Several new multiuser detectors are developed for different direct-sequence codedivision multiple-access (DS-CDMA) application environments. The first detector is based on a semidefinite-programming (SDP) relaxation technique. In this detector, maximum likelihood (ML) detection is achieved by 'relaxing' the associated combinatorial problem into an SDP problem, which leads to a detector of polynomial complexity. It is shown that the SDP-relaxation (SDPR) based detector can be obtained by solving a dual SDP problem which leads to improved efficiency. Computer simulations demonstrate that the SDPR detector offers near-optimal performance with much reduced computational complexity compared with that of the ML detector proposed by Verdu for both synchronous and asynchronous DS-CDMA systems. The second detector is based on a recursive convex programming (RCP) approach. In this detector, ML detection is carried out in two steps: first, the combinatorial problem associated with ML detection is relaxed to a convex programming problem, and then a recursive approach is used to obtain an approximate solution for ML detection. Efficient unconstrained relaxation approach is proposed for the proposed detector to reduce the involved computational complexity. Computer simulations demonstrate that the proposed detectors offer near-optimal detection performance which is superior to that offered by many other suboptimal detectors including the SDPR detector. However, the computational complexity involved in the proposed detectors is much lower relative to that involved in Verdu's ML detector as well as our SDPR detector. The third detector entails a subspace estimation-based constrained optimization approach for channel estimation in DS-CDMA systems with multipath propagation channels. The proposed approach offers an improved approximation for the noise iii subspace compared with that offered by several existing algorithms. Computer simulations show that the performance of the proposed detector offers nearly the same performance as that of existing subspace detectors but leads to a significant reduction in the amount of computation. Relative to some existing constrained optimization methods, the proposed detector offers a significantly improved performance while requiring a comparable amount of computation. The fourth detector is proposed based on a vector constant-modulus (VCM) approach. This detector is designed for DS-CDMA systems with multipath propagation channels where the effective signatures observed at receiver are distorted by multipath propagation and aliasing concurrently. In this detector, detection is carried out by solving a linear constrained optimization problem whose objective function is formulated based on the VCM criterion. Two adaptation algorithms, namely, the constrained stochastic gradient algorithm and the recursive vector constant-modulus algorithm, are developed. Analysis are presented to investigate the performance of the proposed detector. Computer simulations show that the proposed detectors are able to suppress multiuser interference and inter-symbol interference effectively. More importantly, they offer robust detection performance against the effective signature distortion caused by aliasing at the receiver.

In this dissertation, direct-sequence code-division multiple access (DS-CDMA) systems with multiuser detectors used at receiver are investigated and two kinds of multiuser detectors are developed for DS-CDMA systems. In the investigation of DS-CDMA systems using multiuser detectors at receiver, a study on the performance of the system is presented, where heterogeneous traffic with different transmission rates and quality of service (QoS) requirements is supported. The effects of some realistic factors, such as imperfect power control and the existence of multiple cells, on the system performance are studied. In addition, algorithms are proposed to deal with the forward link power allocation problem based on the measurements of random characteristics of the received signals. This power allocation problem is formulated as a constrained optimization problem. To make the problem easy to solve, an additional appropriate constraint is proposed. Two methods are developed to identify the feasible region of this constrained optimization problem. The first proposed multiuser detector is an adaptive minimum mean-squared-error (MMSE) detector. Particularly, it is desirable for the cases where communication channels have severe near-far problem, and thus the convergence rates of adaptive MMSE detectors for users with different power are quite different. To improve the convergence rates of adaptive MMSE detectors for weak power users, the interference effects of the strong power user signals are subtracted from the received signal successively. The method to estimate the parameters required in the proposed detector is also developed. It is shown that the proposed detector achieves fast convergence rates in various near-far scenarios. Other studies conducted include the transient mean-squared-error (MSE) analysis to explain the different convergence rates of adaptive MMSE detectors for users with different power, and the bit-error-rate (BER) performance analysis for the proposed detector. The second proposed multiuser detector is a set of semi-blind linear parallel interference cancellation (PIC) detectors for the reverse link of multiple-cell systems, where only information about intra-cell users is available. To decrease the interference from inter-cell users whose information is unavailable to the receiver, the inter-cell user signal subspace is identified first by making use of the available information about intra-cell users. The eigenvectors and eigenvalues of this signal subspace are then used in the traditional linear PIC structure, in place of the unknown inter-cell users’ signature codes and signal amplitudes. Based on this idea, three detection schemes are proposed. In addition, an efficient adaptation implementation method is developed, and the performance of the proposed detectors is studied. The proposed detectors are shown to be suitable for practical implementations and have satisfactory performance. / Graduate

