Joint bit and power loading for MIMO OFDM based on partial channel knowledge

Multiple antennas at both transmitter and receiver enable the application of spatial multiplexing as a way of increasing transmission rate. However, this technique suffers from severe performance degradation as soon as fading processes in the multiple-input multiple-output (MIMO) channel exhibit significant correlation. If the transmit correlation matrix of the channel is known at the transmitter, performance can be improved by multiplexing signals across beams pointing along the directions of the eigenvectors of the transmit correlation matrix. These eigenbeams constitute a unitary transformation that has already been shown to be optimum in terms of ergodic capacity m Raleigh-fading channels. However, the effectiveness of this technique decisively depends on bit and power loading across eigenbeams. Here, a method for bit and power loading is presented that based on the notion of pairwise error probability (PEP) leads to an optimum distribution of bits and power for a given transmit power and spectral efficiency in an orthogonal frequency-division multiplexing (OFDM) context. Optimality refers to the minimization of a figure of merit that is obtained from adding expected PEPs over single eigenbeams. The effectiveness of this method is verified by means of simulated curves for three channels with different correlation properties.