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Lò vi sóng RF và hệ thống không dây P10

Multiple-Access Techniques Three commonly used techniques for accommodating multiple users in wireless communications are frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). Frequency division multiple access and TDMA are old technologies and have been used for quite a while. Code division multiple access is the emerging technology for many new cellular phone systems. This chapter will brie¯y discuss these techniques. | RF and Microwave Wireless Systems. Kai Chang Copyright 2000 John Wiley Sons Inc. ISBNs 0-471-35199-7 Hardback 0-471-22432-4 Electronic CHAPTER TEN Multiple-Access Techniques 10.1 INTRODUCTION Three commonly used techniques for accommodating multiple users in wireless communications are frequency division multiple access FDMA time division multiple access TDMA and code division multiple access CDMA . Frequency division multiple access and TDMA are old technologies and have been used for quite a while. Code division multiple access is the emerging technology for many new cellular phone systems. This chapter will briefly discuss these techniques. 10.2 FREQUENCY DIVISION MULTIPLE ACCESS AND FREQUENCY DIVISION MULTIPLEXING For the FDMA and frequency division multiplexing FDM systems the available frequency band is split into a specific number of channels and the bandwidth of each channel depends on the type of information to be transmitted. To transmit a number of channels over the same system the signals must be kept apart so that they do not interfere with each other. Figure 10.1 shows an example of the FDM transmitter system with simultaneous transmission of 10 signals from 10 users. Each signal contains video information from 0 to 6 MHz with a guard band of 4 MHz. A double side band DSB modulator is used. The guard band is placed between two adjacent signals to avoid interference. A multiplexer is used to combine the signals and the combined signals are then upconverted and amplified. In the receiver the signals are separated by a multiplexer that consists of many filters. The information is recovered after the demodulator. Figure 10.2 shows a receiver block diagram. The advantage of FDMA is that no network timing 294 10.3 TIME DIVISION MULTIPLE ACCESS AND TIME DIVISION MULTIPLEXING 295 T r . Modulator Information Si vy 0-10 MHz I Access 1 705 MHz s2 - 0- 10 MHz Access 2 1 C2 715 MHz Sio 0-10 MHz Access 10 700-710 MHz 710-720 MHz 790 - 800 MHz 700 - 800 MHz .