Subspace tracking for uplink/downlink array processing in CDMA systems

In antenna array systems, downlink beamforming and uplink maximum likelihood structured channel estimation can be formulated under a common framework related to the algebraic structure of the two problems. The slow variations of the uplink and downlink spatial subspaces, due to moving terminals, can be tracked by using an adaptive structure based on a common processing block, namely a subspace tracker. Simulations for realistic propagation conditions show that the structure is able to efficiently cope with fast-varying fading channels, allowing relevant gains compared to conventional techniques.