This dissertation is focused on multi-service and direct sequence code division multiple access (DS/CDMA) wireless cellular systems. The reverse link performance of a slotted DS/CDMA cellular system with multi-service traffic is analyzed. Services with/without packet retransmission to meet their Quality of Service, share the entire bandwidth. Packet failure probabilities and packet delay are obtained based on analyzing the mutual interaction among services. The impacts of power level allocation and power control error of services on capacity, throughput and delay are analyzed under given Quality of Service. The system capacity is maximized by appropriate power allocation. The impact of power control error on capacity is dependent on whether packet retransmission is allowed or not. Admission control policies for multi-service systems are proposed and analyzed. Both nonprioritized and prioritized admission control are studied. Services difference in terms of resource requirement and degree of importance are considered. Analytical models are developed. Blocking probability of each type of calls are found under given amount of traffic. Fair access by soft capacity is addressed. The cost of protecting certain type(s) of calls on the rest of calls is investigated. The impact of traffic distribution on the performance of the policies is also examined. In a hierarchical cellular system, user mobility estimation helps channel assignment so as to reduce the handoff rate and avoid high mobility users travel among small cells. Two different strategies are compared. It is found that when high mobility users are served by overlay macrocells, call drop rate is reduced. Speed estimation error only has limited impact on the system performance. User membership in a cellular CDMA network is simulated based on the estimation of the local mean value of the pilot signal from surrounding base stations. The base station providing strongest pilot local mean controls the mobile station. Simulation is conducted under different fading environments. Two performance measurements are simulated: the number of membership switchings per second and the probability of wrong base station selection. An optimum window length for filtering out Rayleigh fading is found. Simulation results are in good fit with those of analysis. / Graduate

by Kwok Ming Shan. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves 102-[109]). / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Cellular Radio Systems --- p.3 / Chapter 1.2 --- Code Division Multiple Access (CDMA) --- p.7 / Chapter 1.2.1 --- Direct Sequence CDMA (DS-CDMA) --- p.8 / Chapter 1.2.2 --- Frequency Hopping CDMA (FH-GDMA) --- p.11 / Chapter 1.2.3 --- Time Hopping CDMA (TH-CDMA) --- p.12 / Chapter 1.3 --- Propagation Characteristics --- p.12 / Chapter 1.3.1 --- Signal Strength Prediction - Path Loss --- p.13 / Chapter 1.3.2 --- Signal Variability --- p.17 / Chapter 1.3.3 --- Delay Spread --- p.23 / Chapter 1.3.4 --- Coherence Bandwidth --- p.23 / Chapter 1.4 --- Power Control in Cellular Radio Systems --- p.24 / Chapter 1.4.1 --- Centralized Power Control --- p.24 / Chapter 1.4.2 --- Distributed Power Control --- p.25 / Chapter 1.4.3 --- CDMA Power Control --- p.29 / Chapter 2 --- Contributions --- p.39 / Chapter 3 --- ACI Analysis of the Reverse-Link --- p.41 / Chapter 3.1 --- Adjacent Cell Interference --- p.42 / Chapter 3.2 --- Adjacent Cell Interference Analysis --- p.43 / Chapter 3.2.1 --- Interference Analysis of Hexagonal Cells --- p.43 / Chapter 3.2.2 --- Interference Analysis of Circular Cell Structure --- p.47 / Chapter 3.3 --- Closed-form of Adjacent Cell Interference --- p.51 / Chapter 3.4 --- Generalization to Irregular Cell Structure --- p.54 / Chapter 3.5 --- Conclusions --- p.57 / Chapter 4 --- ACI Analysis of Reverse-Link with Log-normal Shadowing --- p.59 / Chapter 4.1 --- Interference with Shadowing --- p.59 / Chapter 4.2 --- Conclusions --- p.66 / Chapter 5 --- ACI Analysis of Microcell --- p.68 / Chapter 5.1 --- Propagation Characteristics of Microcellular Radio Systems --- p.69 / Chapter 5.2 --- CDMA Microcellular Radio Systems --- p.70 / Chapter 5.3 --- Results and Discussions --- p.74 / Chapter 5.4 --- Conclusions --- p.76 / Chapter 6 --- Outage Probability Analysis of Imperfect Power Control --- p.77 / Chapter 6.1 --- Fast Fading of Signal --- p.78 / Chapter 6.2 --- Imperfect Power Control in CDMA --- p.81 / Chapter 6.3 --- Conclusions --- p.85 / Chapter 7 --- Conclusions --- p.88 / Chapter 8 --- Examples of CDMA Cellular Radio Systems --- p.91 / Chapter 8.1 --- Qualcomm CDMA system --- p.91 / Chapter 8.1.1 --- Forward-link --- p.92 / Chapter 8.1.2 --- Reverse-link --- p.93 / Chapter 8.1.3 --- Reverse-Link Open-Loop Power Control --- p.94 / Chapter 8.1.4 --- Reverse-Link Closed-Loop Power Control --- p.95 / Chapter 8.1.5 --- Forward-Link Power Control --- p.96 / Chapter 8.2 --- Interdigital Broadband CDMA System --- p.96 / Appendix --- p.97 / Chapter A --- Derivation of the PDF of the fast fading signal power --- p.97 / Chapter B --- Derivation of the Mean-to-standard deviation ratio --- p.98 / Chapter C --- Acronyms --- p.100 / Bibliography --- p.102

