Tackling MIMO-OFDMA Feedback Load Through Feedback Encoding

In an FDD MIMO-OFDMA system instantaneous downlink channel knowledge cannot be obtained from uplink pilot signalling, but has to be fed back from the receiver. To exploit tx diversity and tx antenna array gain one tx antenna weight-set is ideally fed back per OFDMA subcarrier. Usually the uplink bandwidth limits the amount of permissible feedback. Thus feedback compression techniques are essential, three of which are explored in this paper within the framework of 3GPP UTRA long term evolution. The tx antenna weight- sets are chosen from Grassmannian code books for 2times2 and 4times2 antenna configurations. Combining subcarriers in subgroups, and feeding back only one antenna weigh set per subgroup is shown to be a simple and effective technique. It reduces the feedback load by up to 96% for a typical urban channel profile, while maintaining the system performance within 3% from the optimum. Methods on the basis of run-length encoding are also shown to decrease the fed back load significantly, however, when forced to reduce the feedback to the same level as with subgroup encoding, their performance turns out to be inferior.