Rotated multidimensional modulation for spatial multiplexing systems

In this paper, we consider a spatial multiplexing (SM) system combined with rotated multidimensional QAM mapping, which can improve system reliability without spending additional power or bandwidth consumption under fast Rayleigh flat fading channels. Based on the pairwise error probability of the spatial multiplexing system, we construct a rotated multidimensional QAM scheme. To reduce the complexity of maximum likelihood decoding subject to increase in the modulation dimension, sphere decoding has been employed. We note that various levels of QoS can be immediately achieved by changing the dimension, and it is a useful feature to implement adaptive modulation under varying channel conditions.