Performanee Research On Wireless Relay Communication Systems

Wireless relay technology has emerged as a promising technique which has the advantage of low cost, extending the coverage at the transmitter, enhancing communication reliability and increasing transmission rate. If the relay can be introduced to the cellular network, it can expand the coverage of base station and improve the Qos of the cell-edge users. It is expected to build an efficient and low-cost mobile communication system with relays. Relay technology is one of the key technologies for LTE-Advanced. The Wireless relay is classified into two main categories, namely, amplify and forward (AF) relay and decode and forward (DF) relay. As a further categorization, AF relay can be classified into two subcategories, namely, channel state information (CSI)-assisted relay and fixed gain relay. This dissertation takes AF relay as the research field.Although the performances of wireless relaying transmissions systems over the short-term fading channels have been extensively studied, there have been few studies over composite multipath/shadowing channels. It is the reason that Nakagami-lognormal fading model has rather complicated mathematically expressions. Fortunately, some papers have verified that the Generalized-k (KG) distribution, which makes mathematical performance analysis much simper to handle as compared to Lognormal-based model, is general enough to model the multipath fading and shadowing phenomena. So, the second chapter of this paper presents the end-to-end performance of dual-hop wireless communication systems with fixed gain relay operating over independent but not necessarily identically distributed (i.n.d) KG fading channels. New closed-form expressions are derived for the moments of the end-to-end signal-to-noise ratio (SNR), while the corresponding moment generating function (MGF) is accurately approximated with the aid of Pade approximants (PA) theory. Useful performance criteria are studied; the average end-to-end SNR and the amount of fading, which are expressed in closed form, the average bit-error probability (ABEP) and the outage probability, which are both accurately approximated. Furthermore, closed-form bound based on Jensen's inequality for the capacity of dual-hop transmissions system is obtained. Numerical results show that the power allocation between the two hops may have effects on the system performance.The semi-blind relay, which is a specific fixed gain relay and have knowledge of statistical CSI about the first hop, has comparable performance to AF systems with CSI-assisted relay. Therefore, it deserves careful study. Novel closed-form expression is obtained for the gain of semi-blind relays over KG fading channels. Simulation results show that is sensitive to the variation ofγ1 for high values. In comparison withγ1 for low values, the increasing ofγ1 for high values will make the transmit power of the relay decrease fasterRecent studies show that the Weibull fading model exhibits an excellent fit to experimental fading channel measurements for both indoor and outdoor environments. It was concluded that the Weibull distribution provides the best fit to the data in some scenes, as compared to Nakagami-m distributions. Therefore, in the third chapter, end-to-end performance of multihop wireless communication systems with CSI-assisted relays and fixed gain relays operating over i.n.d Weibull fading channels is presented. Firstly, with the aid of the inequality between harmonic and geometric mean, the end-to-end SNR is bounded. Then, novel expressions are derived for the moment-generating, probability density, and cumulative distribution functions (CDF) in closed form. Using these results, closed-form bounds are obtained for the average end-to-end SNR, the channel capacity, the average bit-error probability, and the outage probability. Numerical results show that the lower the value of the number of the hops and the average SNR, the tighter the proposed bounds.The outage probability is an important performance criterion of wireless communication system. The third chapter provides a unified mouthed for outage probability of multihop wireless system with CSI-assisted relays. Becausemin(γ1,…,γN) is a upper bound of end-to-end SNR, the bound for the outage probability of multihop systems can be derived easily with the CDF of yi. This makes the analysis of outage probability for multihop system easy.With the help of the above-mentioned bound, closed-form bound for the ABEP of multihop wireless communication systems with CSI-assisted relays over i.n.d Nakagami-m fading channels is derived in the third chapter. Numerical and simulation results are executed to validate that in medium and high SNR, the curves of the proposed bound and the Monte Carlo simulation almost coincide.The destination node in cooperative communication systems can receive multiple independent copies of the same signal and can achieve diversity. If the number of the relays is too much, the complexity of the receiver is high. It can reduce the complexity of the receiver and increase system efficiency by choosing some signals from the best relay channels. The performance of cooperative communication using generalized selection combining (GSC) over Nakagami-m fading channels is obtained in the fourth chapter. With the aid of PA theory, new closed-form expression is derived for the MGF of the GSC output SNR. Then, the ABEP is accurately approximated using the well-known MGF approach. Numerical results show for the more severe fading cases, the GSC receivers are closer to the optimum MRC receivers.Cognitive radio is a hot research field, because it can significantly improve the spectrum efficiency. Spectrum sensing is one of the key technologies. Cooperation sensing is proposed as a solution to problems that arise in spectrum sensing due to multipath fading and shadowing. In fifth chapter, we consider a new cooperative spectrum sensing by allowing the cognitive user operating in the best sensing channel to report the sensing results to the centralized controller firstly in order to optimize the sensing performance. The analyses show that the new cooperative method can improve the agility and sensing probability with the given false alarm probability. In addition, to achieve the same value of the detection probability, it needs the less number of second users in the proposed program.