MMSE techniques for space diversity receivers in OFDM-based wireless LANs

This paper studies the application of the Minimum Mean Square Error beamformer to OFDM-based Wireless LANs. The questions here addressed are mainly the design with finite-length data and the choice of the OFDM signal domain where the beamformer is applied, either frequency or time. As OFDM signals need more samples than other modulations to stabilize the estimations of the signal statistics, how to exploit the finite-length training sequence provided for the design of equalizers becomes an important issue. The paper also shows that the usual frequency processing in OFDM is not always the best choice for the spatial beamforming, mainly for channels with a very high delay spread. Then, time processing turns out to be the best-suited approach in terms of the trade-off between performance and complexity. Additionally, novel modifications of the MMSE spatial filter are proposed to improve the raw BER performance: (i) a temporal semiblind approach that exploits the Cyclic Prefix; (ii) windowing in the frequency domain.