We propose and analyze a low-complexity channel estimator for a multiuser multicarrier code division multiple access (MC-CDMA) downlink in a time-variant frequency-selective channel. MC-CDMA is based on orthogonal frequency division multiplexing (OFDM). The time-variant channel is estimated individually for every flat-fading subcarrier, assuming small intercarrier interference. The temporal variation of every subcarrier over the duration of a data block is upper bounded by the Doppler bandwidth determined by the maximum velocity of the users. Slepian showed that time-limited snapshots of bandlimited sequences span a low-dimensional subspace. This subspace is also spanned by discrete prolate spheroidal (DPS) sequences. We expand the time-variant subcarrier coefficients in terms of orthogonal DPS sequences we call Slepian basis expansion. This enables a time-variant channel description that avoids the frequency leakage effect of the Fourier basis expansion. The square bias of the Slepian basis expansion per subcarrier is three magnitudes smaller than the square bias of the Fourier basis expansion. We show simulation results for a fully loaded MC-CDMA downlink with classic linear minimum mean square error multiuser detection. The users are moving with 19.4 m/s. Using the Slepian basis expansion channel estimator and a pilot ratio of only 2%, we achieve a bit error rate performance as with perfect channel knowledge.

Abstract—In this paper, we propose a time-varying (TV) finite impulse response (FIR) equalizer for doubly selective (time- and frequency-selective) channels. We use a basis expansion model (BEM) to approximate the doubly selective channel and to design the TV FIR equalizer. This allows us to turn a complicated equalization problem into an equivalent simpler equalization problem, containing only the BEM coefficients of both the doubly selective channel and the TV FIR equalizer. The minimum mean-square error (MMSE) as well as the zero-forcing (ZF) solutions are considered. Comparisons with the block linear equalizer (BLE) are made. The TV FIR equalization we propose here unifies and extends many previously proposed serial equalization approaches. In contrast to the BLE, the proposed TV FIR equalizer allows a flexible tradeoff between complexity and performance. Moreover, through computer simulations, we show that the performance of the proposed MMSE TV FIR equalizer comes close to the performance of the ZF and MMSE BLE, at a point where the design as well as the implementation complexity are much lower. Index Terms—Doubly selective channels, equalization, fading channels, orthogonal frequency division multiplexing (OFDM), time-varying finite impulse response (TV FIR), two-dimensional (2-D) equalization, wireless communications. I.

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In this work we present an equalization scheme for a multi-carrier (MC) code division multiple access (CDMA) communication system that is operated in a frequency-selective time-variant (TV) channel. For block oriented data transmission we are interested to describe the TV channel for the duration of a data block with the smallest amount of parameters possible. Slepian showed that time-limited parts of band-limited sequences span a subspace with strongly reduced dimensionality. The discrete prolate spheroidal (DPS) sequences are the basis of this subspace. We exploit this property by using the Slepian basis expansion model (BEM) to describe a TV channel with a minimum amount of parameters. The mean squared error (MSE) of the Slepian BEM is 30 dB smaller than the MSE of the Fourier BEM for a frequency-flat TV channel. We present simulation results in terms of bit error rate (BER) versus E b =N0 for a multi-user MC-CDMA forward link in a frequency-selective TV channel comparing the Fourier BEM and the Slepian BEM for channel equalization.

We consider the effect of mobility on a wideband direct sequence spread spectrum (DSSS) communication system, and study a scale-lag Rake receiver capable of leveraging the diversity that results from mobility. A wideband signal has a large bandwidth-to-center frequency ratio, such that the typical narrowband Doppler spread assump-tions do not apply to mobile channels. Instead, we assume a more general temporal scaling phenomenon, i.e., a dilation of the transmitted signal’s time support. Based on a uniform ring of scatterers model, we determine that the wideband scattering function, which quantifies the average scale spreading, has a “bathtub-shaped” scale profile. We investigate, through frame-theoretic tools, the translation- and dilation-spacing parameters of a scale-lag Rake basis, and compare the performances of a scale-lag Rake and a Doppler-lag Rake, each capable of leveraging the diversity that results from mobility. When the translation spacing of the Rake functions is equal to the minimum resolvable lag, there is no significant performance difference between the receivers. For wider spacings, the receiver is more reliant on dilation diversity; hence, the scale-lag Rake receiver performs relatively better. Such analysis applies, for example, to ultra-wideband (UWB) radio frequency channels and underwater wideband acoustic channels. We study the correlation structure of the scale-lag Rake fingers and show that the normalized scale spread parameter relates directly to the time-variability of the ii channel. We discover that much of the channel energy is concentrated in few eigenmodes and hence propose principal components combining for a reduced-complexity solution. Finally, we perform physical experiments in the air-acoustic channel to demonstrate the applicability of the wideband channel model.

Abstract — In this paper, we propose a per-tone frequencydomain equalization approach for OFDM over doubly-selective channels. We consider the most general case, where the doublyselective channel delay spread is larger than the cyclic prefix (CP), which results into inter-block interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and hence, results into severe intercarrier interference (ICI). In this paper, we propose a novel per-tone frequency-domain equalizer (PTFEQ) that is obtained through transferring a time-varying time-domain equalizer (TV-TEQ) to the frequency-domain. The purpose of the TV-TEQ is to restore orthogonality between subcarriers and eliminate ICI. We use the mean-square error criterion to design the PTFEQ. An efficient implementation of the proposed PTFEQ is also discussed. Finally, we show some simulation results of the proposed equalization technique. I.

This paper presents a unified analysis for the downlink BER performance of convolutionally coded and single-user detected MC-CDMA, MCDS-CDMA and TFL-CDMA. As a special case, an analysis is obtained also for OFDMA. The intra-cell interference is analyzed under the assumption of Hadamard spreading codes and a correlated frequency-selective Rayleigh-fading channel. The presented results assume error-free channel estimates and a perfectly synchronized receiver. It is concluded that the compared schemes achieve essentially the same performance for low system loads. For high loads, and especially in a near-far scenario, OFDMA is found to outperform the others.

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In wireless communications, spectrum is a scarce resource and hence imposes a high cost on the high data rate transmission. It has been demonstrated that multiple antenna system provides very promising gain in capacity without increasing the use of spectrum, reliability, throughput, power consumption and less sensitivity to fading, hence leading to a breakthrough in the data rate of wireless communication systems In this paper, we study the MIMO system architecture, with optimally ordered successive interference cancellation (SIC) receiver in MINIMUM MEAN SQUARE ERROR(MMSE) EQUALIZER and simulate this structure in Rayleigh fading channel. Based on bit error rate, we show the performance of this receiver, indicates that the ordered SIC detector most effectively balances the accuracy of symbol detection. SIC receiver based on MMSE combined with symbol cancellation and optimal ordering improves the performance with lower complexity. Finally, the paper addresses the current questions regarding the integration of MIMO system in practical wireless systems and standards.