Channel Spectral Flattening in Time Domain Equalizer Design for OFDM Systems

Time domain equalization (TEQ) methods based on the maximum shortening signal-to-noise ratio (MSSNR) criterion is one of the most commonly used criterion for shortening impulse responses as it is computationally efficient and simple. However, the MSSNR method tends to design a TEQ with a bandpass-like frequency response, resulting in a spectrally uneven effective channel response. As as result, bit-loading have to be performed to improve the bit rate of the system. Additionally, when residual intersymbol interference (ISI) is low compared to noise power, the spectral shape of the effective channel does not have much impact on the bit rate. However, the negative effect of the channel spectral unevenness on achievable bit rate increases as the noise power decreases. In this paper, a design method is proposed to maximize effective channel spectral flatness to increase the bit rate of the system.