tcom552-lecture-7

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Introduction to MobileIntroduction to Mobile

CommunicationsCommunications

TCOM 552, Lecture #7Hung Nguyen, Ph.D.

23 October, 2006



10/23/2006Hung Nguyen, TCOM 552, Fall 2006 3

AMPS ParametersAMPS Parameters




Bursty control info can also be

transmitted in the voice channel for power

control and/or handoff




Differences Between First (1G) andDifferences Between First (1G) and

Second Generation (2G) SystemsSecond Generation (2G) Systems

Digital traffic channels ± first-generation systemsare analog; second-generation systems are digital

Channel access ± second-generation systems useTDMA or CDMA, first uses FDMA

First in 800-900 MHz band, second also there plus1800-2000 MHz band

Encryption ± all second generation systems provideencryption to prevent eavesdropping

Error detection and correction ± second-generation

digital traffic allows for detection and correction,giving clear voice reception




Cellular vs. PCS CoverageCellular vs. PCS Coverage

Free Space loss is proportional to 20log10f. Difference

between PCS (1900 MHz) and cellular (880 MHz) is around 7 dB

PCS base

stations

Cellular

base station

On average ratio of PCS stations to cellular 3:1




Cellular vs. PCSCellular vs. PCS

Cellular designed for cars

Cellular

± Analog modulation

± portable power: 1/2 to 3watts

FDMA access

Large cell sizes

PCS goal is for a user not a place or vehicle

PCS

± digital modulation

± portable power: 100 to300 milli-watts

TDMA access

± (IS-54/136 and GSM)

CDMA access (IS-95)

Often cells closer together




Steps in Design of TDMA TimeslotSteps in Design of TDMA Timeslot




DIGITAL CELLULAR DAMPSDIGITAL CELLULAR DAMPS --

also called US TDMAalso called US TDMA

IS-54 later renamed IS-136 TDMA, 8 kb/s voice, x2 overhead Three 16 kb/s TDMA channels in 30 kHz - Reuse

factor 7 with sectoring 48 kb/s in 30 kHz = 1.6 bits/sec/Hz

± 3 times more spectrum efficient than analog (AMPS)± Approx. 7 calls/MHz/cell

± Approx. 210 max calls/cell

Used by AT&T, Cingular and others in US See TDMA Tutorial at

± http://www.iec.org/online/tutorials/tdma/

See PCS Tutorial at± http://www.iec.org/tutorials/pcs/index.html or

± http://www.iec.org/online/tutorials/pcs/




US TDMA ArchitectureUS TDMA Architecture

US TDMA started as IS-54, dual mode terminals, after GSM Adopted mobile assisted handoff (MAHO), encryption,

associated control channels (instead of FVC/RVC), but usesthe AMPS forward and reverse control channels to set up callsand for MM

Later established IS-136, with digital control channels (DCCH)

separate from the AMPS control channels, and added sleepmodes, allowing all digital phones, and various supplementaryservices like voice mail, caller ID, and short message service

IS-136 also specifies an air interface, and a base station, MSCand interworking function, and going to public, private or residential networks (PSTN, PBX, or cordless)

Identifiers: AMPS plus others: A-key to each subscriber (for encryption and authentication), location areas (for easier location tracking and registration), IMSI (international mobilesubscriber ID), others




Radio Transmission (1)Radio Transmission (1)

30 KHz, 6 slots per frame, each user 2 slots, 40msec frame

Some time offset between reverse and forward tonot transmit and receive at same time, still do fullduplex

324 bits per slot, 6 slots/frame, in 40 msec = 48.6kbps

Full rate channel is 2 slots/frame = 16.2 kbps; alsohalf rate, 2X, 3X

No fixed assignment of frequencies to controlchannels




ISIS--54 (IS54 (IS--136) TDMA Slot Structure136) TDMA Slot Structure

�6 time slots

�(interleaving of 2 voice samples)

�3 users/ frame�324 bits/ time slot

�6.667 ms/slot

IS-136 Slots




Radio Transmission (2)Radio Transmission (2)

Uses DQPSK with possible 45 degree, 45+90,45+180 and -45 degree shifts from each phaseangle (max phase shift is 135 degree), so 4 possible next symbols, so 2 bits each, called T/4 shifted DQPSK - a 1.62 bps/Hz modulation spectralefficiency

± Differential phase detection, no absolute phase referenceor detector needed

± But not very energy efficient - BER for given Eb/No notgreat, reuse still 7

Mobile transmits 0.25 mw up to 4 w, in 4 dB steps,but only 1/3 the time

Spectral efficiency: in terms of voice calls:± About 3* better than AMPS ( a bit higher, if it uses 21

control channels for one provider in 25 MHz, instead of 21*3), with 7 factor reuse




