Subcarrier Nulling Algorithms for Channel Shortening in Uplink OFDMA Systems

This paper considers a blind channel shortening approach based on subcarrier nulling criteria to mitigate the effect of interblock interference in uplink OFDMA systems. We derive a sufficient condition for a method known as carrier nulling algorithm (CNA) to shorten individual uplink user channels. Unlike several known methods, CNA can shorten channels in time domain even if there are many null subcarriers, and is hence suitable for uplink OFDMA in which individual user signals can be modeled as standard OFDM that includes multiple null subcarriers. In addition, we also develop a semi blind implementation of CNA by exploiting potential pilot symbols. The semi blind approach uses the simple idea of treating known pilot subcarriers in the same way as null subcarriers.