Introduction to FDM, OFDM, OFDMA, SOFDMA

Frequency Division Multiplexing (FDM)

WiMAX air interface is based on OFDM/OFDMA physical layer (PHY). To understand how OFDM and OFDMA work, it is useful to start with its "mother" namely FDM (Frequency Division Multiplexing).

Frequency Division Multiplexing (FDM) sub-carriers

Picture. Frequency Division Multiplexing (FDM)
Spacing is put between two adjacent sub-carriers.

In FDM system, signals from multiple transmitters are transmitted simultaneously (at the same time slot) over multiple frequencies. Each frequency range (sub-carrier) is modulated separately by different data stream and a spacing (guard band) is placed between sub-carriers to avoid signal overlap.

Orthogonal Frequency Division Multiplexing (OFDM)

subcarriers in Orthogonal Frequency Division Multiplexing (OFDM)

Picture. Orthogonal Frequency Division Multiplexing (OFDM)
Sub-carriers are closely spaced until overlap.

Like FDM, OFDM also uses multiple sub-carriers but the sub-carriers are closely spaced to each other without causing interference, removing guard bands between adjacent sub-carriers. This is possible because the frequencies (sub-carriers) are orthogonal, meaning the peak of one sub-carrier coincides with the null of an adjacent sub-carrier.

In an OFDM system, a very high rate data stream is divided into multiple parallel low rate data streams. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some sorts of PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation). i.e. BPSK, QPSK, 16-QAM, 64-QAM.

OFDM needs less bandwidth than FDM to carry the same amount of information which translates to higher spectral efficiency. Besides a high spectral efficiency, an OFDM system such as WiMAX is more resilient in NLOS environment. It can efficiently overcome interference and frequency-selective fading caused by multipath because equalizing is done on a subset of sub-carriers instead of a single broader carrier. The effect of ISI (Inter Symbol Interference) is suppressed by virtue of a longer symbol period of the parallel OFDM sub-carriers than a single carrier system and the use of a cyclic prefix (CP).

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