Research And Optimum Design Of Energy Efficiency And Spectral Efficiency For HARQ Protocols

As a key technique in modern wireless communication networks,hybrid automatic repeat request(HARQ)protocols,which takes the advantage of automatic repeat request(ARQ)protocols and the forward error correction(FEC),can combat the effects of fading in wireless channels and improve the reliability and efficiency of the data transmission.Meanwhile,with the booming development of mobile communication and the sustained increasing of user requirements,energy efficiency(EE)and spectral efficiency(SE)have become two most important metrics for the next wireless communication networks.Aiming at improving the EE and SE or achieving the EE-SE tradeoff for HARQ protocols in different systems over fading channels,this dissertation studies the optimization schemes in HARQ protocols,such as cross-layer design,optimum power allocation,etc.The main contributions of this dissertation are as follows.Firstly,a cross-layer energy efficient optimization scheme for Chase combining based HARQ(CC-HARQ)protocols over block Rayleigh fading channels is proposed.Specifically,the EE of CC-HARQ protocols over block Rayleigh fading channels is derived in a tractable closed-form expression using a new log-domain linear threshold approximation method.Based on the expression,the optimum payload length with fixed transmission energy and the optimum transmission energy with fixed payload length are identified respectively,then the joint optimum payload length and transmission energy are obtained with a joint iterative optimization algorithm.In addition,in order to flexibly adjust the tradeoff between the EE and SE,a unified EE-SE tradeoff metric with a preference factor is introduced,and the optimum payload length and transmission energy are derived to maximize the metric.It is found that the EE and SE of CC-HARQ protocols have a complimentary feature in terms of payload length,but conflict with each other in terms of transmission energy.Simulation results show that the proposed energy efficient optimization scheme can improve the EE of CC-HARQ protocols over block Rayleigh fading channels,and the proposed EE-SE tradeoff scheme can flexibly adjust the tradeoff between the EE and SE of CC-HARQ protocols by adjusting the preference factor.Secondly,for the truncated CC-HARQ protocols in direct and cooperative systems over block Rayleigh fading channels,the closed-form power allocation schemes are proposed to minimize the average total power consumption under the frame loss rate(FLR)and maximum number of transmission rounds constraints.Due to the complexity of the exact average frame error rate(FER)expression in CC-HARQ and Cooperative CC-HARQ protocols,solving the original optimization problem needs address several complex optimization problems.In order to simplify the optimization problem,a simple approximated expression for the average FER at the high signal-to-noise ratio(SNR)region is further proposed.With the simplified expression,the optimization problem can be approximated as a geometric programming problem,which can be solved with the GPP toolbox.However,the closed-form optimum power allocation(OPA)schemes are further derived to reduce the complexity using the Lagrange multiplier method.Applying the solutions,the impact of transmission parameters on the optimum power sequence is investigated,and the necessary and sufficient condition for an increasing power sequence of the OPA scheme are determined.Results show that the closed-form OPA scheme match the GPP approach results very well,and yields significant performance improvement compared to the conventional equal power allocation(EPA)scheme.Moreover,for the OPA scheme,the performance of cooperative CC-HARQ protocols outperform that of direct HARQ protocols.Thirdly,for three typical truncated CC-HARQ protocols in MIMO systems over block Rayleigh fading channels,the closed-form power allocation schemes that minimize the average total power usage with the constraints of targeted outage probability and that minimize the outage probability with the constraints of targeted average total power usage are proposed.Considering type-I HARQ(TI-HARQ)protocols,CC-HARQ protocols and incremental redundancy based HARQ(IR-HARQ)protocols in MIMO systems,the approximate outage probabilities at the high SNR region are derived and unified as an expression that related to the diversity gain and coding gain from the information-theoretical perspective.Based on the unified approximate outage probability expression,the closedform power sequences that minimize the average total power usage with the constraint of targeted outage probability and that minimize the outage probability with the constraint of targeted average total power usage are derived using the Lagrange multiplier method,it is found that the two optimization problems are equivalent.Then by applying the closedform solutions,the properties of the optimum power sequence are further investigated,it is found that the optimum power sequence is monotonically increasing when the targeted outage probability is smaller than a threshold,which is related to the number of antennas,the number of transmissions and transmission rate,for all the three HARQ protocols.It was also shown that the power allocation gains of OPA over EPA first decrease then increases as the targeted outage probability decreases.Numerical simulations demonstrate that the OPA schemes lead to a significant performance improvement in MIMO HARQ systems,and the properties can serve as the design guidelines for MIMO HARQ protocols.Finally,for the multiple hop wireless networks without HARQ protocols,with TIHARQ protocols and CC-HARQ protocols over block Rayleigh fading channels respectively,the spectral efficient joint transmission power and rate optimization strategy are studied for a given total power consumption.The closed-form expression of the SE for those three cases are derived,then the optimum transmission rates for each hop are determined respectively when the transmission power of each hop are fixed.Especially,for the networks without HARQ protocols,the optimum rates are given by a low complexity one dimension search algorithm;for the networks with TI-HARQ protocols and CC-HARQ protocols,the optimum rates are given by the closed-form solutions.Then the optimum transmission power for each hop are determined respectively when the transmission power of each hop are fixed.Especially,for the networks without HARQ protocols and with CC-HARQ protocols,the optimum power are given by a low complexity one dimension search algorithm;For the networks with TI-HARQ protocols,the optimum power are given by the closed-form solutions.Based on the power and rate optimization strategy,the joint iterative algorithms are proposed to find the joint optimum transmission power and rate.Numerical results show that the joint transmission power and rate optimization strategy can improve the EE of the multiple hop wireless networks for all three cases compared to equal transmission rate and power strategy,transmission rate optimization strategy and transmission power optimization strategy.