5Multi-carrier Modulations

5.1. Introduction

As already mentioned in Chapter 4, the channel may generate distortions on the received symbols. The channel is frequency-selective if its frequency response is uncorrelated on the considered bandwidth [PRO 08, GOL 05]. The coherence band Bc is defined as the frequency band on which two frequency components of the channel can be assumed uncorrelated.

The frequency-selectivity of a channel is equivalently characterized by one of the following definitions: either the maximum delay of the channel’s impulse response Tm is large compared to the symbol period T, or the channel’s coherence band is small compared to its bandwidth B. If the channel is frequency-selective, it generates inter-symbol interference at the receiver. On the contrary, we assume that a channel is non-frequency-selective with an almost flat frequency response in its bandwidth if B ≤ 0.1Bc.

The maximum delay of the channel’s impulse response and the channel’s coherence band are related as follows:


An example of frequency-selective channel is given in Figure 5.1. It represents the modulus of the frequency response of channel gc with impulse response:


in baseband and with bandwidth B = 1 MHz. This channel is frequency-selective, since its maximum delay Tm = 6 × Ts = 6 μ ...

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