by Tsan-Fai Ho. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 64-68). / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Review on spread spectrum communications --- p.1 / Chapter 1.2 --- The spread spectrum techniques --- p.2 / Chapter 1.2.1 --- Direct Sequence (DS) Systems --- p.2 / Chapter 1.2.2 --- Frequency Hopping (FH) Systems --- p.2 / Chapter 1.2.3 --- Time Hopping (TH) Systems --- p.4 / Chapter 1.2.4 --- Hybrid Systems --- p.4 / Chapter 1.3 --- Existing Applications of the spread spectrum systems --- p.5 / Chapter 1.4 --- Organization of the thesis --- p.6 / Chapter 2 --- The Concept of Duality --- p.7 / Chapter 2.1 --- Multi-Carrier Systems - An Overview --- p.7 / Chapter 2.2 --- Orthogonal Frequency Division Multiplexing --- p.8 / Chapter 2.2.1 --- Bandwidth Efficiency --- p.9 / Chapter 2.2.2 --- Spectral Efficiency --- p.10 / Chapter 2.2.3 --- Effects of fading --- p.11 / Chapter 2.3 --- Applications of OFDM in multiple access --- p.13 / Chapter 2.3.1 --- ST-CDMA --- p.13 / Chapter 2.3.2 --- MC-DS-CDMA --- p.14 / Chapter 2.3.3 --- OFDM-CDMA --- p.15 / Chapter 2.4 --- Duality - Time-Frequency Interrelation --- p.16 / Chapter 3 --- Performance of Multi-Carrier CDMA System --- p.17 / Chapter 3.1 --- System Model --- p.17 / Chapter 3.2 --- Performance Analysis --- p.20 / Chapter 3.2.1 --- Gaussian Channel --- p.20 / Chapter 3.2.2 --- Fading Channel --- p.24 / Chapter 3.3 --- Performance with Pulse Shape --- p.33 / Chapter 3.4 --- Appendix --- p.34 / Chapter 4 --- Signal Design Criteria for MC-CDMA System --- p.36 / Chapter 4.1 --- Existence of Signal Distortion --- p.37 / Chapter 4.2 --- Measures of the Signal Envelope Fluctuation --- p.38 / Chapter 4.3 --- Complementary Sequences --- p.41 / Chapter 4.4 --- Crest Factors --- p.42 / Chapter 4.4.1 --- Time-limited Pulse --- p.43 / Chapter 4.4.2 --- Ideally Band-Limited Pulses --- p.43 / Chapter 4.4.3 --- Shaped Pulses --- p.45 / Chapter 4.5 --- Spectrally Efficient Complementary (SEC) Sequences --- p.48 / Chapter 4.6 --- Construction of Spectrally Efficient Complementary(SEC) Sequences --- p.50 / Chapter 4.7 --- Generalized Multiphase Spectrally Efficient Complementary Sequences --- p.55 / Chapter 5 --- Summary and Future Extensions --- p.58 / Chapter 5.1 --- Summary of the Results --- p.58 / Chapter 5.2 --- Topics for Future Research --- p.59 / Appendix / Chapter A --- Exhaustive search of MPSEC sequences --- p.61 / Chapter B --- Papers derived from this thesis --- p.63 / Bibliography --- p.64

