Adaptive Antenna Arrays for Interference Cancellation in OFDM Communication Systems With Virtual Carriers

In this paper, we propose adaptive beamforming schemes for orthogonal frequency-division multiplexing (OFDM) communication systems. After applying the discrete-Fourier-transform operation to the output of each antenna, one estimates the signal-plus-interference spatial correlation matrix by selecting the samples at ldquononzero-valuedrdquo subcarriers and the interference-alone spatial correlation matrix by choosing those samples at ldquozero-valuedrdquo subcarriers (also known as virtual carriers). The beamforming weight vector is the ldquolargestrdquo generalized eigenvector of the resulting matrix pencil, wherein the solution is computed in a batch fashion. By using a limited number of samples, the proposed scheme offers competitive performance compared with that of the scheme using optimal beamforming weight vectors that are calculated for each subcarrier under small-angular-spread scenarios. The adaptive solution for calculating the beamforming weight vector is also derived. The proposed schemes are shown to be suitable in scenarios of narrow angular spread with exponential power delay profile. The simulations that are presented here employ the OFDM signal model to illustrate the efficacy of the proposed algorithms.

I. Introduction

Orthogonal frequency-division multiplexing (OFDM) has been shown to be a very promising technology for high-speed data transmission and to be robust to frequency-selective fading over wireless channels [1]–[3]. Many researchers have put much effort into studying OFDM-related topics in beyond third-generation and fourth-generation wireless communication systems. The demand for wireless local area networks (WLANs) has risen since IEEE 802.11b gained a large share of the market. OFDM-based WLAN standards, i.e., IEEE 802.11a/g [4], [5], can even achieve data rates of up to 54 Mb/s. Since there are other systems that adopt different standards in the same spectrum, the co-channel interference problem may degrade system performance. The technique of adaptive antenna arrays is a good candidate for solving this problem.