Frequency Division Multiple Access

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Frequency Division Multiple Access

FDMA splits the total bandwidth into multiple channels. Each ground station on the earth is allocated a particular frequency group (or a range of frequencies). Within each group, the ground station can allocate different frequencies to individual channels, which are used by different stations connected to that ground station. Before the transmission begins, the transmitting ground station looks for an empty channel within the frequency range that is allocated to it and once it finds an empty channel, it allocates it to the particular transmitting station.

This is the most popular method for communication using satellites. The transmission station on earth combines multiple signals to be sent into a single carrier signal of a unique frequency by multiplexing them, similar to the way a TV transmission works. The satellite receives this single multiplexed signal. The satellite, in turn, broadcasts the received signal to the receiving earth station. The receiving station is also supposed to agree to this carrier frequency, so that it can demultiplex the received signals.

Evolution of mobile communication systems

Basic concept of FDMA

The features of FDMA are as follows:
» The FDMA channel carries only one phone circuit at a time. If an FDMA channel is not in use, then it sits idle and it cannot be used by other users to increase share capacity. After the assignment of the voice channel the BS and the MS transmit simultaneously and continuously.
» The bandwidths of FDMA systems are generally narrow i.e. FDMA is usually implemented in a narrow band system
» The symbol time is large compared to the average delay spread.
» The complexity of the FDMA mobile systems is lower than that of TDMA mobile systems
» FDMA requires tight filtering to minimize the adjacent channel interference.

Features of FDMA


The first U.S. analog cellular system, AMPS (Advanced Mobile Phone System) is based on FDMA/FDD. A single user occupies a single channel while the call is in progress, and the single channel is actually two simplex channels which are frequency duplexed with a 45 MHz split. When a call is completed or when a handoff occurs the channel is vacated so that another mobile subscriber may use it. Multiple or simultaneous users are accommodated in AMPS by giving each user a unique signal. Voice signals are sent on the forward channel from the base station to the mobile unit, and on the reverse channel from the mobile unit to the base station. In AMPS, analog narrowband frequency modulation (NBFM) is used to modulate the carrier.


Using FDMA, CT2 system splits the available bandwidth into radio channels in the assigned frequency domain. In the initial call setup, the handset scans the available channels and locks on to an unoccupied channel for the duration of the call. Using TDD(Time Division Duplexing ), the call is split into time blocks that alternate between transmitting and receiving.

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