Performance And Optimal Power Allocation Of Wireless Cooperative Networks

As the increasing requirements on mobile communication services, mobilecommunication systems develop rapidly towards high-speed, wideband,multiple-services and low cost. Multiple input multiple output (MIMO) technique canincrease the capacity and improve the spectral efficiency and reliability withoutfrequency and power increasing. However, due to price, power and size limitations, it ishard to place multiple antennas on some mobile terminals, which blocks the applicationof MIMO in wireless communication systems. By forming virtual MIMO throughsharing antennas of different terminals, cooperative diversity can achieve significantdiversity gain, which provides a way of application of MIMO in practical networks.This thesis investigates the theoretical performance analysis of wireless cooperativenetworks, and based on the performance analysis, optimal power allocation isperformed to further improve the performance. The main contribution of this work canbe summarized as follows:1Cooperative systems based on amplify-and-forwardWithout decoding the received signal, amplify-and-forward (AF) relay can beapplied in many systems in view of its low complexity and blind access. However, forAF-based cooperative systems, the cascaded fading channels between the source and thedestination nodes bring great difficulties in the theoretical performance analysis. Basedon the published results, we focus on the following two questions:The first question is the design of optimal power allocation for multiple-relaysystems. Firstly, the average symbol error rate (SER) and the asymptotic SER in highSNR regime are derived for M-PSK and M-QAM modulation. Secondly, the closedsolutions of optimal power allocation are obtained by using Lagrange multiplier basedon the asymptotic SER under the total power constraint. Results show that optimalpower allocation can improve SER performance compared with equal power allocation.Given M-PSK or M-QAM, the derived optimal power allocation is identical, which isonly dependent with the channel links connected to the relays and independent with thechannel links between the source and the destination nodes.The second is the low sprectral efficiency of the conventional distributed Alamoutispace-time scheme (AF-DSTC) with amplify-and-forward relaying. A new selectivedistributed Alamouti space-time scheme is proposed, which can increase the spectralefficiency from1/2to2/3by shortening the cooperative period from four time slots tothree slots. Theoretical analysis and simulation results on outage probability and biterror rate show that the proposed protocol is better than AF-DSTC in terms of the total power consumption, outage probability and bit error rate under the same spectralefficiency.2Cooperative systems based on decode-and-forwardSelective decode-and-forward relays can improve performance of cooperativesystems because they can eliminate the cascaded fading channels between the sourceand the destination nodes and decrease error propagation. Since it is easier to analyzethe performance of cooperative systems based on decode-and-forward protocol, manyvaluable theoretical results have been reported. The thesis further investigates theimpacts of imperfect channel estimation error on the cooperative systems based ondecode-and-forward protocol and opportunistic relay, i.e.Firstly, the approximate SER and asymptotic SER for M-QAM are provided byusing the moment generating function (MGF) under imperfect Nakagami-m fadingchannels. Based on the compact asymptotic SER formula, the optimal power allocationcan be transformed to the typical convex optimization problem. Thus, the optimal powerallocation formulas can be reached to minimize the SER. Simulations show that optimalpower allocation scheme can significantly improve the performance of cooperativesystems in the low to medium SNR region. However, it can not improve the error floorunder the imperfect channel estimation when SNR is high.Secondly, opportunistic relaying is adopted to improve the spectral efficiency ofmultiple-relay systems. In order to understand the peformace of opportunistic relayingsystems, the closed form of average SER and outage probability are derived understatistically independent but not-identical Nakagami-m fading channels.3Mobile-to-mobile systems assisted by mobile relaysMobile relay-assisted communication systems are the typical scenarioes in the nextgeneration mobile cell networks, Ad Hoc networks and vehicle-to-vehiclecommunications. Due to the different channel models in these scenarios, few resultshave been reported for the theoretical analysis of mobile relay-assisted M2M systems.Based on the assumption of cascaded Nakagami-m channels, this thesis investigates theimpacts of mobile relays with M-PSK modulation. By using MGF method, SERexpressions are derived for AF and DF mobile relays. For AF protocol, a more conciseapproximated SER expression is also derived by using an approximated probabilitydensity function. Simulation SER results verify the rightness of our derived lowerbound and approximated SER expressions.