Effect of Doppler frequency on pilot arrangements in wireless OFDM systems

With the rapid growth of wireless communications in recent years, the need for high speed data transmission is increased. OFDM is a promising candidate for achieving high data rates in mobile environment due its resistance to ISI, which is a common problem found in high speed data communication. Channel Estimation for wireless OFDM systems requires transmission of pilot tones. The success of channel estimate at a desired BER and Doppler frequency is highly dependent on how the pilot signals are arranged. This paper describes in detail the effect of Doppler frequency on different pilot arrangements in Wireless OFDM systems, under Rayleigh Faded channel conditions. This paper also addresses the issue, of how to arrange the location of pilot tones, and propose a new pilot insertion scheme and compare it with existing schemes. It is shown, that for lower Doppler frequencies the proposed scheme performs better than other existing pilot schemes. It is also shown here via simulations that over Rayleigh faded channels it is more efficient to use fewer pilot tones in all symbols, instead of using all tones as pilot tones in some symbols. We also compare existing schemes on different Doppler frequencies, one that uses some tones as pilot tones in each symbol (comb) than a scheme that uses all tones as pilot tones in some symbols (block). In comb arrangement one would form an instantaneous estimate of the channel impulse response in each symbol, and use it to update average channel estimate in some fashion. In block arrangement, where one periodically uses all the tones in a symbol as pilot tones, i.e. periodically one entire symbol would be used just to transmit training data, and channel estimate would be obtained from this.