Chapter 14 Cellular Wireless Networks
Evolu&on of Wireless Communica&ons 1901 Marconi: Trans-Atlantic wireless transmission 1906 Fessenden: first radio broadcast (AM) 1921 Detroit Police Dept wireless radio Mobile telephone introduced in 1946 in 25 cities. Since then both fixed and mobile wireless evolved to cover 4 classes of communications. 2
Fixed vs Mobile Wireless Name the major fixed wireless systems. Until 1980 s, main wireless were microwave and satellite communications systems Recently Wimax & WLAN Name the major mobile wireless systems. Cellular mobile What are the main differences between fixed and mobile wireless systems? Power Antenna 3
Four Classes of Wireless Communica&ons
A rough wireless design exercise Q1. What frequency band? Q2. What is the coverage area (range)? Q3. How much power should be transmitted? Q4. How much power should be received? Q5. How many users can this system support? 5
First Genera&on Cellular Systems Bell Labs developed the cellular concept in 1960 s and 1970 s. Existing technology could not support the concept. Late 70 s saw the development of practical RF circuits which made cellular telephony possible. 6
What are the central concepts of cellular systems? 1. Instead of one big cell, cover the same area using several small cells. 2. Re-use the available frequency band as many times as practical 7
Frequency Reuse Example
Overview of Cellular System
Cellular Telephone Systems MSC PSTN Base sta&on Mobile -Each mobile communicates via radio to one of the base stations and may be handed off a number of times throughout the call. -Mobile terminal contains a transceiver, an antenna, and control circuitry and is usually a handheld device. PSTN: Public Switched Telephone Network MSC: Mobile Switching Centre 10
Base Stations Consists of several transmitters and receivers which simultaneously handle full duplex communications. Generally have towers which support several receiving and transmitting antennas. It serves as a bridge for all mobiles in its cell and connects all simultaneous calls via telephone or microwave links to the MSC. 11
Mobile Switching Center The MSC coordinates the activities of all base stations and connects the entire cellular network to the PSTN. A typical MSC handles 100,000 cellular subscribers and 5,000 simultaneous calls. It handles all billing and system maintenance functions. In large cities, a carrier will employ several MSCs. 12
Communication Between Base and Mobile Defined by a standard common air interface that specifies (at least) 4 different channels. Forward traffic channels for voice and data transmission from base to mobile. Reverse traffic channels for voice and data transmission from mobile to base. Forward control channel(s) and reverse control channel(s) for setting up calls. Also carry service requests and are monitored by a mobile when not active. 13
Frequency Reuse 1 2 4 3 7 5 6 Reuse Distance 2 7 1 3 6 4 5 Interference from cells employing same frequencies: Co- channel interference 14
Frequency Re-use Suppose a cellular provider is allocated 333 channels, and its coverage area is served by 28 cells. No frequency re-use: Each cell can support 11 or 12 channels only. Frequency reuse with cluster size of 7: All cells are grouped into clusters of 7 cells and each cluster employs all of the frequencies: Each cell can now support 47 or 48 channels. Capacity is increased by a factor of 4. 15
Frequency Reuse PaQerns
Evolution of N. A. Cellular Radio First generation (1G). ANALOG! In 1983 FCC allocated 666 duplex channels for US Advanced Mobile Phone System (AMPS). Each duplex channel had bandwidth of 60 khz, for a total bandwidth of 40 MHz in the 800 MHz band. Each city was allowed 2 cellular providers. In 1989, an additional 10 MHz was granted. Employed FDMA with frequency re-use (a carrier frequency is reused every 7th cell). 17
AMPS Call Sequence 1. subscriber ini&ates call keying in number 2. MTSO validates telephone number and checks user authorized to place call 3. MTSO issues message to user's phone indica&ng traffic channels to use 4. MTSO sends ringing signal to called party 5. when called party answers, MTSO establishes circuit and ini&ates billing informa&on 6. when one party hangs up MTSO releases circuit, frees radio channels, and completes billing informa&on
Call Stages
2G in North America 2G, DIGITAL In 1991, the first US digital cellular (USDC) system was introduced (also called DAMPS) Each 30 khz single user channel is to be replaced by a three user channel employing TDMA. Predic&ve speech coding is used (4.8 kbps) - each 20ms burst of data is encoding a 60 ms burst of speech - thus speech appears con&nuous. In the 90s, Qualcomm has introduced a CDMA standard (IS- 95) which reuses frequencies in each cell. The minimum required bandwidth of 1.25 MHz is shared by several users.
2G in Europe and elsewhere In late 80 s, GSM was introduced in Europe. In GSM 8 users share 200 khz bandwith using TDMA. Each user transmit in one of 8 &me slots at a rate of 192 kbps (at an average rate of approx. 192/8 kbps).
2G using CDMA (IS- 95) provide higher quality signals, higher data rates, support digital services, with overall greater capacity key differences include Full frequency reuse BeQer security Poten&al of higher data rates
Reuse Patterns TDMA or FDMA CDMA 23
CDMA In CDMA signals of different users overlap in &me and frequency. The separa&on is achieved by assigning different codes to each user. 24
25 User 1,2&3 data before and after TDMA
CDMA-data 1 User 1 Data User 1 code Spread data 26
27 CDMA-data 2
28 CDMA-data 3
Third Genera&on Systems high- speed wireless communica&ons to support mul&media, data, and video in addi&on to voice 3G capabili&es: voice quality comparable to PSTN 144 kbps available to users over large areas 384 kbps available to pedestrians over small areas support for 2.048 Mbps for office use symmetrical and asymmetrical data rates packet- switched and circuit- switched services adap&ve interface to Internet more efficient use of available spectrum support for variety of mobile equipment allow introduc&on of new services and technologies