Smoothing Beamformer for MIMO-FBMC Based on Frequency Spreading

For filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) and singular value decomposition (SVD)-based beamforming, the beamformers may experience significant changes between adjacent subchannels, therefore destroy the orthogonality among FBMC/OQAM realvalued symbols, even under channels with moderate frequency selectivity. In this paper, we address this problem by propose a smoothing criterion and method under the structure of frequency spreading FBMC (FS-FBMC), namely phase factor optimization, to smooth the beamformers from tone to tone (FS-FBMC tones). Simulations are conducted under the scenario of IEEE 802.11n wireless LAN. Results show that the proposed smoothing method greatly improves the smoothness of beamformers, therefore effectively suppress the leaked ICI/ISI.

I. Introduction

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) [1] –[10] is considered as a promising alternative to the conventional orthogonal frequency division multiplexing (OFDM) technique [11], [12]. However, integration of multiple-input multiple-output (MIMO) techniques with FBMC/OQAM, in general, is more complicated than with OFDM. Thanks to the adding of cyclic prefix (CP), a subchannel (subcarrier band) is exactly flat and independent from other subchannels in OFDM systems. Thus, MIMO precoding and equalization can be taken on each subchannel independently, without leading to any inter-carrier interference (ICI) or inter-symbol interference (ISI). However, without CP, the MIMO precoding and equalization of FBMC systems are more complicated and could lead to considerable ICI/ISI under frequency selective channels, due to the fact that FBMC/OQAM is a non-orthogonal waveform (FBMC/OQAM symbols are orthogonal with each other only in the real domain [1] –[3]).