. Succeeding development of the OFDMA physical layer

focused on adding features to support low-speed mobile operation.

In this white paper, we will confine the capacity discussion to the OFDM physical

layer in order to avoid a lengthy discussion of the additional complications that

mobility introduces; including handover and dynamic cell occupancy. Capacity formobile applications will be an important topic for a later white paper.

The objective of this white paper is to better understand WiMAX system capacity.

But first, what do we mean by the capacity of a WiMAX system? Simply put, the

system capacity refers to the number of connections that the wireless channel can

support without unduly degrading the data services carried on the channel.1. We

focus on the wireless channel rather than other system resources because normally

the airlink is the most expensive and therefore the controlling system element related

to capacity in wireless access networks.

Operators care deeply about the system capacity because of the nature of wirelessaccess network deployments. WiMAX access networks are often deployed in point-

to-multipoint cellular fashion where a single base station provides wireless coverage

to a collection of subscriber stations within the coverage area. The base station in

turn is linked to external wide area networks via wired, fiber, or wireless point-to-

point backhaul infrastructure. Normally, the radio spectrum that is available to a

deployment is a scarce and often expensive resource. During the planning phase of

a deployment, once an operator has determined the radio spectrum channel size for

each base station, the next question becomes: how many data connections can the

channel support? The question is doubly important since it is often prohibitively

expensive to later overlay additional wireless capacity into the same coverage area.

Further, it is central to understanding how may base stations are required for adeployment region. And without a firm understanding of the system capacity an

operator has no way of estimating the recovery time for the up-front costs of

deploying the access network. Understanding the system capacity is therefore key to

deploying a commercially successful access network.

1As we will see, each WiMAX connection carries data traffic characterized by a set of QoS parameters, so that a set of

connections has an associated aggregate total bandwidth. The capacity of the system can therefore equivalently be thought of

in terms of the aggregate total bandwidth required to support a set of connections.



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In wired networks, such as legacy voice, the capacity of a channel (trunk) is

intuitively obvious because each active voice conversation requires fixed dedicated

bandwidth (e.g. t = 64 kbits/s plus a small signaling overhead) and the capacity is

simply the number of conversations that the channel (trunk) bandwidth, T, can

support: capacity = (T / t).

In a WiMAX wireless channel, the situation is considerably more complex as we shall

see. To begin with, the channel is not necessarily of fixed size but can vary with time

as environmental conditions change. This is particularly relevant in NLOS channels.

Also, a WiMAX channel can be configured in a number of different ways depending

on operator preferences, regulatory constraints, and performance requirements.

Many of these configuration choices affect the channel capacity, often in non-

obvious ways. Finally, the nature of mixed application broadband data services

defies easy classification of an average connection. Accurate capacity analysis

therefore presupposes detailed specification of the number and type of the data

services sharing the channel. As we will see, the manner in which the traffic is

mapped onto the WiMAX channels QoS model also affects the capacity. The

capacity of a WiMAX system therefore depends on environmental conditions,

configuration, and the nature of the data traffic that is transported by the system.

The structure of this white paper begins with a discussion of the 802.16 protocol

model. Next we examine the system capacity at the physical layer including a

discussion of the overhead needed to support the wireless channel. Moving up the

protocol stack we next look at the capacity at the media access layer and examine

the overhead introduced there. We next examine the WiMAX QoS model and

discuss how the mapping of data services influences the system capacity. Bringing

this information together, we next illustrate the resultant system capacity that arises

in several hypothetical deployment scenarios. The white paper concludes with a briefsummary of the main points.



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2 Protoc o l ModelThe IEEE 802.16 protocol is a member of the IEEE 802 family of standards and

addresses the media access and physical layers. The protocol reference model is

shown in Figure 1.

Figure 1 - IEEE 802.16 Protocol Reference Model

A standard protocol-layering model is used. MAC peers communicate by

sending/receiving data to/from the PHY layer. The PHY layers communicate via the

802.16 airlink.

In the point-to-multipoint mode of operation, a base station transmits on the downlink

channel to a collection subscriber stations by broadcasting the data to the stations

that then select data that is addressed to them. Each subscriber station

communicates with a single base station and a collection of subscriber stations

share an uplink channel for transmitting to the base via a multiple access scheme

that is controlled by the base.



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2.1 Data/Control Plane

User data flows into an 802.16 node at the MAC CS SAP and is passed to the

underlying PHY layer at the PHY SAP. The PHY layer then transports the data over

the 802.16 airlink. When we speak of the capacity of an 802.16 system we are

referring to capability to pass the user data that flows vertically through the

Data/Control Plane shown in the figure.

2.2 Management Plane

Although not part of the standard, management traffic can also enter the system via

the Management Plane. Some of this traffic may be destined for remote stations and

is transported via the PHY layer via the airlink. Strictly speaking, capacity analysis

should account for management traffic as part of the overall system overhead.

However, because management traffic is ordinarily a negligible portion of the overall

traffic load, it will mostly be ignored in this white paper.



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3 Physical LayerWiMAX protocol adds overhead to user traffic starting with the physical layer, which

transmits MAC PDUs to physical layer peers over the airlink. This section presents

the overhead incurred at the PHY layer including mandatory and optional elements.

Variable factors including adaptive modulation and code rate are discussed.

The physical (PHY) layer takes MAC PDUs input at the PHY SAP and arranges

them for transport over the airlink. The WiMAX protocol adds overhead to user traffic

starting with the physical layer, which transmits to peer physical layers over the

airlink. This section presents the overhead incurred at the PHY layer including

mandatory and optional elements. Variable factors including adaptive modulation

and code rate are discussed.

3.1 Channel Size

A basic understanding of a WiMAX systems capacity begins with knowing howmuch radio spectrum is available. The available radio spectrum ultimately constrains

the size of the channels in frequency bandwidth. The channel size in turn fixes the

raw capacity of the channel double the channel size and the capacity doubles

(albeit range in general will decrease).

The 802.16 MAC/PHY standard attempts to avoid constraining the carrier frequency

(below 11 GHz) for OFDM/OFDMA radios and places very general limits on the

channel size (from 1.25 to 20 MHz). There are currently no worldwide spectrum

allocations for WiMAX systems.

The obvious issue is interoperability between hardware implementations, which theWiMAX Forum has addressed by developing equipment profiles that specify

licensed/unlicensed carrier frequency and channel sizes, as well as long lists of

interrelated mandatory and optional features from the 802.16 standard. WiMAX

certification of compliant implementations is based on these profiles in order to

insure basic interoperability between vendors.

Many large operators have strong motivation, either competitive or regulatory, to

operate in licensed spectrum bands. Of the available bands in the global patchwork

of regulated spectrum the licensed 3.5 GHz ITU FDD blocks are the most widely

available. For this reason, the majority of initial WiMAX equipment has been certified

for use in the 3.5 GHz band. The available profile channel sizes are integer multiplesof 1.75 MHz (1.75, 3.5, 7 MHz). The channel size is driven by the size of an

operators allocation from their countrys regulator. For example, assuming cell reuse

of one between four-sector base stations, an operator needs 14+14 MHz for 3.5

MHz channels and 7+7 MHz for 1.75 MHz channels. Oftentimes clustered base

station deployments will require even more spectrum to increase the reuse factor

and mitigate interference between coverage cells.



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3.2 OFDM

In Orthogonal Frequency Division Multiplexing, the occupied spectrum is broken up

into many discrete narrowband channels known as sub carriers (alternatively

known as tones). Each data carrier is modulated over a symbol time that is

inversely related to the carrier frequency spacing so that sub carriers have minimal

mutual interference between them. In this sense the sub carriers are orthogonal or

independent from one another. The capacity of each sub carrier depends on themodulation order, which can be BPSK (1 bit per sub carrier), QPSK (2 bits per sub

carrier), 16QAM (4 bits per sub carrier), or 64QAM (6 bits per sub carrier) in the case

of the OFDM PHY. In general more power is required for using higher order

modulation in order to achieve the same range performance.

