Chapter 16 Wireless WANs Cellular Telephone and Satellite

Chapter 16 Wireless WANs: Cellular Telephone and Satellite Networks Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.

16 -1 CELLULAR TELEPHONY Cellular telephony is designed to provide communications between two moving units, called mobile stations (MSs), or between one mobile unit and one stationary unit, often called a land unit. Topics discussed in this section: Frequency-Reuse Principle Transmitting Receiving Roaming First Generation Second Generation Third Generation 16. 2

Wireless Communications n When? n n n Mobile communications is needed Terrain makes wired communication difficult Communications must be set up quickly Communications must be installed at low cost Same information broadcast to many locations

Wireless Disadvantages n n More susceptible to interference, noise, signal loss, and eavesdropping Generally lower data rate than wired Frequencies interfere in close proximity Less connection stability

Cellular Network Organization n Multiple low power transmitters n n 100 w or less Area divided into cells n n n Each with own antenna Each with own range of frequencies Served by base station n n Transmitter, receiver, control unit Adjacent cells on different frequencies to avoid crosstalk

Shape of Cells n Square n n Width d cell has four neighbors at distance d and four at distance d Better if all adjacent antennas equidistant n n Simplifies choosing and switching to new antenna Hexagon n n Provides equidistant antennas Radius defined as radius of circum-circle n n n Distance from center to vertex equals length of side Distance between centers of cells radius R is R Not always precise hexagons n n n Topographical limitations Local signal propagation conditions Location of antennas

Cellular Geometries

Frequency Reuse n Power of base transceiver controlled n n n Allow communications within cell on given frequency Limit escaping power to adjacent cells Allow re-use of frequencies in nearby cells Use same frequency for multiple conversations 10 – 50 frequencies per cell E. g. n n The pattern consists of N cells K total number of frequencies used in systems Each cell has K/N frequencies Advanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average

Characterizing Frequency Reuse n n D = minimum distance between centers of cells that use the same band of frequencies (called cochannels) R = radius of a cell d = distance between centers of adjacent cells (d = R) N = number of cells in repetitious pattern n Hexagonal cell pattern, following values of N possible n n Reuse factor Each cell in pattern uses unique band of frequencies N = I 2 + J 2 + (I x J), I, J = 0, 1, 2, 3, … Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, … D/R= D/d =

Frequency Reuse Patterns

Figure 16. 2 Frequency reuse patterns 16. 11

N=7, 32 cells, R=1. 6 km, in total 336 channels

Operation of Cellular Systems n Base station (BS) at center of each cell n n Controller handles call process n n n One MTSO serves multiple BS MTSO to BS link by wire or wireless MTSO: n n n Number of mobile units may in use at a time BS connected to mobile telecommunications switching office (MTSO) n n Antenna, controller, transceivers Connects calls between mobile units and from mobile to fixed telecommunications network Assigns voice channel Performs handoffs Monitors calls (billing) Fully automated

Overview of Cellular System

Call Stages

Three Generations n 1 st Generation n n 2 nd Generation n n based on analog voice using frequency modulation digital techniques and time-division (TDMA) or code-division multiple access (CDMA) 3 rd Generation n broadband access for personal communications services (PCS)

Note AMPS is an analog cellular phone system using FDMA. 16. 25

Figure 16. 5 Second-generation cellular phone systems 16. 26

Advanced Mobile Phone Service n n n 1 st Generation most common mobile phone service since early 80’s developed by AT&T

AMPS Spectral Allocation n Two 25 -MHz bands n n n Each split in two to allow competition n n base to mobile (869 -894 MHz) mobile to base (824 -849 MHz) each operator allocated 12. 5 MHz bands 416 channels per operator n 395 for calls, 21 for control data

Figure 16. 3 Cellular bands for AMPS 16. 29

Figure 16. 4 AMPS reverse communication band 16. 30

AMPS Spatial Allocation n n n Limited channels dictate frequency reuse in nearby cells Generally 10 to 50 frequencies assigned to cell Pattern of 7 cells smallest allowing sufficient isolation 57 frequencies per cell 6. 5 to 13 km per cell May be split with lower power