Logical Channels (1)Logical Channels (1)

Digital traffic channels± Data (including voice), associated control channels, sync

and other information

± Typically all in one slot, e.g., 28 bit sync, 260 data bits, 12

SACCH (Slow Associated Control Channel), some guard

time

SYNC does frame sync, and is training sequence for

equalizer

SACH is control, at 600 b/s per user, like FVC/RVC in AMPS

± Mobile only transmits on its slot, power off rest of time

± FACCH (Forward Access Control Channel) does a blankand burst on the traffic channel

Faster rate control for handoffs (about x6), with rate 1/4 code




Logical Channels (2)Logical Channels (2)

DCCH (Digital Control Channel)± Forward are both broadcast as well as

addressed to one, reverse are random access -all have SYNC, some preamble, control data

±Organized hierarchically in half frames (blocks),super-frames (32 frames) and hyper-frames (64 frames) - control data is MUXed in into super-frames Different types of control data are called logical

channels

e.g., SPACH is Short message service, Paging andAccess response Channel

Terminals listen to a specific paging sub-channels inthe SPACH, sleep otherwise




Messages and AuthenticationMessages and Authentication

On AMPS logical channels, on SACCH andFACCH, on DCCH¶s

Table 5.5 for SACCH and FACCH - includes callmanagement RRM, authentication, handoff, etc

± e.g., Handoff: includes new frequency for handoff, power to radiate, half rate or full rate, time slot number, color code

of new BS, other On DCCH: system info on broadcast channels, call

management messages, message waiting andpaging on SPACH, authentication, etc

Authentication and privacy in IS-136: due to A-key,in phone and in authentication center (AC)

± Used by both mobile and AC to generate a shared secretkey, SSD, from A-key and a random number generator (random number is transmitted) - but can not be reversedto A-key - used for authentication and privacy




MAHO MAHO -- MACA MACA -- Some RRM (RadioSome RRM (Radio

Resource Management)Resource Management)

Terminal measures signal quality on the active traffic channel During time slots it is not active it monitors other BS¶s

Transmits channel quality information to its BS on the SACCH

Mobile is told which other channels to monitor by BS - 6 or 12

Signal quality is from power level and BER

± BER is better than just power levels: interference could give goodpower levels, but bad BER - better than AMPS

BS also measures signal quality on active traffic channel

Since in TDMA the BS knows signal quality at nearby BS¶s itknows who to handoff to

In TDMA most of the processing done at BS, in AMPS at MSC

Mobile - assisted channel allocation (MA

CA

) is similar, for channel allocation, helping the BS assign channels; themobiles measure idle channels and tell the BS

SACCH and FACCH have also power adjustment and timealignment messages




GSM (Europe/US))GSM (Europe/US))

Global System for MobileGlobal System for Mobile

Agreed TDMA standard devised for European environment

200 kHz channels with 270.833 kbits/s.

eight TDMA users

13kb/s vocoder, 20kb/s w/overhead Reuse factor 3-4

About 5 calls/MHz/cell with sectoring, or 150calls/cell (30 MHz)

See GSM Tutorial± Available at

http://www.iec.org/tutorials/gsm/index.html




Mobile Wireless TDMA DesignMobile Wireless TDMA Design

ConsiderationsConsiderations -- for GSMfor GSM

Number of logical channels (number of timeslots in TDMA frame): 8

Maximum cell radius (R): 35 km

Frequency: region around 900 MHz

Maximum vehicle speed (Vm):250 km/hr Maximum coding delay: approx. 20 ms

± Really, this is also max. speech sample delay sothat one can not distinguish breaks

Maximum delay spread ((m): 10 Qs Bandwidth: Not to exceed 200 kHz (25 kHz

per channel)




Logic for GSM Rate and ModulationLogic for GSM Rate and Modulation

Max. delay of 20 msec >>> How much data in 20 msec? If 12 kbps speech codec, that¶s 260 bits

Add rate 1/2 convolutional code, that¶s 480 bits

Put in 8 speech slots, that¶s 8*480 bits, all in 20 msec

That¶s 192 kbps

±Notice that data rate is high enough that 20 msec worth of speech is included, multiplexed in with 7 other users, for eachuser¶s sample

Really with 13 kbps and other overhead it turns into 270.8kbps

It uses GMSK modulation - Gaussian weighted Minimum ShiftKeying - like FSK, but changes frequency while maintaining

continuous phase, and shifts the minimum possible - usedbecause more spectrally efficient than PSK or FSK, and fitsdata rate into 200 KHz BW, but power efficient (see later)




