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Orthogonal Frequency Division Multiple Access (OFDMA) uplink systems have been adopted generally in wireless communication in recent years. By combining Orthogonal Frequency Division Multiplexing (OFDM) with Frequency Division Multiple Access (FDMA), OFDMA systems divide all subcarriers in frequency domain into several mutually exclusive subbands, and assign the subbands to different user, transmitting different kinds of data by the orthogonality between subcarriers. The users¡¦ Carrier Frequency Offsets (CFO) caused by Doppler effects and/or misadjusted local oscillators may destroy the orthogonality between subcarriers, then lead to Multiple Access Interference (MAI) for all users, the MAI may degrade the performance of bit error rate. Several MAI suppression or cancellation schemes have been investigated to mitigate the effect of MAI, however, these schemes work on the hypothesis of known CFOs. As the result, it is the crucial issue for the multiuser CFOs estimation in OFDMA uplink systems. In this paper, we propose the Minimum Mean Square Error Criterion (MMSEC) estimation scheme to achieve multiuser CFOs estimation in Additive White Gaussian Noise (AWGN) environment. By modifying the MAI suppression weighting in [9], the MMSEC scheme search desired user¡¦s true CFO with CFO trial parameter, while the CFO trial parameter is equal to true CFO, the modified MAI suppression weighting will suppress the MAI ideally and then separate the desired user¡¦s estimated signal from received signal. Thus the distribution of estimated signal is highly concentrated in each of quadrant in the signal constellation, and it will have the minimum mean square error in each of quadrant. Compare with the MUtiple SIgnal Classification (MUSIC) scheme [12] and the Estimation of Signal Parameter via Rotational Invariance Technique (ESPRIT) scheme [13], they deal with the fully loaded problem by extending cyclic prefix, that may cause the redundant power consumption or degrade data throughput, the advantage of the MMSEC scheme is that it can works on fully load without extending cyclic prefix. Because despite fully loaded, the CFO trial parameter search desired user¡¦s true CFO in reasonable range until it causes minimum mean square error. The simulation result shows that the non-fully loaded system mean square error (MSE) performance is superior to the MUSIC and ESPRIT schees at high signal to noise ratio, while system is fully loaded, the system MSE performance is also acceptable.

Multicarrier code division multiple access (MC-CDMA) is a very promising candidate for the multiple access scheme in fourth generation wireless communi- cation systems. During asynchronous transmission, multiple access interference (MAI) is a major challenge for MC-CDMA systems and significantly affects their performance. The main objectives of this thesis are to analyze the MAI in asyn- chronous MC-CDMA, and to develop robust techniques to reduce the MAI effect. Focus is first on the statistical analysis of MAI in asynchronous MC-CDMA. A new statistical model of MAI is developed. In the new model, the derivation of MAI can be applied to different distributions of timing offset, and the MAI power is modelled as a Gamma distributed random variable. By applying the new statistical model of MAI, a new computer simulation model is proposed. This model is based on the modelling of a multiuser system as a single user system followed by an additive noise component representing the MAI, which enables the new simulation model to significantly reduce the computation load during computer simulations. MAI reduction using slow frequency hopping (SFH) technique is the topic of the second part of the thesis. Two subsystems are considered. The first sub- system involves subcarrier frequency hopping as a group, which is referred to as GSFH/MC-CDMA. In the second subsystem, the condition of group hopping is dropped, resulting in a more general system, namely individual subcarrier frequency hopping MC-CDMA (ISFH/MC-CDMA). This research found that with the introduction of SFH, both of GSFH/MC-CDMA and ISFH/MC-CDMA sys- tems generate less MAI power than the basic MC-CDMA system during asyn- chronous transmission. Because of this, both SFH systems are shown to outper- form MC-CDMA in terms of BER. This improvement, however, is at the expense of spectral widening. In the third part of this thesis, base station polarization diversity, as another MAI reduction technique, is introduced to asynchronous MC-CDMA. The com- bined system is referred to as Pol/MC-CDMA. In this part a new optimum com- bining technique namely maximal signal-to-MAI ratio combining (MSMAIRC) is proposed to combine the signals in two base station antennas. With the applica- tion of MSMAIRC and in the absents of additive white Gaussian noise (AWGN), the resulting signal-to-MAI ratio (SMAIR) is not only maximized but also in- dependent of cross polarization discrimination (XPD) and antenna angle. In the case when AWGN is present, the performance of MSMAIRC is still affected by the XPD and antenna angle, but to a much lesser degree than the traditional maximal ratio combining (MRC). Furthermore, this research found that the BER performance for Pol/MC-CDMA can be further improved by changing the angle between the two receiving antennas. Hence the optimum antenna angles for both MSMAIRC and MRC are derived and their effects on the BER performance are compared. With the derived optimum antenna angle, the Pol/MC-CDMA system is able to obtain the lowest BER for a given XPD.

