Frequency-selective Multipath Fading Channels Research Articles

The Effect of Narrowband Interference on Wideband Wireless Communication Systems

This paper evaluates the performance of wideband communication systems in the presence of narrowband interference (NBI). In particular, we derive closed-form bit-error probability expressions for spread-spectrum systems by approximating narrowband interferers as independent asynchronous tone interferers. The scenarios considered include additive white Gaussian noise channels, flat-fading channels, and frequency-selective multipath fading channels. For multipath fading channels, we develop a new analytical framework based on perturbation theory to analyze the performance of a Rake receiver in Nakagami-m channels. Simulation results for NBI such as GSM and Bluetooth are in good agreement with our analytical results, showing the approach developed is useful for investigating the coexistence of ultrawide bandwidth systems with existing wireless systems.

Semi-analytical approach of asynchronous MC-CDMA systems with a cyclic prefix

In this letter, asynchronous multicarrier-code division multiple access (MC-CDMA) systems with a cyclic prefix (CP) are analyzed over frequency-selective multipath fading channels, and the average bit error rate is evaluated. The derived results show that a CP is required for MC-CDMA systems in order to mitigate intersymbol interference (ISI) and fully obtain the achievable path diversity as the frequency diversity.

Distributed Versus Centralized Multiuser Detection With Antenna Arrays in Frequency-Selective Multipath Channels

We have analyzed and compared two different multiuser detection receiver structures that are appropriate for code-division multiple access (CDMA) systems with multiple antennas in frequency-selective multipath channels. Distributed decorrelation detection (where signal decorrelation is performed at each receiving antenna) and centralized decorrelation detection (where decorrelation is performed once collectively) are compared. We show that they provide the same performance in additive Gaussian channels. Centralized decorrelation detection provides better performance in general frequency-selective multipath fading channels. Therefore, the hardware cost, complexity, and processing at the receiver can be optimized using centralized decorrelation detection. We employ decorrelation detection to demonstrate our results, although the work can be easily extended to other linear multiuser detection configurations.

Turbo decoding for precoded systems over multipath fading channels

A combined precoding and turbo decoding strategy for multi-path frequency-selective fading channels is presented. The precoder and multi-path fading channel are jointly modeled as a finite-state probabilistic channel to provide the multistage turbo decoder with its statistics information. Both a priori and a posteriori probabilities are used in the metric computation to improve the system performance. Structures of the combined turbo-encoder, interleaver, and precoder in the transmitter and two-stage turbo decoder in the receiver are described. Performance of the proposed scheme in fixed, Rician and Rayleigh multi-path fading channels are evaluated by simulation. The results indicate that the combined precoding and two-stage turbo decoding strategy provides a considerable performance improvement while maintaining the same inner structure of a conventional turbo decoder.

Performance Analysis and Optimization of a Space–Time Selective PIC for CDMA Systems

This paper deals with an efficient receiver for applications in direct sequence-code-division multiple access wireless communication systems. This receiver combines a modified scheme of a parallel interference cancellation detector with antenna arrays with optimum beamforming and is used at the base station for the detection of asynchronous user signals. Each user's transmission channel is assumed to be a multipath frequency-selective independent Rayleigh fading channel. The receiver operates by dividing the signals into reliable and unreliable sets following space-time combining. The reliable signals are detected and canceled from the whole signal received at each sensor. Unreliable signals are then detected to improve the decision reliability. The receiver performance in terms of bit-error probability is analytically derived and optimized. According to the analytical and simulation results, this receiver outperforms previously proposed schemes and, thanks to its low implementation complexity, real-time operation is achieved.

Performance of asynchronous multicarrier CDMA multiuser receiver over frequency selective multipath fading channels

Architecture of a cross-coupled decision feedback detector is suggested to reduce both inter-symbol-interference and multiple-access interference for an asynchronous multicarrier CDMA scheme for the reverse link of a mobile communication system. Results show that an enhancement in capacity and good interference resistance are obtained.

