Cooperative Diversity And Hybrid Combining Techniques For Wireless Diversity Systems

Wireless communication has received people’s widespread favors by its unique superiority of breaking the bound of cables, and then has been developed gradually to be one of the core braches in modern communications driven by more and more application requirements. Unfortunately, we have to face many disadvantageous factors brought by wireless communication while we enjoy its convenience. Of course, it is just for solving lots of challenging problems, especially the channel fading, which inspires the interest and stirs the emotions of people and then make wireless communication the fastest-growing part in the communication industry. Among numerous measures that combat the channel fading, diversity technique is no doubt the most effective means, which has been widely applied in large numbers of wireless systems, such as mobile internet, Internet of Things, trunking communications and satellite communications. Actually, the idea of the diversity is very simple, which first separates the copies of the transmitting signal and then manages to combine them together again at the receiving end. It is just between the separation and the combination that people can think out many methods to increase the overall performance of wireless systems. The separation can be realized through time, space, frequency, etc., which has been developed into the cooperative diversity(CD) stage; on the other hand, the combination can be realized by means of maximal ratio combining(MRC), equal gain combining(EGC), selection combining(SC), etc., which has been advanced into the hybrid combining(HC) stage. This thesis just focuses on the cooperative diversity technique and the hybrid combining technique of the diversity systems, and the main contributions are summarized as follows.1. Cooperative diversity techniqueDiversity technique is the important and basic technique for combating channel fading in wireless communications and, recently, is further expanded to cooperative diversity(CD), i.e., treating different users as antennas to obtain the diversity gain. Compared with the conventional diversity techniques, CD technique has distinct advantages(e.g., being able to enjoy the powerful processing capability of the relay node), and yet has brought some transmission efficiency loss resulted by the receiving-forwarding half duplex(HD) mode. In recent years, a kind of novel full duplex(FD) cooperative diversity schemes based on quadrature signaling(QS) was proposed, which effectively solved the inefficiency of conventional schemes. Unfortunately, this kind of schemes has been facing a dilemma in engineering practice, because the various techniques among the schemes cannot balance both performance and complexity of systems.In order to overcome the problem above, this thesis proposes a cooperative diversity scheme based on threshold-based quadrature signaling(TQS) for a wireless network of three nodes, in which the two nodes(i.e., the users) cooperate to transmit information to the other node(i.e., the destination). By assigning different components of the quadrature signaling to different cooperative users and letting each user node decide whether to relay information for the partner in the assigned component based on a threshold or not, independently, the TQS scheme effectively integrated the advantages of both quadrature signaling and threshold techniques. Based on the qualitative analysis of the performance of the TQS scheme, we investigate its end-to-end bit error rate(BER) performance and obtain its approximate solution. According to our theoretical analysis, the TQS scheme can enhance the diversity gain of a wireless cooperative system and, meanwhile, reduce the complexity of its implementation to a great extent. In addition, the communication system, using TQS scheme, has the feature of easily switching between cooperative and non-cooperative patterns. Finally, the simulation results show that TQS scheme has excellent performance as expected and is able to achieve full diversity gain in the condition of the appropriate threshold value.2. Hybrid combining techniqueAlthough the conventional diversity combining technique has been well studied in the past, it has intrinsic limitations, i.e., being unable to achieve the tradeoff of the performance and the complexity of systems. Hence, people recently turn to the hybrid combining technique, which can solve the above problem well. By integrating at least two conventional combining techniques into together, hybrid combining technique has become a kind of wireless communications technology with the balanced overall performance, which can maximize their advantages and, meanwhile, compensate their deficiency. In addition, it has the feature of strong adaptability and wide application. Therefore, hybrid combining technique has developed into the mainstream direction of the diversity combining field. Base on the intensively studing of the present various diversity combining techniques, this thesis proposes several hybrid combining schemes for different wireless scenarios. And then, theoretical analyses and simulation verifications are also made in detail.1) Hybrid selection/threshold-switched/maximum-ratio combining(HS/TS/MRC) scheme for dual-branch systems. Presented is a novel hybrid selection/threshold switched/maximum-ratio combining(HS/TS/MRC) scheme with balance of overall performance for dual-branch systems. The main idea is, by designing proper switching logic, to integrate the three conventional combining techniques(i.e., SC, TSC and MRC) into an organic whole. Theoretical analysis shows that the HS/TS/MRC scheme, compared to the available schemes which merely use two of the three techniques above, can not only improve the performance of systems but also reduce the complexity of the systems implementations to some degree. Simulation results validate the analysis and further show that, comparing with the existing solutions, the proposed scheme also widens the beneficial scope of outage threshold and is especially applicable to the scenarios with power constraint, revealing that it has more extensive application potential and is more attractive for the modern green communication trend.2) Selection-and-stay combining for multi-branch(SSTC-MB) scheme. Selection-and-stay combining for multi-branch(SSTC-MB) scheme is proposed and investigated. The scheme upgrades the available SSTC scheme from dual-branch scenarios into diversity-rich scenarios, with both improving the outage performance and expanding the applied scope of wireless diversity systems. Based on the Markovian property of the received signal-to-noise ratio(SNR), the authors analyze the average number of estimated branches and the outage performance respectively, and then obtain the corresponding closed-form expressions. Simulation results show that, compared with the prior relevant techniques, the scheme can effectively improve the system performance and even achieve full diversity gain under the condition of low outage threshold value, of course, at the cost of increasing system complexity to a certain extent.3) Group minimum estimation and combining generalized selection combining(Gr MEC-GSC) scheme for diversity-rich systems. For diversity-rich wireless systems, we also propose a group minimum estimation and combining generalized selection combining(Gr MEC-GSC) scheme, which expands an excellent MEC-GSC scheme into the general form, making its system model more consummated and its function stronger. After designing the system model and the corresponding operation mode, we analyze the average number of estimated branches and achieve its closed-form expression. Next, we establish the mathematical expression of its operation mode and use it as the basis for further research of the system outage probability. Unfortunately, due to the complexity of the correlation among the Markovian chain, its closed-form expression cannot be attained, and so Monte Carlo method is introduced. Simulation results illustrate that, compare to the existing scheme, Gr MEC-GSC scheme not only enhances the system performance but also overcomes the disadvantage of fixed usage pattern(i.e., it can flexibly adjust the parameters of diversity systems to meet the channel states and the service requirements).