Exact BER analysis and design of Prerake combining schemes for direct sequence ultra-wideband multiple access systems

Ph.DDOCTOR OF PHILOSOPH

Exact modeling of multiple access interference, ber derivation and a method to improve the performance of UWB communication systems

Master'sMASTER OF ENGINEERIN

Transceiver design and system optimization for ultra-wideband communications

This dissertation investigates the potential promises and proposes possible solutions to the challenges of designing transceivers and optimizing system parameters in ultra-wideband (UWB) systems. The goal is to provide guidelines for UWB transceiver implementations under constraints by regulation, existing interference, and channel estimation. New UWB pulse shapes are invented that satisfy the Federal Communications Commission spectral mask. Parameters are designed to possibly implement the proposed pulses. A link budget is quantified based on an accurate frequency-dependent path loss calculation to account for variations across the ultra-wide bandwidth of the signal. Achievable information rates are quantified as a function of transmission distance over additive white Gaussian noise and multipath channels under specific UWB constraints: limited power spectral density, specific modulation formats, and a highly dispersive channel. The effect of self-interference (SI) and inter-symbol interference (ISI) on channel capacity is determined, and modulation formats that mitigate against this effect is identified. Spreading gains of familiar UWB signaling formats are evaluated, and UWB signals are proved to be spread spectrum. Conditions are formulated for trading coding gain with spreading gain with only a small impact on performance. Numerical results are examined to demonstrate that over a frequency-selective channel, the spreading gain may be beneficial in reducing the SI and ISI resulting in higher information rates. A reduced-rank adaptive filtering technique is applied to the problem of interference suppression and optimum combining in UWB communications. The reduced-rank combining method, in particular the eigencanceler, is proposed and compared with a minimum mean square error Rake receiver. Simulation results are evaluated to show that the performance of the proposed method is superior to the minimum mean square error when the correlation matrix is estimated from limited data. Impact of channel estimation on UWB system performance is investigated when path delays and path amplitudes are jointly estimated. Cramér-Rao bound (CRB) expressions for the variance of path delay and amplitude estimates are formulated using maximum likelihood estimation. Using the errors obtained from the CRB, the effective signal-to-noise ratio for UWB Rake receivers employing maximum ratio combining (MRC) is devised in the presence of channel path delay and amplitude errors. An exact expression of the bit error rate (BER) for UWB Rake receivers with MRC is derived with imperfect estimates of channel path delays and amplitudes. Further, this analysis is applied to design optimal transceiver parameters. The BER is used as part of a binary symmetric channel and the achievable information rates are evaluated. The optimum power allocation and number of symbols allocated to the pilot are developed with respect to maximizing the information rate. The optimal signal bandwidth to be used for UWB communications is determined in the presence of imperfect channel state information. The number of multipath components to be collected by Rake receivers is designed to optimize performance with non-ideal channel estimation

Enhancing the bit error rate performance of ultra wideband systems using time-hopping pulse position modulation in multiple access environments

Ultra-Wide Band (UWB) technology is one of the possible solutions for future short-range indoor data communication with uniquely attractive features inviting major advances in wireless communications, networking, radar, imaging, and positioning systems. A major challenge when designing UWB systems is choosing a suitable modulation technique. Data rate, transceiver complexity, and BER performance of the transmitted signal are all related to the employed modulation scheme. Several classical modulation schemes can be used to create UWB signals, some are more efficient than others. These schemes are namely, Pulse Position Modulation (PPM), Pulse Amplitude Modulation (PAM), Binary Phase Shift Keying (BPSK), and On-Off Keying (OOK) are reviewed. In the thesis, the performance of PPM system, combined with Time Hopping Spread Spectrum (THSS) multiple access technique is evaluated in an asynchronous multiple access free space environment. The multiple access interference is first assumed to be a zero mean Gaussian random process to simulate the scenario of a multi user environment. An exact BER calculation is then evaluated based on the characteristic function (CF) method, for Time Hopping-Pulse Position Modulation Ultra Wide Band (TH-PPM UWB) systems with multiple access interference (MAI) in AWGN environment. The resulting analytical expression is then used to assess the accuracy of the MAI Gaussian Approximation (GA) first assumed. The GA is shown to be inaccurate for predicting BERs for medium and large signal-to-noise ratio (SNR) values. Furthermore, the analysis of TH-PPM system is further extended to evaluate the influence of changing and optimising some of the system or signal parameters. It can be shown how the system is greatly sensitive to variations in some signal parameters, like the pulse shape, the time-shift parameter associated with PPM, and the pulse length. In addition, the system performance can be greatly improved by optimising other system parameters like the number of pulses per bit, Ns, and the number of time slots per frame, Nh. All these evaluation are addressed through numerical examples. Then, we can say that, by improving signal or system parameters, the BER performance of the system is greatly enhanced. This is achieved without imposing exact complexity to the transceiver and with moderate computational calculations

