Capacity of a mobile multiple-antenna communication link in Rayleigh flat fading

We analyze a mobile wireless link comprising M transmitter and M receiver a ntennas operating in a Rayleigh flat-fading environment. The propagation co efficients between pairs of transmitter and receiver antennas are statistic ally independent and unknown; they remain constant for a coherence interval of T symbol periods, after which they change to new independent values whi ch they maintain for another T symbol periods, and so on. Computing the lin k capacity, associated with channel coding over multiple fading intervals, requires an optimization over the joint density of T.M complex transmitted signals, We prove that there is no point in making the number of transmitte r antennas greater than the length of the coherence interval: the capacity for M > T is equal to the capacity for M = T. Capacity is achieved when the T x M transmitted signal matrix is equal to the product of two statistical ly independent matrices: a T x T isotropically distributed unitary matrix t imes a certain T x M random matrix that is diagonal, real, and nonnegative, This result enables us to determine capacity for many interesting cases, W e conclude that, for a fixed number of antennas, as the length of the coher ence interval increases, the capacity approaches the capacity obtained as i f the receiver knew the propagation coefficients.