Channel estimation and equalization for CE-OFDM in multipath fading channels

Abstract

In OFDM, the orthogonal subcarriers can be assumed to undergo flat fading in a frequency selective channel permitting operation without an equalizer especially when differential data encoding is used. However, in CE-OFDM, the data symbols are no longer confined to orthogonal subcarriers and, therefore, an equalizer is required for operation in frequency selective channels. A frequency domain equalizer (FDE) is known to perform well for OFDM as well as CE-OFDM [3]. In this paper, we study the performance of CE-OFDM in multipath fading channels while employing a FDE. The quality of the channel estimate which is obtained using a training signal directly affects the performance of the equalizer. We investigate channel estimation using both the least squares (LS) as well as the linear minimum mean square error (LMMSE) techniques and compare the equalizer performance for both these cases. It is desirable to have a training signal that has a low PAPR in time domain while also having a flat power spectrum in the frequency domain. While CE-OFDM has a constant envelope, it does not have a flat spectrum in the frequency domain. Therefore, we also investigate various training signals for channel estimation and compare their performance in the presence of an amplifier.