In the OFDM PHY there are 256 sub carriers spanning the sampling spectrum which

is defined as:

Eq. 1) Fs = FLOOR(n BW / 8000) 8000 ,

Where n is the sampling factor, a constant dependent on the channel size, and BW

is the channel size in units of Hz. The number of sub carriers corresponds to the sizeof the FFT/IFFT used to receive and transmit the OFDM symbols. To reduce the

complexity of the digital processing algorithms it is desirable to use FFT sizes that

are powers of 2.

For channels in the 3.5 GHz band the licensed channels are multiples of 1.75 MHz

and n = 8/7. For a channel width of 3.5 MHz the sampling spectrum is 4.0 MHz. The

256 sub carriers are equally distributed across the sampling spectrum implying a

spacing of:

Eq. 2) f = Fs/256 .

For example f = Fs/256 = 15,625 Hz for a 3.5 MHz channel.

Notice that changing the channel width changes both the sub carrier spacing and the

symbol time. This implies a range of practical channel sizes for fixed applications but

quickly becomes unworkable for mobile applications where the design approach of

scaling the FFT size to the channel width is used with the OFDMA PHY.

In order to provide increased inter-channel interference margin and ease the radio

filtering constraints, not all of the 256 sub carriers are energized. There are 28 lower

and 27 upper guard sub carriers plus the DC sub carrier that are never energized.

Of the 256 total sub carriers therefore, only 200 are used which leaves a total

occupied spectrum off 200 = 3.125 MHz for a 3.5 MHz channel. This example

implies a raw, occupied bandwidth efficiency of 89% (3.125/3.5 = 89%), but thenumber varies for other channel bandwidths and sampling factors. This is the first

example we have encountered of what can be considered to be channel overhead

that decreases the channel capacity, in this case it is required by design to improve

the channel quality when adjacent spectrum is occupied.



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Not all of the 200 occupied sub carriers are used to carry data traffic. There are eight

pilot sub carriers that are dedicated for channel estimation purposes, leaving 192

data sub carriers for user and management traffic. In order to calculate the raw

channel capacity it is useful to understand how many bits each data sub carrier can

carry.

The raw sub carrier capacity, before taking out the overhead added by redundant

error correction bits, is given by the modulation order: 6 bits/sub carrier for 64QAM, 4

bits/sub carrier for 16 QAM, and so on. For example, a channel able to support

64QAM modulation could send six bits for each data carrier per symbol. But how

long is a symbol?

As we noted, the orthogonality of the sub carriers is achieved by maintaining an

inverse relationship between the sub carrier spacing and the symbol time. So the

useful symbol time is just the inverse of the sub carrier spacing:

Eq. 3) Tb = 1/f.

For example, a 3.5 MHz channel has a useful symbol time of 1/15625 = 64 us.

However for multi-path channels, we must make allowances for variable delay

spread and time synchronization errors. In OFDM, this is accomplished by repeating

a fraction of the last portion of the useful symbol time and appending it to the

beginning of the symbol for a resulting symbol time of:

Eq. 4) Ts = Tb + G Tb,

Where G is a fraction:

Eq. 5) G = 1/2m

, m = {2,3,4,5}.

The repeated symbol fraction is called the cyclic prefix. Larger cyclic prefix implies

increased overhead (decreased capacity since the cyclic prefix carries no new

information) but larger immunity to ISI from multi-path and synchronization errors.

For a 3.5 MHz channel the useful symbol time is 64 us and the minimum total

symbol time is Ts = 64 us + 64/32 us = 66us. The raw channel capacity per symbol

is:

Eq. 6) Craw = 192 k / Ts,

Where k is the bits per symbol for the modulation being used.

Assuming 64QAM modulation (6 bits per symbol): 192 data sub carriers x 6 bits/sub

carrier / 66 us = 17.45 Mbps.

But in any practical wireless system we can expect to have occasional errors

introduced by imperfect transmission, the airlink, or imperfect detection. The solution

is to send redundant bits with the information bits in each symbol to aid in error

detection and correction, a technique known as Forward Error Correction (FEC). In

the OFDM PHY FEC is done using a combination of a Reed-Solomon outer code

combined with a convolutional inner code. Adding redundant bits adds overhead and



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reduces the channel capacity. The design goal is to balance the added overhead

against the improvement in link performance and residual error rate.

The useful capacity of the combined 192 data sub carriers therefore depends on the

overall coding rate as given by the following table reproduced from the standard

(Table 215 in IEEE 802.16-2004).

Table 1 - Mandatory Channel Coding Per Modulation

Notice that the modulation rates are designed so that an FEC coded block just fits in

one symbol time when all 192 sub carriers are used. For instance for 64QAM, 144

Bytes = 1152 bits / 6 bits/symbol = 192 sub carriers.

The useful channel capacity per symbol is:

Eq. 7) C = Craw x OCR,

Where OCR is the overall coding rate given in the table. For example, for a 3.5 MHz

channel the useful channel capacity per symbol assuming the highest rate

modulation and coding is: C = 17.45 Mbps x 3/4 = 13.1 Mbps.2

It is useful to summarize the discussion of the channel capacity is terms of the

spectral efficiency. Spectral efficiency is expressed in units of bits per second per Hz

and is obtained by dividing the channel capacity by the channel width:

Eq. 8) E = C / BW.

We can see that our 3.5 MHz channel has a spectral efficiency (so far) up to 13.1

Mbps / 3.5 MHz = 3.74 b/s/Hz. The spectral efficiency is a useful figure of merit to

keep in mind because it lets you quickly calculate the capacity for other channel

sizes that WiMAX supports.

2 By now at least some readers must be wondering what happened to the often-hyped 75 Mbps channel capacity for WiMAX?

Taking the very largest channel size, 20 MHz, highest coding rate, and minimum cyclic prefix, the raw channel size using

equation 6 is: Craw = 192 x 6 b/sub carrier / 11.3 us = 102.0 Mbps. The useful channel size from equation 7 is: C = Craw x =

76.5 Mbps. Of course we have said nothing about the (short) range of such a hypothetical channel, and we should be aware

that this is before taking out other PHY and MAC layer overhead that, as we will see, is significant. To be blunt, talking about 75

Mbps WiMAX channels for MAN applications is about as meaningful as quoting the top end speed marked on the speedometer

of a family minivan.



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3.3 Frame and Symbol Size

So far we have been concerned with the combined capacity of the sub carriers

referenced to the OFDM symbol time. In fact, the symbols are not sent in an endless

stream but are formatted into a TDMA frame with a whole number of symbols per

frame. Since the symbol time varies with channel width there is no way to have a

whole number of symbols fit into a fixed frame length in every case. This means that

there can be a small gap at the end of each frame that is unused. This overhead,

amounting to less than one symbol period in a frame, depends on the selected frame

size and channel width and is of greatest impact for shorter frame lengths.

In the OFDM PHY specification the allowed frame sizes are: Tf = {2.5, 4, 5, 8, 10,

12.5, 20 ms}. For a 3.5 MHz channel width with a 1/8 cyclic prefix, the symbol length

is 72 us. Assuming a 10 ms frame length the whole number of symbols per frame is:

Eq. 9) N = FLOOR(Tf / Ts)

Where Tf is the frame length. In this case, N = FLOOR(10ms/72us) = 138 symbols.

The gap at the end of the frame is therefore Tf (138 x Ts) = 64 us, equivalent to

about 0.6% reduction is the sustainable channel capacity.

