## Wireless FDMA operation

FDMA is widely used in wireless communications systems where the radio environment creates several challenges for any multiple access method owing to the unpredictable and time varying nature of the communications channel. One of the biggest challenges is the very large variations in received signal power that arise from users in different frequency slots due to the so-called near-far effect. A radio user that is very near to a base-station receiver will produce a much stronger signal than that from a distant (far) user operating on the extreme of the communication range. Typical variations in power can be up to 100 dB. If the strong signal is producing any out-of-band radiation in the slot occupied by the weak signal, this can easily swamp the weak signal corrupting the communications. Much of the discussion in this book on controlling the bandwidth and side-lobe energy of digital modulation formats, such as CPFSK, and on designing modulation formats that are not overly sensitive to amplifier distortion, such as p/4 QPSK, are all driven by this near-far problem in the wireless application. |

Other challenges in the radio environment include dealing with the frequency uncertainty for any individual user caused by Doppler shift and local oscillator error. This inevitable error requires guard-bands to be allocated between frequency slots, thus sacrificing some of the efficiency of the FDMA scheme.