A New Algorithm for Blind Adaptive Multiuser Detection in Frequency Selective Multipath Fading Channel

This paper deals with a modified version of a blind adaptive multiuser detector for direct sequence code-division multiple-access (DS-CDMA) wireless communication systems which is named precombining blind adaptive multiuser detector (PBA-MUD). The main properties of this receiver are low complexity, multiple-access interference mitigation and remarkable near-far resistance in time-varying multipath fading scenarios. In particular, the PBA-MUD is based on a blind adaptive algorithm that allows the detector to avoid deep fading impairments and the cancellation of the desired signal due to signature sequence mismatch. Nevertheless, such a receiver experiences a performance degradation in fast fading channels. In order to overcome this problem, a window reprocessing technique (Del Re et al., 2001) was introduced which yielded a new blind detection algorithm. It will be shown that this receiver, together with this new algorithm [precombining reprocessing window blind adaptive multiuser detector (PWBA-MUD)] allows a high performance without requiring training sequences or the knowledge of the interfering signature waveforms in multipath fast fading channels.

Group-Orthogonal Multicarrier CDMA

In the presence of frequency-selective multipath fading channels, code-division multiple access (CDMA) suffers from multiuser interference (MUI) and intersymbol interference (ISI); but when properly designed, it enjoys multipath diversity. Orthogonal frequency-division multiple access (OFDMA) is MUI-free, but it does not enable the available channel diversity without employing error-control coding. On the other hand, coded OFDMA may achieve lower diversity than a CDMA system employing the same error-control codes. In this paper, we merge the advantages of OFDMA and CDMA to minimize MUI effects, and also enable the maximum available diversity for every user. In our group orthogonal multicarrier CDMA (GO-MC-CDMA) scheme, groups of users share a set of subcarriers. By judiciously choosing group subcarriers, we guarantee that every user transmits with maximum diversity. MUI is only present among users in the same group, and is suppressed via multiuser detection, which becomes practically feasible because we assign a small number of users per group. Performance is analyzed, and simulations are carried out to illustrate the merits of GO-MC-CDMA relative to existing alternatives.

Comparison of cdma and ofdm techniques for downstream power-line communications on low voltage grid

This paper provides a fair comparison between direct sequence code division multiple access (DS-CDMA) and orthogonal frequency division multiplexing (OFDM) systems for broadband downstream power-line communications (PLCs). The considered schemes seem particularly suitable for high bit rate broadcast flexible communications on low voltage grid in order to guarantee local access. The performance of the considered systems is expressed in terms of bit error rate (BER), derived by simulations under the assumptions of frequency-selective multipath fading channel and additive colored Gaussian noise according to the in-building networks model, under the same overall working conditions of bandwidth occupation, transmitted power, and global data rate.

Capacity of RAKE reception with energy randomization

We consider a RAKE receiver for coherent binary orthogonal signaling over a frequency selective multipath Rayleigh fading channel. The receiver uses maximal-ratio combining such that the weight estimation errors are not independent of the additive noise. We find the capacity-achieving energy randomization scheme with two energy levels for the cases of imperfect and perfect channel estimates. We observe that the capacity-achieving probability gets closer to 1/2 with increase in the number of paths. We also show that the capacity is higher if channel estimates are perfect and that the channel estimation errors have more pronounced effect on the capacity at low signal-to-noise ratios.

Error performance analysis of an energy sequence estimation receiver for binary FSK on frequency-selective fading channels

Energy detection of frequency-shift keying (FSK) signals is the optimum noncoherent detection technique yielding a minimum bit-error rate (BER) on the additive white Gaussian noise channel. It is usually used when carrier phase estimation is difficult. When FSK signals are passed through a frequency-selective multipath fading channel, the multipath delay in the channel results in a multitone waveform being received during each signaling interval. An effective practical sequence estimation receiver for this situation is proposed. It makes use of energy detectors followed by a Viterbi (1979) or sequence estimator that uses an energy difference metric. Statistical properties of the detection variable are derived. Analytical upper and lower bounds on the BER are derived and the results compared with computer simulations to show the effectiveness of the technique.

