Conference Paper

Wireless OFDM-OQAM with a Small Number of Subcarriers

Univ. degli Studi di Palermo, Palermo
DOI: 10.1109/WCNC.2008.38 Conference: Wireless Communications and Networking Conference, 2008. WCNC 2008. IEEE
Source: IEEE Xplore

ABSTRACT Orthogonal frequency division multiplexing based on offset quadrature amplitude modulation (OFDM-OQAM) is a multicarrier signaling technique which trades off robustness for spectral efficiency when compared to conventional OFDM with a cyclic prefix. In this paper, a novel matrix model for passband OFDM-OQAM signaling with a small number of subcarriers over a multipath frequency selective fading channel is presented. Specifically, in OFDM-OQAM a frequency selective channel is divided into many smaller but still frequency selective overlapping channels, so approximating the frequency response of a subchannel by the channel frequency response sampled at the subcarrier frequency may be inadequate. Channel effects may be better characterized if the frequency response of each subchannel is represented as a Taylor expansion at the subcarrier frequency. Simulation results show how the matrix model implemented by means of this approximation is a suitable model for OFDM-OQAM with a small number of subcarriers.

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    ABSTRACT: Filterbank orthogonal multicarrier transmission schemes, such as OFDM/OQAM, are currently under intense study for their spectral efficiency and suitability for the physical layer of Cognitive Radio. Nonetheless, the mathematical description of the signal is cumbersome, and closed form expressions for performance figures are not available, so performance analysis on fading channels has to be performed by means of simulations. We propose an algorithm for simulating the received signal over multipath frequency selective fading channels, based on Taylor series expansion of the channel transfer function. The proposed method is compared to the state-of-the-art FFT-based method in terms of computational complexity.
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    ABSTRACT: Filter bank-based multicarrier modulation (FBMC) using offset quadrature amplitude modulation (OQAM), known as FBMC/OQAM, provides an attractive alternative to the conventional cyclic prefix-based orthogonal frequency division multiplexing (CP-OFDM), especially in terms of increased robustness to frequency offset and Doppler spread, and high bandwidth efficiency. However, channel equalization in FBMC/OQAM is a nontrivial task, mainly because of the fact that the subchannels are no longer flat, in general. In multiple-antenna (MIMO) time-varying systems, equalizing the channel becomes even more challenging. This paper presents an adaptive T/2-spaced decision-feedback equalization (DFE) algorithm for MIMO-FBMC/OQAM systems, that is both computationally efficient and numerically stable. Its structure follows the V-BLAST idea and the algorithm is applied in a per subcarrier fashion. Simulation results are reported that demonstrate its effectiveness in time-varying MIMO channels with high frequency selectivity.
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    ABSTRACT: Filter bank-based multicarrier (FBMC) systems based on offset quadrature amplitude modulation (FBMC/OQAM) have recently attracted increased interest due to their enhanced flexibility, higher spectral efficiency, and better spectral containment compared to conventional OFDM. FBMC/OQAM suffers, however, from an imaginary inter-carrier/inter-symbol interference that complicates signal processing tasks such as channel estimation. Most of the methods reported thus far in the literature rely on the assumption of (almost) flat subchannels to more easily tackle this problem. However, this assumption may be often quite inaccurate, due to the high frequency selectivity of the channel and/or the small number of subcarriers employed to cope with frequency dispersion in fast fading environments. In such cases, severe error floors are exhibited at medium to high signal-to-noise ratio (SNR) values. Moreover, the existing methods provide estimates of the subchannel responses, most commonly in the frequency domain. The goal of this paper is to revisit this problem through an alternative formulation that focuses on the estimation of the channel impulse response itself and makes no assumption on the degree of frequency selectivity of the subchannels. The possible gains in estimation performance offered by such an approach are investigated through the design of optimal (in the MSE sense) preambles, of the smallest possible duration of only one pilot FBMC symbol. Existing preamble designs for flat subchannels are then shown to result as special cases. Simulation results are presented, for both mildly and highly frequency selective channels, that demonstrate the significant improvements in performance offered by the proposed approach over both OFDM and the optimal flat subchannel-based FBMC/OQAM method. Most notably, no error floors appear anymore over a quite wide range of SNR values.