Promising feedback methods for transmit beamforming in broadband mobile OFDM

This paper details three methods for closed-loop operation in the downlink of a broadband mobile OFDM system: uplink sounding, codebook feedback, and direct channel feedback (DCFB). Uplink sounding is intended for TDD systems and works by the base translating the uplink channel estimates into downlink channel estimates. DCFB is a means of obtaining full channel knowledge in either FDD or TDD systems. In DCFB, the mobile replaces normal transmitted symbols with a DCFB waveform constructed from downlink channel estimates to multiple base antennas. Another feedback method for TDD or FDD is codebook feedback where the mobile chooses a best transmit weight out of a codebook of 2^B vectors and then signals this weight back to the base. To track the frequency-selective channel, multiple codebook vectors are sent back corresponding to different parts of the band. Simulation results show that uplink sounding is the best method for obtaining channel information for transmit beamforming, but sounding is only effective in TDD systems. For FDD, DCFB is more flexible, computationally efficient (at the mobile), and effective than codebook feedback for broadband transmit beamforming at the base