Feedback Schemes for Multiuser MIMO-OFDM Downlink

We consider a MIMO-OFDM broadcast channel and compare achievable ergodic rates under three channel state feedback schemes: analog feedback, direction quantized feedback and "time-domain" channel quantized feedback. The third scheme is new, and it is inspired by rate-distortion theory of Gaussian correlated sources. For each scheme we derive the conditions under which the system achieves full multiplexing gain. The first two schemes are direct extensions of previously proposed schemes to the OFDM case. The key difference with respect to the widely treated frequency-flat case is that in MIMO-OFDM the "frequency-domain" channel is a Gaussian correlated source. The new "time-domain" quantization scheme takes advantage of the channel frequency correlation structure and outperforms the other schemes. Furthermore, it is by far simpler to implement than complicated spherical vector quantization. In particular, we observe that no structured codebook design and vector quantization is actually needed for efficient channel state information feedback.(1)