Fractionally sampled decorrelating detectors for time-varying rayleigh fading CDMA channels

In this dissertation, we propose novel decorrelating multiuser detectors in DSCDMA time-varying frequency-nonselective and frequency-selective fading channels and analyze their performance. We address the common shortcomings of existing multiuser detectors in a mobile environment, such as detector complexity and the error floor. An analytical approach is employed almost exclusively and Monte Carlo simulation is used to confirm the theoretical results. Practical channel models, such as Jakes\u27 and Markovian, are adopted in the numerical examples. The proposed detectors are of the decorrelating type and utilize fractional sampling to simultaneously achieve two goals: (1) the novel realization of a decorrelator with lower computational complexity and shorter processing latency; and (2) the significant reduction of the probability of error floor associated with time-varying fading. The analysis of the impact of imperfect power control on IS-95 multiple access interference is carried out first and the ineffectiveness of IS-95 power control in a mobile radio environment is demonstrated. Fractionally-spaced bit-by-bit decorrelator structures for the frequency-nonselective and frequency-selective channels are then proposed. The matrix singularity problem associated with decorrelation is also addressed, and its solution is suggested. A decorrelating receiver employing differentially coherent detection for an asynchronous CDMA, frequency-nonselective time-varying Rayleigh fading channel is proposed. A maximum likelihood detection principle is applied at the fractionally spaced decorrelator output, resulting in a significantly reduced error floor. For coherent detection, a novel single-stage and two-stage decision feedback (DF) maximum a posteriori (MAP) channel estimator is proposed. These estimators are applicable to a channel with an arbitrary spaced-time correlation function. The fractionally-spaced decorrelating detector is then modified and extended to a frequency-selective time-varying fading channel, and is shown to be capable of simultaneously eliminating MAI, ISI, and path cross-correlation interference. The implicit equivalent frequency diversity is exploited through multipath combining, and the effective time diversity is achieved by fractional sampling for significant performance improvement. The significance of the outcome of this research is in the design of new lower complexity multiuser detectors that do not exhibit the usual deficiencies and limitations associated with a time-varying fading and multipath CDMA mobile environment

Performance Analysis of Cognitive Radio Systems under QoS Constraints and Channel Uncertainty

In this paper, performance of cognitive transmission over time-selective flat fading channels is studied under quality of service (QoS) constraints and channel uncertainty. Cognitive secondary users (SUs) are assumed to initially perform channel sensing to detect the activities of the primary users, and then attempt to estimate the channel fading coefficients through training. Energy detection is employed for channel sensing, and different minimum mean-square-error (MMSE) estimation methods are considered for channel estimation. In both channel sensing and estimation, erroneous decisions can be made, and hence, channel uncertainty is not completely eliminated. In this setting, performance is studied and interactions between channel sensing and estimation are investigated. Following the channel sensing and estimation tasks, SUs engage in data transmission. Transmitter, being unaware of the channel fading coefficients, is assumed to send the data at fixed power and rate levels that depend on the channel sensing results. Under these assumptions, a state-transition model is constructed by considering the reliability of the transmissions, channel sensing decisions and their correctness, and the evolution of primary user activity which is modeled as a two-state Markov process. In the data transmission phase, an average power constraint on the secondary users is considered to limit the interference to the primary users, and statistical limitations on the buffer lengths are imposed to take into account the QoS constraints of the secondary traffic. The maximum throughput under these statistical QoS constraints is identified by finding the effective capacity of the cognitive radio channel. Numerical results are provided for the power and rate policies

Novel Approach for Modeling Wireless Fading Channels using a Finite State Markov Chain

yesEmpirical modeling of wireless fading channels using common schemes such as autoregression and thefinitestate Markov chain (FSMC) is investigated. The conceptual background of both channel structures and the establishment of their mutual dependence in a confined manner are presented. The novel contribution lies in the proposal of a new approach for deriving the state transition probabilities borrowed from economic disciplines, which has not been studied so far with respect to the modeling of FSMC wireless fading channels. The proposed approach is based on equal portioning of the received signal-to-noise ratio, realized by using an alternative probability construction that was initially highlighted by Tauchen. The associated statistical procedure shows that afirst-order FSMC with a limited number of channel states can satisfactorily approximate fading. The computational overheads of the proposed technique are analyzed andproven to be less demanding compared to the conventional FSMC approach based on the levelcrossing rate. Simulations confirm the analytical results and promising performance of the new channel modelbased on the Tauchen approach without extracomplexity costs