Adaptive loading for multitone version of V-BLAST

The demand for high data rate communication systems and the lack of available bandwidth has led to much research effort into MIMO systems using multiple transmit and receive antennas. It has been shown that MIMO systems can improve the capacity of rich Rayleigh fading channels by a factor which is linearly proportional to the minimum number of transmit and receive antennas. The V-BLAST architecture has been proposed for realizing high spectral efficiencies over flat Rayleigh fading indoor wireless channels. Multitone transmission, or OFDM, has been used in a number of systems. OFDM eliminates the need for a complex equalizer for overcoming ISI caused by delay spread or frequency-selective fading. However, it has been shown that for a single-input-single-output (SISO) system operating over a frequency-selective channel, if the transmitter "knows" the channel characteristics, the "water-pouring" technique can be used to optimize the transmitted bit rate and approach the channel capacity. In this paper, we use the "water-pouring" approach for combined OFDM-MIMO systems, and show that it is possible to achieve very good bandwidth efficiencies for un-coded systems.