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).
Picture. Frequency Division Multiplexing (FDM)
Spacing is put between two adjacent subcarriers.
In FDM system, signals from multiple transmitters are transmitted simultaneously
(at the same time slot) over multiple
frequencies. Each frequency range (subcarrier) is modulated
separately by different data stream and a spacing (guard band)
is placed between subcarriers to avoid signal overlap.
Orthogonal Frequency Division Multiplexing (OFDM)
Picture. Orthogonal Frequency Division Multiplexing (OFDM)
Subcarriers are closely spaced until overlap.
Like FDM, OFDM also uses multiple subcarriers but the subcarriers
are closely spaced to each other without causing interference,
removing guard bands
between adjacent subcarriers. This is possible because the frequencies
(subcarriers) are orthogonal, meaning the peak of one subcarrier
coincides with the null of an adjacent subcarrier.
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 subcarrier and modulated using
some sorts of PSK (Phase Shift Keying) or QAM (Quadrature
Amplitude Modulation). i.e. BPSK, QPSK, 16QAM, 64QAM.
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 frequencyselective fading caused by multipath because equalizing is done on
a subset of subcarriers 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 subcarriers than a single carrier system and
the use of a cyclic prefix (CP).
