Channel Estimation for Per-Subcarrier Maximal Ratio Transmission in OFDM

Frequency-selective maximal ratio transmission (MRT) weights applied by the base station to each subcarrier in OFDM can greatly increase the range and significantly improve the link and system performance over a single transmit antenna. However, applying these frequency-selective MRT weights causes the composite channel seen by the mobile (the composite channel is the combination of the transmit weights and the RF downlink channel) to be significantly different from the single antenna transmission. By accurately modeling the composite downlink channel in beamforming, the best channel estimation of the composite channel can be obtained. This paper gives a derivation of the power delay profile of the composite channel as a function of Doppler frequency and channel measurement delay (i.e., the delay from when the downlink channel information is obtained by the base station and when the beamforming occurs on the downlink). The power delay profile of the composite channel is used to derive an MMSE channel estimator for the composite channel. In addition, because the power delay profile of the composite channel has a strong Ricean component and thus channel estimation using this power delay profile is very sensitive to the synchronization point, the paper also discusses means to compensate for a small synchronization error at the mobile. Simulation results show that the channel estimator derived from the power delay profile of the composite channel is very effective at estimating the downlink beamformed channel.