Composite performance and availability analysis in wireless communications systems

With the increasing popularity of wireless communications systems, customers are expecting the same level of service, reliability and performance from the wireless communications systems as the traditional wireline networks. Due to the dynamic environment, such as the roaming of mobile subscribers, maintaining a high radio frequency (RF) availability is one of the most challenging aspects in wireless networks. In this work, we analyze the above challenges and propose reactive/proactive algorithms through which the end-to-end performance is significantly improved. In addition, state-of-the-art modeling techniques are applied to evaluate the performance of these algorithms. Because of the coexistence of different multiple access techniques in wireless networks, there is no single panacea that can cure all the diagnosed syndromes. As a consequence, based on the nature of each multiple access technique, we develop a series of solutions that combat the above problems. These solutions include: (1) A recovery method is proposed for single permanent channel failures in frequency division multiple access (FDMA) wireless systems. This method is quite similar to the Automatic Protection Switching (APS) scheme, which is widely used to enhance the network integrity in the asynchronous transfer mode (ATM) networks. Three stochastic reward net (SRN) models are devised to evaluate the performance of the recovery method. Fixed-point iteration method is applied to capture the dynamic behavior of the handoff arrivals. (2) A channel recovery method is proposed for both permanent and transient failure recoveries in time division multiple access (TDMA) wireless systems. Like the above single channel recovery method, the basic idea of this recovery method is similar to the APS scheme. However, the design and the evaluation issues become more complex due to the possible existence of multiple ongoing calls on a failed base repeater. Two-level hierarchical SRN models are constructed to analyze the performance of the recovery method. Fixed-point iteration is employed to resolve the dependency between the higher and lower level models. (3) A call admission control (CAC) algorithm is developed with emphasis on reducing dropped calls in code division multiple access (CDMA) cellular systems. The dropping of a handoff call is considered more disturbing than the blocking of a new call. In our work, we introduce the idea of ‘soft’ guard channels to prioritize handoff calls. Unlike traditional CAC algorithms which are based only on the effective traffic load for the target cell if one call is accepted, our CAC algorithm is based on the effective traffic loads for both the target cell and the neighboring cells. SRN models based on a decomposition approach are developed to evaluate the performance of our CAC algorithm. Fixed-point iteration schemes are devised to analyze an interrelated collection of subsystems, either symmetric or asymmetric. Both homogeneous and hot-spot traffic loads for our CAC algorithm have been analyzed.;In addition to developing specific strategies discussed above, we also derive closed-form solutions for both transient and steady state probabilities of minimum duration outage in wireless networks. In order to explicitly accommodate the deterministic distribution in minimum duration outage, our model is based on a semi-Markov process (SMP). The results would provide a valuable aid in selecting optimal system parameters and consequently allow an enhancement in network capacity and performance.