Frequency Division Multiple Access (FDMA)
In this technique the bandwidth is divided into a number of channels and distributed among users. The channels are assigned only when demanded by the users.Therefore when a channel is not in use it becomes a wasted resource.
Features of FDMA
- FDMA channels have narrow bandwidth (30 KHz) and therefore they are usually implemented in narrowband systems.
- Since the user has his portion of the bandwidth all the time, FDMA does not require synchronization or timing control,which makes it algorithmically simple.
- Even though no two users use the same frequency band at the same time, guard bands are introduced between frequency bands to minimize adjacent channel interference
- FDMA achieves simultaneous transmission and reception by using Frequency division duplexing (FDD).
- In order for both the transmitter and the receiver to operate at the same time, FDD requires duplexers which make the FDMA system expensive.
Nonlinear Effects in FDMA
- In a FDMA system, many channels share the same antenna at the base station. The power amplifiers or the power combiners
- When operated at or near saturation for maximum power efficiency, are nonlinear.
- The nonlinearities cause signal spreading in the frequency domain and generate intermodulation (IM) frequencies.
- IM is undesired RF radiation which can interfere with other channels in the FDMA systems
- Spreading of the spectrum results in adjacent-channel interference
- Intermediation is the generation of undesirable harmonics
Figure allocation of bands
Advantages of FDMA
- FDMA can be used for both analog and digital signals.
- A guard band, in which no signal is transmitted, must usually be allocated between each pair of channels to ensure that interference does not occur. As shown in Figure
- The allocation of capacity is simple.
- No network synchronization or timing is required.
- Baseband signal can be recovered using simple and inexpensive receiver equipment.
Disadvantages of FDMA
- FDMA include the need for guard bands between signals, which reduces the available bandwidth.
- FDMA is the relatively expensive and complicated band pass filters required.
- FDMA is the rather strict linearity requirement of the medium.
- Need for power balancing to avoid capturing of the receiver by a strong signal.
- The requirement for amplifier back off to eliminate intermodulation products and distortion due to the amplifier being driven into its non-linear stage.