Channel estimation in wireless LANs with transmitter diversity

Three pilot symbol architectures are compared in an orthogonal frequency-division multiplexing (OFDM) system for a down link diversity environment. The first architecture is called a basic pilot scheme, in which two long pilot symbols (similar to 802.11a) are transmitted on interleaved sub-carriers from Q transmit antennas. The second architecture extends the basic scheme by interleaving the two pilot symbols in the time domain as well as the frequency domain. Each pilot symbol is preceded with a single cyclic prefix (CP) of length 800 ns. The third architecture interleaves a single pilot symbol with a CP of length 1600 ns, over twice the number of sub-carriers than the two preceding architectures. The modified architectures always outperform the basic pilot scheme in HIPERLAN/2 channel environment, since they reduce the interpolation error. No additional pilot overhead (compared to 802.11a) is necessary if the diversity order is low (less than or equal to2) and the maximum excess delay of a channel is confined to a CP of length 800 ns.