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New Robust Beamforming Method for Frequency Offsets in Uplink Multiuser OFDM-MIMO
Yasushi TAKATORI Riichi KUDO Atsushi OHTA Koichi ISHIHARA Kentaro NISHIMORI Shuji KUBOTA
IEICE TRANSACTIONS on Communications
Publication Date: 2007/09/01
Online ISSN: 1745-1345
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on 2006 International Symposium on Antennas and Propagation)
Category: Smart Antennas
multiuser MIMO, uplink, beamforming, frequency offset,
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Multiuser multiple input multiple output (MU-MIMO) systems are attracting attention due to their frequency efficiency. However, in uplink MU-MIMO systems, different frequency offsets among multiple mobile stations (MSs) significantly degrade the transmission quality, especially when orthogonal frequency division multiplexing (OFDM) is used. In this paper, the influence of these frequency offsets is first analyzed in a frequency selective fading environment. Numerical analysis shows that an error floor occurs in the bit error rate and the influence of the frequency offset becomes larger in short delay spread environments. To overcome this problem, a new beamforming method is proposed to compensate for the frequency offset by introducing an auto frequency controller after frequency-space equalization in each data stream. The effect of the proposed method is evaluated in a frequency selective fading environment by computer simulations and measured results.