Adaptive pilot distribution for OFDM systems in time-variant channels

An OFDM-based WLAN system in time-variant environment has been considered. 4-tap and 2-tap channels with a maximum delay spread of 550ns were used. Uncoded QPSK symbols were used and perfect synchronization was assumed. A more realistic assumption is to let the level of variation of the channel changes between fast and very slow. Six threshold levels of the Doppler spread have been chosen for the adaptive pilot arrangement. The simulation results in Fig. 2 show an increase in the bit rate when the channel is static or slowly varying. Whereas in the rapidly varying cases the bit rate falls down to the conventional one (24 Mbps). In Fig. 2 the bit rate has been averaged every 500 symbols. It was shown that the adaptive method increases the total capacity of the system. The gain in the throughput is noticeable for slow varying and static channels. We noticed that, by choosing proper threshold levels, the adaptive system is comparable in the BER performance with the conventional one.