Viterbi decoder aided equalization and sampling clock tracking for OFDM WLAN

Abstract

IEEE 802.11a is a standard for the high-speed wireless local area network (WLAN), supporting from 6 up to 54 Mbit/s in a 5 GHz band. There are training sequences for the IEEE802.11a standard in the preamble for channel equalization and sampling timing tracking. However, the preamble is only at the initial transmission. Thereby, it is not easy to track channel variations and sampling phase errors even though there are four pilot samples in each symbol of 52 subcarriers. We propose a channel equalization algorithm and a sampling clock tracking algorithm by utilizing the Viterbi decoder output of the IEEE 802.11a WLAN standard. The proposed channel equalizer adaptively compensates channel variations. The proposed system uses re-encoded Viterbi decoder outputs as reference symbols for the adaptation of the channel equalizer. It also extracts sampling phase information with the Viterbi decoder outputs for fine adjustment of the sampling clock. The proposed sampling timing recovery and equalizer are more robust to noise and frequency selective fading environments than conventional systems using only four pilot samples.