Blind multi-user combining at the base station for asynchronous CDMA systems

This paper studies the potential benefits of antenna arrays in cellular CDMA communications and proposes a powerful scheme to undertake the array processing at the base station in CDMA mobile systems. The proposed technique exploits the temporal structure of CDMA signals. The necessary information is extracted directly from the received signals, thus no training signal orPeer ReviewedPostprint (published version

Transceiver design and system optimization for ultra-wideband communications

This dissertation investigates the potential promises and proposes possible solutions to the challenges of designing transceivers and optimizing system parameters in ultra-wideband (UWB) systems. The goal is to provide guidelines for UWB transceiver implementations under constraints by regulation, existing interference, and channel estimation. New UWB pulse shapes are invented that satisfy the Federal Communications Commission spectral mask. Parameters are designed to possibly implement the proposed pulses. A link budget is quantified based on an accurate frequency-dependent path loss calculation to account for variations across the ultra-wide bandwidth of the signal. Achievable information rates are quantified as a function of transmission distance over additive white Gaussian noise and multipath channels under specific UWB constraints: limited power spectral density, specific modulation formats, and a highly dispersive channel. The effect of self-interference (SI) and inter-symbol interference (ISI) on channel capacity is determined, and modulation formats that mitigate against this effect is identified. Spreading gains of familiar UWB signaling formats are evaluated, and UWB signals are proved to be spread spectrum. Conditions are formulated for trading coding gain with spreading gain with only a small impact on performance. Numerical results are examined to demonstrate that over a frequency-selective channel, the spreading gain may be beneficial in reducing the SI and ISI resulting in higher information rates. A reduced-rank adaptive filtering technique is applied to the problem of interference suppression and optimum combining in UWB communications. The reduced-rank combining method, in particular the eigencanceler, is proposed and compared with a minimum mean square error Rake receiver. Simulation results are evaluated to show that the performance of the proposed method is superior to the minimum mean square error when the correlation matrix is estimated from limited data. Impact of channel estimation on UWB system performance is investigated when path delays and path amplitudes are jointly estimated. Cramér-Rao bound (CRB) expressions for the variance of path delay and amplitude estimates are formulated using maximum likelihood estimation. Using the errors obtained from the CRB, the effective signal-to-noise ratio for UWB Rake receivers employing maximum ratio combining (MRC) is devised in the presence of channel path delay and amplitude errors. An exact expression of the bit error rate (BER) for UWB Rake receivers with MRC is derived with imperfect estimates of channel path delays and amplitudes. Further, this analysis is applied to design optimal transceiver parameters. The BER is used as part of a binary symmetric channel and the achievable information rates are evaluated. The optimum power allocation and number of symbols allocated to the pilot are developed with respect to maximizing the information rate. The optimal signal bandwidth to be used for UWB communications is determined in the presence of imperfect channel state information. The number of multipath components to be collected by Rake receivers is designed to optimize performance with non-ideal channel estimation

Multi-carrier CDMA using convolutional coding and interference cancellation

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN016251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Enhanced Wireless Access Technologies and Experiments for W-CDMA Communications

This article reviews enhanced wireless access technologies and experimental evaluations of the wideband DS-CDMA physical layer employing intercell asynchronous operation with a three-step fast cell search method, pilot symbol-assisted coherent links, signal-to-interference plus background noise power ratio-based fast transmit power control, site diversity (soft/softer handover), and transmit diversity in the forward link. The article also presents link-capacity-enhancing techniques such as using an interference canceller and adaptive antenna array diversity receiver/transmitter, and experimental results in a real multipath fading channel. The laboratory and field experiments exemplify superior techniques of the W-CDMA physical layer and the potential of the IC and AAAD transceiver to decrease the mobile transmit power in the reverse link and multipath interference from high-rate users with large transmit power in the forward link

Fractionally sampled decorrelating detectors for time-varying rayleigh fading CDMA channels

In this dissertation, we propose novel decorrelating multiuser detectors in DSCDMA time-varying frequency-nonselective and frequency-selective fading channels and analyze their performance. We address the common shortcomings of existing multiuser detectors in a mobile environment, such as detector complexity and the error floor. An analytical approach is employed almost exclusively and Monte Carlo simulation is used to confirm the theoretical results. Practical channel models, such as Jakes\u27 and Markovian, are adopted in the numerical examples. The proposed detectors are of the decorrelating type and utilize fractional sampling to simultaneously achieve two goals: (1) the novel realization of a decorrelator with lower computational complexity and shorter processing latency; and (2) the significant reduction of the probability of error floor associated with time-varying fading. The analysis of the impact of imperfect power control on IS-95 multiple access interference is carried out first and the ineffectiveness of IS-95 power control in a mobile radio environment is demonstrated. Fractionally-spaced bit-by-bit decorrelator structures for the frequency-nonselective and frequency-selective channels are then proposed. The matrix singularity problem associated with decorrelation is also addressed, and its solution is suggested. A decorrelating receiver employing differentially coherent detection for an asynchronous CDMA, frequency-nonselective time-varying Rayleigh fading channel is proposed. A maximum likelihood detection principle is applied at the fractionally spaced decorrelator output, resulting in a significantly reduced error floor. For coherent detection, a novel single-stage and two-stage decision feedback (DF) maximum a posteriori (MAP) channel estimator is proposed. These estimators are applicable to a channel with an arbitrary spaced-time correlation function. The fractionally-spaced decorrelating detector is then modified and extended to a frequency-selective time-varying fading channel, and is shown to be capable of simultaneously eliminating MAI, ISI, and path cross-correlation interference. The implicit equivalent frequency diversity is exploited through multipath combining, and the effective time diversity is achieved by fractional sampling for significant performance improvement. The significance of the outcome of this research is in the design of new lower complexity multiuser detectors that do not exhibit the usual deficiencies and limitations associated with a time-varying fading and multipath CDMA mobile environment

Transmitter precoding for multi-antenna multi-user communications

Emerging wireless sensor networks and existing wireless cellular and ad hoc networks motivate the design of low-power receivers. Multi-user interference drastically reduces the energy efficiency of wireless multi-user communications by introducing errors in the bits being detected at the receiver. Interference rejection algorithms and multiple antenna techniques can significantly reduce the bit-error-rate at the receiver. Unfortunately, while interference rejection algorithms burden the receiver with heavy signal processing functionalities, thereby increasing the power consumption at the receiver, the small size of receivers, specifically in sensor networks and in downlink cellular communications, prohibits the use of multiple receive antennas. In a broadcast channel, where a central transmitter is transmitting independent streams to decentralized receivers, it is possible for the transmitter to have a priori knowledge of the interference. Multiple antennas can be used at the transmitter to enhance energy efficiency. In some systems, the transmitter has access to virtually an infinite source of power. A typical example would be the base station transmitter for the downlink of a cellular system. The power consumption at receivers can be reduced if some of the signal processing functionality of the receiver is moved to the transmitter.;In this thesis, we consider a wireless broadcast channel with a transmitter equipped with multiple antennas and having a priori knowledge of interference. Our objective is to minimize the receiver complexity by adding extra signal processing functions to the transmitter. We need to determine the optimal signal that should be transmitted so that interference is completely eliminated, and the benefits that can be obtained by using multiple transmit antennas can be maximized. We investigate the use of linear precoders, linear transformations made on the signal before transmission, for this purpose