by Yin-Man Lee. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 82-[87]). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Overview of MC-CDMA --- p.1 / Chapter 1.2 --- System Model --- p.3 / Chapter 1.3 --- Receiver Optimization --- p.7 / Chapter 1.4 --- Transmitter Optimization --- p.9 / Chapter 1.5 --- Nonlinearly Constrained Optimization --- p.10 / Chapter 1.6 --- Outline of Thesis --- p.11 / Chapter 2 --- Centralized Transmitter Optimization for MC-CDMA Systems --- p.13 / Chapter 2.1 --- Introduction --- p.13 / Chapter 2.2 --- Problem Development --- p.15 / Chapter 2.3 --- Lagrangian Optimization Approaches --- p.16 / Chapter 2.3.1 --- Penalty Function Method --- p.17 / Chapter 2.3.2 --- Barrier Function Method --- p.19 / Chapter 2.3.3 --- Powell's Method and Augmented Lagrangian Method --- p.21 / Chapter 2.4 --- Optimal FDMA System --- p.23 / Chapter 2.5 --- Modified Centralized Optimization Schemes --- p.25 / Chapter 2.6 --- Performance --- p.27 / Chapter 2.6.1 --- Typical Behavior --- p.27 / Chapter 2.6.2 --- Average Performance --- p.32 / Chapter 2.7 --- Summary --- p.38 / Chapter 3 --- Decentralized Transmitter Optimization for MC-CDMA Sys- tems --- p.39 / Chapter 3.1 --- Introduction --- p.39 / Chapter 3.2 --- System Model --- p.41 / Chapter 3.3 --- Optimization --- p.42 / Chapter 3.3.1 --- Receiver Optimization --- p.43 / Chapter 3.3.2 --- Single-user Transmitter Optimization --- p.44 / Chapter 3.3.3 --- Decentralized Transmission Scheme --- p.45 / Chapter 3.3.4 --- Multirate Transmission with Decentralized Transmission Scheme --- p.47 / Chapter 3.4 --- Performance --- p.48 / Chapter 3.5 --- Summary --- p.57 / Chapter 4 --- Performance Evaluation of Various Adaptive Transmission Schemes --- p.59 / Chapter 4.1 --- Introduction --- p.59 / Chapter 4.2 --- Comparison of Different Adaptive Transmission Schemes --- p.61 / Chapter 4.3 --- Adaptive Transmission Schemes with K > M --- p.64 / Chapter 4.4 --- Modified Adaptive Transmission Scheme with Graceful Degrada- tion in the SNR --- p.68 / Chapter 4.5 --- Summary --- p.71 / Chapter 5 --- Conclusions and Future Work --- p.73 / Chapter 5.1 --- Conclusions --- p.73 / Chapter 5.2 --- Future Work --- p.75 / A The Hungarian Method for Optimal Frequency Assignment --- p.76 / Bibliography --- p.82

Kwan Ho-yuet. / "April 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (p. 98-[103]). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