This discussion assumes that Frequency Division Duplex (FDD) channels are used,

which uses separate spectrum for the transmit and receive channels. WiMAX also

supports Time Division Duplex (TDD) channels, which uses the same spectrum for

the receive and transmit channels. TDD is used primarily in unlicensed spectrum

deployments. With TDD, the transmit and receive frames are adjustable in length

and there is a mandatory guard time gap between them which can increase the

overhead slightly. The constraint that the transmit and receive frames must have a

whole number of symbols remains.

3.4 Preambles

3.4.1 Synchronization

Receivers need a way of synchronizing to the beginning of the TDMA frame and

symbol time.

On the downlink (base station talking to a subscriber station) synchronization is

provided by a fixed preamble pattern of bits that is transmitted at the beginning of

each frame. Since the preamble transmits no actual data its presence reduces the

channel capacity. The downlink preamble takes two OFDM symbols out of each

frame. Therefore, after accounting for transmission gaps at the end of the frame, the

overhead is increased by 2/N where N is the number of whole symbols in a frame.

Obviously this impacts shortest frame lengths most since N is smaller. For our

example 3.5 MHz channel with a 1/8 cyclic prefix and a 10ms frame, the downlink

preamble overhead amounts to 2/138 = 1.4%.



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The uplink on the other hand is shared between a collection of subscriber stations

and the base station receiver similarly needs to synchronize to each subscriber

station transmitter when they begin to transmit a burst of data consisting of one or

more consecutive symbols in a frame.3

This requires each subscriber station to

prepend a preamble to the beginning of each burst. The preambles occupy one

symbol at the beginning of each and every burst. The overhead this presents is

variable depending on the burst size in terms of the number of symbols. This is our

first example of overhead that depends on the type of user traffic that is being carried

by the channel. In the worst case, subscriber stations are sending bursts of very

short data requiring only a single symbol per frame. For example, referring to the

coding table, suppose each subscriber station was using 64QAM-3/4 and sent 108

Bytes (uncoded) or less in each burst, that is one symbols worth of data assuming

all data carriers are used. In this case, the overhead amounts to an alarming 50%

because each one symbol burst would be sent with a one-symbol preamble.4

Fortunately many types of user traffic have characteristically longer burst sizes or

can be buffered to group data requests together to form longer bursts.

3.4.2 Ranging

In a WiMAX system consisting of a base station communicating with a collection of

subscriber stations at different ranges, we need a method compensating for the

variable transit delay over the airlink so that the base station can coordinate selective

use of the uplink and avoid receiving symbols that overlap in time. This is

accomplished by measuring the distance (delay) between each subscriber station

and their base station. The goal is to make each subscriber station appear to be

collocated with the base station in terms of the transmission alignment.

The details of the methods used (part of the initial ranging and periodic ranging

processes) are unimportant to our capacity discussion except that periodically, abase station will allocate one or more uplink symbols for listening to new subscriber

stations joining the network and reporting their delay compensation value and other

data needed to communicate with the base station. These ranging opportunities are

allocated on the uplink assuming a two-symbol preamble. The total allocation is

therefore at least three symbols long.5

How often the base station listens for ranging

information is configurable but in steady state an allocation every few hundred

OFDM frames is reasonable.6

The overhead is therefore normally quite negligible

and affects the uplink frame only.

3In the 802.16 OFDM PHY protocol a burst is a consecutive group of symbols in the time axis by all of data sub carriers in the

frequency axis. For the uplink, if optional sub-channels are supported, multiple simultaneous bursts can be supported by

dividing the data sub carriers into groups. A burst is confined to a TDMA frame and must use the same channel parameters

such as modulation and coding, transmitted power, etc., during the burst.4

Granted 108 Bytes is not a lot of data but the example is not entirely academic, many compressed VoIP implementations

generate packet sizes less than 100 Bytes every 10 to 20 ms. Aggregating the packets together implies increasing end-to-end

latency that may not be acceptable5

It could be longer at the base stations discretion to allow for more subscriber stations to join at once with smaller chance of

colliding with each others requests. Normally this is only an issue for initial base station startup where there could be a large

number of subscriber stations trying to join at once.6

Section 10.1 in IEEE 802.16-2004 defines the maximum interval between initial ranging opportunities as 2 seconds.



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Once a subscriber station has joined a network, it periodically needs to refresh the

ranging values (periodic ranging process). This is configurable and done on a per

subscriber station basis but similarly requires negligible added overhead because it

is performed relatively infrequently.

To summarize, preambles required for receiver synchronization and ranging are

examples of required overhead at the PHY layer. For now, simply note that the

uplink has higher overhead than the downlink because of the shared nature of the

channel and that burst preamble overhead can be significant.

3.4.3 Midambles

Midambles are allowed for OFDM PHY bursts under certain circumstances. The goal

is to improve channel estimation particularly in mobile scenarios.

For the uplink, midambles, if enabled, are inserted every 8, 16, or 32 data symbols.

For the downlink, midambles are optional if downlink sub-channels are implemented

as specified in the IEEE 802.16e-2005 amendment.

Because the focus of midambles is on mobility where channel estimation is more

dynamic, we will ignore their overhead contribution here.

3.5 Sub-channels

The OFDM PHY allows the uplink channel to be subdivided into 16 sub-channels in

order to allow subscriber stations to concentrate their transmission power into fewer

data sub carriers in each symbol.7

This also lets multiple subscriber stations share

the channel simultaneously, which increases the flexibility (and scheduler

complexity) for efficiently using the uplink channel. Support of sub-channels bysubscriber stations is optional.

Sub-channels affect the channel capacity indirectly by changing the minimum

allocation unit on the uplink. The standard supports sub-channel grouping in powers

of two so that a subscriber station can transmit in 1, 2, 4, 8, or all 16 sub-channels

(the default). The smallest allocation unit is one sub-channel, which consists of

192/16 = 12 sub carriers in frequency by one OFDM symbol in time. In that case the

coded and uncoded block size is 1/16 of the value shown in the coding table. In

general, the coded and uncoded block sizes shown in the coding table are reduced

by a fraction determined by the number of allocated sub-channels divided by 16.

This allows very granular data allocations on the uplink, which can improve theairlink utilization by matching the allocation to the amount of data being sent. For

instance, without sub-channels a subscriber station using 64QAM-2/3 code rate has

an uncoded minimum allocation unit of 96 Bytes. If the subscriber station needs to

send 6 Bytes in a frame then 90 Bytes of the allocation are wasted. However if a

single sub-channel is allocated the uncoded minimum allocation unit is 96 x 1/16 = 6

Bytes. The allocation perfectly matches the data payload in that case.

7Sub-channels can be useful for balancing the uplink and downlink link margins since ordinarily subscriber stations, compared

to the base station, have much lower radiated power capability due to cost and antenna constraints. Regulatory power density

limits, as always, must be observed when transmitting in fewer sub-channels.



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Sub-channels therefore influence a capacity analysis depending on the size of the

payload in relationship to the minimum allocation unit. In order to make an estimate

of the amount of wasted capacity (overhead) in an allocation we need to have an

idea of the average payload size which in turn depends on the types of data

applications supported by the subscriber stations.

3.6 Minimum Allocation Unit

One of the strengths of OFDM technology is its ability to send very small amounts of

information using as few as a single data sub carrier for one symbol time. For

example, using the highest order modulation (64QAM), a single data sub carrier

could be used to send as few as six bits at a time. The channel usage is therefore, at

least in principle, highly granular. In terms of the channel capacity this granularity

helps reduce the amount of wasted bandwidth in sending packets over the channel,

because the allocation can be closely fitted to the size of the packet. In that case the

aggregate capacity of the channel increases because it can be used more efficiently.

