Study On Effective Hybrid Automatic Repeat Request In Wireless Communication Systems

In recent years, the transmission technologies in wireless communication system have been developed rapidly. Some new topics,e.g., network coding, energy efficiency and physical layer security, have attracted increased attention of researchers. Hybrid Automatic Repeat Request (HARQ) plays important role in error control, thus it is widely applied in current systems. How to introduce and analyze the above-mentioned issues in HARQ system is hotly discussed. In this thesis,by considering the applied FEC, these analyses are of more practical significance while current research pays less attention to the influence of FEC codes. Based on network coding, this thesis designs two novel HARQ schemes; then analyzes the characteristics of energy efficiency,and proposes two HARQ schemes with improved energy efficiency.Additionally, the impact of HARQ on physical layer security is discussed.The main innovations and contributions in this thesis are summarized as follows:1. In the HARQ system with Low-Density Parity-Check (LDPC)codes, two network coding methods which construct and reconstruct retransmitted data are proposed. (i) For the retransmission scheme that two errorneous codewords are combined, this thesis proposes an iterative decoding algorithm. Iteration and updating of external information are separated into two parts with different setting respectively. Simualtion results show that the metrics of throughput and average transmission number are improved compared with conventional HARQ scheme, (?)This idea is extended into N channel stop-and-wait HARQ system.According to the properties of short codes, a multi-rate coding method is designed to construct retransmitted codewords. This method has such features as good flexibility and high efficiency. At the receiver, a combined decoding algorithm with reduced complexity is proposed. The parity check relationship and corresponding iterations are simplified without affecting the performance.2. The energy efficiency of HARQ is analyzed theoretically both in Additive White Gaussian Noise (AWGN) channel and block fading channel, and two improved energy efficient HARQ schemes are proposed.(i) In AWGN channel, based on the features of practical codes, this thesis drives the asymptotic value of maximal energy efficiency, which equals to the reciprocal of the threshold signal-to-noise ratio of the FEC code. (?)The analysis is extended into block fading channel, while considering the energy consumption of circuitry processing. The upper bound of energy consumption is derived in equal power transmission, and a sub-optimal algorithm is proposed for variable power transmission with reduced complexity and near-optimal performance. (?) A simple and energy efficient HARQ scheme is proposed, in which conditional Word Error Ratio (WER) of each transmission is fixed and the transmit power is adjusted correspondingly. Three performance metrics are analyzed including average transmission number, throughput and energy efficiency.Compared with the conventional equal power HARQ scheme, the proposed scheme can significantly improve the energy efficiency and other two metrics. (iv) The energy efficiency of Energy Harvesting Node(EHN) in HARQ system is improved. Since this optimization problem is difficult to derive for EHN in block fading channel, this thesis maximizes local energy efficiency in order to reduce the consumed energy and improve system performance. A utility function is defined as the conditional energy efficiency when the former transmissions are failure.In addition to the searched optimal solution, a sub-optimal close-form one is derived by using approximated Q function. Simulation results demonstrate the proposed HARQ scheme has improved energy efficiency compared with conventional schemes.3. The physical layer security of HARQ system is investigated. By the theoretical analysis, it is proved that HARQ is helpful for improving physical layer security. We firstly derive that achievable diversity order of main channel equals to maximum transmission number ( K ) while this order of wiretap channel is only one. That is to say, the diversity gain of main channel increases with retransmissions. However, this increasement is extremely limited for wiretap channel. Furthermore, we evaluate the metric of secure gap which demonstrates the difference between main channel and wiretap channel with reliable and secure constraints. As K increases, the secure gap decreases monotonously which means the improved secure performance.