Recently, it has been shown that Multiple Access Interference (MAl) cancellation is a promising technique for improving the performance and capacity of the reverse link in a Code Division Multiple Access (CDMA) cellular system. However, it has beep observed that indiscriminate cancellation of all received signals can degrade performance. This thesis explores the use of selective cancellation to improve the performance of practical CDMA systems. First, this thesis considers the performance of adaptive interference cancellation applied to a CDMA micro cellular environment. This thesis employs a circular geometry and a closed form expression for the Bit Error Rate of a CDMA system with interference cancellation to analyze the effect of out-of-cell interference. Results are presented which indicate that out-of-cell interference will severely limit the benefits of interference cancellation in a multicellular system. Attempts to cancel all out-of-cell interference will further degrade performance. However, the use of selective interference cancellation in which only the strongest out-of-cell interferers are cancelled may result in significant performance enhancement. These results are shown to agree closely with those obtained using a hexagonal geometry. The MAl is modeled using both the simple and an improved Gaussian approximation. This thesis also investigates the use of selective cancellation with bit averaging. Amplitude estimates over several consecutive symbols can be averaged to improve the accuracy of the estimate. An expression for the BER of the interference cancellation receiver with hard decisions is developed. Results show that averaging power estimates leads to considerable improvement in capacity. Results are also presented for the case of perfect power estimates. / Master of Science

Ultra-Wideband impulse radio (UWB-IR) technology is an attractive method on multi-user for high data rate transmitting structures. In this thesis, we use the ultra wideband (UWB) signal that is modulated by the time-hopping spread spectrum technique in a wireless multiple access environments, and discuss the influences of multiple access interference. We discuss two parts of the influences of multiple access interference in this thesis. The first, we analyze the multiple access interferences on the conventional correlation receiver, and discuss the influences by using the time hopping code on different multiple access structures. The second, we know that the performances of user detection and system capacity would be degraded by the conventional correlation receiver in the multiple access channels. The Probabilistic Data Association(PDA) multi-user detection technology can eliminate multiple access interferences in this part. We will use this method to verify the system performance through the computer simulations, and compare to other multi-user detectors with convention correlation receivers. Finally, the simulation results show that the performance of the PDA multi-user detections is improved when the system is full loaded.

