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An orthogonal frequency division multiplexing system suffers performance degradation when the length of the cyclic prefix is less than the channel impulse response. The root cause of this degradation is the inter-carrier interference (ICI) and inter-symbol interference (ISI) introduced by the excessive multipath delay. Generally, multiple-input multiple-output (MIMO) beamforming is helpful in mitigating such interference because it can spatially suppress some of the multipath. However, the effectiveness of this suppression is very limited. In this paper, we propose an ICI/ISI-aware beamforming algorithm which explicitly takes into account the multipath characteristic of the channel. Optimal steering vectors are derived to maximize the signal-to-interference-plus-noise ratio. This technique not only achieves the beamforming benefit, but also significantly mitigates the ICI and ISI. We show, via simulations, that the proposed algorithm can dramatically reduce the block error rate, permitting good performance for channel delay profiles that would break conventional links. This is vitally important for the extension of indoor wireless LAN designs to outdoor uses.
Date of Publication: January 2012