OFDM
(Orthogonal frequency-division multiplexing) is a method of coding digital signals by orthogonal frequency division as multiple subcarriers. This technique allows to fight against frequency-selective channels allowing equalization of low complexity. These channels are particularly apparent in the presence of multipath and are particularly disadvantageous when the transmission rate is high. This technique is widely adopted in most applications with very high speed.
There exist several variants of OFDM. DMT (Discrete Multi Tone) refers to a system for transmitting OFDM baseband. The COFDM (Coded Orthogonal Frequency Division Multiplexing) introduces error correcting code. WCP-OFDM (Cyclic Prefix Weighted Orthogonal Frequency-Division Multiplexing) includes the addition of a cyclic prefix and the weighting of the output signal of the transmitter to adapt the mobile multipath channels.
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In the presence of a multipath channel, the reception of multiple echoes of phase may lead to fading (severe attenuation of a portion of the frequency band). As part of an OFDM system, it is generally impossible to reconstruct the symbols transported by subcarriers affected by these fainting phenomena. This can be explained by the fact that the non-precoded OFDM does not introduce redundancy (or frequency diversity). This drawback can be faded using the COFDM at the cost of reduced spectral efficiency.
OFDM as a block transmission system is generally introduced a guard interval between them. This eliminates interference between the successive blocks in the presence of multipath channels and especially facilitates the equalization, provided that the guard interval duration is greater than the arrival time of the last path. Two types of guard intervals are commonly used: the cyclic prefix, which consists of copying the last samples of the block to the top thereof and stuffing zeroes of inserting leading zeros of the block. Both techniques naturally lead to a decrease in spectral efficiency.
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