The wireless communications industry has experienced tremendous growth worldwide in the past decade. Numerous cellular communication systems have been developed to meet this need. In North America, AMPS, IS-54, 1S-95 and GSM are the most popular cellular systems ruling the market. In developing nations, Wireless Local Loop (WLL) services will be in great demand in the coming years. While some service providers may adapt existing cellular standards to this application, WLL does not require the support for mobility which is available in a full cellular system. As a result, substantial cost savings may be obtained through dedicated WLL implementations. In this thesis, a new wireless communication system is investigated. The system combines the low cost and flexibility of analog frequency modulation with the capacity and multipath advantages of a Code Division Multiple Access (CDMA) system. This system aims to provide WLL telephone services at low cost, with wireline grade voice quality, fast infrastructure deployment and ease of planning. In this thesis, a theory for FM/CDMA system performance is established. Closed form analytical expressions of signal to multiple. access interference ratio are obtained using both upper bounds and lower bounds. The hybrid system is also optimized for the optimum combination of modulation index and processing gain. In addition, a software test-bed is developed to model different FM demodulation schemes, evaluate the tradeoff of FM modulation index versus CDMA processing gain and the system robustness, compare the forward link and the reverse link system performance and investigate the effect of power control schemes. / Master of Science

Ever since the FCC allocated 7.5 GHz (from 3.1 GHz to 10.6 GHz) for ultra wideband (UWB) technology, interest has been renewed in both academic and industrial circles to exploit this vast spectrum for short range, high data rate wireless applications. The great potential of UWB lies in the fact that it can co-exist with the already licensed spectrum users and can still pave the way for a wide range of applications. However, this wide bandwidth complicates the circuit level implementation of key RF blocks like the power amplifier (PA), transmit/receive switch, low noise amplifier (LNA) and mixers in an UWB transceiver. Though expensive technologies like SiGe or GaAs have been used for transceiver realizations, the ultimate goal is to have a single-chip, low-cost solution which can only be achieved by using CMOS technology. Nevertheless, some of the inherent limitations of CMOS like lower fT of transistors make the design of UWB circuits in CMOS an extremely challenging task. Two proposals- Multi-Band OFDM and Direct-Sequence CDMA have been put before the IEEE 802.15.3a task group to decide on the industry standard for the commercial deployment of this technology. Though the debate on which standard is better has not been resolved, proponents of both the groups have already begun to develop prototypes of their respective proposals. This thesis describes the design of a key RF block in the UWB transceiver - the Power Amplifier. For the first part of this work, a PA suitable for MB-OFDM specifications was designed and fabricated in TSMC 0.18um CMOS technology. The class-AB PA is able to cover the lower UWB frequency band from 3.1 GHz to 4.75 GHz and delivers an output power of -2 dBm at 4 GHz. Simulated results show a gain of 19±2 dB achieved over the entire band and the PA consumes 36.54 mW from a 1.8V supply. In the second part of this work, a PA that meets the DS-CDMA specifications was designed and fabricated. This PA operates in the class-AB regime, delivering an output power of -4.2 dBm with input-1dB compression point at -22 dBm. Complete design and implementation was done using TSMC 0.18um CMOS technology and it consumes a very low power of 25 mW, while realizing a flat gain of 19±1 dB across the whole band of operation. All the above mentioned results are from simulations in SpectreRF and measurements are yet to be taken. Additional features like power ON/OFF scheme and output impedance control has also been incorporated in the design. / Master of Science

Recently there has been a significant amount of interest in the area of wideband and ultra-wideband (UWB) signaling for use in indoor wireless systems. This interest is in part motivated by the notion that the use of large bandwidth signals makes systems less sensitive to the degrading effects of multipath propagation. By reducing the sensitivity to multipath, more robust and higher capacity systems can be realized. However, as signal bandwidth is increased, the complexity of a Rake receiver (or other receiver structure) required to capture the available power also increases. In addition, accurate channel estimation is required to realize this performance, which becomes increasingly difficult as energy is dispersed among more multipath components. In this thesis we quantify the channel response for six signal bandwidths ranging from continuous wave (CW) to 1 GHz transmission bandwidths. We present large scale and small scale fading statistics for both LOS and NLOS indoor channels based on an indoor measurement campaign conducted in Durham Hall at Virginia Tech. Using newly developed antenna positioning equipment we also quantify the spatial correlation of these signals. It is shown that the incremental performance gains due to reduced fading of large bandwidths level off as signals approach UWB bandwidths. Furthermore, we analyze the performance of Rake receivers for the different signal bandwidths and compare their performance for binary phase modulation (BPSK). It is shown that the receiver structure and performance is critical in realizing the reduced fading benefit of large signal bandwidths. We show practical channel estimation degrades performance more for larger bandwidths. We also demonstrate for a fixed finger Rake receiver there is an optimal signal bandwidth beyond which increased signal bandwidth produces degrading results. / Master of Science

