Research On Energy-Efficient Physical-Layer Network Coding

Physical-layer Network Coding (PNC) in wireless relay networks is considered as one of the most promising key technologies of the future mobile communications. PNC makes use of the additive nature of simultaneously arriving electromagnetic waves for equivalent network coding operation at physical layer. Hence, the harmful superimposed interference signal is converted to a useful network coding signal, which implies that PNC can improve both spectral efficiency and energy efficiency. This thesis studies the energy efficiency transmission technologies of PNC in two-way multi-relay system, multi-user multicast system as well as OFDM/OFDMA systems, when the Quality of Service (QoS) is guaranteed. The main works are given as follows:1) Energy-efficient transmission technology of PNC in two-way multi-relay system.The joint power allocation and relay selection with partial Channel Status Information (CSI) was studied when the transmission rates are constrained. By optimization theory, the closed-form expressions of power allocation with guaranteed transmission rates were derived for Amplify-and-Forward (AF), Decode-and-Forward (DF), Compress-and-Forward (CF) and DeNoise-and-Forward (DNF) protocols of PNC. Then, an energy-efficient joint power allocation and relay selection schemes were proposed. Analysis shows that DNF protocol with Lattice coding and decoding has the best energy efficiency followed by the CF protocol, and then the AF protocol. For the DF protocol, energy efficiency fluctuates with system demand, it only obtains well energy efficiency with low-rate or large gap between two-way rates. Distributed beamforming with full CSI was studied when the transmission rates are constrained. The closed-form expression of phase information was derived for the weighting matrix of beamforming, and then a Maximum Received Power (MRP) suboptimal beamforming algorithm was proposed with low computational complexity. Analysis shows that limited performance loss between MRP algorithm and optimal beamforming scheme. Especially, they almost have the same performance when all the relay nodes place to the middle area between two users.2) Energy-efficient transmission technology of PNC in multi-user multicast system.Based on the link-oriented energy efficiency model that minimizes energy per bit with constrained rates for every link, energy-efficient resource allocation of DNF protocol was considered as a joint power allocation and relay selection problem in both multiple access and broadcast stages. From the viewpoint of energy efficiency, we proved that the optimal number of relay nodes participating in transmission is one or two. Also, closed-form expressions of the power allocation were given. To further reduce the complexity, a suboptimal algorithm with single relay selection and power allocation was proposed. Analysis shows that the suboptimal algorithm has limited performance loss, and even approximates to the optimal solution when the gap between link’s rates is large.3) Energy-efficient transmission technology of PNC in OFDM/OFDMA systems.With the given Bit Error Rate (BER) and bits per OFDM symbol, a greedy-based adaptive bit and power allocation algorithm was proposed and then proved to have almost the same performance with the exhaustive search algorithm. In addition, the performance advantage of proposed algorithm increases with the bits per OFDM symbol compared with the equal bit allocation algorithm.In OFDMA system, the energy-efficient resource allocation with guaranteed rates is considered as a joint power allocation, subcarrier assignment and relay selection problem. With the mathematical analysis, the problem has been decomposed into independent sub-problems on the subcarriers. Then, we proved that the optimal subcarrier assignment criterion in energy efficiency sense is an opportunistic subcarrier assignment, in which a subcarrier is assigned to a unique relay node. Based on the result, a heuristic resource allocation scheme with opportunistic subcarrier assignment was proposed. Analysis shows that the proposed scheme provides diversity gain compared to the single relay network, and PNC gain relative to the scheme without PNC.4) Deployment of PNC with practical power modelBased on the practical power model with transmit power, power amplifier efficiency and the power independent of transmit power, the expressions of the relationship among energy efficiency, transmission rates and relay location were derived for direct and relay transmission modes. Through analysis, the energy-efficient region of different protocols and relay deployment with different two-way rates requirement were given. Simulations show that the PNC can achieve the best energy efficiency when the two-way rates requirement is larger than a certain threshold. In addition, relay node should be deployed close to the user, while rate from base station to user is fast than reverse link.