7 - Wireless Network Access Technologies

Abstract

This chapter discusses three multiple access methods: frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). It also draws comparison between these three access methods. FDMA assigns individual channels to individual users. These channels are assigned on demand to subscribers who request service. Guard bands are maintained between adjacent signal spectra to minimize cross talk between channels. During the period of the call, no other user can share the same frequency band. FDMA is used by analog systems, such as AMPS, NMT, or Radiocom 2000. TDMA relies on the fact that the audio signal is digitized. It allocates a single frequency channel for a short period of time and then moves to another channel. The digital samples from a single transmitter occupy different time slots in several bands at the same time. GSM uses a TDMA technique, where the carrier is 200 kHz wide and supports eight full rate channels. A channel (roughly) consists of the recurrence every 4.6 ms of a time slot of 0.58 ms. In CDMA, each user is assigned a unique code sequence for encoding an information-bearing signal. The receiver, knowing the code sequences of the user, decodes a received signal after reception and recovers the original data. This is possible since the cross correlations between the code of the desired user and the codes of the other users are small.