On The Design And Performance Analysis Of Transmission Strategies In Buffer-aided Wireless Cooperative Networks

With the rapid development and wide application of wireless communication networks,people’s demand for wireless communication service is increasing day by day.Next-generation wireless communication systems,including the fifth generation(5G),need to provide higher throughput,wider coverage area,lower latency and massive connections.In recent years,buffer-aided systems has been widely concerned and researched for its ability to improve the throughput and coverage area of multinode cooperative networks.In order to further meet the higher transmission performance requirements for the next generation of wireless communication networks,this paper designs the buffer-aided cooperative transmission strategies that can achieve higher throughput for the buffer-aided cooperative communication networks.On the other hand,the inherent broadcasting nature of wireless communications makes it increasingly urgent to ensure communication security.As an effective way to enhance the security of wireless transmissions,physical layer security(PLS)has a great prospect for guaranteeing completely secure communications in wireless networks.In this paper,data buffers and key buffers are incorporated into wireless cooperative networks,and the buffer-aided transmission schemes are designed.The secrecy throughput and secrecy outage probability of these proposed schemes are analyzed respectively.And tradeoffs,such as the tradeoff between the secrecy throughput and the number of relays under the total power constraint,are revealed,to better balance the performance indexes such as total power consumption and throughput in the design of wireless systems.Main works and contributions are summarized as follows.·Firstly,a novel transmission policy referred as alternate distributed beamforming(ADB)for data buffer-aided multi-antenna half-duplex(HD)multi-relay systems is proposed,in which the relays are divided into two groups and activated alternately in their transmitter and receiver modes,with one group receiving the same information broadcast from the source while the other group simultaneously transmitting the common messages to the destination via distributed beamforming in each time slot.It is worth noting that the relays used for reception and transmission are determined without the instantaneous channel state information(CSI).Theoretical analysis of the achievable throughput of the proposed scheme in Rayleigh fading channels is provided and the approximate closed-form expressions are derived for multi-antenna relay scenario,with single-antenna relay scenario as a special case.Moreover,the asymptotic expressions for the maximum achievable throughput of the proposed scheme and corresponding optimal power allocation in the high-and low-SNR regimes are derived as well.Numerical results in accordance with theoretical analysis demonstrate that the proposed policy achieves a significant improvement in the maximum achievable throughput compared with benchmark schemes.·Secondly,secure communications in data buffer-aided multi-relay wireless cooperative systems in the presence of one eavesdropper are considered,which can intercept the data transmissions from both the source and relay nodes.Two cooperative secure transmission schemes,i.e.,hybrid imitating full-duplex maxratio relay selection(Hy IFD)scheme and threshold-based link selection(TBLS)scheme are proposed for adaptive-and fixed-rate transmissions aiming at improving the secrecy throughput and secrecy outage probability,respectively.For adaptive-rate transmissions(ART),the proposed scheme switches among three sub-strategies according to different conditions such as the number of relays and transmit power.Different relays are chosen for reception and transmission according to the ratio of the legitimate channels to the eavesdropper channels to imitate the full-duplex transmission mode.For fixed-rate transmissions(FRT),a hybrid HD/FD transmission mode is designed to increase the transmission probabilities of two hops under the transmission quality constraint.Two parameters are introduced and optimized to minimize the secrecy outage probability.A suboptimal TBLS(SO-TBLS)scheme is also given.Theoretical analysis of the secrecy throughput and the secrecy outage probability are provided and the closed-form expressions are derived,and verified by numerical results.It is shown that the proposed schemes outperform benchmark schemes in terms of secrecy throughput and secrecy outage probability.·Finally,the performance improvement of secure communications assisted by destination node is revisited for the traditional secure communication systems between source and destination in the presence of one eavesdropper.It is assumed that the CSI of all links is available at the source.By incorporating key buffers into the system,a destination-aided hybrid secure communication scheme where destination helps source via sending keys when destination channel is stronger than the eavesdropper channel,which can be used to encrypt data when source’s channel is bad,is proposed.A parameter is introduced to adjust the priority of the message transmission from the source and the key sharing from the destination when they are both available.The closed-form expressions of the average secrecy rate of the proposed scheme for given transmit power levels at source and destination are derived.In comparison with the conventional physical layer secure communications under the same total power consumption model,it is shown that the proposed scheme achieves significant improvement in terms of average secrecy rate.