DSP algorithm and system design for UWB communication systems

Master'sMASTER OF ENGINEERIN

Contribution to the Rapid Acquisition of Signals for UWB Communication Systems

Ultra Wide-band is a promising technology for future short-range wireless communications with high data rate. In generally, one of the biggest difficult tasks for researchers today is the acquisition task of signals, where they are looking through different tools for getting a good quality of transmission; the phenomenon of multipath always stands up in the front as the first problem to be faced. When we talk about the Ultra Wide Band (UWB) signals, the problem becomes more complicated due to ultrashort impulses duration used by this kind of signals that causes the generation of paths by huge numbers. In this thesis, to address the task mentioned above, the study is subdivided into two aspects. The first one is the UWB channel estimation that we have done to have information about the amplitudes and the delays of the paths. For this purpose, a maximum likelihood method is used to find the amplitudes and the delays estimate using two estimation contexts: Data Aided (DA) and Non-Data-Aided (NDA). In the second aspect, various parameters affecting the acquisition of signals are evaluated. Furthermore, several contributions in the framework of a new strategy based on an Intelligent Controlling System (ICS) are done and detailed in this thesis for the first once. This system is characterised by its flexibility through two techniques, one that allows to users to communicate even with different M-ary PPM levels at the same time. Another technique that gives the flexibility for dealing with the phenomenon of multipath, where this latter is combated through manipulating the modulation’s levels via the ICS to achieve a rapid acquisition of UWB signals

Cross-Layer Design for Multi-Antenna Ultra-Wideband Systems

Ultra-wideband (UWB) is an emerging technology that offers great promises to satisfy the growing demand for low cost and high-speed digital wireless home networks. The enormous bandwidth available, the potential for high data rates, as well as the potential for small size and low processing power long with low implementation cost, all present a unique opportunity for UWB to become a widely adopted radio solution for future wireless home-networking technology. Nevertheless, in order for UWB devices to coexist with other existing wireless technology, the transmitted power level of UWB is strictly limited by the FCC spectral mask. Such limitation poses significant design challenges to any UWB system. This thesis introduces various means to cope with these design challenges. Advanced technologies including multiple-input multiple-output (MIMO) coding, cooperative communications, and cross-layer design are employed to enhance the performance and coverage range of UWB systems. First a MIMO-coding framework for multi-antenna UWB communication systems is developed. By a technique of band hopping in combination with jointly coding across spatial, temporal, and frequency domains, the proposed scheme is able to exploit all the available spatial and frequency diversity, richly inherent in UWB channels. Then, the UWB performance in realistic UWB channel environments is characterized. The proposed performance analysis successfully captures the unique multipath-rich property and random-clustering phenomenon of UWB channels. Next, a cross-layer channel allocation scheme for UWB multiband OFDM systems is proposed. The proposed scheme optimally allocates subbands, transmitted power, and data rates among users by taking into consideration the performance requirement, the power limitation, as well as the band hopping for users with different data rates. Also, an employment of cooperative communications in UWB systems is proposed to enhance the UWB performance and coverage by exploiting the broadcasting nature of wireless channels and the cooperation among UWB devices. Furthermore, an OFDM cooperative protocol is developed and then applied to enhance the performance of UWB systems. The proposed cooperative protocol not only achieves full diversity but also efficiently utilizes the available bandwidth