Single-carrier space-time block-coded transmissions over frequency-selective fading channels

We study space-time block coding for single-carrier block transmissions over frequency-selective multipath fading channels. We propose novel transmission schemes that achieve a maximum diversity of order N/sub t/N/sub r/(L+1) in rich scattering environments, where N/sub t/ (N/sub r/) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. We show that linear receiver processing collects full antenna diversity, while the overall complexity remains comparable to that of single-antenna transmissions over frequency-selective channels. We develop transmissions enabling maximum-likelihood optimal decoding based on Viterbi's ( 1998) algorithm, as well as turbo decoding. With single receive and two transmit antennas, the proposed transmission format is capacity achieving. Simulation results demonstrate that joint exploitation of space-multipath diversity leads to significantly improved performance in the presence of frequency-selective fading channels.

Performance of Reverse-Link Synchronous DS-CDMA System on a Frequency-Selective Multipath Fading Channel with Imperfect Power Control

We analyze the performance for reverse-link synchronous DS-CDMA system in a frequency-selective Rayleigh fading channel with an imperfect power control scheme. The performance degradation due to power control error (PCE), which is approximated by a log-normally distributed random variable, is estimated as a function of the standard deviation of the PCE. In addition, we investigate the impacts of the multipath intensity profile (MIP) shape and the number of resolvable paths on the performance. Finally, the coded bit error performance is evaluated in order to estimate the system capacity. Comparing with the conventional CDMA system, we show an achievable gain of from 59% to 23% for reverse-link synchronous transmission technique (RLSTT) in the presence of imperfect power control over asynchronous transmission for . As well, the effect of tradeoff between orthogonality and diversity can be seen according to the number of multipaths, and the tendency is kept even in the presence of PCE. We conclude that the capacity can be further improved via the RLSTT, because the DS-CDMA system is very sensitive to power control imperfections.

Performance of various multistage interference cancellation schemes for asynchronous QPSK/DS/CDMA over multipath Rayleigh fading channels

The performance of multistage interference cancellation (MIC) and three combining techniques, i.e., multipath decorrelating (MIC-DECO), optimum combining (MIC-OPTM), and RAKE combining (MIC-RAKE) for asynchronous quadrature phase-shift keying/direct-sequence code-division multiple access over frequency-selective multipath Rayleigh fading channels is studied. The analytical bit-error probabilities of the MIC-DECO and MIC-OPTM are derived and shown to be in a good agreement with simulation results. Both analytical and simulation results show that the MIC-DECO, MIC-OPTM, and MIC-RAKE in a multiuser environment provide a good performance close to the ideal performance in a single-user system even in the presence of channel estimation error.

Serial acquisition of DS-CDMA signals in multipath fading mobile channels

This paper compares the mean acquisition time (MAT) performance of two serial search acquisition schemes over frequency-selective multipath Rayleigh fading channels. Both the conventional cell-by-cell detection and the novel joint two-cell detection are characterized. In contrast to the majority of published results considering only one correct timing state (H/sub 1/ cell) in the uncertainty region of the pseudonoise (PN) sequence, this paper analyzes the MAT performance of the serial search acquisition schemes with multiple timing states in the uncertainty region. The MAT performance comparison between the conventional cell-by-cell detection and the joint two-cell detection shows that the latter can achieve an improved acquisition performance at almost the same complexity.

RAKE receiver using blind adaptive minimum output energy detection for DS/CDMA over multipath fading channels

To achieve better tracking of time-varying channel parameters, a structure using blind adaptive minimum output energy (MOE) detection in conjunction with RAKE combining diversity for direct-sequence code-division multiple-access communications over frequency selective multipath fading channels is introduced. In this MOE-RAKE structure, the signal replicas received from multiple fading paths are first independently produced by a bank of adaptive MOE detectors and then combined by a RAKE receiver for final symbol decision. A normalised, complex-valued stochastic gradient descent algorithm is developed for the blind adaptation used in the MOE-RAKE. Simulation results show that, in the time-invariant channel, the MOE-RAKE has a performance comparable to the previously proposed MOE and outperforms the conventional RAKE. In the time-varying channel, the MOE-RAKE performs better than the other blind multiuser detection techniques. In practice, to further improve the detector performance, a hybrid scheme started with blind adaptation for the initial updates and then switched to the decision-directed algorithm is desired.

