Complexity reduction techniques for blind adaptive beamforming in OFDM antenna arrays

In this paper, we propose two complexity reduction techniques for blind adaptive beamforming in orthogonal frequency division multiplexing (OFDM) antenna array systems. For each sub-carrier, a beamformer with generalized sidelobe canceller (GSC) structure is designed according to the constrained constant modulus (CCM) criterion with weight vectors adaptation based on the recursive least squares (RLS) method. The two techniques that we propose for complexity reduction rely on frequency domain interpolation and spatial domain clustering, respectively. The former exploits the coherence bandwidth of the radio channel, so that only the weight vectors at selected frequencies need to be adapted, while the remaining weight vectors are obtained via interpolation. The latter relies on the partitioning of the receiving array into sub-arrays of smaller size. In addition, a combination of these two techniques is proposed to further reduce the complexity. A complexity analysis is presented to quantify the computational savings offered by the proposed techniques. Finally, simulation results are provided to illustrate that these savings can be obtained at the price of only a minor, acceptable loss in performance when compared to the direct application of the conventional CCM-RLS-GSC beamforming algorithm to the wideband OFDM antenna array systems.