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: Orthogonal frequency division multiplexing based on offset quadrature amplitude modulation (OFDM-OQAM) signaling over frequency selective multipath channels shows inter-symbol interference (ISI) and Inter-Channel Interference (ICI) that degrade its performance. Channel equalization and channel estimation are needed to combat these intrinsic interferences. In this paper a novel modal channel estimator based on the MUltiple signal classification (MUSIC) and least squares (LS) algorithms for a wideband passband OFDM-OQAM signaling over static multipath channels is presented. The effects of the frequency selective channel on the received signal are described considering a wideband OFDM-OQAM system model based on Taylor expansion of the channel transfer function. The spatial smoothing technique is employed to obtain time delay estimates based on the MUSIC algorithm from a single observation of the output data. The least squares estimate for the path gains is based on the knowledge of the estimated path delays. Numerical simulation results for the performance of the novel MUSIC-LS modal channel estimator are presented.
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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.
    06/2013;
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    ABSTRACT: A novel equalization structure for filter bank based multicarrier (FBMC) based modulations is proposed in this paper. The equalizer architecture is derived by assuming that the number of carriers is asymptotically large, and it consists of multiple parallel stages that are linearly combined on a per-subcarrier basis. The traditional single-tap per-subcarrier equalizer is obtained as a special case of the proposed architecture when the number of stages is fixed to one. An analytical characterization of the output signal to noise plus distortion ratio is provided, which can be used in practice to fix the number of parallel stages of the equalizer in order to obtain a certain performance level at the minimum complexity. Both simulations and analytical predictions indicate that high performance gains can be obtained with respect to more conventional FBMC equalization approaches. Furthermore, the proposed architecture is also shown to present important advantages in terms of computational complexity when compared with classical FBMC equalization architectures.
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