Ultra-Wideband has recently gained great interest for high-speed short-range communications (e.g. home networking applications) as well as low-speed long-range communications (e.g. sensor network applications). Two flavors of UWB have recently emerged as strong contenders for the technology. One is based on Impulse Radio techniques extended to direct sequence spread spectrum. The other technique is based on Orthogonal Frequency Division Multiplexing. Both schemes are analyzed in this thesis and modifications are proposed to increase the performance of each system. For both schemes, the issue of simultaneously operating users has been investigated. Current MAC design for UWB has relied heavily on existing MAC architectures in order to maintain backward compatibility. It remains to be seen if the existing MACs adequately support the UWB PHY (Physical) layer for the applications envisioned for UWB. Thus, in this work we propose a new MAC scheme for an Impulse Radio based UWB PHY, which is based on a CDMA approach using a code-broker in a piconet architecture. The performance of the proposed scheme is compared with the traditional CSMA scheme as well as the receiver-based code assignment scheme. A new scheme is proposed to increase the overall performance of the Multiband-OFDM system. Two schemes proposed to increase the performance of the system in the presence of simultaneously operating piconets (namely Half Pulse Repetition Frequency and Time spreading) are studied. The advantages/disadvantages of both of the schemes are discussed. / Master of Science

Deployment of antenna arrays is a very promising solution to reduce the Multiple Access Interference (MAI) from high data rate users in the Wideband Code Division Multiple Access (W-CDMA) system. Combining the antenna array with a RAKE receiver, both of which exploits multipath diversity, can significantly improve the system performance. In this research, we investigate the performance of these beamformer-RAKE receivers, also known as two-dimensional (2-D) RAKE receiver, for the reverse link of the W-CDMA system. We consider three different Pilot Symbol Assisted (PSA) beamforming techniques, Direct Matrix Inversion (DMI), Least-Mean Square (LMS) and Recursive Least Square (RLS) adaptive algorithms. Two different Geometrically Based Single Bounce (GBSB) statistical channel models are considered, one, which is more suitable for array processing, and the other is conductive to RAKE combining. The performances of the 2-D RAKE receivers are evaluated in these two channel models as a function of the number of antenna elements and RAKE fingers. It is shown that, in both the cases, the 2-D RAKE receiver outperforms the conventional RAKE receiver and the conventional beamformer by a significant margin. Also, the output SINR expression of a 2-D RAKE receiver with the general optimum beamformer is derived. / Master of Science

In recent years, research on the design and performance evaluation of suboptimal receiver implementations has received considerable attention owing to complexity in the realization of the optimal receiver algorithms over wireless channels. This thesis addresses the effects of using reduced complexity receivers for the Satellite Digital Audio Radio (SDAR), Code Division Multiple Access (CDMA) and UltraWideband (UWB) communications technologies. A graphical-user-interface simulation tool has been developed to predict the link reliability performance of the SDAR services in the continental United States. Feasibility study of receiving both satellite and terrestrial repeater signals using a selection diversity (single antenna) receiver has also been performed. The thesis also develops a general mathematical framework for studying the efficacy of a sub-optimal generalized selection combining (GSC) diversity receiver over generalized fading channel models. The GSC receiver adaptively combines a subset of M diversity paths with the highest instantaneous signal-to-noise ratios (SNR) out of the total L available diversity paths. The analytical framework is applicable for rake receiver designs in CDMA and UWB communications. / Master of Science

The wireless market of the future will demand inexpensive hardware, expandability, interoperability, and the implementation of advanced signal processing functions--i.e. a software radio. Configurable computing machines are often ideal software radio platforms. In particular, the Stallion reconfigurable processor's efficient hardware reuse and scalability fulfill these radios' demands. The advantages of Stallion-based design inspired an FPGA-based software radio - the proto-Stallion receiver. This thesis introduces the proto-Stallion architecture and details its implementation on the SLAAC-1V FPGA platform. Although this thesis presents a specific radio implementation, this architecture is flexible; it can support a variety of applications within its fixed framework. This implemented single-user DS-CDMA receiver utilizes an LMS adaptive filter that can combat MAI and constructively combine multipath; most notably, this receiver employs an adaptive tracking algorithm that harnesses the LMS algorithm to maintain symbol synchronization. The proto-Stallion receiver demonstrates the dependence of adaptive tracking on channel noise; the algorithm requires significant noise levels to maintain synchronization. / Master of Science