Abstract The present dissertation deals with initial synchronization in Code Division Multiple Access (CDMA) networks. In the first part of this thesis an extensive and up-to-date review of the literature is presented. The basic theory of code acquisition as well as different techniques and structures used to achieve the initial synchronization are discussed. A survey of the most common theoretical approaches allowing performance characterization of the acquisition process is included. The effect of noise, interference, carrier Doppler, multipath propagation, fading and data modulation on system performance are reviewed. Advanced code acquisition approaches exploiting interference suppression techniques and multiple antennas are also described. A summary of the results obtained within the area of code acquisition in CDMA networks is also embraced by this thesis. The distinctive assumption is to consider the actual variable effect of multiple access interference on the delay-domain search process, instead of the usual constant approximation. Three directions of research are followed. Models for code acquisition in quasi-synchronous and asynchronous CDMA networks are first developed and analysed. Closed-form expressions for the main performance figures of the acquisition process are derived and analysed. Results show a strong dependence of the mean acquisition time with the nature of the multiple access interference. In the second area of research the previous results are extended to consider code acquisition with a multi-branch (Rake) receiver in a multipath channel. A generic model for Rake receiver code acquisition is considered and developed, in which the synchronization takes place in two phases. The first detected path is allocated to the first finger during the initial synchronization phase, whereas the remaining fingers are successively allocated to other available paths in the postinitial synchronization phase. Performance measures for this acquisition process are also derived and analysed. Finally, based on the use of an antenna array and beamforming techniques, conventional delay-domain code acquisition is extended to the angular domain, resulting in a two-dimensional (delay-angle) search. This technique is found to be feasible, outperforming the synchronization approach exploiting a single-antenna. It is found that there exists an optimal number of antennas that minimises the mean acquisition time. Two-dimensional code acquisition is studied in a variety of scenarios, including single and multipath channels, fixed and fading channels, and with uniform and nonuniform spatial distributions of interference. Different two-dimensional search strategies are studied. A clear dependence of acquisition performance with the search strategy and the particular distribution of interference is pointed out. The performance of two-dimensional code acquisition is found to be seriously deteriorated by the presence of spatially nonuniform interference. Schemes based on search strategy and adaptive detector structures are considered and analysed to combat the performance degradation in the mentioned case. A comparative study of code acquisition exploiting multiple antennas is also presented.

Abstract Conventional versions of linear multiuser detectors (MUD) are not feasible in the wideband code division multiple access (WCDMA) downlink due to the use of long scrambling sequences. As an alternative, linear channel equalizers restore the orthogonality of the spreading sequences lost in frequency-selective channels, thus, suppressing multiple access interference (MAI) in the WCDMA downlink. In this thesis, linear channel equalizers in WCDMA terminals are studied. The purpose of the thesis is to develop novel receivers that provide performance enhancement over conventional rake receivers with an acceptable increase in complexity, and to validate their performance under WCDMA downlink conditions. Although the WCDMA standard is emphasized as the candidate system, the receivers presented are suitable for any synchronous direct sequence code division multiple access downlink employing coherent data detection and orthogonal user or channel separation. Two adaptive channel equalizers are developed based on the constrained minimum output energy (MOE) criterion and sample matrix inversion method. An existing equalizer based on the matrix inversion lemma is also developed further to become a prefilter-rake equalizer. Performance analysis is carried out for equalizers trained using a common pilot channel and for the channel response constrained MOE (CR-MOE) and sample matrix inversion (SMI) based equalizers developed in the thesis. The linear minimum mean square error (LMMSE) channel equalizer, which assumes a random scrambling sequence, is shown to approximate the performance of the LMMSE MUD. The adaptive CR-MOE, SMI-based, and prefilter-rake equalizers are observed to attain performance close to that of an approximate LMMSE channel equalizer. The equalizers considered are also shown to be suitable for implementation with fixed-point arithmetic. The SMI-based equalizer is shown to provide good performance and to require an acceptable increase in complexity. It is also well suited for symbol rate equalization after despreading, which allows for computationally efficient receiver designs for low data rate terminals. Hence, the SMI-based equalizer is a suitable receiver candidate for both high and low data rate terminals. Adaptive equalizers are considered in conjunction with forward error correction (FEC) coding, soft handover, transmit diversity and high speed downlink packet access (HSDPA). The adaptive equalizers are shown to provide significant performance gains over the rake receiver in frequency selective channels. The performance gains provided by one antenna equalizers are noted to decrease near the edges of a cell, whereas the equalizers with two receive antennas achieve significant performance improvements also with soft handover. The performance gains of one or two antenna equalizers are shown to be marginal in conjunction with transmit antenna diversity. Otherwise the equalizers are observed to attain good signal-to-noise-plus-interference ratio performance. Therefore, they are also suitable receiver candidates for HSDPA.

