Subspace Tracking Based Blind MIMO Transmit Preprocessing

In this contribution projection approximation subspace tracking using deflation (PASTD) is investigated in the context of MIMO transmit preprocessing systems by exploiting the specific property of time division duplexing (TDD) techniques that the uplink and downlink channels are similar, since they both use the same carrier frequency. Hence the channel estimated from the received signal can also be used for transmit preprocessing. More explicitly, based on the received signal, the PASTD algorithm is used for tracking both the left and the right singular vectors of the MIMO channel matrix, which are required by eigenmode transmissions, instead of periodically reestimating the MIMO channel matrix and performing the singular value decomposition (SVD), which would impose a high computational complexity. A specific deficiency of the family of subspace tracking algorithms is their phase ambiguity imposed by the non-unique nature of the SVD, which is resolved in this treatise by employing differential encoding. The efficiency of the proposed subspace tracking scheme is demonstrated by our performance results, indicating that the advocated technique preforms within 1 dB from the BER curve of the perfect channel estimation aided benchmarker.