Wireless Communications Systems Design

9Channel Estimation and Equalization

As we discussed in Chapter 3, a wireless channel is no longer a black box. Wireless communication systems can obtain Channel State Information (CSI) such a fading effect and power delay. The mathematical models of the wireless channels enable us to understand how a received signal is affected by specific environments. Channel estimation is used to obtain CSI and has become a critical part of wireless communication systems. Many communication techniques use this information and improve system performances. Especially, the performance of equalizers and MIMO techniques depends on accuracy of channel estimation.

9.1 Channel Estimation

Channel estimation is simply defined as characterizing a mathematically modeled channel. The mathematical channel model is characterized by long-term CSI (or statistical CSI) and short-term CSI (or instantaneous CSI). Long-term CSI simply means statistical information such as channel statistical distribution and average channel gain. Short-term CSI simply means channel impulse response. Channel estimation algorithms typically find channel impulse response by time domain channel estimation (before DFT processing in OFDM systems) or channel frequency response by frequency domain channel estimation (after DFT processing in OFDM systems). While short-term CSI can be accurately estimated in a slow fading channel, it is difficult to obtain accurate short-term CSI in a fast fading channel. Thus, an adaptive channel ...