Frequency Division Multiple Access (FDMA)

Of the common
multiple access techniques used in mobile communications systems,FDMA
is the simplest.With FDMA, the available spectrum is divided into a number
of radio channels of a specified bandwidth, and a selection of these channels
is used within a given cell. In analog AMPS, for example, the available
spectrum is divided into blocks of 30 kHz. A number of 30-kHz channels
are allocated to each cell, depending on the expected traffic load for the cell.
When a subscriber wants to place a call, one of the 30-kHz channels is allocated
exclusively to the subscriber for that call.
In most FDMA systems, separate channels are used in each direction—
from network to subscriber (downlink) and from subscriber to network
(uplink). For example, in analog AMPS, when we talk about 30-kHz channels,
we are actually talking about two 30-kHz channels, one in each direction.
Such an approach is known as Frequency Division Duplex (FDD) and
normally a fixed separation exists between the frequency used in the uplink
and that used in the downlink. This fixed separation is known as the duplex
distance. For example, in many systems in North America, the duplex distance
is 45 MHz.Thus, in such a system, channel 1 corresponds to two channels
(uplink and downlink) with a separation of 45 MHz between them. An
FDD FDMA technique can be represented as shown in Figure 1-7.
FDD is not the only duplexing scheme, however. Another technique
known as Time Division Duplex is also used. In such a system, only one
channel is used for both uplink and downlink transmissions.With TDD, the
channel is used very briefly for uplink, then very briefly for downlink, then
very briefly again for uplink, and so on. TDD is not very common in North
America, but it is widely used in systems deployed in Asia.