On spreading codes for the down-link in a multiuser MIMO/OFDM system

We propose a method for multiple access using multiple antennas at both ends of the communication link (MIMO) and orthogonal frequency division multiplexing (OFDM). The MIMO component of the system serves to increase bandwidth efficiency, whereas the OFDM component tackles the frequency selectivity of most practical channels. The unique feature of the proposed design is that it is based on direct sequence spread spectrum, except that the spreading codes are defined in the space and frequency domains, rather than the time domain. With second-order channel knowledge at the transmitter, this allows for power allocation schemes over spatial and frequency channels; without any knowledge of the channel, exploitation of frequency diversity is still an inherent feature. Lastly, OFDM is known to be simpler to implement than time-domain equalization, and therefore the proposed scheme is more practically feasible than competing schemes based on equalization.