The rapid evolution of current and upcoming high speed and complex communications networks often necessitates flawless time synchronization among the network nodes in order to guarantee performance. GPS based synchronizers have long been used for synchronizing telecommunications equipment, currently providing an accuracy of up to 10ns. Such high accuracy demands excellent operation from GPS timing receivers. Interference is an important threat to GPS performance. Any degradation in performance, due to the introduction of interference, can cause these receivers to provide a low quality timing solution, or to lose lock with incoming GPS signals altogether. This consideration motivates the study of the performance of GPS timing receivers in the presence of harmful interference. This work is devoted to the theoretical and practical investigations of the effects of RF interference on GPS-based synchronizers and their impacts on communications networks. Contributions made during this work include: a) Identification of the processes and the parameters involved in producing a timing solution which are vulnerable to interference; b) experimentbased confirmation of a hypothesis about the effects of interference on GPS timing receivers; c) identification of the effects of degraded synchronization on the performance of communications networks, especially CDMA and GSM cellular networks, which rely upon GPS based synchronizers; and d) proposal of a method to predict and avoid communications network performance degradation.

The objective of this thesis is to develop tools for the analysis and optimization of an iterative receiver. These tools can be applied to most soft-in soft-out (SISO) receiver components. For illustration purposes we consider a multi-user DS-CDMA system with forward error correction that employs iterative multi-user detection based on soft interference cancellation and single user decoding. Optimized power levels combined with adaptive scheduling allows for efficient utilization of receiver resources for heavily loaded systems.¶ Metric transfer analysis has been shown to be an accurate method of predicting the convergence behavior of iterative receivers. EXtrinsic Information (EXIT), fidelity (FT) and variance (VT) transfer analysis are well-known methods, however the relationship between the different approaches has not been explored in detail. We compare the metrics numerically and analytically and derive functions to closely approximate the relationship between them. The result allows for easy translation between EXIT, FT and VT methods. Furthermore, we extend the $J$ function, which describes mutual information as a function of variance, to fidelity and symbol error variance, the Rayleigh fading channel model and a channel estimate. These $J$ functions allow the \textit{a priori} inputs to the channel estimator, interference canceller and decoder to be accurately modeled. We also derive the effective EXIT charts which can be used for the convergence analysis and performance predictions of unequal power CDMA systems.¶ The optimization of the coded DS-CDMA system is done in two parts; firstly the received power levels are optimized to minimize the power used in the terminal transmitters, then the decoder activation schedule is optimized such that the multi-user receiver complexity is minimized. The uplink received power levels are optimized for the system load using a constrained nonlinear optimization approach. EXIT charts are used to optimize the power allocation in a multi-user turbo-coded DS-CDMA system. We show through simulation that the optimized power levels allow for successful decoding of heavily loaded systems with a large reduction in the convergence SNR.¶ We utilize EXIT chart analysis and a Viterbi search algorithm to derive the optimal decoding schedule for a multi component receiver/decoder. We show through simulations that decoding delay and complexity can be significantly reduced while maintaining BER performance through optimization of the decoding schedule.

Doctor of Philosophy(PhD) / In this thesis, we propose and investigate promising approaches for interference mitigation in multiple input multiple output (MIMO) and code division multiple access (CDMA) systems. Future wireless communication systems will have to achieve high spectral efficiencies in order to meet increasing demands for huge data rates in emerging Internet and multimedia services. Multiuser detection and space diversity techniques are the main principles, which enable efficient use of the available spectrum. The main limitation for the applicability of the techniques in these practical systems is the high complexity of the optimal receiver structures. The research emphasis in this thesis is on the design of a low complexity interference suppression/cancellation algorithm. The most important result of our research is the novel design of interference cancellation receivers which are adaptive and iterative and which are of low computational complexity. We propose various adaptive iterative receivers, based on a joint adaptive iterative detection and decoding algorithm. The proposed receiver can effectively suppress and cancel co-channel interference from the adjacent antennas in the MIMO system with a low computation complexity. The proposed adaptive detector, based on the adaptive least mean square (LMS) algorithm, is investigated and compared with the non-adaptive iterative receiver. Since the LMS algorithm has a slow convergence speed, a partially filtered gradient LMS (PFGLMS) algorithm, which has a faster convergence speed, is proposed to improve the convergence speed of the system. The performance and computational complexity of this receiver are also considered. To further reduce the computational complexity, we apply a frequency domain adaptation technique into the adaptive iterative receivers. The system performance and complexity are investigated. It shows that the computational complexity of the frequency domain based receiver is significantly lower than that of the time domain based receiver with the same system performance. We also consider applications of MIMO techniques in CDMA systems, called MIMO-CDMA. In the MIMO-CDMA, the presence of the co-channel interference (CCI) from the adjacent antennas and multiple access interference (MAI) from other users significantly degrades the system performance. We propose an adaptive iterative receiver, which provides the capability to effectively suppress the interference and cancel the CCI from the adjacent antennas and the MAI from other users so as to improve the system performance. The proposed receiver structure is also based on a joint adaptive detection and decoding scheme. The adaptive detection scheme employs an adaptive normalized LMS algorithm operating in the time and frequency domain. We have investigated and compared their system performance and complexity. Moreover, the system performance is evaluated by using a semi-analytical approach and compared with the simulation results. The results show that there is an excellent agreement between the two approaches.

