Performance Analysis And Optimization For Wireless Two-way Relaying Systems

In wireless communications systems, cooperative relaying technique have received extensive research interest due to it provides a simple solution to extend radio coverage and improve link quality. Traditional One-Way Relaying System (OWRS) suffers from obvious loss in spectrum efficiency due to the practical constraint of half-duplex mode.At the same time, Two-Way Relaying System (TWRS) achieves better spectrum efficiency compared to OWRS as it completes one round of information exchange between two source nodes when provided with the same amount of channel realizations, such as code, time slot, frequency,etc. Amplify-and-forward is a popular protocol for cooperative relaying networks as it does not require decoding and re-encoding signal, instead only amplification and broadcasting operations are needed. It is widely noted that the fading characteristics of wireless channels have a major impact on system performance, so thorough analysis and discussion about crucial metrics such as Outage Probability (OP) and Ergodic Capacity(EC), as well as Power Allocation Schemes (PAS) that optimizes system performance are of obvious significance. The main contributions of this paper can be concluded as follows:Firstly, we studied the end-to-end (E2E) performance in AF-TWRS systems when different Amplification Policies (APs) are adopted at the relay. There are mainly four different There are mainly four different kinds of Aps in AF protocol, namely the Variable Gain (VG) policy, the Fixed Gain (FG) policy, the Mixed Amplification Type-1 (MA-1) policy and the Mixed Amplification type-2 (MA-2) policy. Based on detailed analysis, we found that the equivalent Signal-to-Noise Ratio (SNR) at source nodes with different Aps can be written in a uniform expression with, of course, different parameters. Starting from this observation, it is possible to establish a uniform approach for the analysis of almost any performance metric for different Aps. To validate this hypothesis, we derived a uniform formula for the E2E-OP, considering a AF-TWRS system operating over Mixed Fading Scenario of Rician and Rayleigh channels (MFS-Ri/Ra), which significantly reduced the complexity of system performance evaluation.Secondly, we carried out performance analysis and optimization concerned discussions, considering asymmetric VG-TWRS system operating over MFS-Ri/Ra scenario. Asymptotic expressions for E2E-OP at high SNR region were given. Both exact expressions and high SNR asymptotic expression for the Overall OP (O-OP) were provided. By carrying out standard analysis of Lagrangian method with KKT conditions, Closed-Form Solutions (CFS) for PAS that minimizes the O-OP under different categories of constraints were derived. As for EC,we presented the tight lower bound based on a clever manipulation of the Jensen's Inequality, as well as the high SNR approximation based on a proximal analysis. Furthermore, we extend the MFS-Ri/Ra scenario to a MFS-Ri/Na scenario, replacing the second hop Rayleigh channel with Nakagami-m channel. For now No CFS expressions can be acquired for the global maximizers of EC, so we resort to exhaustive search type algorithm to search for the numerical solutions.Thirdly, we redirect our attention to FG-TWRS system operating over MFS-Ri/Ra scenario. The basic performance metrics studied in this section are similar with that of the VG-TWRS system as previously mentioned, only for that the analysis gets more complicated. Multiple folds of infinite series were involved. Complex special functions were defined to simplify the notations. Numerical solutions such as Gauss-Chebyshev approximations were presented to reduce complexity.By enlarge the equivalent SNR that source nodes experience, upper bounds and high SNR approximations of O-OP were given, based on a concrete analysis of integration region partitions and special function manipulations. For now no CFS for FG-TWRS system performance optimizers can be concluded, similarly we resort to exhaustive search algorithms to search for numerical solutions, trying our best to guarantee an desirable accuracy.Fourthly, we conducted research for performance analysis considering AF-TWRS systems operating over fading channels characterized by the insightful Two-Wave with Diffuse Power (TWDP)model. To begin with, exact expressions for E2E-OP in FG-TWRS and VG-TWRS systems sere presented, in the form of multiple folds of infinite series. Although finite-term truncation does not affect the accuracy, it suffers from heavy computation burden so numerical solutions based on Gauss-Chebyshev approximations were presented as alternatives to facilitate the evaluation. Next, considering VG-TWRS systems, upon utilizing the reciprocals of channel Random Variables(RVs), a geographical based approach were presented to calculate the upper and lower bounds of system O-OP by slightly enlarging or decreasing the integration region. In nature, this approach eliminates the bundling and correlation between RVs, thus direct multiplication of Cumulative Distribution Functions (CDFs) can be achieved, which significantly reduced the computation complexity. Furthermore, based on a novel expression for the CDF function of TWDP model, a new tight expression for the high SNR approximation was presented, which is quite efficient in calculation and helpful in getting more insights into system performance.To sum up, in this research, the exact performance analysis of TWRS system operating over specific scenarios laid solid foundation, the high SNR asymptotic analysis achieved crucial advancements, and ultimately we were able to carry out meaningful discussions on how to design optimal PASs in order to optimize system performance, of course,under practical constraints. Extensive simulation experiments were performed to validate the theoretical analysis and to provide intuitive understandings of system performance.