Researchers have shown that by combining multiple input multiple output (MIMO) techniques with CDMA then higher gains in capacity, reliability and data transmission speed can be attained. But a major drawback of MIMO-CDMA systems is multiple access interference (MAI) which can reduce the capacity and increase the bit error rate (BER), so statistical analysis of MAI becomes a very important factor in the performance analysis of these systems. In this thesis, a detailed analysis of MAI is performed for binary phase-shift keying (BPSK) signals with random signature sequence in Raleigh fading environment and closed from expressions for the probability density function of MAI and MAI with noise are derived. Further, probability of error is derived for the maximum Likelihood receiver. These derivations are verified through simulations and are found to reinforce the theoretical results. Since the performance of MIMO suffers significantly from MAI and inter-symbol interference (ISI), equalization is needed to mitigate these effects. It is well known from the theory of constrained optimization that the learning speed of any adaptive filtering algorithm can be increased by adding a constraint to it, as in the case of the normalized least mean squared (NLMS) algorithm. Thus, in this work both linear and non-linear decision feedback (DFE) equalizers for MIMO systems with least mean square (LMS) based constrained stochastic gradient algorithm have been designed. More specifically, an LMS algorithm has been developed , which was equipped with the knowledge of number of users, spreading sequence (SS) length, additive noise variance as well as MAI with noise (new constraint) and is named MIMO-CDMA MAI with noise constrained (MNCLMS) algorithm. Convergence and tracking analysis of the proposed algorithm are carried out in the scenario of interference and noise limited systems, and simulation results are presented to compare the performance of MIMO-CDMA MNCLMS algorithm with other adaptive algorithms.

The inadequacy of conventional CDMA receivers in a multiple access interference-limited mobile radio environment has spurred research on advanced receiver technologies. This research investigates the use of adaptive receivers for single user demodulation to overcome some of the deficiencies of a conventional receiver and, hence, enhance the system capacity. Several new adaptive techniques are proposed. The new techniques and some existing schemes are analyzed. The limitation of existing blind algorithms in multipath channels is analyzed and a new blind algorithm is proposed that overcomes this limitation. The optimal receiver structure for multi-rate spread spectrum systems is derived and the performance of this receiver in various propagation channels is investigated. The application of coherent and differentially coherent implementations of the adaptive receiver in the presence of carrier frequency offsets is analyzed. The performance of several new adaptive receiver structures for frequency offset compensation is also studied in this research. Analysis of the minimum mean-squared error receiver is carried out to provide a better understanding of the dependence of its performance on channel parameters and to explain the near-far resilience of the receiver. Complex differentially coherent versions of the sign algorithm and the signed regressor algorithm, algorithms that have a much lower computational complexity than the least-means square algorithm, are proposed and applied for CDMA interference rejection. / Ph. D.

The direct-sequence code division multiple access (DS-CDMA) is one of the significant techniques for wireless communication systems with multiple simultaneous transmissions. The main concern of this thesis is to propose a new linearly constrained constant modulus modified conjugate gradient (LCCM-MCG) adaptive filtering algorithm to deal with problem of channel mismatch associated with the multiple access interference (MAI) in DS-CDMA system over multipath fading channel. In fact, the adaptive filtering algorithm based on the CM criterion is known to be very attractive for the case when the channel parameters are not estimated perfectly. The proposed LCCM-MCG algorithm is derived based on the so-called generalized sidelobe canceller (GSC). It has the advantage of having better stability and less computational complexity compared with conventional recursive least-squares (RLS) algorithm, and can be used to achieve desired performance for multiuser RAKE receiver. Moreover, with the MCG algorithm it requires only one recursive iteration per incoming sample data for updating the weight vector, but still maintains performance comparable to the RLS algorithm. From computer simulation results, we show that the proposed LCCM-MCG algorithm has fast convergence rate and could be used to circumvent the effect due to channel mismatch. Also, the performance, in terms of bit error rate (BER), is quite close to the LCCM-RLS algorithm suggested in [18], and is superior to the stochastic gradient descent (SGD) algorithm proposed in [7].