Rural telephony has historically been a recurring subject of concern for most large developing countries. It is generally considered that rural telephone users do not generate the same level of telephone traffic and, thus, revenue as urban users, lowering the incentives to invest in rural telecommunications. The financial implications of wiring a vast area for low telephone traffic causes most telephone service providers to ignore or delay offering telephone service to those regions. Still, it is known that telecommunications are essential to the economic development of a region, and that traffic increases rapidly as soon as the service is available. A satellite-based telephone network can provide efficient long distance telephone service to remote rural communities at a lower cost than land-based wired networks in most cases. Mobile satellite systems already provide this service, but are limited in capacity and charge high per- minute fees for the satellite link. Small earth stations and GEO satellites can provide this service more efficiently and at lower cost. A methodology to optimize the network performance has been developed. A set of economic models to evaluate different combinations of network topologies and multiple access techniques have been implemented, and a technical-economic assessment has been performed for the different technologies under different traffic scenarios. Traffic intensity, network size and per-minute user costs have been optimized to achieve the network's economic break-even point under different conditions and constraints. The general behavior of fixed-assignment Single Channel per Carrier SCPC), fixed-assignment Multiple Channel per Carrier (MCPC) and demand-assignment SCPC star networks, as well as demand-assignment SCPC mesh networks has been analyzed. Important parameters have been identified in order to improve the process of effective and cost-efficient satellite rural telephone network design. / Ph. D.

Recent accomplishments in link-level and radio technologies have significantly improved the performance of wireless links. Wireless mobile ad hoc networks, however, typically only take limited advantage of these enhancements. In this research, the medium access control protocol and ad hoc routing protocol are extended to take advantage of radios offering multi-user interference cancellation and direct-sequence spread-spectrum functionality, by encouraging multiple simultaneous connections and adaptively changing communication parameters on a per-packet basis. Through its environment characterization techniques, the adaptive direct sequence spread spectrum MAC protocol for non-broadcast multiple access networks (ADIM-NB) improves several aspects of the wireless mobile ad hoc network performance, including throughput, delay, stability, and power consumption, through its use of spread-spectrum multiple access and four different adaptive algorithms. The four adaptive algorithms change processing gain, forward error correction coding rate, transmit power, and number of simultaneous connections. In addition, the ad hoc routing protocol is extended with the clustering algorithm for mobile ad hoc network (CAMEN). With ADIM-NB in mind, CAMEN discourages the use of broadcast messages, supplements ADIM-NB's functionality at the network level, and improves the network scalability by aggregating nodes into clusters. Both protocols are intended to lead to more powerful and flexible communication capabilities for wireless nodes. Simulation models have been developed and simulated to verify the performance improvements of both protocols at the network-level as well as provide a means to perform trade-off analysis. Results indicate that the network capacity is increased between 50% in a moderately loaded network to 100% in a heavily loaded network over a non-adaptive MAC protocol. The delay also improve significantly in most scenarios of interest. / Ph. D.

The focus of this dissertation is the study of advanced processing techniques for multiuser interference cancellation in direct sequence code division multiple access communications. Emphasis is placed on the development of efficient techniques that are practical to implement. The work begins with a study of several sub-optimal multiuser detection techniques under a variety of conditions. Multistage parallel interference cancellation is identified as a practical and robust approach for mitigating multiple access interference. In order to reduce the effect of biased decision statistics inherent to parallel cancellation, a low-complexity modification to parallel interference cancellation that significantly improves performance is derived. Based on this approach, two real-time DSP implementations are devised, one fully coherent and one non-coherent. Multi-symbol differential detection is then explored as an alternative for improving the performance of the non-coherent approach. Additionally, dual-antenna diversity techniques are also investigated as a means for improving performance in multipath environments. / Ph. D.

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.

Parallel interference cancellation for DS-CDMA has been shown to suffer from biased amplitude estimates if a matched-filter estimator is used. The bias magnitude is proportional to the number of interfering users. For heavy system loads, the bias has been shown to adversely effect the accuracy of the interference cancellation process, thereby impairing BER after cancellation. Empirical simulation work has demonstrated that weighting down interference estimates can improve BER performance. This thesis substantiates these BER improvements by modelling and analyzing a soft interference cancellation technique which mitigates the effects of the bias by minimizing BER after cancellation in a bit-synchronous parallel interference cancellation CDMA receiver. We analyze system decision metrics with down-scaled interference estimates and determine both the mean and variance of the biased decision statistics. From these two metric moments, system BER is evaluated, and the optimal interference scaling function which minimizes BER is derived. We demonstrate BER performance enhancements by simulating this soft interference cancellation technique in systems under perfect power control and in the near-far situation. We further discuss the applicability of the results to asynchronous systems. / Master of Science

This thesis proposes a multistage RAKE receiver for use at the base station of a Code Division Multiple Access (CDMA) cellular communications system. This receiver combines the interference rejection capability of multiuser receivers with the robust performance of a RAKE receiver in the presence of multipath interference. Unlike previous multiuser receiver designs, this multistage RAKE receiver requires no a priori information about the time-vary multipath radio channel. The thesis presents a mathematical description of the multistage receiver, and a software simulation of the receiver performance. A wide range of channel models are considered which include the effects of Gaussian noise, multipath propagation, imperfect power control and multiple access interference for the reverse link CDMA channel. Under a wide range of conditions, the multistage RAKE receiver is able to support two to three times as many users as a conventional single stage RAKE receiver. The receiver is also shown to be robust to the near-far problem. / Master of Science

