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CS 117 Winter 2004Lecture #9

March 9, 2004

• Cellular Wireless Networks– AMPS (Analog)

– D-AMPS (TDMA)

– GSM

Reference: Tanenbaum Chpt 2 (pg 153-169)

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Cellular Wireless Network Evolution

First Generation: Analog Voice (1946)

AMPS: Advance Mobile Phone Systems (1982) Residential cordless phones

Second Generation: Digital Voice (1990)

IS-54; IS-136: North American Standard - TDMA (1996)

IS-95: CDMA (Qualcomm) (1993)

GSM: Pan-European Digital Cellular (1991)

Iridium , 2002: Mobile Communication Satellite (1998)

Third Generation: Digital Voice and Data (2000) IMT-2000 (cdma2000, W-CDMA/UMTS-Combines the functions

of: cellular, cordless, wireless LANs, paging, Bluetooth, etc. Supports multimedia services: data, voice, video, image)

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4

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Cellular Concept• Geographical separation• Capacity (frequency) reuse• Backbone connectivity

BS BSBS

BSBSBS

Backbone Network

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Invented by Bell Labs; installed In US in 1982; in Europe as TACS

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1G; Advanced Mobile Phone System (AMPS)

• Frequencies are not reused in in a group of 7 adjacent cells• To add more users, smaller cells can be used.• In each cell, 57 channels each for A-side and B -side carrier• about 800 channels total (across the entire AMPS system)• FDMA: one frequency per user channel

10-20 km

100 cells

(1982)

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Cellular Concepts• Geographical separation• Capacity (frequency) reuse• Backbone connectivity

BS BSBS

BSBSBS

Publ. Swit. Tel. Net.MTSO (Mobile Telephone

Switching office)- MSC (Mobile

Switching Center).

Hand-held telephone 0.6 watts; Car transmitters are 3 watts, Maximum. by the FCC.

The base station consists of a computers and transmitter/receiver connected to an antenna.

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Packet Switching Telephone Network (PSTN)

;ConceptsIf MS leave a cell, its BS asks the surrounding BS how much

power they are getting from it. The BS then transfers ownership to the cell getting the strongest

BSMS

Nerve center of the system

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1G; Channel Categories

832 channels are divided into four categories:

1. Control (base to mobile) to manage the system (PSK) (=21 channels are reserved for control, and are wired into a Programmable Read-Only Memory (PROM)).

2. Paging (base to mobile) to alert users to calls for them.

3. Access (bidirectional) for call setup and channel assignment.

4. Data (bidirectional) for voice, fax, or data. (FM)

Since the same frequencies cannot be reused in nearby cells, the actual number of voice channels available per cell is much smaller than 832, about 45.

AM

PS

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To register and make a phone call

• When phone is switched on , it scans a preprogrammed list of 21 control channels, to find the most powerful signal.

• It transmits its ID number on it to the MSC – which informs the home MSC (registration is done every 15 min)

• To make a call, user transmits dest Ph # on random access channel; MSC will assign a data channel

• At the same time MSC pages the destination cell for the other party (idle phone listens on all page channels)

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Handoff (Cellular Concepts)Handoff: Transfer of a MS from one cell to another (300 msec.) Each BS constantly monitors the received power from each MS. When power drops below given threshold, BS asks neighbor station (with stronger received power) to pick up the MS, on a new channel

(the old is not reused in adjacent cells) - handoff.

Hard handoff: User must switch from one frequency to another. The old BS drops the MS before the new one acquires it. If the new one is unable to acquire it, the cell is disconnected abruptly.

Soft Handoff: The MS is acquired by the new BS before the previous one drops, the MS needs to be able to tune to two frequencies at the same time (available only with CDMA).

Neither first nor second generation devices can do this.

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Soft - Handoff (Cellular Concepts)

• Soft Handoff: simultaneous radio link between MS and

different BSs

• Hard handoff: The old BS drops the MS before the new one

acquires it.

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(Freq DivisionDuplex)

AM

PS

Each simplex channels is 30 KHz wide

(A) 832 channels (B) 832 channels

AMPS uses FDM to separate the channels.

