Chapter 8
Reference Signals and Channel Estimation
8.1 Introduction
A simple communication system can be generally modelled as in Figure 8.1, where the transmitted signal x passes through a radio channel H and suffers additive noise before being received. Mobile radio channels usually exhibit multipath fading, which causes Inter-Symbol Interference (ISI) in the received signal. In order to remove ISI, various kinds of equalization and detection algorithms can be utilized, which may or may not exploit knowledge of the Channel Impulse Response (CIR). Orthogonal Frequency Division Multiple Access (OFDMA) is particularly robust against ISI, thanks to its structure and the use of the Cyclic Prefix (CP) which allows the receiver to perform a low-complexity single-tap scalar equalization in the frequency domain, as described in Section 5.2.1.
Figure 8.1: A simple transmission model.
As explained in Section 7.3, when the detection method exploits channel knowledge, it is generally said to be ‘coherent’; otherwise it is called ‘non-coherent’. Coherent detection can make use of both amplitude and phase information carried by the complex signals, and not of only amplitude information as with non-coherent detection. Optimal reception by coherent detection therefore typically requires accurate estimation of the propagation channel.
The main ...
