Cellular Wireless Networks: Mostly Voice

• First Generation voice: Analog– AMPS: Advance Mobile Phone Systems– Residential cordless phones

• Second Generation: Digital– IS-54: North American Standard - TDMA– IS-95: CDMA (Qualcomm)– GSM: Pan-European Digital Cellular– DECT: Digital European Cordless Telephone

Cellular Wireless Networks: Mostly Data

• Wide-area data networks– ARDIS– RAM Mobile Data– CDPD

• Wireless LANs– InfraRed LANs– Radio LANs

• Paging systems

• Satellite systems (GEOs, MEOs, LEOs)

Third Generation Wireless Networks

PCN (Personal Communications Networks)– will merge: cellular, cordless, wireless LANs, paging

etc.– will support multimedia services (data, voice, video,

image)– several on going efforts:

(a) GPRS

(b) EDGE

(c) UMTS

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

BS BSBS

BSBSBS

Backbone Network

Characteristics of Radio MediumPath Loss

– Attenuation increases with respect to frequency and distance

• Free space loss = square of distance• Indoor mobile radio = quadrature of distance

Transmitter

R1 Distance

10 100 1000

Pow

er40 dB per decade

20 dB per decade

Distance

Characteristics of Radio Medium (cont’d)• Fading

– Multipath fading

– Shadowing

• Delay spread– Intersymbol interference

• Interference

Transmitter

Object

Receiver

Pow

er d

B

Deep fade - 40 dB

AMPS: Channel Assignment

In each cell, 57 channels each for A-side carrier and B -side carrier

Channels are divided into 4 categories:

1. Control (base to mobile) to manage the system.

2. Paging (base to mobile) to alert mobile users to incoming calls.

3. Access (bidirectional) for call set up and channel assignment.

4. Data (bidirectional) for voice, FAX, or data

B

A

ED

C

F

G

B

A

ED

C

F

G

B

A

ED

C

F

G Frequencies are not reused in adjacent cells

Handoff• Handoff: Transfer of a mobile from one cell to

another

• Each base station constantly monitors the received power from each mobile.

• When power drops below given threshold, base station asks neighbor station (with stronger received power) to pick up the mobile, on a new channel.

• The handoff process takes about 300 msec.

Digital Cellular: IS-54 TDMA System• Second generation: digital

• Same frequency as AMPS• Each 30 kHz RF channel is used at a rate of 48.6 kbps

– 3 TDM slots/RF band– 16.2 kbps TDM digital channel– 8 kbps voice coding

• 4 cell frequency reuse• Capacity increase per cell per carrier

– 3 x 416 / 4 = 312 (instead of 57 in AMPS)– Additional factor of two with speech activity detection.

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

GSM (Group Special Mobile)Pan European Cellular StandardSecond Generation: DigitalFrequency Division duplex (890-915 MHz Upstream; 935-960 MHz

Downstream)125 frequency carriersCarrier spacing: 200 Khz8 channels per carrier (Narrowband Time Division)

Speech coder: linear predictive coding (Source rate = 13 Kbps)

Modulation: phase shift keying (Gaussian minimum shift keying)

Multilevel, time division frame structure

Slow frequency hopping to overcome multipath fading

CDMA (Code Division Multiple Access): IS-95

QUALCOMM, 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)

CDMA (cont’d)• One of 64 PS (Pseudo Random) codes assigned

to subscriber at call set up time

• RAKE receiver (to overcome fading)

• Pilot tone inserted in forward link for:– power control– coherent reference

• Speech activity detection

• Voice compression to 8 kbps (16 kbps with FEC)

Data transmissions in cellular networks• modems

• CDPD (Cellular Digital Packet Data)• CDPD is built on top of AMPS• Packet switched digital datagram service• Any idle 30 kHz channel can be temporarily grabbed for

sending data frames at 19.2 kbps • Base station broadcasts on a dedicated down link channel

the data packets, along with info on idle uplink channels• Mobiles access the idle uplink channels in random access

mode

Limitations of cellular systems• wired backbone network infrastructure required. What if there

is no infrastructure, e.g.:– search and rescue

– disaster relief

– battlefield

• voice only (+ limited data, e.g. CDPD); not suitable for multimedia (data, voice, compressed video, image)

• only one channel rate (up to 20-30 kbps). In multimedia, more flexible bandwidth allocation is required, with different data rates; mix of datagram and stream traffic; admission control; QoS support

Packet Data: ARDIS• Nationwide packet radio• Partnership of IBM and Motorola• Coverage

– 400 metropolitan areas– 1300 base station

• Data rate– Original system

• 4800 bps(raw), 50% overhead due to FEC• MAX packet length 240 bytes

– New system• 19.2 kps• MAX packet length 512 bytes

• Frequency range– 855-865 MHz– RF channel bandwidth, 25 MHz