In cellular and personal communications services (PCS) systems based on code division multiple access (CDMA), a pilot signal is used on the forward link for synchronization, coherent detection, soft handoff, maintaining orthogonality between base stations, and, in the future, position location. It is critical that the percentage of power allocated to the pilot signal transmitted by each base station be fixed properly to ensure the ability of the CDMA network to support subscriber demand. This thesis reports on the design and implementation of a prototype receiver for measuring pilot signals in CDMA PCS systems. Since the pseudonoise (PN) signal of the pilot channel is a priori information, the receiver searches for pilot signals by digitally correlating the received signal with this known, locally generated pilot signal. By systematically changing the phase of this locally generated pilot signal, the receiver scans the received signal to identify all possible signs of pilot signal activity. Large values of correlation indicate the presence of a pilot signal at the particular phase of the locally generated pilot signal. The receiver can also detect multipath components of the pilot signal transmitted from a given base station. One issue associated with this receiver is its ability to keep the signal power within the dynamic range of the analog-to-digital (A/D) converter at its input. This necessitated the design of an automatic gain control (AGC) mechanism, which is digitally implemented in this receiver. Simulation studies were undertaken to assist in the design and implementation of the pilot signal scanning receiver. These simulations were used to quantify how various non-idealities related to the radio frequency (RF) front-end and A/D converter adversely affect the ability of the digital signal processing algorithms to detect and measure pilot signals. Because the period of the pilot signal is relatively long, methods were developed to keep the receiver's update period as small as possible without compromising its detection ability. Furthermore, the high sampling rate required strains the ability of the digital logic to produce outputs at a rate commensurate with real-time operation. This thesis presents techniques that allow the pilot signal scanning receiver to achieve real-time operation. These techniques involve the judicious use of partial correlations and windowing the received signal to decrease the transfer rate from the A/D converter to the digital signal processor. This thesis provides a comprehensive discussion of these and other issues associated with the actual hardware implementation of the pilot signal scanning receiver. / Master of Science

Research efforts have shown that capacity in a DS/CDMA cellular system can be increased through the use of digital signal processing techniques that exploit the nature of the multiple access interference (MAI). By jointly demodulating the users in the system, this interference can be characterized and reduced thus decreasing the overall probability of error in the system. Numerous multiuser structures exist, each with varying degrees of complexity and performance. However, the size and complexity of these structures is large relative to a conventional receiver. This effort demonstrates a practical approach to implementing parallel interference cancellation applied to DBPSK DS/CDMA on an FPGA-based configurable computing platform. The system presented acquires, tracks, cancels, and demodulates four users independently and performs various levels of interference cancellation. The performance gain of the receiver in a four-user environment under various levels of noise and cancellation are presented. / Master of Science

The capacity of a direct-sequence spread-spectrum code division multiple access (DSSS-CDMA) system is limited by multiple access interference (MAJ) and the near-far problem. There are two approaches to mitigating these problems: multiuser detection and single-user detection techniques. Multiuser detection techniques cancel the interference and enhance system capacity, but have large computational requirements and require the knowledge of MAI parameters. Single-user detection techniques require only the knowledge of the desired user’s spreading code and timing, and have a complexity comparable to the conventional receiver. This thesis reviews a wide range of DSSS-CDMaA single-user detectors found in the literature. The receivers are explained with a common approach using an adaptive antenna array perspective and noting that single-user detectors exploit spectral redundancy, while adaptive arrays exploit spatial redundancy. Commonly used trained adaptation algorithms for single-user detection are first presented, and are followed by newly proposed blind adaptation algorithms. These new blind algorithms are Griffiths’ algorithm, and the linearly constrained constant modulus algorithm (LCCMA ). Through simulation, a blindly-adapted single-user detector is shown to greatly outperform the conventional receiver in terms of bit-error-rate (BER) performance, and to perform almost as well as in the case of trained adaptation. The receivers are shown to be near-far resistant, and are computationally attractive for a mobile receiver. Both receivers have good convergence properties and don’t suffer from catastrophic failure. / Master of Science