Investigation On ARQ And Adaptive Modulation & Coding For Wireless Communications

Link adaptation technique, which can effectively increase data transmission rate and spectrum efficiency, is one of the key techniques of mobile communication systems for both nowadays and in the future. Adaptive Modulation and Coding (AMC) and Hybrid Automatic Repeat reQuest (HARQ) are two main link adaptation techniques. Along with the higher demands of data rate and quality of service, the system performance can be well improved by employing AMC and HARQ. AMC is able to choose modulation and coding scheme flexibly according to the channel condition but is sensitive to channel estimation error and feedback time delay. However, HARQ can adapt to the instantaneous channel condition automatically and seldom subject to channel estimation error and delay. Multilevel Quadrature Amplitude Modulation (MQAM) and M-ary Phase Shift Key (MPSK) have been widely utilized in various non-adaptive and adaptive data communication systems due to their high spectrum efficiency. Therefore, the core of this thesis is on MQAM/MPSK, with emphases on both HARQ and AMC.First of all, based on the analysis of the unequal error protection performance of Gray coded 16QAM and 64QAM constellations, a modified HARQ scheme is proposed. It is applicable for MPSK or MQAM in which the modulation order M is bigger than 4. The symbol-bits rearrangement rules and modulation constellation adjustment rules are proposed for 8PSK, 16QAM, 32QAM and 64QAM respectively. It is shown that the performance of the proposed scheme is pretty good (especially when SNR is low), and the performance gain obtained by modified MQAM/MPSK HARQ scheme enhances along with the increase of modulation order M. Meanwhile, an AMC scheme associated with modified MQAM/MPSK HARQ is proposed based on the simulation results of fixed modulation.In succession, aiming at modified 16QAM-HARQ scheme, the probability density function of 16QAM soft bit values after each retransmission combining and system throughput is derived by threshold receiver over the AWGN channel. Numerical analysis and simulation results show that the modified HARQ scheme can effectively improve the reliability of important bits in poor channel condition. Together with the symbol-bits rearrangement and packet combining in the retransmissions, the reliability of each bit will be averaged and increased gradually in the receiver, thus the ability of Viterbi decoder is improved, the bit-error-rate is reduced and the system throughput is enhanced.This thesis analyzes the performances of multi-channel stop-and-wait ARQ associated with packet combining in the Rayleigh fading channel in respect of the number of subchannels, combining scheme, packet loss rate and round-trip delay, etc. From the viewpoints of packet error rate, average transmission number and throughput, the performance of AM-ARQ system with packet combination is analyzed by discrete first-order finite-state Markov model. Combining with the modified ARQ proposed in Chapter two, the performance of AM-modified ARQ system is evaluated. The research results show that the performance of adaptive modulation associated with modified ARQ turns to be very good. By satisfying the target frame error rate and considering the ARQ retransmission gain, the modulation switching thresholds are defined and the throughput of AM-modified ARQ system can be maximized. The combination of adaptive modulation and ARQ can provide more stable performance even if there is channel state information error.Thereafter, using MGF function, the formulas of exact symbol error rate for MQAM/MPSK are derived in the multi-branch two-hop cooperation diversity system adopting amplify-and-forword and decode-and-forward, respectively. The symbol-error-rate performance is analyzed and compared among different transmission modes, modulations and SNR. A coded and modulation cooperation transmission strategy is proposed aiming at 16QAM. Based on conventional coded cooperative strategy between source and relaying nodes, the modulation cooperation is introduced. Two different 16QAM symbol-bits sequences are used in source and relaying node respectively. In destination node, the reliability of each data bit is equalized and increased by diversity combining. It is shown that both the average retransmission number of source nodes and the average cooperative transmission number of relaying nodes are decreased, the packet loss rate is reduced and the throughput is increased. The proposed scheme can be realized easily since what is needed is just two bits ARQ signal and four bits buffer for symbol bits rearrangement. In the end, the implicit training sequence assisted channel estimation based on first-order statistic is studied.The performance of 16QAM system, channel estimation and prediction in time-varying channel and block fading channel are investigated on different factors such as the length of training sequence, power allocation between training and data sequence, the length of the first-order statistics, etc. The research results demonstrate that, in order to minimize channel estimation error and system bit-error-rate, adaptive technique is required to adjust correlative parameters according to channel condition. In time-varying fading channel, if the variance of channel estimation error is less than 10^-4, the 16QAM system can achieve the preferred BER performance similar to the one with perfect channel estimation.