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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Available from: Stefano Mangione, Mar 30, 2015
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    • "The motivation behind this simplification is that it allows one to address the problem in a way similar to OFDM. However, and especially in propagation conditions envisaged in future networks, this assumption may be often quite inaccurate, due to the high frequency selectivity of the channel [8] and/or the small number of subcarriers employed to cope with frequency dispersion in fast fading environments [9]. In such cases, relying on the above assumption results in severe error floors at medium to high signal-to-noise ratio (SNR) values, which cancel the advantage of the FBMC modulation over OFDM [7]. "
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    ABSTRACT: Filter bank-based multicarrier (FBMC) techniques have attracted a lot of interest in the last decade as a competitive alternative to the long established OFDM, particularly in wireless applications. Their potential stems from their increased ability to carrying a flexible spectrum shaping together with a major increase in spectral efficiency and robustness to synchronization requirements, features of fundamental importance in future mobile networks. A particular type of FBMC, the so-called FBMC/OQAM system, consisting of pulse shaped OFDM carrying offset QAM (OQAM) symbols, has received increasing attention due to, among other features, its potential for maximum spectral efficiency. It suffers, however, from an inherent inter-carrier/inter-symbol interference that complicates signal processing tasks such as channel estimation. The goal of this paper is to concisely present the state-of-the-art in channel estimation for FBMC/OQAM, putting emphasis on the problems that are still open and outlining related on-going research. Preamble-based channel estimation is given special attention in the realistic scenario of highly frequency/time selective channels and some new results are reported in this context.
    Full-text · Conference Paper · May 2014
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    • "Most of the methods reported thus far in the literature rely on the assumption of (almost) flat subchannels to more easily tackle this problem, with the aim of addressing it in a way similar to OFDM; see [16] and references therein. However, this assumption may be often quite inaccurate, due to the high frequency selectivity of the channel [25] [4] and/or the small number of subcarriers employed to cope with frequency dispersion in fast fading environments [8]. "
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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.
    Full-text · Article · Jun 2013
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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.
    No preview · Article · Dec 2008
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