Physical Layer Network Coding Design In Multiple Access Relay Channels

In pace with the development and improvement of mobile networking techniques, wireless communication technologies, such as WIFI, cellular communications and bluetooth, have become essential parts of our daily life. However, compared to the wireline networks, wireless networks are more vulnerable to the unreliable effects caused by fading, pathloss, shadowing and co-channel interference, which greatly degrade the quality of transmitted signals, thus can only provide very limited data transmission rate. Recently, for the widely applications of key techniques in wire-less communication networks, e.g., MIMO system, cooperative communication and network coding, the data transmission rate and spectral efficiency of wireless system have been highly boosted, which ultimately realize the efficiency of the network re-sources. In the thesis, we comprehensively consider the requirement of diversity gain, error performance and resource allocation in wireless communication, and design the network coding protocols from the perspective of both analog and digital domains in non-orthogonal multiple access relay channels. All the techniques and contributions are summarized as follows:Firstly, due to the amplification of the received noise of analog network coding scheme at the relay node, the received constellation points at destination distribute too close to each other. To address this problem, we design a pairwise error probability optimal generalized analog network coding scheme, which keeps the simplicity of con-ventional analog network coding and effectively improve the error performance of the system. Based on the relationship between pairwise error probability and Euclidean distances of neighboring points on destination received constellation, we propose a relay function according to the maximizing the minimum Euclidean distance crite-ria to adjust the transmission power and phase of the relay received signal. Then we obtain the optimal solution of transformation matrix and sources transmission power with Lagragian multiplier method, which decrease the system pairwise error probabil-ity. Specially, when the transformation matrix is equivalent to a second-order identity matrix, the generalized analog network coding is degraded to conventional analog net-work coding.Secondly, in the light of problems caused by detection ambiguity and error prop-agation due to relay detection for conventional digital network coding, we propose a power adaptive network coding scheme, which prompts the system to achieve full diversity gain and obtain higher coding gain. Specifically, we express the relay trans-mission power as the multiplication of a power scaling factor and a two-level power adaptive factor, which mitigates the error propagation from sources-relay channels and realizes the one-to-one mapping between sources signal pair and network coded signal to solve the detection ambiguity problem, respectively. Thus, the system is proved to achieve full diversity. Moreover, based on the characteristics of the received constel-lations at both relay and destination nodes, we derive the closed-form expression of symbol pair error rate with wedge probability model and coordinate transformation method. According to the relationship between symbol error rate and Euclidean dis-tance between constellation points, we obtain the optimal power adaptation factors, which improve the coding gain of the system.Thirdly, aiming at the error propagation of the relay detection and improving the efficiency of cooperative links, we propose an opportunistic network coding scheme. The relay node determines the types of forwarded signal based on the outage events of sources-to-relay multiple access channels, which including network coded signal, repetition coded signal of individual source or not cooperate. Through the analysis of mutual information between transmission signal and received signal with binary input, we derive the outage probability of the opportunistic network coding scheme. Based on the the received constellations at destination corresponding to different types of forwarded signal from relay node, we determine the expression of decision regions of detection and analyze the system bit error rate.In summary, we investigate the noise amplification phenomenon, detection ambi-guity and error propagation issues in a non-orthogonal multiple access channels from the point of views of both analog and digital network coding. We propose three dif-ferent physical-layer network coding schemes, namely, generalized analog network coding scheme, power adaptive network coding scheme and opportunistic network coding scheme, which effectively solves the problems of not achieving full diversity and low coding gain for conventional network coding strategies. Based on the theo-retical analysis and simulation results, all the proposed network coding schemes can achieve full diversity. Generally speaking, generalized analog coding outperforms the alternative scheme in terms of pairwise error probability. As the distance between re-lay and source nodes getting further, the detection error probability of relay increases, and the opportunistic network coding scheme, which shares the same digital network coding scheme as its counterpart, has better error performance than power adaptive network coding scheme.