However, there are restrictions on how the 802.16 OFDM PHY organizes the data

sub carriers into Minimum Allocation Units (MAU). The MAU is the smallest two-

dimensional quantum of frequency and time that can be allocated for sending

information across the channel. In the OFDM PHY the MAUs useful capacity (Bytes)

is variable and depends on the chosen modulation and coding according to the

following.

Eq. 10) SIZEOF(MAU) = FLOOR(Nc * Nsc * OCR / 16) , where

Nc = coded block size in Bytes (see Table 1), and Nsc = number of allocated sub-

channels (1..16) for the uplink or 16 for the downlink.8

3.6.1 Downlink

The downlink does not support sub-channels for the OFDM PHY with the current

baseline specification.9

The downlink MAU is therefore one symbol by all (192) data

sub carriers. The second column in Table 1 shows the MAU size in number of Bytes,

which varies with coding and modulation. This size allows each symbol to carry

exactly one FEC block, which protects the integrity of the data over the wireless

channel.

8To be completely accurate, each burst should have one Byte subtracted from the total size because of the tail bits required to

flush the convolutional coder. For instance, if an uplink burst consisted of only one MAU, the total size would be one Byte less

than calculated in the equation.9

Downlink sub-channel support in PMP deployments was recently added in the IEEE 802.16e-2005 amendment (see

8.3.5.1.1) for improved frequency reuse, lower overhead, and mobility support. In remains to be seen how widely this feature

will be adopted for fixed applications, which is the focus of this white paper.



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For the downlink, the base station has sole usage and control of the channel. Later

we will see that there are packing and fragmentation features at the MAC layer that

can be used to fit the size of the packets to be sent to the MAU. In addition, the base

station scheduler has considerable freedom in arranging the packets to be sent into

a burst, which can be shared, by multiple subscriber stations. However there can be

cases where the amount of data to be sent in a burst just spills over a MAU

boundary, and in those cases a nearly empty MAU is sent, representing additional

channel overhead.

Clearly, the additional overhead represented by fractionally occupied downlink MAUs

is variable. In order to estimate the impact of the additional overhead on the downlink

from unused portions of the MAU we need to know something about the packet size

and the modulation distributions. A worst-case estimate of the additional overhead

would be to assume that each burst has one additional unused MAU associated with

it. For the downlink the number of bursts depends on number of separate channel

profiles in use but will typically be less than four within the time span of a TDMA

frame.

3.6.2 Uplink

The uplink supports optional sub-channels for the OFDM PHY.

By default, when sub-channels are not used, the MAU is the same as the downlink:

one symbol by all data sub carriers. The second column in Table 1 shows the MAU

size in number of Bytes in that case which varies with coding and modulation. This

size allows each symbol to carry exactly one FEC block.

When sub-channels are implemented, the MAU is as shown in the table but divided

by the number of sub-channels allocated to the subscriber station. For example inthe case of 64QAM-2/3, if a single sub-channel is allocated to a subscriber station,

the MAU is 96 Bytes / 16 = 6 Bytes.

For the uplink, the base station controls access to the channel that is shared

between multiple subscriber stations. Only a single subscriber station can transmit

on a (sub)channel at once. As with the downlink, packing and fragmentation features

at the MAC layer that can be used to fit the size of the packets to be sent to the

MAU. But because an uplink burst can only be used by a single subscriber station,

there is less opportunity for the base station to optimally fit the amount of uplink data

to the MAU quanta. Mitigating this disadvantage is that the MAU is smaller when

sub-channels are supported. However there can be cases where the amount of datato be sent in a burst just spills over a MAU boundary, and in those cases a nearly

empty MAU is sent, representing additional channel overhead.



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As with the downlink, the additional overhead represented by fractionally occupied

MAUs is variable. In order to estimate the impact of the additional overhead on the

uplink from unused portions of the MAU we need to know something about the

packet size and the modulation distributions. A worst-case estimation of the

additional overhead would be to assume that each burst has one additional unused

MAU associated with it. For the uplink the number of bursts depends on number of

active subscriber stations in a frame. The number of uplink bursts per frame will

generally be higher than the downlink for this reason.

3.7 Adaptive Modulation/Coding and Power Control

In wired networks the channel impairments tend to be constant or at least very slowly

varying. Wireless networks in contrast, especially those that support non-line-of-sight

(NLOS) communication, are well known for rapidly fluctuating channel conditions

even when the transmitter and receiver are stationary. Broadly speaking, the lower

the modulation and coding rate, and the higher the transmitted power, the morechannel fading a system can tolerate and still maintain a link at a constant error

level. It is desirable therefore to be able to dynamically change the transmitted power

and coding rate to best match the channel conditions at the moment in order to

continually support the highest capacity channel possible. WiMAX systems,

controlled by the base stations, support adaptive modulation and coding on both the

downlink and uplink and adaptive power control on the uplink.

Adaptive modulation and coding is relevant to our capacity discussion because it

changes the size of the raw channel. Referring to the channel coding table, the

uncoded channel size varies by a factor of nine between the highest and lowest

modulation. This means that the channel capacity can vary by nearly an order ofmagnitude depending on link conditions! This potentially presents a real challenge to

predicting the overall system capacity. Fortunately, in properly designed real

systems consisting of more than a few 10s of stationary subscriber stations, the

overall distribution of modulation and code rates tends to be relatively stable.10

For

analysis purposes, what is important is knowledge of what the modulation

distribution is so that an average channel capacity can be calculated.

10The reason is that most systems adjust the transmitted power or receiver attenuation as a first line of defense against

channel fades, and then the code rate, and finally the modulation if necessary. Diversity also adds increased margin against

channel fades and adds stability.



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3.8 Diversity

So far we have been considering a basic OFDM channel consisting of a single

transmitter and a single receiver at each station. Increased resistance to channel

fades can be accomplished by combining signals from multiple path independent

channels. The general method, called Multiple-In Multiple-Out (MIMO) channel

estimation, combines signals from M transmitters (M-In) and N receivers (N-Out) with

the goal to either enhance the fade resistance, increase the combined channel

spectral efficiency, or some combination of both. MIMO is highly effective but with

the tradeoff of increased hardware cost and signal processing complexity.

In WiMAX diversity is optional but can be supported via Space-Time Coding and

Maximum Ratio Combining.11

. We focus on the added overhead required rather than

implementation details.

3.8.1 Space-Time Coding (STC)

Space-Time Coding (STC) is a form of downlink transmit diversity, and is an optional

PHY capability that can add up to 15 dB link margin in fading NLOS environments.

This substantial benefit is provided via an additional transmit chain at the base

station and decoding logic at the subscriber station. Technically STC is a special

case of MIMO: Multiple-In Single-Out (MISO). Since there are ordinarily many more

subscriber stations than base stations STC is a particularly cost effective way of

adding downlink diversity without requiring additional expensive subscriber station

hardware. The base station generates two differently coded streams for the two

transmit chains so that the subscriber station can use a relatively simple decoding

algorithm to combine the two signals. Since fading will not ordinarily affect both

transmit chains simultaneously, the result is additional link protection and higheravailability in NLOS conditions.

At the PHY layer, implementing STC adds some overhead by requiring an extra

preamble in each OFDM frame. Bursts of data on the downlink that will be sent over

the two transmit chains must be preceded by a one-symbol preamble. The only extra

requirement is that the total number of symbols must be a multiple of two since the

receiver processes them in pairs according to the Alamouti algorithm. Only one STC

group of symbols is allowed per frame, and once it begins the base station must

transmit from both antennas until the end of the frame. The additional overhead is

therefore one symbol in each frame.

3.8.2 MIMO

Generalized MIMO diversity is not support for the OFDM PHY considered in this

white paper. MIMO is supported for the OFDMA PHY, which supports mobile

applications.

