Channel Tracking for Fast Time-Varying Channels in IEEE802.11p Systems

We address the problem of channel tracking in fast vehicular environments for OFDM systems. In modern vehicular OFDM systems such as the IEEE802.11p, the preamble-based channel estimation is not sufficient to guarantee a good equalization until the end of the burst. Indeed, at urban and highway vehicular speeds, a conventional OFDM receiver generates a large number of errors after only a few OFDM symbols and the bit and packet error rate curves show unacceptable flooring. In addition, the number of pilot sub-carriers during the burst is too small to accurately track the channel variations. To alleviate this, we propose an advanced receiver scheme that updates the channel during the burst in a decision-directed fashion. We show that this technique alone is not sufficient and that it must be complemented with channel smoothing to perform satisfactorily at low SNRs. Finally, we also analyze how the system can be further improved with spatial diversity. Simulation results are provided to illustrate the performance of our novel receiver design and some complexity reduction techniques are described.