The introduction of cellular wireless systems in the 1980s has resulted in a huge demand for personal communication services. This demand has made larger capacity systems necessary. This has been partially satisfied by the introduction of second generation digital systems. New third generation systems are now under going standardisation and will require even more efficient utilisation of the spectrum if the high bandwidth features and larger capacity are to become a reality. Motivated by these growing requirements we discuss methods of achieving large improvements in spectral efficiency and performance. Multiple-user communications over a channel can only be achieved with some form of diversity. In this work we point out that the efficient utilisation of the dimensions of space, time, and frequency will ultimately maximise the system capacity of a multiple-user system. We apply our receiver techniques solely to the base-station design where capacity limitations are currently present. We note however, that some of these techniques could also be applied at the mobile terminal receiver. We primarily focus our attention on the direct-sequence code-division multiple-access (DS/CDMA) channel, since this channel is inherently interference limited by other users in the cell of interest. We exploit a new powerful channel coding technique named " turbo coding" for its iterative decoding approach. We show how we can substitute the inner convolutional code of a turbo code encoder with the CDMA channel. By " iterative detection/decoding" or " turbo equalisation" at the receiver we achieve performance results which show the interference from other users to approach complete removal. We develop and analyse a new, low complexity, iterative interference canceller/decoder. This receiver has complexity per user linear with the memory of the channel and independent of the number of users in the system. We extend this receiver to more realistic channels that are asynchronous and include multi-path, and include spatial diversity by using an antenna array at the receiver. The CDMA channel we study exclusively uses randomly generated spreading codes. With this channel model we still achieve single user performance (no interference from other users) with a 10logL gain from L antenna elements and a gain of up to 10logP from P multi-path components. With any new receiver design, sensitivity to channel parameter errors is of paramount interest. We find that the sensitivity of our receiver is low with respect to the parameter errors induced. This is as we desire for a realisable receiver design. Finally we investigate the application of this new iterative interference canceller/decoder receiver to a number of other interference channels. These include the intersymbol interference (ISI) channel, partial response signalling (PRS), and continuous phase modulation (CPM). For these channels excellent performance improvement is generally achieved by the utilisation of the iterative interference canceller/decoder solution. / Thesis (PhD)--University of South Australia, 1999

Interference from other adjacent users in wireless applications is a major problem in direct-sequence code-division multiple-access (DS-CDMA). This is also known as the near-far problem where a strong signal from one user interferes with other users. The current approach to deal with the near-far problem in DS-CDMA systems is to use strict transmitter power control. An alternative approach is to use near-far resistant receivers. The practical near-far resistance receiver structure is the adaptive decorrelating detectors since it avoids complex matrix inversion. The existing CDMA standard known as IS-95 uses a long signature code sequence. However for simplicity, the adaptive multi-user receiver uses short signature code sequence. The problem is that adaptive receivers lose near-far resistance as the number of users increases in the system. This thesis describes a novel method of multistage decision feedback cancellation (DFC) scheme immune from the near-far problem. The performance of the new DFC structure is constructed using three different adaptive algorithms: the least mean squared (LMS), the recursive least squared (RLS) and the linearly constraint constant modulus (LCCM) adaptive algorithms. It is found that LMS adaptive algorithm provides the best result considering its simple hardware complexity. It is also found that the LMS adaptive receiver along with the DFC structure provides a better bit synchronization capability to the over all system. Since the receiver is near-far resistant, the LMS adaptive receiver along with the decision feedback cancellation structure also performs better in the presence of Rayleigh fading. / Graduation date: 1998

IS-95, an interim standard proposed for future digital personal communications systems, uses two levels of encoding of digital data for error control and compatibility with code-division multiple access (CDMA) transmission. The data is first convolutionally encoded and the resulting symbols are interleaved and then groups are encoded as orthogonal Walsh sequences. Decoding these two separate encodings is traditionally done in separate sequential steps. By combining the decoding and applying feedback of the final decision of the second level of decoding to the first level decoder it is possible to reduce the error rate of the decoder. Each Walsh sequence encodes six non-adjacent symbols of the convolutional code. The receiver computes an estimate of the probability that each of the sixty-four possible Walsh sequences has been sent, and uses this estimate as an estimate for each of the convolution symbols which specified the Walsh sequence. Since the convolution symbols are non-adjacent, it is likely that the actual value of some of the earlier symbols will have been determined by the final decoder before later symbols specifying the same Walsh sequence are used by the convolution decoder. The knowledge of the values of these symbols can be used to adjust the probability estimates for that Walsh sequence, improving the likelihood that future convolutional symbols will be correctly decoded. Specific metrics for estimating probabilities that each convolutional symbol was sent were tested with and without the proposed feedback, and error rates were estimated based on extensive computer simulations. It was found that applying feedback does improve error rates. Analytical methods were also applied to help explain the effects. / Graduation date: 1996