<p>The purpose of this thesis work is to examine the possibility of building a software-defined radio (SDR) for the VHF-band. The goal is to accomplish this with as few components as possible, thus cutting down the size and the production cost.</p><p>An SDR solution means that the sampling of the signal is done as close to the antenna as possible. The wide bandwidth needed in such a product is achieved by using SP Devices algorithm for time-interleaved ADCs. Two hardware prototypes and two versions of the software were designed and implemented using this technology.</p><p>They were also analyzed within this thesis work. The results proved to be good, and the possibilities to produce a commercial software-defined radio receiver for the VHF-band are good.</p> / <p>Syftet med det här examensarbetet är att utreda möjligheten att bygga en mjukvarustyrd radiomottagare (SDR) för VHF-bandet. Målet är att göra detta genom att använda så få komponenter som möjligt, och därigenom minska storleken och produktionskostnaden.</p><p>En SDR lösning ger att samplingen kommer att ske så nära antennen som möjligt. Den stora bandbredd som behövs för en sådan produkt uppnås genom att använda SP Devices algoritm för att ''tidsinterleava'' höghastighets ADC:er. Två hårdvaruprototyper och två versioner av mjukvaran har designats och implementerats.</p><p>Analyserna har visat bra resultat, och möjligheterna att bygga en komersiell mjukvarudefinierade radiomottagare för VHF-bandet ses som goda.</p>

Recently, many multi-receiver identity-based encryption schemes have been proposed in the literature. However, none can protect the privacy of message receivers among these schemes. In this thesis, we present an anonymous multi-receiver identity-based encryption scheme where we adopt Lagrange interpolating polynomial mechanisms to cope with the above problem. Our scheme makes it impossible for an attacker or any other message receiver to derive the identity of a message receiver such that the privacy of every receiver can be guaranteed. Furthermore, the proposed scheme is quite receiver efficient since each of the receivers merely needs to perform constant times (twice in fact) of pairing computation, which is the most time-consuming computation in pairing-based cryptosystems, to decrypt the received message. Furthermore, we prove that our scheme is secure against adaptive chosen plaintext attacks and adaptive chosen ciphertext attacks. Finally, we also prove that the receivers of the scheme can be anonymous.

We consider the uplink of a multicell multiuser single-input multiple-output system where the channel experiences both small and large-scale fading. The data detection is done by using the linear zero-forcing technique, assuming the base station (BS) has perfect channel state information of all users in its cell. We derive  new, exact analytical expressions for the uplink rate, symbol error rate, and outage probability per user, as well as alower bound on the achievable rate. This bound is very tight and becomes exact in the large-number-of-antennas limit. We further study the asymptotic system performance in the regimes of high signal-to-noise ratio (SNR), large number of antennas, and large number of users per cell. We show that at high SNRs, the system is interference-limited and hence, we cannot improve the system performance by increasing the transmit power of each user. Instead, by increasing the number of BS antennas, the effects of interference and noise can be reduced, thereby improving the system performance. We demonstrate that, with very large antenna arrays at the BS, the transmit power of each user can be made inversely proportional to the number of BS antennas while maintaining a desired quality-of-service. Numerical results are presented to verify our analysis.

