Cross polarisation based extension of closed loop transmit diversity

An extension to Release99 closed loop transmit diversity is proposed by combining uplink based beamforming and polarisation diversity. A generic expression for the polarisation covariance matrix is derived based on the assumption of uncorrelated fading processes for different polarisation states. It becomes evident that the product of the open loop beamforming gain and the two antenna polarisation covariance matrix gives the overall covariance matrix for the proposed transmitter structure. Specialising to 2M±45° cross polarised transmit antennas it is shown that the overall covariance matrix shows partly correlated fading depending on the cross polarisation discrimination (XPD) and the receive antenna orientation. Simulation results for a system with eight transmit antennas show gains of 7 dB in terms of required transmit power in highly spatially correlated scenarios compared to a spatial diversity based system with two transmit antennas using space-time codes.