11Maximal Ratio Combining (MRC) is typically used only at the base station for the uplink since it increases the cost of the

receiver hardware by requiring a second receiver chain. Received signal quality is improved by combining the two signals in

proportion to the ratio of the signal to noise levels. MRC adds no additional coding overhead.



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3.9 Adaptive Antenna System (AAS)

Another strategy for improving system capacity is to spatially overlay coverage areas

by adding additional independent antennas systems. So far, we have only

considered fixed-gain-pattern antennas, which, if their coverage areas overlap,

require separate spectrum allocations to avoid interference. But suppose at the base

station we could electronically adapt the directional antenna gain profiles to

selectively point to particular subscriber stations while excluding others? By

manipulating multiple antenna patterns in time and space, independent antenna

systems could simultaneously access different subscriber stations in the same

overlapping coverage area without interfering with each other. This optional

capability is supported by WiMAX and implemented via multi-element phased array

base station antennas.

The benefits of AAS are enhanced system capacity, in theory scaling linearly with

the number of base station antennas assuming randomly located subscriber stations.

In addition there are SNR gains available arising from coherent antenna elementsignal detection, and directing gain towards subscriber stations of interest while

simultaneously placing nulls on interfering transmitters. All this comes at the expense

of additional base station antenna complexity and processing.

Because this white paper focuses on fixed (no-AAS) infrastructure we will not include

them in our capacity discussion except to note that the added complexity of

managing the space-time channel access on the downlink implies additional

management overhead that would need to be accounted for.



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4 Media Acc ess Cont ro l LayerThe WiMAX interface to user traffic is the Media Access Control layer, which

transmits MAC SDUs to MAC peers over the PHY layer. This section presents the

overhead incurred at the MAC layer including mandatory and optional elements.

In the last section we looked at the overhead added by the physical layer to a

WiMAX channel. In this section we move up to the next layer in the protocol stack

and examine the Media Access Control (MAC) layer. In contrast to the physical

layer, where much of the overhead is fixed, the MAC layer introduces many variable

overhead elements that are either configuration-dependent, traffic-dependent, or

both. We begin by examining the structure of the MAC PDU.

4.1 MAC Header

Referring to Figure 1, the 802.16 MAC layer consists of three parts: the

Convergence Sub layer (CS), the Common Part Sub layer (CPS), and the SecuritySub layer (SS). The MAC layer accepts higher layer PDUs and places them in the

payloads of one or more MAC PDUs prior to sending them to the PHY layer for

transport over the airlink. In the reverse direction, the MAC layer receives MAC

PDUs from the PHY layer and reforms the original upper layer PDU before passing

them up the stack for transport over external interfaces. Data payloads may flow into

or out of the MAC layer via either user traffic interfaces, or the management plane.

Figure 2 - MAC PDU Formats

he structure of the MAC PDU is shown in Figure 2. The MAC header comes in one

he structure of the GMH is shown in the following figure reproduced from the

a

T

of two forms: the Generic MAC Header (GMH), or a Bandwidth Request Header

(BRH). Both GMH and BRH are fixed length and six Bytes long.

T

standard. Notice that the length field (LEN) is 11 bits and therefore can specify

MAC PDU including the header up to 2047 Bytes. The Connection Identifier (CID)

field identifies the virtual connection/service-flow of the MAC PDU.

Generic MAC Header Payload (optional) CRC (optional)

MSB

LSB

Bandwidth Request Header

MSB

LSB

0 - 2041 B 4 B6 B

Subheaders (optional)

(variable)

6 B

Generic MAC Header Payload (optional) CRC (optional)

MSB

LSB

Bandwidth Request Header

MSB

LSB

0 - 2041 B 4 B6 B

Subheaders (optional)

(variable)

6 B



23/46

Figure 3 - Generic MAC Header Format

The GMH may optionally have one or more appended sub-headers as follows:

Fragmentation Sub header (2B, optionally 1B)

Packing (3B, optionally 2B)

Grant Management (2B)

Mesh Sub header (2B)

Fast-Feedback-Allocation (1B)

Extended Sub header (variable length).

The sub headers can occur only once per MAC PDU except for the Packing subheader, which may be inserted before each MAC SDU packed into the payload.

Following the GMH and optional sub headers comes the optional payload which is

variable length up to (2047 6) = 2041 Bytes but with the restriction that the entire

MAC PDU including header, sub headers, payload and CRC must be less than 2048

Bytes.

Following the GMH, optional sub headers, and optional payload, is the optional CRC,

which is four Bytes long.

The BRH is six Bytes long.

MAC PDUs will always begin with either a GMH or a BRH and are therefore at least

six Bytes long. MAC PDUs transporting data payloads will always begin with a

GMH.

With this background we can understand the overhead added by the MAC headers,

sub headers, and checksum to the payload being transported. From the figure we

see that MAC PDUs are variable in length and can be as short as six Bytes, or as

long as 2047 Bytes (211

- 1). The overhead for transporting the payload therefore

depends on the size of the payload. For example, a 1514B Ethernet frame (preamble

and CRC removed) has a minimum per-PDU MAC overhead of 10 / (1514+10) =

0.7% assuming no sub-headers but including a MAC CRC. On the other hand a

single short packet such as an 40B TCP/IP ACK over Ethernet would have a

minimum per-PDU MAC overhead of 10 / (54+10) = 15.6% overhead. No wonder the

standard allows multiple user packets to be packed into a single MAC PDU to

improve the efficiency of the wireless channel!



24/46

For analyzing the channel capacity, the 802.16 MAC structure presents some

challenges because of the number of optional fields. Each PDU carrying a data

payload must have a fixed-size GMH, but what about the optional sub-headers?

4.1.1 Sub headers

Although the additional overhead of the sub headers (typically two Bytes) is small,

we should pay attention to it because it potentially affects each MAC PDUthat

transits the airlink.

4.1.1.1 Fragmentation

Fragmentation refers to splitting a MAC SDU across multiple MAC PDUs.12

The idea

is to allow better packing of MAC SDUs into the available OFDM frequency-time

resources by using all data sub carriers in each OFDM symbol. Use of fragmentation

is optional but encouraged to improve link efficiency.

For capacity analysis, it is reasonable to assume that some fraction of the MAC

PDUs will be fragmented. The variable overhead is an additional three Bytes added

to the 802.16 MAC header for each packed SDU. A worst-case assumption is to

assume that each MAC PDU includes a sub header when fragmentation is

supported. Both downlink and uplink channels are affected.

4.1.1.2 Packing

Packing refers to combining two or more MAC SDUs into a single MAC PDU. Like its

converse, fragmentation, this allows better packing of MAC SDUs into the available

OFDM frequency-time resources by using all data sub carriers in each OFDM

symbol. Use of packing is optional but encouraged to improve link efficiency.

For capacity analysis, it is reasonable to assume that some fraction of the MAC

PDUs will be packed. The variable overhead is an additional three Bytes added to

the 802.16 MAC headerfor each packed SDU.13

A worst-case assumption is to

assume that each MAC PDU includes a one or more sub headers when packing is

supported; the exact number depending on the relative sizes of the SDUs and PDU.

Both downlink and uplink channels are affected.

Normally packing and fragmentation are either both supported or not at all. Since

packing and fragmentation are mutually exclusive operations for a given MAC SDU

we can conservatively estimate that, on average, one packing sub header will be

present in each MAC PDU increasing the total header overhead by three bytes.

12The PDUs however must still be part of the same transmission burst and cannot be split across TDMA frames.

13An exception is made for connections with fixed-length SDUs where a packing sub header is not required.