GSM Speech Signal ProcessingGSM Speech Signal Processing

RPE-LPC (Linear Predictive Coding)

In 20 msec, 260 bits,turned into (with rate ½coding+other)

189*2+78=456 bits, in 20msec is 22.8 kbps (trafficchannel)

Interleaved over multipleslot timeperiods, within20 msec protects against

bursts Encrypted 114 bits at a

time Into time slots or bursts

GMSK modulation




Radio TransmissionRadio Transmission ±± GSM (1)GSM (1)

200 KHz carriers, so fewer transmitters andreceivers at a BS

GMSK does 1.35 bps/Hz, worse than US TDMA,but has better BER for a given Eb/No (so better frequency reuse), and has constant envelopemodulation which allows more efficient amplifiersand is better on battery drain than US TDMA

Can do slow frequency hoping - network directed

Slot is 0.577 msec, then a frame is 8 slots at 4.615

msec Slot has 2*57 bits of data, 26 bits training sequence

(8 different ones, also used as SAT/DCC function),guard time and tail bits, flags




Spectrum EfficiencySpectrum Efficiency

GMSK is more power efficient than USTDMA, providing good voice quality at S/I of about 7 dB

Thus allows frequency reuse of 3-4-5

With 4 it is 5 calls/cell/MHz

± 8 calls/200 KHz or 40 in 1 MHz, one way

± 20 two ways, and with 4 reuse it¶s 5

calls/MHz/cell

Actually one carrier left as guard, slightly smaller (4.96)




TDMA FormatTDMA Format ±± Time Slot FieldsTime Slot Fields -- GSMGSM

Trail bits ± 3 - allow synchronization of transmissions from mobile units

Encrypted bits ± encrypted data, same number of bits - 114 in two groups of 57

Stealing bit - indicates whether block contains data

or is "stolen´ for control signaling Training sequence ± used to adapt parameters of

receiver to the current path propagationcharacteristics - in the middle

± Ground rule is that it 6*max. delay spread for equalizer

training - that¶s 60usec, at 270 kbps or so it¶s about 16 bits- actually 26

Guard bits ± used to avoid overlapping with other bursts




Logical ChannelsLogical Channels

Traffic channels, half and full rate

Signaling channels

± Broadcast

e.g., frequency correction (pure sine wave, used to

match the BS, SYNC, some control

± Common Control Channels

Paging, Random access, Access

± Dedicated Control Channels

Slow, fast, stand-alone




GSM Network ArchitectureGSM Network Architecture




Mobile StationMobile Station

Mobile station communicates across Uminterface (air interface) with base stationtransceiver in same cell as mobile unit

Mobile equipment (ME) ± physical terminal,

such as a telephone or PCS± ME includes radio transceiver, digital signal

processors and subscriber identity module (SIM)

GSM subscriber units are generic until SIM

is inserted± SIMs roam, not necessarily the subscriber

devices




Base Station Subsystem (BSS)Base Station Subsystem (BSS)

BSS consists of base station controller andone or more base transceiver stations (BTS)

Each BTS defines a single cell

± Includes radio antenna, radio transceiver and a

link to a base station controller (BSC)

BSC reserves radio frequencies, manageshandoff of mobile unit from one cell toanother within BSS, and controls paging




Network Subsystem (NS)Network Subsystem (NS)

NS provides link between cellular networkand public switched telecommunicationsnetworks

± Controls handoffs between cells in different

BSSs± Authenticates users and validates accounts

± Enables worldwide roaming of mobile users

Central element of NS is the mobile

switching center (MSC)




Mobile Switching Center (MSC)Mobile Switching Center (MSC)

DatabasesDatabases

Home location register (HLR) database ±stores information about each subscriber that belongs to it

Visitor location register (VLR) database ±

maintains information about subscriberscurrently physically in the region

Authentication center database (AuC) ±

used for authentication activities, holdsencryption keys

Equipment identity register database (EIR) ±keeps track of the type of equipment thatexists at the mobile station




GSM Signaling Protocol ArchitectureGSM Signaling Protocol Architecture

(m - modified/mobile from ISDN)

(Uses CRC, ARQ)




Functions Provided by ProtocolsFunctions Provided by Protocols

Protocols above the link layer of the GSM signalingprotocol architecture provide specific functions:

± Radio resource management: Does radio channel

management, including for handoffs

± Mobility management: Roaming, location databases,

authentication

± Connection management: sets up calls between users

± Mobile application part (MAP) - Core Network functions,

like IS-41 in US systems

± BTS management

± SCCP (Signal connection control part) and MTP (message

transfer part) are from SS7, for control signaling

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