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Introduction To OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is a popular modulation scheme that is used in wireless LAN standards like 802.11a, g, HIPERLAN/2 and in the Digital Video Broadcasting standard (DVBT). It is also used in the ADSL standard, where it is referred to as Discrete Multitone modulation. OFDM modulation divides a broadband channel into many parallel subchannels. This makes it a very efficient scheme for transmission in multipath wireless channels. The use of an FFT/IFFT pair for modulation and demodulation make it computationally efficient as well.
Figure 1 A broadband channel divided into many parallel narrowband channels
The transmitted signals arrive at the receiver after being reflected from many objects. Sometimes the reflected signals add up in phase and sometimes they add up out of phase causing a "fade". This causes the received signal strength to fluctuate constantly and during times of fade can be significantly low. Also, different subchannels are distorted differently as shown in Figure 1. An OFDM receiver has to sense the channel and correct these distortions on each of the subchannels before the transmitted data can be extracted. OFDM is effective in correcting such frequency selective distortions.
OFDM has many advantages over other transmission techniques. One such advantage is high spectral efficiency (measured in bits/sec/Hz). The "Orthogonal" part of the name refers to a precise mathematical relationship between the frequencies of the subchannels that make up the OFDM system. Each of the frequencies is an integer multiple of a fundamental frequency. This ensures that even though the subchannels overlap they do not interfere with each other. This results in high spectral efficiency.
The use of IFFT and FFT for modulation and demodulation results in computationally efficient OFDM modems. The block diagram of an OFDM modulator and demodulator are shown in Figure 2 An OFDM (a) Modulator and (b) Demodulator.
Figure 2 An OFDM (a) Modulator and (b) Demodulator
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