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5-1

Cellular Telephone

This chapter contains some slides from chapter 17 of Data Communications and networking, 3rd ed. by B. Forouzan. The borrowed slides contain the copyright notice of McGraw-Hill at the bottom of the slide. Most of these slides are either modified or supplemented by additional information. Slides without any copyright notice were taken from the web.

Part II : Wireless Communication

Chapter 5

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5-2Cellular System

Mobile station

Base station(each cell has one BS)Also called Mobile Telephone Switching Office (MTSO)

Coordinates communication between all base stations and the telephone central office

Size of cell depends on population(1-12 miles radius)

This network contains telephone central offices (COs)

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5-3Cellular System (Cont.)

The neighboring cells can not use the same set of frequencies in order to avoid interference between MS close to cell boundaries. Since the available frequencies are a limited resource, they have to be reused. Patterns below show variousfrequency reuse factors (cells with same numbers can use the same set of frequencies.)

One cell separation Two cells separation

Frequency Reuse

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5-4AMPSCellular Bands for AMPS

25 MHz forward and reverse bands832 full-duplex channels (42 for control, 790 for voice) Frequency reuse factor = 7 !!!! 790/7 channels available per cell

25 MHz 25 MHzUplink

Downlink

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5-5

Figure 17.4 AMPS reverse communication band

AMPS (Cont.)Reverse Communication Band

Uplink

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5-6Second Generation Cellular Systems

Digital AMPS1991

.

824-849 MHz (upload)869-894 MHz (download)

Global System for Mobile Communication

(European standard)1990

.

890-915 MHz (upload)935-960 MHz (download)

Code Division Multiple Access

1993.

824-849 MHz (upload)869-894 MHz (download)

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5-7

Total subscribers in 2002 = 249.1 million

Second Generation Cellular Systems

(Published by Lucent Technologies)

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5-8D-AMPS

VSELPVector Sum Excited Linear Predictive speech coding

Frequency reuse factor = 7

(Originally defined as IS-54, later revised by IS-136)

D-AMPS uses the same band and channels as AMPS

25 1944-bit frames per second

Control Voice FEC64 bits 159 bits 101 bits

Reverse communication band

Uplink

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5-9GSM

= =

Frequency Bands

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5-10GSM (Cont.)

RPE-LTPRegular Pulse Excited Long

Term Prediction speech coding(Excellent speech quality at high

complexity, noise immune)

GMSKGaussian Minimum

Shift Keying(used in Europe)

24 traffic + 2 control frames

890-915 MHz(935-960 MHz downlink)

260-bit samples each 20ms = 13 kbps

Uplink

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5-11GSM (Cont.)Multiframe Components

A lot of overhead in TDM framesBecause of complex error correctionGSM allows a r frequency reuse factor of 3

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5-12IS-95 (Cont.)

Downlink

Forward Transmission

CELPCode excited linear predictive speech coding (very complex, very high quality)

1 bit of 64(not DSSS, for privacy)

64 chip Walsh code (Orthogonal spreading)Code assigned to MS during the call setup

For synchronization of demodulator/CDMAand for signal strength monitoring for handoff

64 CDMA channels(9 control and sync, 55 voice)

Channels 1-7 for pagingchannel 32 for info to MS about the system

869-894 MHz

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5-13

Electronic Serial Number (ESN).

The unique 32-bit identification number embedded in a wireless phone by the manufacturer. Each time a call is placed, the ESN is automatically transmitted to the base station so the wireless carrier's mobile switching office can check the call's validity. The ESN cannot easily be altered in the field. The ESN differs from the mobile identification number (MIN), which is the wireless carrier's identifier for a phone in the network. MINs and ESNs can be electronically checked to help prevent fraud.

Mobile Identification Number (MIN).

10-digit number (which is represented by 34 bits) that uniquely identifies a mobile unit within a wireless carrier's network. The MIN often can be dialed from other wireless or wireline networks. The number differs from the electronic serial number (ESN), which is the unit number assigned by a phone manufacturer. MINs and ESNs can be checked electronically to help prevent fraud (Example: 6195947898, meaning (619) 594-7898.)

IS-95 (Cont.)

MFR code reserved Serial Number31 24 23 18 17 0

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5-14IS-95 (Cont.)Reverse Transmission

Uplink

The diagram for access channels is identical, only the data bit rate is fixed to 4.8 kbps

242-1 period length

94 channels (32 access, 62 voice)Control channels use for:

- call setup- response to paging message- location update

824-849 MHz

32-bit

Similar to CCK, only with Walsh codes-each group of 6 bits from the 28.8 kbps streamreplaced by a 64-bit Walsh code from the lookup table(28.8 ksps*64/6 = 307.2 kcps, 307.2/64 = 4.8 ksps)Improves reception at BS since the Walsh codes are orthogonal. ( Form of block error-correcting code)

Long code mask- unique to MS(4 PN chips per one Walsh chip)

Rates 4.8, 2.4 and 1.2 also available

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5-15IS-95 (Cont.)Multiple Access

Forward transmission uses CDMA (based on Walsh codes) to resolve the contention.This is possible because BS transmits to all MSs simultaneously and therefore does multiplexing of all channels. In addition, BS transmits the pilot signal for all MS, so they can synchronize their CDMA demultiplexers.