25/46

If packing and fragmentation are notsupported then the overhead associated with

fractionally used MAUs (see 3.6) will obviously be higher because the scheduler will

have fewer options to size the packets to the bandwidth allocations. This will be

particularly true for the uplink where only a single station can use a burst. Depending

on the traffic characteristics, this increased overhead can often be larger than the

small fractional overhead associated with the packing/fragmentation sub headers.

4.1.1.3 Grant Management

The Grant Management (GM) sub header is an optional method for subscriber

stations to communicate bandwidth management needs to the base station including

poll requests, slip indications, and bandwidth requests. Of these, bandwidth requests

are the only form of the sub header of consequence to capacity analysis since they

can occur relatively frequently. This method of bandwidth requesting is called a

piggyback request since it uses an existing uplink MAC PDU to signal the base

station that further data remains to be sent. The GM sub header adds two Bytes to

the MAC header and only affects the uplink channel overhead.

For capacity analysis the worst-case assumption is that each uplink MAC PDU

contains a GM sub header. A more realistic assumption is that 10% of the uplink

MAC PDUs carry the additional GM sub header overhead.

4.1.1.4 Mesh

The Mesh sub header is intended to support mesh operation where the subscriber

stations are allowed to communicate directly without a base station to relay the data.

Although the foundations are present in the 802.16 specification, the mesh portion

has received comparatively little attention and is relatively immature, reflecting the

current market focus on cell coverage deployments with base stations. Accordingly,

we will ignore mesh operation in this white paper.

4.1.1.5 Fast-Feedback-Allocation

The Fast-Feedback-Allocation (FFA) sub header is intended as a low-overhead and

low-latency method for allocating a small temporary uplink channel for the subscriber

station to communicate link information to the base station. It is presently supported

only for the OFDMA PHY for mobility and is therefore out of the scope of this white

paper.

4.1.1.6 Extended Sub header

The Extended sub header is a new feature added during development of the mobility

portion of the 802.16 standard. It supports sleep mode control, sequence number

control for handover, and another channel feedback mechanism. Its intended use is

mobility support applicable to the OFDMA PHY, and is therefore out of the scope of

this white paper.



26/46

4.2 Channel Usage Maps

The base station controls its sole access to the downlink channel and coordinates

multiple access of the uplink to one or more active subscriber stations. Since the

downlink is formed by one or more bursts of data, the subscriber stations must have

some way of learning the timing of the bursts and whether they need to listen to a

given burst to receive their traffic. Similarly, on the uplink the subscriber stations

need to have some way of learning when they can transmit on the (sub)channel and

for how long. Both of these needs are satisfied in 802.16 by broadcasting channel

allocation and usage information in each downlink frame. The information carries no

user traffic and is considered part of the downlink channel overhead. We therefore

need to take this into account in capacity analysis.

4.2.1 Frame Control Header

Immediately after the downlink preamble (see section 3.4), each downlink frame

must have a Frame Control Header (FCH) which is sent at the lowest modulation

and coding rate (BPSK 1/2) to ensure all subscriber stations in the coverage cell canreceive it. The FCH takes up one MAU, i.e. one OFDM symbol by all data sub

carriers. The FCH is used to describe up to four separate broadcast bursts in the

downlink frame. Each broadcast burst can use a different modulation and coding

profile in order from lowest to highest. The broadcast bursts must begin immediately

after the FCH and finish before any other bursts begin.

The overhead associated with the FCH is fixed i.e. one MAU each downlink frame.

The percentage overhead only changes if the frame length is changed.

Examples of data that may be in the first broadcast burst; includes, maps, burst

profile descriptions (UCD, DCD), grant allocations for initial ranging, grant allocationsfor contention bandwidth requests, and so on. Although all subscriber stations listen

to the broadcast bursts, there may also be ordinary MAC PDUs with broadcast,

multicast, or unicast CIDs sent within a burst.14

It is up to the subscriber station to

classify the CIDs in each MAC PDU header to identify traffic destined for it.

4.2.2 Downlink Map

In the first broadcast burst following the FCH, the base station may insert a downlink

map to describe other bursts that follow the FCH broadcast burst(s). The size of the

downlink map is variable but can be sent with the most efficient modulation and

coding acceptable to the intended subscriber station recipients.

14The waters are a bit murky here. Presumably a subscriber station must listen to all FCH referenced broadcast bursts that it is

capable of receiving, i.e. at the modulation and coding it uses and all lower combinations. On the other hand, it is impossible for

a subscriber station to decode bursts that are using a higher modulation and coding combination. It is the base stations

responsibility to ensure that all broadcast information, including the maps, DCD, UCD, and so on, are sent in FCH referenced

bursts that use the lowest common modulation and coding.



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The downlink map begins with eight Bytes of header information followed by one or

more information elements (DL-MAP_IE). Each DL-MAP_IE is four Bytes long but

can optionally contain variable length extensions. The basic size of the downlink map

is therefore 8 + N*4 Bytes, where N is the number of DL-MAP_IE using the frame.

There is one information element for each active connection using the downlink

frame and the map must terminate with an IE marking the end of the map. A

connection is typically associated with one subscriber station (unicast), but can be

configured to be shared between multiple stations.

The downlink map IE references a specific MAC connection (CID) and a burst profile

code (DIUC) so that a subscriber station can know whether a burst contains traffic

destined for it or not. This capability allows a subscriber station to skip bursts in the

downlink frame that contain no relevant traffic, thus reducing the processing load.

Multiple downlink map IE may point to the same burst, so that a burst can be shared

between one or more subscriber stations. The subscriber station must therefore be

capable of classifying the MAC PDU header CIDs to select the traffic destined for it.

How often downlink maps are sent, and therefore the amount of downlink overhead

they represent, depends on the channel configuration and how the base station

schedules the traffic. In cases where four or fewer burst profiles are in use, the FCH

alone could be used to reference all traffic without the need for a downlink map. On

the other hand, maps can be used to reduce the amount of downlink processing at

the subscriber station at the expense of increased map overhead. Normally this is a

design choice made by the base station designers.

For worst-case capacity estimation, each downlink frame includes a downlink map

whose size is determined by the number of active connections sharing the frame.For example, if there were ten active connections, the size of the basic downlink map

would be 8 + 11*4 = 52 Bytes (excluding any extended information elements). How

much overhead this takes up in a frame depends on the modulation and coding used

to send the map. A worst-case assumption is that there will be some subscriber

stations capable of only BPSK modulation and the burst containing the maps will

therefore be forced to use this. In our example, a 52 Byte downlink map would

occupy five MAUs of the downlink frame using BPSK.

Notice that there is a recursive element to the size of the downlink map, set by the

number of active stations, which influences the channel capacity, which determines

the possible number of active stations! This suggests that accurate capacity analysisneeds to iterate to a self-consistent solution.



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4.2.3 Uplink Map

Following the FCH and downlink map (if present) the base station may insert an

uplink map to indicate how the upcoming uplink channel frame should be allocated.

Practically speaking, an uplink map is essential, since without it the subscriber

stations would have no way of coordinating their uplink access. The size of the uplink

map is variable but can be sent with the most efficient modulation and coding

acceptable to the intended subscriber station recipients.

The uplink map begins with 11 Bytes of header information followed by one or more

information elements (UL-MAP_IE). Each UL-MAP_IE is six Bytes long but can

optionally contain variable length extensions. The basic size of the uplink map is

therefore 11 + N*6 Bytes, where N is the number of UL-MAP_IE using the frame.

There is one information element for each active connection using the frame and the

map must terminate with an IE marking the end of the map. Unless contention

access is intended, the subscriber station referenced in an UL-MAP_IE by CID must

refer to a unique uplink burst since each burst can only be used by a single station.

How often uplink maps are sent, and therefore the amount of downlink overhead

they represent, depends on the channel configuration and how the base station

schedules the traffic.

