Efficient Cooperative Transmission Techniques For Relaying Systems Over Generalized Fading Channels

As a key method to achieve cooperative diversity,the relaying technique can overcome the fading of the wireless channel and improve the reliability of-data transmission.However,along with the increasing rate requirements of wireless communication networks,it has become a hot research topic about how to improve the effectiveness of communication with limited spectrum resources.A conventional cooperative relaying system emphasizes the orthogonality between channels,which results in low spectrum efficiency.At the same time,due to the complexity of the wireless communication scenario,the channel model needs to reflect the real signal propagation characteristics.Therefore,it is of great importance to design both reliable and effective cooperative transmission schemes over generalized channel model,which can be used for promoting the application of cooperative relaying technique in future wireless communication networks.In this dissertation,the relaying system based on different cooperative schemes were investigated,in terms of the orthogonal relaying system,the single frequency network(SFN)-based relaying system,and the buffer-aided relaying system.The physical layer performance of different cooperative schemes was analyzed,and some useful results were achieved.The main contributions of this dissertation include:(1)Considering the shadowing effect and complex fading scenarios of the wireless channel,physical layer performance of orthogonal relaying systems was analyzed under generalized channels and mixed fading scenarios.The moment generating function-based method was used to derive the expression of the ergodic capacity with either amplify-and-forward and decode-and-forward cooperative protocol.Then,under the assumption of large signal-to-noise ratio(SNR),the closed-form expression of the asymptotic ergodic capacity was derived,and the multiplexing gain and coding gain of the relaying system were obtained.Based on the obtained coding gain,a relay selection method was proposed.The results showed that the transmission efficiency of orthogonal relaying system heavily decreased along with the increase of the cooperative nodes.The proposed relay selection method can significantly increase the ergodic capacity while resulting in lower communication overhead.Because the proposed relay selection method was insensitive to time-variant channel,it can be applied in fast fading scenarios.(2)Aiming at the transmission efficiency problem of orthogonal relaying system in the multi-node scenario,an SFN-based cooperative transmission scheme was proposed.The mechanism can save transmission time slots and improve transmission efficiency by enabling multiple relay nodes to transmit identical symbols at the same time and frequency band.Considering the multi-node transmission delay,the closed-form expressions of ergodic capacity and outage probability under generalized channel model were derived.The results showed that the SFN-based relaying system achieved significantly larger ergodic capacity compared with the orthogonal relaying system.However,the diversity gain of SFN system was limited.In order to improve the diversity gain of SFN system,a power control method based on statistical channel state information was proposed.The proposed method first established an optimization problem with the goal of minimizing the asymptotic system outage probability,and then transformed the original problem into a simple form,which could be easily solved.The results showed that the proposed method could bring diversity gain for the SFN system.Under the symmetric channel scenario,the system could achieve lower outage probability with little extra power consumption.The proposed SFN-based multi-relay cooperative transmission mechanism is suitable for scenarios when a high communication rate is required.The proposed power control method improves the communication reliability while preserving the transmission efficiency.(3)Aiming at the bottleneck effect caused by the fixed link scheduling scheme,which is commonly adopted in relaying systems,this dissertation investigated the buffer-aided relaying system to achieve non-fixed channel access.For a multi-antenna full-duplex relay,a hybrid duplex scheduling strategy was proposed without considering the transmission rate priori information.The closed-form expressions of the link outage probability and the system throughput were derived under the proposed strategy.The results show that the throughput of hybrid duplex relaying system was sensitive to the node transmission rate.When the transmission rate was given,the transmission efficiency of the buffer-aided relaying system was higher than that of the bufferless relaying system.(4)To overcome the transmission efficiency problem faced by physical layer security techinique,a buffer-aided relaying system with a multi-antenna relay and multiple eavesdroppers was studied,and the closed-form expression of the secrecy outage probability was derived.The results showed that the fixed-rate relaying system cannot achieve security in the high SNR regime.Based on the security performance analysis,a rate control method was proposed for the buffer-aided relaying system.This method optimized the system secrecy throughput by designing the transmission rate and secrecy rate under a given secrecy outage probability constraint.The results showed that,through the proposed rate control method,the buffer-aided relay system overcame the security problem under high SNR,and achieved higher transmission efficiency than the conventional bufferless transmission method.The cooperative transmission technology of relaying system over generalized fading channels was investigated in this dissertation.The research results can provide theoretical guidance and technical support for the application of cooperative relaying technology in future wireless communication systems.