Performance of forward-link cellular DS-CDMA over frequency-selective multipath fading channels

A new frequency-selective multipath-fading model is studied for a forward-link direct-sequence code-division multiple-access (DS-CDMA) system with a multi-cell configuration. In the performance evaluation of this model, the probability density function (pdf) of the interference-to-signal ratio is first derived in closed form. This pdf is then employed to form a compact expression for efficient bit-error rate (BER) evaluation. Numerical results show that more RAKE fingers are important to ensure good forward-link cellular performance when the mobiles travel through different topographical environments.

Error probability of multicell CDMA over frequency selective fading channels with power control error

It is well known that power control is critical in CDMA mobile systems. In this paper, the error probability of a multicell CDMA system, operating with imperfect power control over a frequency selective multipath fading channel, is derived. Asymptotic expressions (for high signal-to-interference-plus-noise ratios) of the error probability show that the use of diversity combining exacerbates the effect of the power control error (PCE), since the power control is identical on each diversity channel; also, the error probability increases exponentially with PCE, while (asymptotically) increasing linearly with the number of users and decreasing inverse linearly with the processing gain. In addition to the asymptotic results mentioned above, both Chernoff bounds on, and approximations to, the error probability are found. Numerical results reveal the tightness of the Chernoff bounds, and show that the approximations are actually much tighter than the bounds, at least for PCE standard deviations of 1.5 dB or less.

Adaptive equalisation of frequency selective multipath fading channels based on sample selection

The behaviour of the frequency selective multipath fading channel has been studied for normalised RMS delay spread over the range from 0.01 to 1.0. This corresponds to 1 ns-100 ns RMS delay spread for a system operating at 10 Mb/s. Over this range, occur in bursts because of fading, intrasymbol interference (IaSI) when the channel spread is low and intersymbol interference (ISI) when the channel spread is high. When the SNR is sufficiently high, the channel is rarely in deep fade, and equalisers in such channels may improve the system performance. It is shown that conventional decision feedback equalisers (DFE) with fixed midpoint sampling do not perform well in the range of RMS delay spread considered for realising a BER/spl les/10/sup -3/. A new equalisation technique is introduced, namely, the sample-diversity (SD) DFE. To realise sample diversity at the DFE input, N samples are taken from each symbol and these samples are used to drive N different DFEs. A simple selection mechanism is introduced to select the best sampling phase and DFE to be used in the tracking mode. Through simulations, the SDDFE is shown to have much better performance than conventional DFE. The SD technique is also applied to coherent-QPSK (CQPSK) detection where N parallel detectors are used. Being a combination of diversity and equalisation, the SD approach is shown as a promising technique for symbol detection in multipath fading channels.

Adaptive MMSE maximum likelihood CDMA multiuser detection

The well-known code division multiple access maximum likelihood receiver (MF-ML) uses a bank of matched filters as a generator of sufficient statistics for maximum likelihood detection of users transmitted symbols. In this paper, the bank of matched filters is replaced by a bank of adaptive minimum mean squared error (MMSE) filters as the generator of sufficient statistics. This formal replacement of the MF bank by the adaptive MMSE filter bank has significant conceptual consequences and provides improvement by several performance measures. The adaptive MMSE-ML receiver's digital implementation is significantly computationally simplified. The advantages of the proposed adaptive MMSE-ML receiver over the MF-ML receiver are: (1) ability to perform joint synchronization, channel parameter estimation, and signal detection where the signal is sent over an unknown, slowly time-varying, frequency-selective multipath fading channel; (2) increased information capacity in a multicellular environment; and (3) significantly improved bit error rate (BER) performance in a multicellular mobile communications environment. The information capacity and the BER of the proposed MMSE-ML receiver are analyzed. Numerical results showing the BER performance of the MMSE-ML receiver in a multipath channel environment are presented.