Obtaining channel knowledge for closed-loop multi-stream broadband MIMO-OFDM communications using direct channel feedback

This paper introduces direct channel feedback in broadband multi-stream MIMO-OFDM communications as an efficient means for providing the base station with complete broadband channel information. The mobile station first estimates the complete (frequency selective) downlink channel responses between all transmit and receive antenna combinations and then transmits these responses back to the base in the frequency domain in place of the usual data symbols. The base station equalizes the received feedback signal and recovers the channel information by using standard channel estimation techniques. Using some signal processing tricks, multiple channel responses can be encoded together and sent back in a single feedback message, making the feedback very bandwidth efficient. Additional gains in bandwidth efficiency are provided by allowing multiple mobile stations to transmit their feedback on the same time-frequency resource in a spatial division multiple access (SDMA) fashion. Simulation results demonstrate that in IEEE 802.16 type systems, up to 16 complete broadband channels can be sent in one OFDM symbol period using the direct channel feedback method