Research On Wireless Communications Based On Cooperative Diversity

Multiple-input multiple-output (MIMO) techniques can improve the receivedsignal quality remarkably and increase the data transmission rate. However, manywireless devices are limited by size or hardware complexity to a single antenna,examples include most handsets or the nodes in a wireless sensor network, then MIMOtechniques can not be applied directly. In the foregoing scenario, the concept ofcooperative diversity has been proposed, including the amplify-and-forward (AF)protocol and the decode-and-forward (DF) protocol. Cooperative diversity techniquescan enable single-antenna mobiles in a multi-user environment to share their antennasand generate a virtual multiple-antenna transmitter that allows them to achieve transmitdiversity. So a wireless communication system based on cooperative diversity can reapsome of the benefits of MIMO systems.In cooperative communication systems, improving the spectrum efficiency andincreasing the diversity gain are two primary problems. The common method toimprove the spectrum efficiency is the application of two-way cooperativecommunication systems. At present, two unidirectional links are consideredindependently in the existing performance analysis of two-way cooperative systems.But in fact, a two-way cooperative communication system is a multi-user system, it isinappropriate to consider two unidirectional links independently. The well-knowntechniques to increase the diversity gain of cooperative communication systems includetransmit antenna selection (TAS), relay selection, and beamforming, etc. However, theexisting papers about TAS techniques have focused on AF cooperative systems, and theexisting papers about relay selection and beamforming techniques have focused onone-way cooperative systems. To overcome these drawbacks, some new cooperativeschemes are proposed in this dissertation, and the main contributions are outlined asfollows:1. TAS schemes in DF cooperative communicationsIn a two-hop DF cooperative communication system with a source-to-destinationdirect link, two TAS schemes denoted by TAS-I and TAS-II are proposed andinvestigated over fading channels. Exact closed-form expressions for the outageprobabilities of these two TAS schemes are derived, respectively, and the asymptoticexpressions at high signal-to-noise ratio (SNR) for the outage probabilities of two TASschemes are further obtained, then the diversity gains of these two TAS schemes areascertained and compared. The theoretical analysis and Monte Carlo simulation results show that both TAS-I and TAS-II can achieve full diversity gain over Rayleigh fadingchannels.2. Relay selection schemes in two-way AF cooperative communicationsFirst, in a variable-gain two-way AF cooperative communication system, the exactintegral expressions and approximate closed-form expressions are derived for theoverall outage probability (OOP) and average symbol error probability (SEP), and thena relay selection scheme is proposed and analyzed over fading channels. Both thetheoretical analysis and Monte Carlo simulation results show that the proposed relayselection scheme can achieve full diversity gain over Rayleigh fading channels.Second, in a fixed-gain two-way AF cooperative communication system, a relayselection scheme is investigated and analyzed over fading channels. The exact integralexpression for the OOP is derived, and a tight closed-form lower bound on the OOP isobtained too. Monte Carlo simulation results are presented to verify the theoreticalanalysis, and show that the relay selection scheme can also achieve full diversity gainover Rayleigh fading channels.3. Beamforming schemes in two-way AF cooperative communicationsFirst, a beamforming scheme in a two-way AF cooperative communication systemis proposed and its performance is investigated over fading channels. Exact closed-formexpressions for the outage probabilities of two unidirectional links of the two-waysystem are derived, respectively. Furthermore, both a lower bound and an upper boundin simple closed forms are derived for the outage probabilities, and then the diversitygain are ascertained with the aid of the the squeeze theorem. Finally, Monte Carlosimulation resluts are presented to validate the theoretical analysis.Second, in a multi-user two-way AF cooperative network based on beamforming, aperformance analysis of the optimal user selection criterion is performed over fadingchannels. Both a lower bound and an upper bound in closed forms are derived for theoutage probabilities of two unidirectional links, and then the diversity gain isascertained by the use of the the squeeze theorem. Finally, Monte Carlo simulationresults are presented to verify the theoretical analysis.