Reverse transmission can not use CDMA forcontention resolution because there is difficultto synchronize reverse receivers in BS (the pilot signal must be transmitted from only one device). Instead, MS transmitters use DSSSbased on spreading sequences that are uniquefor each MS (ESN).

MS 1

MS n

BS

Forward

Reverse

(DSSS)

Reverse(DSSS)

CDMA

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5-16

1100330338Users per cell

137Frequency reuse factor

199319901991Year introduced

9.6 kbps13 kbps7.95 kbpsSpeech coding bit rate

5583Users per channel

CELPRPE-LTPVSELPSpeech coder

20124790Number of duplex channels

1,228 kHz200 kHz30 kHzChannel bandwidth

IS-95GSMIS-136

Summary of 2G

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5-17Connection Management in Cellular Systems

Transmitting1. User enters the telephone number (MIN) for destination telephone. 2. MS scans the band for an analog channel that has the strongest signal.3. MS sends the MIN to the closest BS.4. BS relays MIN to MSC5. MSC sends MIN to telephone CO. If the called party is available the

connection is made and CO relays the result back to MSC.6. MSC assigns an unused voice channel to the call.7. MS tunes to the new channel and communication can begin.

Receiving1. When a MS is called, the telephone CO sends MIN to MSC2. MSC searches for the location of MS by sending query signal to each

cell (paging).3. When MS is found, the MSC transmits the ringing signal to MS4. When MS answers, MSC assigns a voice channel to the call, voice

communication begins

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5-18Connection Management in Cellular Systems (Cont.)

HandoffMS can move (during conversation) from one cell to another, resulting in a weaker signal. Therefore MSC monitors the level of the signal every few seconds. If the level of the signal diminishes, the MSC seeks a new cell that can better accommodate the communication. MSC then changes the channel carrying the call.

Hard HandoffIn early systems a MS can communicate with only one BS. When MS moves from one BS to another, the communication must first be broken, then established with new BS. Rough transmission.

Soft HandoffNew systems. MS can communicate with two BS at the same time. MS can continue communication via new BS before breaking with the old BS.

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5-19

Personal Communication Services(PCS)

The term PCS refers to a wide variety of wireless access and personal mobility services provided through a small terminal, with the goal of enabling communications at any time, at any place and in any form. (PCS doesnt refer to a single technology or standard such as IS-136, GSM, IS-95.)

Some services are short message service (SMS) and limited access to Internet.

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5-20

Wireless hand held devices (cell phones, pagers, PDA,) have limitations:

Small displaySmall keypadSmall CPUSmall memoryShort battery lifeLimited data bit rateLonger latencyLower reliability (stability)Browser must be simple (nave users)

WAP is designed to integrate lightweight WEB browser into hand held devices which are bearer independent (D-AMPS, GSM, IS-95, 3G,).

WAP was designed by WAP Forum, established by Ericsson, Nokia, Motorola and Phone.com in 1997. Today, the forum has hundreds of members.

Wireless Application protocol (WAP)

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5-21WAP (Cont.)WAP Programming Model

WAEuser agent

Client WAP Gateway Original Server

Encodersdecoders

CGIScripts

Encoded requestsEncoded requests RequestsRequests

Response (content)Response (content)Encoded responseEncoded response

Proxy server for the wireless domain

(DNS, WAP-WWW protocol conversion,HTML-WML translation, encoding, caching)

HTTP content intext format

Requests in compact(binary) format

WML contentin compact

(binary) format

Requests in textformat

Has microbrowser

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5-22WAP (Cont.)

GSMD-AMPS IS-95 3G Bluetooth

Wireless Datagram Protocol (WDP)IP

UDP

Wireless Transport Layer Security (WTLS)

Wireless Transaction Protocol (WTP)

Wireless Session Protocol (WSP)

Wireless Application Environment (WAE)

Wireless Markup Language (WML) WML Script

WAP Protocol Stack

Similar to HTMLbut smaller,

less demanding

Similar to JavaScriptbut smaller,

less demanding

Tools for WAP application

development

Based on HTTP(Session

establishment/termination)

Provides end-to-end security between the client and

the WAP gateway( data integrity, privacy,

authentication and denial-of-service detection

- based on TLS)

Bearer

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5-23

HTTP Hypertext Transfer ProtocolSSL Secure Sockets Layer

WAP (Cont.)WAP gateway Protocol Stack