A simple multiplexing scheme for MIMO systems using multiple spreading codes

There has been a growing interest in the use of multiple transmit and receive antennas for wireless communications, due to the enormous increase in data rate that they promise over single antenna systems. Spatial multiplexing is a scheme that aims to achieve such high data rates by transmitting an independent substream of data from each transmit antenna. However when the receive spatial signatures are poor (i.e., when the channel is not favorable), one or more transmit antennas can not be used to transmit an independent stream. In fixed modulation systems this results in some loss in data rate. We consider such fixed modulation systems and propose a scheme that uses multiple spreading codes as in CDMA MIMO (multiple input multiple output) systems to smartly combine code multiplexing with spatial multiplexing in order to mitigate this loss in data rate. We also show that, in the presence of a low bandwidth feedback path, the proposed scheme, unlike the spatial-only multiplexing scheme, allows the transmitter to adapt the data, rate smoothly based on the receive spatial signatures.