Iterative processing has been shown to be very effective in multiuser space-time block coding (STBC) systems. The complexity and efficiency of an iterative receiver depend heavily on how the log-likelihood ratios (LLRs) of the coded bits are computed and exchanged at the receiver among its three major components, namely the multiuser detector, the maximum a posterior probability (MAP) demodulators and the MAP channel decoders. This thesis first presents a method to quantitatively measure the system complexities with floating-point operations (FLOPS) and a technique to evaluate the iterative receiver's convergence property based on mutual information and extrinsic information transfer (EXIT) charts.<p>Then, an integrated iterative receiver is developed by applying the sigma mappings for M-ary quadrature amplitude modulation (M-QAM) constellations. Due to the linear relationship between the coded bits and the transmitted channel symbol, the multiuser detector can work on the bit-level and hence improves the convergence property of the iterative receiver. It is shown that the integrated iterative receiver is an attractive candidate to replace the conventional receiver when a few receive antennas and a high-order M-QAM constellation are employed.<p> Finally, a more general two-loop iterative receiver is proposed by introducing an inner iteration loop between the MAP demodulators and the MAP convolutional decoders besides the outer iteration loop that involves the multiuser detection (MUD) as in the conventional iterative receiver. The proposed two-loop iterative receiver greatly improves the iteration efficiency. It is demonstrated that the proposed two-loop iterative receiver can achieve the same asymptotic performance as that of the conventional iterative receiver, but with much less outer-loop iterations.

Till recently, no significant attempts have been made of developing Aircraft Based Augmentation System (ABAS) architectures capable of generating integrity signals suitable for safety-critical GNSS applications and no commercial ABAS products are available at present. The aim of this research is to support the design a system that generates integrity signals suitable for GNSS application. The conceptual design and key mathematical models were recently developed by the Italian Air Force Experimental Flight Test Centre (CSV-RSV) [1, 2]. Such a system, would be able to provide steering information to the pilot, allowing for real-time and continuous integrity monitoring, avoidance of safety/mission-critical flight conditions and fast recovery of the required navigation performance in case of GNSS data losses. The key elements addressed in this thesis are the development of a CATIA model for military and civil aircraft, supporting antenna obscuration and multipath analysis. This is to allow the ABAS system to generate suitable integrity flags when satellites signals are lost. In order to analyse the GNSS signal loss causes, the GNSS constellation models, the flight dynamics models, fading models, multipath models, Doppler shift models, and GNSS receiver tracking technology previously developed by CSV-RSV, are considered in this research.

The purpose of this thesis work is to examine the possibility of building a software-defined radio (SDR) for the VHF-band. The goal is to accomplish this with as few components as possible, thus cutting down the size and the production cost. An SDR solution means that the sampling of the signal is done as close to the antenna as possible. The wide bandwidth needed in such a product is achieved by using SP Devices algorithm for time-interleaved ADCs. Two hardware prototypes and two versions of the software were designed and implemented using this technology. They were also analyzed within this thesis work. The results proved to be good, and the possibilities to produce a commercial software-defined radio receiver for the VHF-band are good. / Syftet med det här examensarbetet är att utreda möjligheten att bygga en mjukvarustyrd radiomottagare (SDR) för VHF-bandet. Målet är att göra detta genom att använda så få komponenter som möjligt, och därigenom minska storleken och produktionskostnaden. En SDR lösning ger att samplingen kommer att ske så nära antennen som möjligt. Den stora bandbredd som behövs för en sådan produkt uppnås genom att använda SP Devices algoritm för att ''tidsinterleava'' höghastighets ADC:er. Två hårdvaruprototyper och två versioner av mjukvaran har designats och implementerats. Analyserna har visat bra resultat, och möjligheterna att bygga en komersiell mjukvarudefinierade radiomottagare för VHF-bandet ses som goda.

Iterative processing has been shown to be very effective in multiuser space-time block coding (STBC) systems. The complexity and efficiency of an iterative receiver depend heavily on how the log-likelihood ratios (LLRs) of the coded bits are computed and exchanged at the receiver among its three major components, namely the multiuser detector, the maximum a posterior probability (MAP) demodulators and the MAP channel decoders. This thesis first presents a method to quantitatively measure the system complexities with floating-point operations (FLOPS) and a technique to evaluate the iterative receiver's convergence property based on mutual information and extrinsic information transfer (EXIT) charts.<p>Then, an integrated iterative receiver is developed by applying the sigma mappings for M-ary quadrature amplitude modulation (M-QAM) constellations. Due to the linear relationship between the coded bits and the transmitted channel symbol, the multiuser detector can work on the bit-level and hence improves the convergence property of the iterative receiver. It is shown that the integrated iterative receiver is an attractive candidate to replace the conventional receiver when a few receive antennas and a high-order M-QAM constellation are employed.<p> Finally, a more general two-loop iterative receiver is proposed by introducing an inner iteration loop between the MAP demodulators and the MAP convolutional decoders besides the outer iteration loop that involves the multiuser detection (MUD) as in the conventional iterative receiver. The proposed two-loop iterative receiver greatly improves the iteration efficiency. It is demonstrated that the proposed two-loop iterative receiver can achieve the same asymptotic performance as that of the conventional iterative receiver, but with much less outer-loop iterations.