It is desirable to develop a universal digital receiver which is capable of operating with many different wireless communication systems. Time Dependent Adaptive Filter (TDAF) Processing represents an effective approach for CDMA interference rejection. This technique is especially appropriate at the mobile receiver where there is little prior knowledge of the spreading codes of the other users. This thesis presents enhanced forms of the Time Dependent Adaptive Filter (TDAF), which is an optimal periodically time-varying adaptive filter. It is able to fully exploit the cyclostationarity and spectral correlation in certain DS-SS signals. It is shown how multiple AVM service providers can exist in adjacent channels within the ISM band. It is demonstrated that using the realistic channel models the TDAF can provide substantial improvement in interference rejection over the conventional correlation receiver. Computationally efficient TDAF structures have been also proposed to handle the case of different code repeat intervals for different systems. Results of CDMA overlay system simulation have shown that TDAF processing can achieve tremendous gain in narrowband interference rejection compared to notch/matched filter processing. An input signal which has 45 dB narrowband interference-to-signal power ratio (J/S), the SIR of the TDAF processed output signal can be 20 dB, a 65 dB improvement. The use of TDAF processing and a spectral shaping filter can minimize the impact of adjacent channel interference. Even in an environment which has a 30 dB adjacent channel J/S, the SIR at output is demonstrate to be a 50 dB improvement. System performance shows little degradation by adjacent channel and narrowband interference. / Master of Science

Lately, spread spectrum systems are being increasingly used for commercial wireless communications because of their ability to reject various types of interference. This ability allows them to be used in multiple access systems. Direct sequence and frequency hopping systems have been the primary spread spectrum techniques used in practice. One technique which has not received much attention until recently is time hopping. In time hopping, a symbol is transmitted at a random position within the symbol period using a pulse width which is much smaller than the symbol period. Ultra-wideband (UWB) technology is a radar technology which shows promise for an relatively simple implementation of a time hopping system. This thesis looks at the error probability performance of a UWB time hopping multiple access system. Previous work has led to an estimate of the performance using a Gaussian approximation similar to that used for direct sequence systems. Through the use of a fast simulation technique, it will be shown that in certain situations, the Gaussian approximation fails to accurately predict the performance. A numerical analysis which uses characteristic functions is developed and shown to correctly predict the system’s performance under a wide range of situations. This numerical analysis also contributes to the understanding of the system. / Master of Science

Our society is undergoing a wireless revolution in which there is a rapidly increasing demand for personal communication services (PCS). Several alternate technologies have been proposed to meet this demand, including a 1.25 MHz CDMA system. It is important to provide system designers with tools to accurately predict system performance. Analytical techniques are useful for providing general information on system performance, but may not fully capture the complexities of the mobile radio channel needed to accurately predict the performance of complex system designs. This thesis presents a novel simulation technique in which a CDMA system is modeled by software, but the channel impulse response model is derived from actual measurements. This thesis describes a bit error simulation tool for a 1.25 MHz bandwidth CDMA system. The simulator incorporates measurement-based channel impulse response data with the simulated CDMA system. Bit error rate for COMA is simulated under a variety of actual channel conditions, including heavy urban, urban, suburban and rural environments. Simulation results for both 915 MHz and 1920 MHz are presented. Vertical and circular antenna polarization are compared. In the simulation, a RAKE receiver is implemented, the advantage of this RAKE receiver implementation is demonstrated by comparison to a single correlation receiver. Results for outage probability are also presented. / Master of Science

After Qualcomm's proposal of the IS-95 standard, <i>code-division multiple access</i> (CDMA) gained popularity as an alternative multiple-access scheme in cellular and <i>personal communication systems</i> (PCS). Besides the advantage of allowing asynchronous operation of the users, CDMA <i>direct-sequence spread spectrum</i> (DS-SS) offers resistance to frequency selective fading and graceful degradation of the performance as the number of users increases. Orthogonality of the signals in time-division multiple access and frequency-division multiple access is inherent from the nature of the multiple access scheme. In a CDMA system, orthogonality of the signals is not guaranteed in general. Consequently, the performance of conventional correlation receivers suffers. Sub-optimum receivers which use knowledge of the interfering signals have been investigated by other researchers. These receivers attempt to cancel the multi-user interference by despreading the interfering users. Hence, these receivers require knowledge about all the spreading codes, amplitude levels, and signal timing, and are, in general, computationally intensive. In this thesis, a technique is presented for which a high degree of interference rejection can be obtained without the necessity of despreading each user. It is shown that exploiting spectral correlation can help mitigate the effects of the multiple-access interference. If <i>code-on-pulse</i> DS-SS modulation is used, a cyclic form of the Wiener filter provides substantial improvements in performance in terms of bit error rate and user capacity. Furthermore, it is shown, that a special error-criterion should be used to adapt the weights of the filter. The computational complexity of the receiver is equivalent to that of conventional equalizers. / Master of Science