1G; Channels

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FDD & TDD duplexing

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AM

PS

1G;

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AM

PS 1G;

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Channel multiplexing FDMA+TDM

Uplink 824-849 MHz-832 channels

Downlink 869-894 MHz-832 channels

Channel Bandwidth 30 kHz

FDD separation 45 MHz

Modulation FM (traffic, voice); FSK (control)

Channels Control, Paging, Access, Data

AMPS

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2G-USDigital Cellular: (Int.Stand:IS-54; IS-136) =USDC

• Initially, Same frequency as AMPS (simplex Transmit:

824 to 849 MHz; simplex receive: 869 to 894 MHz). • Each 30 kHz band RF channel is used at a rate of 48.6

kbps (to co-exist with AMPS, one channel can be analog and the adjacent ones can be digital-MTSO determines) – 3 TDM slots/RF band– 8 kbps voice coding– 16.2 kbps TDM digital channel

• 4 cell frequency reuse (instead of 7 as in AMPS)

– 3 x 416 / 4 = 312 channels (57 in AMPS)

D-

AM

PS

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Conceptually, it works like AMPS

D-

AM

PS

its successor

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2G; Phones: Digital Voice The second generation was digital and four systems are in use now: D-AMPS, GSM, CDMA, and PDC (Japan) FDD: (1850-1910 MHz Upstream; 930-1990 MHz Downstream)Waves are 16 cm long, so ¼-wave antenna is only 4 cm long.

D-AMPS can use both the 850 MHz and 1900MHz bands to get a wider range of available channels

• D-AMPS mobile phone, the voice signal picked up by the microphone is digitized and compressed. Compression is done from the standard 56-kbps PCM encoding to 8 kbps, or less. The compression is done in the telephone. • In D-AMPS, three users can share a single frequency pair using TDM. Each frequency pair supports 25 frames/sec of 40 msec each. Each frame is divided into six time slots of 6.66 msec each,

(6.66msec x 6=3.996 msec); 25 frames/sec x 6 slots = 150 slots

Data Modulation: Pulse Code Modulation

D-

AM

PS

26 Narrow single-user bandwidth

f

f1

s1(t)

sN(t)

S1(f) S2(f) SN(f)

Buser

f2

+f1 fN

fN

FDMA

TDM

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(a) D-AMPS Data channel with three users. (b) D-AMPS channel with six

users

213 543216

1 65432

3 1

1 2 3 1 2 3 1850.01 MhzMS-BS

1930.05 MhzBS-MS

TDM frame40 msec

Upstrm

Downst

TDM frame40 msec

2

1850.01 Mhz

MS-BS

1930.05 Mhz

BS-MS

324 bit slot=64 bits of control+101 bits of error correction+159 bits of speech data.Speed=50 slots/sec

D-

AM

PS

1/3 of time a MS is idle = line quality measurement

1/6 of time a MS is idle = line quality measurement

users

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D-

AM

PS

G-Guard Time; R-Ramp Time

RSVD-Reserved for Future Use

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IS-54 slot and frame structure

BASE TO MOBILE

SLOT 1 SLOT 2 SLOT 3 SLOT 4 SLOT 5 SLOT 6

Frame1944 bits in 40 ms( 48600 b/s)

G6

R6

DATA16

SYNC28

DATA122

SACCH12

DVCC 12

DATA122

MOBILE TO BASE

DATA130

DATA130

DVCC 12

SACCH 12

SYNC28

RSVD 12

G:GUARD TIME R:RAMP TIMEDVCC: DIGITAL VERIFFICATION COLOR CODERSVD: RESERVE FOR FUTURE USE

D-

AM

PS

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D-

AM

PS

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D-AMPS Control structure, conceptually, like AMPS

Groups of 16 frames form a superframe, with control information used: 1. system configuration, 2. real-time control,3. non real-time control, 4. paging, 5. access response, 6. short messages. When a mobile is switched on, it makes contact with the base station toannounce itself and then listens on a control channel for incoming calls.Having picked up a new mobile, the MTSO informs the user’s homebase where he is, so calls can be routed correctly.One difference between AMPS and D-AMPS is how handoff is handled. A. In AMPS, the MTSO manages it completely without help from MS;B. D-AMPS, 1/3 of the time a mobile is neither sending nor receiving. It uses these idle slots to measure the line quality. As in AMPS, it stilltakes about 300 msec to do the handoff. This technique is called: MAHO (Mobile Assisted Handoff).

