On Resource Optimization In Wireless Cooperative Communications

By deploying multiple antennas at terminals, multiple input multiple out-put(MIMO) technique can combat wireless channel fading induced by multiple paths,and therefore holds the potential to drastically improve the capacity and reliability infuture wireless communication systems. However, due to the limitation of size, com-plexity and power consumption of Mobile terminals or network nodes, it is difficult toapply MIMO directly in some practical systems. In cooperative communications, vir-tual antenna arrays will be formed between the source and destination as a result ofthe processing and forwarding by the relay nodes. So cooperative relaying is regardedas effective communication paradigm to enhance coverage and error performance forwireless systems. This dissertation deals with resource optimization for cooperativenetworks, including relay selection, power allocation and subcarrier pairing in OFDMsystems. The main achievements and results of this dissertation are listed as follows:1. We discuss the network lifetime optimization issue for bidirectional cooperativeOFDM systems recruiting multiple relays, and formulate a model to maximize thenetwork lifetime. In the maximization model, direct treatment is not feasible asthe source and relay selection nested with each other. Then a suboptimal strate-gy is proposed, which takes energy pricing concept for each node into account.Specifically, the power allocation for each subcarrier, relay, and source selec-tion is optimized gradually rather than deriving jointly optimal solutions. And theoptimization minimizes the total energy cost subject to network throughput con-straints. Two practical scenarios are considered, i.e., a direct source-destinationlink is available or not while implementing the proposal. Moreover, when the di-rect link can be fully exploited, the impact of two diversity combining techniquesincluding Maximal Ratio Combining (MRC) and Selective Combining (SC) onthe power allocation optimization is theoretically derived. Simulation results in-dicate that, the network lifetime by utilizing the proposed algorithm outperformsthe existing approaches significantly.2. In cooperative OFDM systems with a single source and multiple relays, the op-timization of network lifetime based on subcarrier pairing and independent se-lection of subcarrier is investigated. we first propose an exhaustive algorithm,named EALM(Exhaustive Approach for Lifetime Maximization)+MEC(Minimum Energy Cost). Specifically, all possible combinatorial strategies of subcarrierpairing and relay selection are firstly formulated and by applying standard La-grange technique, the optimal power assignment can be theoretically derived.Accordingly, the candidate with smaller energy sacrifice is designated as theoptimum solution. Since the real world applications of the proposed approachare restricted by prohibitively high computational complexity, two low complexi-ty alternatives, JRSSP(Joint Relay Selection and Subcarrier Pairing)+MEC andSPRS(Subcarrier Pairing and then Relay Selection)+MEC, are consequently de-signed based on the price of unit Signal-to-Noise Ratio (SNR). Simulation result-s indicate that, compared with the exhaustive algorithm, the two low complexityapproaches perform small lifetime losses and are applied.3. Combining OFDM with multiuser based cooperative relaying strategy, resourceoptimization to maximize the lifetime of multi-source multi-relay cooperativeOFDM networks is firstly researched. Since the formulation is a mixed binaryinteger programming problem, it is difficult to obtain the optimal solution. Thuswe propose a centralized method in two steps. Specifically, during each trans-mission interval, source-relay selection and subcarrier pairing can be decidedaccording to channel state information of the relay links and residual energy in-formation at each terminal. Next, energy-aware power loading can be solvedby employing standard Lagrange techniques. Numerical examples verify theeffectiveness of our proposal.4. Resource assignment with maximizing received SNR as objective for two-pathAF relaying networks is studied for the first time. we generalize the expression ofthe received SNR over Rayleigh fading channel for two-path relaying networkswith partial and full interference cancellation respectively, and propose relay s-election and power allocation strategies. Theoretical analysis shows that, sincedifferent relay nodes forward information symbols alternatively in adjacent timeslots, the power allocation strategy in two-path AF relaying networks is muchdifferent with that in the conventional one-path relaying networks. Simulationresults indicate that the proposed mechanisms outmatch existing approacheswith little overhead.5. we discuss the power allocation and relay selection strategy for two-way relaying systems using physical-layer network coding (PNC). The objective is to improvetransmission rate of the system, including the minimum rate of the two direc-tions and the sum rate of the system, by designating resources. Since both ofthese two optimization objectives are not continuously differentiable functions,the suboptimal solution is firstly solved based on the results in one-way relayingsystems. Then the original problem is re-modeled on the basis of the suboptimalsolution. Specially, avoiding bottleneck introduced by MAC is necessary towardsmaximizing sum rate. Numerical results reveal that, by exploiting the proposeddistributed algorithm, the objective rate of the two-way relaying system signifi-cantly outmatches that in one-way relay system or other existing approaches.