Note D-AMPS, or IS-136, is a digital cellular phone system using TDMA and FDMA. 16. 32

Figure 16. 6 D-AMPS 16. 33

AMPS Components n Mobile Units n n n Base Transceiver n n contains a modem that can switch between many frequencies 3 identification numbers: electronic serial number, system ID number, mobile ID number full-duplex communication with the mobile Mobile Switching Center

AMPS Mobile Units n n n Modem that can switch between frequencies Power output of unit controlled to match size of cell Three identification numbers n n n electronic serial number - 32 bits system operator identification number - 15 bits mobile identification number - 34 bits - phone #

AMPS Logon n n When mobile becomes operational, it senses control channels to determine channel and base station received best Exchanges information via base station Announces its system id # to identify its home carrier Home carrier contacted for authorization and to locate mobile for incoming calls

AMPS Handoffs n Roaming operator must move between cells n n Different cells have different frequencies and power levels Choice of handoff depends on received power from base stations and controlled by mobile switching center

Global System for Mobile Comm. n n n 2 nd Generation First appeared in 1991 in Europe Similar to working of AMPS Designed to support phone, data, and image Rates up to 9. 6 kbps GSM transmission is encrypted using secret keys

Global System for Mobile Communication n n Developed to provide common 2 ndgeneration technology for Europe 200 million customers worldwide, almost 5 million in the North America GSM transmission is encrypted Spectral allocation: 25 MHz for base transmission (935– 960 MHz), 25 MHz for mobile transmission (890– 915 MHz)

GSM SIM n n n Subscriber Identity Module Smart card or plug-in module to activate unit stores n n subscriber’s identification number networks subscriber is authorized to use encryption keys Can use any unit anywhere with your SIM

Multiple Access n Four ways to divide the spectrum among active users n n frequency-division multiplexing (FDM) time-division multiplexing (TDM) code-division multiplexing (CDM) space-division multiplexing (SDM)

GSM Access Methods n n FDM too wasteful TDMA - time-division multiple access n n early lead - more successful experience CDMA - code-division multiple access n n n theoretical advantages increased range choice for 3 rd generation

Figure 16. 7 GSM bands 16. 45

Figure 16. 8 GSM 16. 46

Figure 16. 9 Multiframe components 16. 47

Note GSM is a digital cellular phone system using TDMA and FDMA. 16. 48

Figure 16. 10 IS-95 forward transmission 16. 49

Figure 16. 11 IS-95 reverse transmission 16. 50

Note IS-95 is a digital cellular phone system using CDMA/DSSS and FDMA. 16. 51

Choice of Access Methods n n FDM, used in 1 st generation systems, wastes spectrum Debate over TDMA vs CDMA for 2 nd generation n n TDMA advocates argue there is more successful experience with TDMA. CDMA proponents argue that CDMA offers additional features as well, such as increased range. TDMA systems have achieved an early lead in actual implementations CDMA seems to be the access method of choice for thirdgeneration systems

3 rd Generation Wireless n n Provide high speed wireless for voice, data, video and multimedia ITU’s view n n n voice quality of wired 144 kbps high-speed roaming / 384 kbps low-speed adaptive interface to internet for asymmetric speed more efficient use of spectrum support wide variety of equipment, services, etc

PCS & PCN n Personal Communications Services (PCS) n n n find person easily use communication system anywhere with single account Personal Communications Network (PCN) n use terminal in wide variety of environments to connect to information services

WAP n Wireless Application Protocol n n n universal, open standard - WAP forum provide mobile users access to information services, including internet and web Works with wireless network technologies Based on existing internet standards such as TCP, IP, HTTP, HTML, XML Support limited resources in and variety of mobile devices

WAP Specs n Include n n n programming model Wireless Markup Language (adhering to XML) Microbrowser Lightweight protocol stack Framework for wireless telephony applications

Note The main goal of third-generation cellular telephony is to provide universal personal communication. 16. 57

Figure 16. 12 IMT-2000 radio interfaces 16. 58