D-

AM

PS

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Channel multiplexing FDM+6 TDM slots

Uplink (initially)

(later)

824-849 MHz; 832 channels (x6=4992)

1850-1910 MHz

Downlink (initially)

(later)

869-894 MHz; 832 channels

1930-1990 MHz

Channel Bandwidth 30 kHz

FDD separation 45 MHz

Modulation FM (traffic, voice); FSK (control) PCM

Channels Control, Paging, Access, Data

Channel Rate 48.6 kbps

Voice compression 56 kbps to 8 kbps; PCM

TDM frames 25 frames, 40 msec each

Time slots 6 slots, 6.67 msec each; (25x6=150)

D-AMPS; IS-54, IS-136 D

-A

MP

S

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2G; GSM (Group Special Mobile)

(Global System for Mobile) • Pan-European Cellular Standard: 2G; Digital• FDD: (890-915 MHz Upstr; 935-960 MHz Downstr.)

• 124 frequency carriers; 8 channels per carrier • Carrier spacing: 200 KHz (Narrowband TDM)• Speech coder: linear coding (rate = 13 Kbps)• Modulation: PSK • Slow FHSS modulation (217.6 hops/s) to

overcome multipath fading.• First approximation, GSM is similar to D-• AMPS.

GS

M

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First approximation, 2G; GSM is similar to 2G; D-AMPS.

GSM D-AMPS

FDM FDM

Dual simplex channels Dual simplex channels

Single frequency pair is split by TDM

Single frequency pair is split by TDM

Channel 200 kHz wide Channel 30 kHz wide

Higher data rate

D-AMPS is used in the U.S. and in Japan (modified). Everywhere else in the world used GSM

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GSM-The Global System for Mobile Communications

959.8 MHz

935.4 MHz

935.2 MHz

914.8 MHz

890.4 MHz 890.2 MHz

Frequency

Simplex. Chann.124

BS to2 MS

1

124

MS To2 BS

1

Time

TDM frame

GSM uses 124 frequency channels, each of which uses an 8-Slot TDM system. (In the Figure one TDM frame is absent). 992 Ch/cell.

GS

M

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Broade single-user bandwidth (1/Tb)

GSM = FDMA 200 KHz + TDMA 8 slot/frame

t

ts1(t)

sN(t)1 2 N

t

N *T frame

T frame

TDMA

T frame

T frameT frame

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A portion of the GSM framing structure

11 14Ctl 13 15 23 24 1 2 3 4

32,500 bit multiframe sent in 120 msec.

0 1 2 3 4 5 6 7

148-Bit data frame sent in 4.615 msec

8.25-bit30 mk. secguard time

000 Information Sync Information 000

148-Bit frame sent in 547 mk.sec

Bits 3 57 26 57 3

Voice/data bit

GS

M

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GS

M

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GS

M

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GSM system• A GSM system has 124 pairs of simplex channels; • Each 200 kHz wide.• Supports eight separate connections on it, using TDM. • Each active station is assigned one time slot on one channel

pair.• 992 channels can be supported in each cell, but many of them

are not available, to avoid frequency conflict with neighboring cells.

• The eight shaded time slot all belong to the same connection.• Four of them in each direction. • Transmitting and receiving does not happen in the same time

slot because the GSM radios cannot transmit and receive at the same time and it takes time to switch from one to the other.

GS

M

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GSM system (Cont)• A data frame is transmitted in 547 mksec, but a transmitter is only

allowed to send one data frame every 4.615 msec, since it is sharing the channel with seven other stations.

• The gross rate of each channel is 270,833 bps, divided among eight users. This gives 33.854 kbps gross, more than double D-AMPS, 324 bits, 50 times per second for 16.2 kbps.

• With AMPS, the overhead cats up a large fraction of the bandwidth, ultimately leaving 24.7 kbps worth of payload per user before error correction.

