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.
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 6MHz 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 ...