FDMA

FDMA assigns individual channels to individual users. It can be seen from Figure 7.1 that each user is allocated a unique frequency channel. 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. In frequency division duplex (FDD) systems, the users are assigned a channel as a pair of frequencies; one frequency is used for the upward channel, while the other frequency is used for the downward channel.

FDMA is used by analog systems, such as AMPS, NMT, or Radiocom 2000. The advantages of FDMA are the following:

The complexity of FDMA systems is lower when compared to TDMA and CDMA systems, though this is changing as digital signal processing methods improve for TDMA and CDMA.

·         FDMA it is technically simple to implement.

·         A capacity increase can be obtained by reducing the information bit rate and using efficient digital codes.

·         Since FDMA is a continuous transmission scheme, fewer bits are needed for overhead purposes as compared to TDMA.

The disadvantages of FDMA include the following:

v  Only modest capacity improvements could be expected from a given spectrum allocation.

v  FDMA wastes bandwidth. If an FDMA channel is not in use, then it sits idle and cannot be used by other users to increase or share capacity.

v  FDMA systems have higher cell site system costs compared to TDMA systems because of the need to use costly bandpass filters to eliminate spurious radiation at the base station.

v  The FDMA mobile unit uses duplexers since both the transmitter and receiver operate at the same time. A duplexer adds weight, size, and cost to a radio transmitter and can limit the minimum size of a subscriber unit.

v  FDMA requires tight RF filtering to minimize adjacent channel interference.

v  The maximum bit rate per channel is fixed and small, inhibiting the flexibility in bit-rate capability that may be a requirement for 3G applications in the future.