Studies On Resource Allocation And Physical Layer Security Techniques For Wireless Cooperative Communications

Due to the rapid development and wide application of the information technology,there are great requirements about efficiency，reliably and security for the nextgeneration wireless communications network. Cooperative communication, which canachieve spatial diversity gain and drastically improve transmission performance byallowing users to share their resource, has become the research hotspot in wirelesscommunication area. In cooperative system, spectrum efficiency can be improvedsignificantly by resource allocation, including time schedule, relay selection, subcarrierassignment and power allocation. Physical layer security technology, which can fullyutilize the characteristics of wireless communications including broadcasting and fading,and improve the security, is the important complementarity for the existing securitysystem. Thus, this dissertation focuses on the resource allocation and physical layersecurity technology in the cooperative communication. The main contributions aresummarized as follows.Orthogonal frequency division multiple access (OFDMA) is a preferred radioaccess technique in4G networks, due to its high spectrum efficiency and significantpotential to mitigate the problem of frequency-selective fading. This dissertationinvestigates the resource allocation problem in OFDMA systems. We use the artificialintelligence algorithm and consider the cross layer design and muti-service support. Ourobjective is to maximize the sum utility of all MSs (Mobile Stations) under per-relaypower constraint (PPC). We propose an asymptotic optimal resource allocationalgorithm based on multi-value discrete particle swarm optimization (MDPSO). InMDPSO resource allocation scheme is coded by multi-value discrete vector whichdenotes particle’s position. Different from the traditional discrete particle swarmoptimization (DPSO) algorithm, we develop new probability based operations forcomputing particle velocity and updating particle positions according to thecharacteristic of discrete space. Analysis and simulation results show that the proposedmethod achieves larger throughput and higher degree of user fairness than the existingmethods.In practical systems, resource allocation often combines with adaptivemodulation-and-coding (AMC). And modulation-and-coding mode is finite. So we can’ttransmit information by any needed rate. Thus resource allocation algorithm shouldconsider discrete transmit rate. We propose joint subcarrier assignment, relay selection,adaptive modulation-and-coding and power allocation algorithm based on combinedcontinual and discrete particle swarm optimization (CDPSO). In the proposed method,resource allocation scheme and AMC mode are coded by mixed discrete and continuousvector which denotes particle’s position. Particles move to find the optimal solution. Analysis and simulation results show that it achieves higher performance in practicalscenario.The real cooperative network is characteristic of dynamic. There are three dynamicfactors in the cooperative network: time-varying fading channel, MSs states change, andrelay stations (RSs) states change. The existing resource allocation algorithms do notconsider the dynamic factors. In fact, the dynamic factors can be utilized to reduce thecomputational complexity and improve the performance. We construct a dynamicoptimization framework for the resource allocation problem, with the aim to maximizethe average utility of all users with multi-service. Different from the exiting works, ourobjective optimizing function is under time-varying situations constraints including thefading of the time-varying channel, the changes of the user states, and the changes ofthe relay stations. We propose a PSO based resource allocation algorithm for dynamiccooperative OFDMA network. The proposed method reduces computational complexitydramatically and has higher performance than the static algorithms.We also investigate the robust resource allocation algorithm for the multi-cellcooperative OFDMA systems. We analyze the channel for the multi-cell OFDMAcooperative systems and found the uncertain model. And we consider three uncertainfactors: channel estimate error, channel quantized error, and the delay for feedback ofchannel information. We formulate the probability density function of the uncertainchannel, and propose a robust distributed resource allocation algorithm for the multi-cellcooperative OFDMA systems. The proposed algorithm obtains the cochannelinterference from cognitive measure, and it does not need to exchange informationamong the adjacent cells. So the feedback of channel information is saved. Differentfrom the traditional algorithms, we consider the uncertain of channels and aim tomaximize the expectation of the sum throughput of the cell. Through numericalexperiments, we see that the proposed algorithm outperforms the existing algorithmswhen the channels are uncertain.For the physical layer secrecy performance analyses, the existing concept (secrecycapacity), can not be calculated when the channel of eavesdropper is unknown. And thesecrecy capacity cannot also describe the secrecy performance for differentgeographical area. Thus we propose the new concepts of secrecy region and outagesecrecy region to evaluate the secrecy performance from a geometrical perspective. Thesecrecy region is defined as region where the eavesdropper cannot decode the secrecymessage.The outage secrecy region is defined as region where the eavesdropper can notdecode the secrecy message over a given probability. This should be useful if we needto know what zone should be protected (or militarized). And the proposed concepts canbe well utilized to evaluate the secrecy performance and guide the design of physicallayer secrecy strategy. They would be important both in military and business communications.In traditional artificial noise (AN) based physical layer secrecy methods, AN isgenerated by the transmitter. Thus the additional power is needed for the transmitter.These methods are not suitable for some cases such as when the transmitter is a handset.In order to overcome this disadvantage we propose to generate AN by the receiver. Inthe proposed method the legitimate receiver use one of its antennas to generate ANwhen receive the signal using the other antennas. Through interference cancellation theAN can be counteracted and does not affect the legitimate receiver. The proposedmethod is robust because it does not need the feedback of channel state information(CSI) to the transmitter and can resist multi-antenna eavesdropper. Further more, anovel approach for ensuring confidential wireless communication is proposed andanalyzed from an information-theoretic standpoint. In this method, both the legitimatereceiver and transmitter generate artificial noise (AN) to impair the intruder’s channel.We use the concept of insecure region to characterize the security performance whenthe eavesdropper’s channel is unknown. With the aim of minimizing the size of theinsecure region, an optimum power allocation strategy between the transmittedinformation and the artificial noise is proposed. We also give the condition of perfectsecrecy (the insecure regions are reduced to zero), which means that wherever theeavesdropper is located, she cannot decode the secret message. Analyses and simulationresults show that the proposed method can achieve high security in practical scenarios.We also address physical layer security in MIMO relay system in the presence ofpassive eavesdroppers, i.e., the eavesdroppers’ channels are unknown to the transmitter.Different from the existing works, we consider that the relay works in full duplex modeand transmits artificial noise (AN) in both stages of the decode-and-forward (DF)cooperative strategy. We proposed two optimal power allocation strategies for powerconstrained and power unconstrained systems respectively. For power constrainedsystem, our aim is to minimize the secrecy rate outage probability. We also consider thesecrecy outage probability for different positions of eavesdropper. For powerunconstrained systems, we obtain the optimal power allocation to minimize the totalpower under the quality of service and secrecy constraints. Simulation results show thatthe proposed method achieves a good security performance.