For capacity estimation, each downlink frame includes an uplink map whose size is

determined by the number of active connections that will share the uplink frame. For

example, if there were ten active connections the size of the basic uplink map would

be 11 + 11*6 = 77 Bytes (excluding any extended information elements). Again, how

much overhead this takes up in a frame depends on modulation and coding used to

send the map. A worst-case assumption is that there will be some subscriberstations capable of receiving only BPSK modulation and the burst containing the

maps will therefore be forced to use this. In our example, a 77 Byte uplink map

would occupy seven MAUs of the downlink frame using BPSK.

4.2.4 Downlink and Uplink Channel Descriptors

After any downlink and uplink maps in the first broadcast burst the base station may

insert a Downlink Channel Descriptor (DCD) and/or an Uplink Channel Descriptor

(UCD). The purpose of the DCD/UCD is to define downlink/uplink burst profiles

specifying parameters such as modulation type, FEC, scrambler seed, cyclic prefix,

and transmit diversity type. Once defined, burst profiles are referred to in later

downlink maps via a numerical index called the Downlink Interval Usage Code(DIUC) or Uplink Interval Usage Code (UIUC), which is associated with the profile.



29/46

The size of the DCD/UCD is variable depending on the number of downlink/uplink

burst profiles elements (Downlink/Uplink_Burst_Profile). Each DCD/UCD begins with

three/six bytes of header information followed by one or more channel descriptor

information elements. There are many possible options for configuring a channel so

the size of the channel descriptor depends on the configuration. For basic licensed

FDD operation a DCD/UCD might have 36/32 bytes of channel description and 15/6

bytes for each burst description. There is currently a downlink/uplink limit of 12/8

active burst types at any time but recall that multiple subscriber stations can share a

burst profile.

How often downlink or uplink channel descriptors are sent, and therefore the amount

of downlink overhead they represent, depends on the channel configuration and how

often conditions change, thus forcing updates. For example the types of modulation

and coding in use could vary over time. In general however the burst profiles will be

relatively static and we can ignore them for capacity estimation.

4.3 Bandwidth Requests

The 802.16 MAC protocol specifies several different ways that a subscriber station

can inform the base station that it has data to send on the uplink. Most of these

methods involve sending a BRH (see Figure 2) for which the subscriber station must

first obtain an uplink channel access grant from the base station. Since there is

overhead associated with getting the grants as well as sending the requests we are

interested in the specifics of the bandwidth request mechanism.

In this section we will describe the basic mechanisms without explaining why they

exist. Later we will see how these are used for providing quality of service (QoS) and

further discuss their ramifications on channel overhead in the context of QoSconnection profiles.

4.3.1 Contention

Contention based uplink access is a method where the base station periodically

allocates part of the uplink channel capacity (grants a transmit opportunity) to

specified stations that might have data to send. The stations in the contention group

are identified by their CID. There are two ways used to manage the process.



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4.3.1.1 Full Contention

The mandatory method for issuing the grant is for the base station to include a REQ

Region Full uplink map information element in the uplink map, which is sent to the

subscriber stations on the downlink. There is one information element for each

subscriber station CID in the contention group. The information element allocates

some whole number of MAU to the stations to use for sending BRH PDUs in the

pending uplink frame. Request collisions are handled in the usual exponential back-

off fashion so that eventually a station can have its request heard. When the base

station receives a BRH it responds by allocating an uplink allocation for the

requested CID. A subscriber station that does not see an uplink allocation response

from the base station assumes that there has been a collision and retries the

request.

The overhead associated with this method is in the small addition to the size of

uplink map (see 4.2.3), but mainly in the contention allocation itself, which must allow

for the most robust modulation and coding combination. The interval between

contention allocations is configurable.

The uplink contention allocation includes a one-symbol preamble followed by one or

more symbols configured for the allocation. The number of symbols depends on the

most robust modulation and coding and the number of sub-channels used for the

allocation. The size of the allocation should be sufficient to send one BRH. For

example, since the BRH is 6 + 4 = 10 Bytes assuming a CRC is used, the number of

required symbols is 10 / sizeof(MAU). If there are four sub-channels in the allocation,

then from Eq. 10 and Table 1, and assuming BPSK 1/2 modulation and coding, then

four OFDM symbols are required to hold one BRH. If desired, the base station can

allocate multiple contention allocation opportunities in a frame.

4.3.1.2 Focused Contention

A second optional method for issuing the grant is for the base station to include a

REQ Region Focused uplink map information element in the uplink map, which is

sent to the subscriber stations on the downlink. There is one information element for

each subscriber station CID in the contention group. The information element

allocates some whole number of OFDM symbols to the stations to use for sending

special coded requests for a non-contention bandwidth request allocation in the

pending uplink frame. The base station, upon detecting a coded request, sends back

an allocation in the uplink map, coupled with its corresponding code index so that

subscriber stations can know their request was heard. The subscriber station then

uses the non-contention uplink allocation to send a BRH to the base station that

responds by granting an uplink allocation for the requested CID in an uplink map.

Focused contention is therefore a three-way handshake: subscriber requests a

grant, base allocates a grant, and subscriber sends a BRH.15

15

Whether a base station supports Focused Contention is an implementation decision. The mandatory Full Contention method

suffers from congestion collapse beyond a knee in the curve of the access latency versus number of active subscriber

stations. Where this point is varies according to the uplink traffic patterns of the subscriber stations. Focused Contention on the

other hand is much more robust and can handle situation where there are a large number of simultaneously contending

subscriber stations. The tradeoff is the increased overhead and latency of the three-way handshake.



31/46

Request collisions are handled in the usual exponential back-off fashion so that

eventually a station can have its request heard. To reduce the chance of collisions

Focused Contention subdivides the total uplink data carriers into sub-channel groups

of four carriers each and further uses one of eight available CDMA codes for

transmitting the request. The codes are transmitted for two consecutive symbols. A

subscriber station randomly selects a sub-channel and code index pair. A collision

occurs only if two subscriber stations happen to choose the same index pair.16

A

subscriber station that does not see an uplink allocation response from the base

station assumes that there has been a collision and retries the request.

The overhead associated with this method is in the small addition to the size of the

uplink map (see 4.2.3), but mainly in the uplink contention allocation itself, plus the

following bandwidth request allocation. The interval between contention allocations is

configurable.

The uplink contention allocation is two OFDM symbols, by all data carriers. This is

shared by all requesting stations as explained above.

The subsequent uplink bandwidth request allocation, one for each requesting

subscriber station, includes a one-symbol preamble followed by one or more

symbols configured for the allocation. The number of symbols depends on the

modulation and coding and the number of sub-channels used for the allocation. The

size of the allocation should be sufficient to send one BRH. For example, since the

BRH is 6 + 4 = 10 Bytes, and assuming a CRC is used, then the number of required

symbols is 10 / sizeof(MAU). If there are four sub-channels in the allocation, then

from Eq. 10 and Table 1, and assuming BPSK 1/2 modulation and coding, four

OFDM symbols are required to hold one BRH.

4.3.2 Polling

Polling is a process where the base station periodically allocates part of the uplink

channel capacity (issues a grant or transmit opportunity in the uplink map) to each

participating subscriber station that might have data to send. The transmit

opportunity itself is the poll, there is no explicit message type. The subscriber

stations use the transmit opportunity to send a BRH to request uplink bandwidth. The

grants must therefore be at least large enough to send one BRH.

Polls may be unicast or multicast or broadcast according to the CID specified in the

uplink map transmit opportunity information element. If a poll is multicast or

broadcast then one of the contention bandwidth request methods (full or focused) isspecified to collect the bandwidth request responses. Unicast polls are directed

towards a single CID associated with a single subscriber station.