• After error correction, 13 kbps is left for speech, giving better voice quality than D-AMPS (using correspondingly more bandwidth). See pp.30, eight data frames make up a TDM frame and 26 TDM frames make up a 120-msec multiframe. Of the 26 TDM frames in a multiframe, slot 12 is used for control and slot 25 is reserved for future use, so only 24 are available for user traffic.

GS

M

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Control Channels (CC)• CC used to manage the system. • The broadcast control channel (BCC) is a continuous stream

of output from the BS containing the BS’s identity and the channel status. All MS monitor their signal strength to see when they moved into a new cell.

• The dedicated control channel (DCC) is used for location updating, registration, and call setup. In particular, each BS maintains a database of MS. Information needed to maintain this database and is sent on the dedicated control channel.

• The common control channel (CCC), which is split up into three logical sub-channels:

1. Is the paging channel CC3a), which the BS uses to announce incoming calls. Each MS monitors it continuously to watch for call it should answer.

2. Is the random access channel (CC3b). This allows users to request a slot on the dedicated control channel. If two requests collide, they are garbled and have to be retried later.

3. Is the access grant channel (CC3c). The announced assigned slot.

GS

M

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Some of these slots are used to hold several control channels used to manage the system

Broadcast control channel

(BCC)

Continuous stream of output from the BS, containing the BS’s identity and the channel status.

All MS monitor their signal strength to see when they moved

into a new cell

Dedicated control channel (DCC)

For location updating, registration, and call

setup

Common control channel (CCC)

1. paging channel To announce incoming

calls

2. random access channel To request a slot on the dedicated

control channel

3. access grant channel announced assigned slot

3 logical subchannels:

GS

M

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BCCH: Broadcast Control Channel

point-to-multipoint unidirectional control

channel broadcasting system information

to MS

CCCH: Common Control Channel

up-link: RACH (Random Access Channel)

down-link: PCH (Paging Channel)

AGCH (Access Grant Channel)

DCCH: Dedicated Control Channel

point-to-point bidirectional control channel

SACCH (Slow Associated Control Channel)

FACCH (Fast Associated Control Channel)

SDCCH (Stand Alone Dedicated Control

Channel)

GSM Signalling channels

GS

M

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GS

M

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GS

M

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GS

M

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Channel multiplexing FDM+8 TDM slots

Uplink (GSM900)

(DCS1800)

890-915 MHz; 125 channels (x8=1000)

1710-1785 MHz

Downlink (900)

(DCS1800)

935-960 MHz; 125 channels (x8=1000)

1805-1880 MHz

Channel Bandwidth 200 kHz

FDD separation 45 (900) / 95 (1800) MHz

Modulation FSK

Channels Brdcst. Cont; Ded. Cont; Comn. Cont.= =Paging.+Rndm. Access+Acc. Grnt.

Channel Rate 13 kbps

TDM frames 24 frames, 120 msec each

Time slots 8 slots, 0.577 msec each; (24x8=192)

GSM;

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CDMA (Code Division Multiple Access): IS-95QUALCOMM, San Diego

• Based on DS spread spectrum

• Two frequency bands (1.23 Mhz), one for forward channel (cell-site to subscriber) and one for reverse channel (sub to cell-site)

• CDMA allows reuse of same spectrum over all cells. Net capacity improvement:

– 4 to 6 over digital TDMA (eg. GSM)– 20 over analog FM/FDMA (AMPS)

Access techniques for mobile communications

P - PowerT - TimeF - Frequency

P

T

P

T

F

P

T

F

FDMA (TACS)

TDMA (GSM, DECT)

CDMA (UMTS)

F

ATDMA (UMTS)

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CDMA (Code Division Multiple Access)

• unique “code” assigned to each user; i.e., code set partitioning

• all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

• Note: chipping rate >> data rate (eg, 64 chips per data bit)

• encoded signal = (original data bit) X (chipping sequence)

• decoding: inner-product of encoded signal and chipping sequence

• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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CDMA Encode/Decode

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CDMA: two-sender interference

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CDMA (cont’d)

• One of 64 PS (Pseudo Random) codes assigned to subscriber at call set up time

• RAKE receiver (to overcome mpath-fading)• Pilot tone inserted in forward link for:

– power control

– coherent reference

• Speech activity detection • Voice compression to 8 kbps (16 kbps with FEC)

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