published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

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The objective of this research is to develop a Markovian model in the form of a discrete-time queueing network to assess the performance of random access code division multiple access networks (CDMA). An approximation method called equilibrium point analysis (EPA) has been used to solve the model. The CDMA protocol IS an important application of spread spectrum communications that allows simultaneous transmission of multiple users to occupy a wideband channel with small interference. This is done by assigning each user a unique pseudo noise code sequence. These codes have low cross-correlation between each pair of sequences. Both slotted direct sequence CDMA (DS) and frequency hopping CDMA (FH) are considered with an emphasis on DS-CDMA systems. The EPA method has previously been used to evaluate the performance of other random access systems such as the ALOHA protocol, but has not previously been used in the context of a CDMA protocol. Throughput and mean packet delay of random access CDMA networks are evaluated, since these two measures are usually used in the study of the performance assessment of mUltiple access protocols. The analytical results of the random access model are validated against a discrete-event simulation which is run for large number of slots. The study then proceeds by using the model to examine the effect on performance of introducing error correcting codes to the DS-CDMA systems. Optimum error correcting codes that give the best performances in terms of the throughput and the delay are determined. The perfonnance of random access CDMA systems applied to radio channels, as in packet radio networks, is then studied, and the effect of multipath fading on the perfonnance is evaluated. Finally, the perfonnance of DS-CDMA with different user classes (non-identical users case) is investigated. An extended equilibrium point analysis (EEPA) method has been used to solve the Markovian model in this situation. This extended model is used to assess the effects on perfonnance of the unequal powers due to varying distances of the users to an intended receiver or to a base station (near-far problem).

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