The overhead associated with this method is in the small addition to the size of

uplink map (see 4.2.3), but mainly in the request allocation itself, which must allow

16The odds of choosing the same Focused Contention sub-channel are 1 in 50 and choosing the same code are 1 in 8. The

overall collision probability for a given contention opportunity is therefore 1/(50*8) = 0.25%. The tradeoff is the additional

complexity, latency, and overhead of the methods three-way handshake.



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for the modulation and coding combination in use by the requesting subscriber

station.17

The interval between polls is configurable for a given CID.

The uplink contention allocation includes a one-symbol preamble followed by one or

more symbols configured for the allocation. The number of symbols depends on the

modulation and coding and the number of sub-channels used for the allocation. The

size of the allocation should be sufficient to send one BRH. For example, since the

BRH is 6 + 4 = 10 Bytes assuming a CRC is used, then the number of required

symbols is 10 / sizeof(MAU). If there are four sub-channels in the allocation, then

from Eq. 10 and Table 1, assuming BPSK 1/2 modulation and coding, five OFDM

symbols by four sub-channels are required to hold one BRH plus the mandatory

preamble. For this example, if the polling interval for a connection is configured to be

once a frame, and the frame length is 10ms, this amounts to 3B/MAU * 5 MAU /

10ms = 12 kbps in bandwidth request overhead.

4.3.3 Piggyback

A piggyback bandwidth request is a method of using a previously granted uplinkchannel access opportunity to inform the base station that a subscriber station

requires another allocation to send pending data. The idea is that once a subscriber

station obtains uplink channel access it can use the channel for future bandwidth

requests without incurring the overhead associated with contention or polling. This is

most useful when a subscriber station connection has long consecutive trains of data

packets to send.

To improve efficiency, the method uses a GM sub header of the GMH (see 4.1.1.3).

The overhead associated with piggybacking adds two bytes to the length of a MAC

PDU. Support for piggyback requests by a subscriber station is optional.

17In cases where there are a large number of inactive subscriber stations to poll, supporting unicast polling to each of them

would be an inefficient use of the downlink frames uplink map, and the uplink frames bandwidth request opportunities.

Multicast polling was designed with this case in mind to improve bandwidth efficiency. The collection of subscriber stations are

assigned to a special bandwidth request multicast group and only those that have traffic to send respond to the multicast polls.

Broadcast polling is similar, except that all subscriber stations that have traffic to send respond to the polls.



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5 Channel Bandwidt h Calcu la t ionAt this point in the white paper it should be readily apparent that there is a complex

interrelated array of factors in the WiMAX MAC and PHY layers that influence the

channel capacity in terms of the useful available bandwidth. It can be confusing how

the various factors should be combined, and in what order, to determine the channel

bandwidth capacity. This sections aims to clarify that process.

Our approach is to summarize the list of factors in a step-by-step sequence diagram

in order to illustrate the process of calculating the useful channel bandwidth. Putting

the sequence diagram into practice, we will present an example spreadsheet

channel model for exploring various configuration scenarios. In order to keep the

discussion to a reasonable length we will focus on a FDD system where the uplink

and downlink TDMA frames are independent of one another. Analysis of TDD

systems would be similar except user input of the time partitioning of the single

uplink/downlink TDMA frame would be required.

input channel size

(MHz)

input cyclic prefix(1/4 to 1/32)

calc useful channel bw

input mod and

coding distribution

input frame length

(2.5 to 20 ms)

calc frame o/h

calc frame bw

calc preamble o/h

calc FCH o/h

calc DL map o/h

calc UL map o/h

calc useful frame bw

calc MAC hdr o/h

calc MAC subhdr o/h

calc MAC CRC o/h

input avg user pk

(B)

calc useful MAC bw

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

input channel size

(MHz)

input cyclic prefix(1/4 to 1/32)


input mod and

coding distribution

input frame length

(2.5 to 20 ms)

calc frame o/h

calc frame bw

calc preamble o/h

calc FCH o/h

calc DL map o/h

calc UL map o/h


calc MAC hdr o/h

calc MAC subhdr o/h

calc MAC CRC o/h

input avg user pk

(B)

calc useful MAC bw

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Figure 4 - Downlink Bandwidth Calculation Sequence

Beginning with the downlink, Figure 4 presents the sequence diagram for accounting

for the various PHY and MAC overhead contributions in order to calculate the useful

channel bandwidth. In the figure there are four intermediate results represented in

the four columns. In column one the raw channel bandwidth is determined. Incolumn two the basic TDMA framing overhead is accounted for. In column three the

required preamble and channel management overhead is taken out. In column four

the various per-packet overhead is accounted for resulting finally in the useful user

bandwidth available at the input of the MAC protocol layer.



34/46

Various user inputs are required to complete the calculation. As indicated in Figure

4, steps 1, 2, 3, 5, 13 call out those inputs which we have already discussed in

sections 3 and 4.

calc preamble o/h

calc ranging o/h


calc MAC hdr o/h

calc MAC subhdr o/h

calc MAC CRC o/h

input avg user pk(B)

calc useful MAC bw

calc contention o/h

input subch size

input burst size

calc MAU

calc subch o/h

input channel size(MHz)

input cyclic prefix

(1/4 to 1/32)


input mod andcoding distribution

input frame length(2.5 to 20 ms)

calc frame o/h

calc frame bw

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Figure 5 - Uplink Bandwidth Calculation Sequence

Figure 5 presents the companion sequence diagram for the uplink, which similarly

accounts for the various PHY and MAC overhead contributions leading to the useful

channel bandwidth. As with the downlink there are four intermediate results

represented in the four columns. The first two and last columns are identical to the

downlink. The third column accounts for the required preambles and required

channel management overhead, which differs from downlink.

Again, user inputs are required to complete the calculation. These inputs are

indicated in steps 1, 2, 3, 5, 9, 12, 16 of Figure 5 as previously discussed in sections

3 and 4.



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Figure 6 presents a screen shot of a spreadsheet model which implements the

sequence steps of Figure 4 for the downlink and Figure 5 for the uplink on separate

tabs (downlink tab shown).18

The cells in blue are the user inputs to the bandwidth

calculation. The other cells are formulas or constants that should not be changed.

The result of the calculation is shown in the last two rows of the model, which

present the aggregate useful channel bandwidth, Cmac, in units of Mbits/s and the

equivalent spectral efficiency, Seff, of the channel. By changing the inputs various

channel scenarios can be investigated.

Item Value Units Comments

ch_size 3.5 MHz Channel size (sets sampling rate)

cyclic_exp 3 # Cyclic prefix exponent N (2 to 5 where 5 represents the shortest prefix), Tg = Tb/2^N

BPSK-1/2 5 % Fraction of SS using the modulation

QPSK-1/2 2.5 % Fraction of SS using the modulation

QPSK-3/4 2.5 % Fraction of SS using the modulation

16QAM-1/2 5 % Fraction of SS using the modulation




n 1.143 # Sampling factor

Fs 4.0 MHz Sampling frequencyTb 64.0 us Useful symbol time

Tg 8.0 us Guard time (cyclic prefix)

Ts 72 us Symbol Time (Tb+Tg)

C 9.97 Mbps Useful channel bandwidth

Tf 10 ms Frame length {2.5, 4, 5, 8, 10, 12.5, 20 ms}

N 138 symbols Frame length is whole symbols

Cf 9.90 Mbps Frame bandwidth

sc 8 subch Configured subch's per SS (1,2,4,8,16), 16 means that SS uses all data subcarriers during uplink access

Nfull 1 # / frame Number of Full Contention allocations per frame

Nfocused 0 # / frame Number of Focused Contention allocations per frame (optional), (set 0 if no

wimax capacity fixed

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