Research On Key Technologies Of Device-To-Device Wireless Communications

In the modern cellular systems, the data is transmitted between base-station and users. However, in practical applications, some problems arise, such as the low efficiency of resource utilization, the inadequate coverage of base-station and the fact that users communication cannot be guaranteed when the base-station is down. In order to deal with the above problems, the Third Generation Partner Project(3GPP) proposes the Device-to-Device(D2D) communication technology. The core idea of D2D technology is the direct data transmission between users. Based on D2D technology, the efficiency of resource utilization and base-station coverage can be improved,and moreover, the user communication can be guaranteed without base-station. Therefore, the research on the D2D technology has a great theoretical and practical value.This thesis firstly analyzes some key technologies related with D2D technology, and then introduces the research status of D2D technology in 3GPP LTE-A. In the research of 3GPP LTE-A,three D2D scenarios are considered, namely, D2D technology within the cellular network coverage, D2D technology for partial cellular network coverage and D2D technology out of the cellular network coverage. According to the scenario classification of 3GPP, this thesis introduces the current situation of D2D technology, and some important directions are selected for the research in this thesis.For the D2D technology within the cellular network coverage, this thesis studies the resource allocation mechanism in the resource pool decided by the base-station. The cellular network users and D2D users select the resources in the resource pool, and therefore the resource allocation mechanism design must ensure that D2D users’ interference on the cellular network users is less than a given threshold. Moreover, the delay of D2D data transmission must be bounded, and the transmission rate of D2D users should be improved as much as possible. Specifically, this thesis mainly focuses on the static single-hop D2D networks and static multi-hop D2D networks, uses the Lyapunov optimization and virtual collision queue, and finally improves the D2D transmission rate on the premise of the two given requirements. According to the theory analysis and simulation experiment, the validity of the proposed scheme is proved. Since the users in this thesis are allowed to equip with multiple radio and the multi-hop scenario is considered, the proposed scheme has more widely applied area comparing with the current works.For the D2D technology with partial cellular network coverage, this thesis studies the joint pre-coding design for base-station and D2D relay. When some D2D users are allocated outside the base-station coverage, the message can be send to them from the base-station with the help of D2D relays within the base-station coverage. However, since there may be more than one D2D receiver outside the base-station coverage, the interference between different D2D transmissions must be reduced. This thesis considers the channel estimation inaccuracy between D2D relays and D2D receiver of other cell. A robust pre-coding is designed for base-station and D2D relays,and the effectiveness of the proposed pre-coding is verified by simulation. Since the pre-coding is designed especially for the target scenario, it is outperforms most of the existing schemes.For the D2D scheduling design out of the cellular network coverage, this thesis studies impact of network coding on the performance of the D2D networks. Specifically, this thesis introduces the random linear network coding in large-scale D2D networks, and studies the optimal network coding configuration for some transmission schemes in order to improve the accurate delay/goodput tradeoff and goodput performance. The theoretical analysis results show that the network coding can improve the goodput of the network but cannot improve the accurate delay/goodput tradeoff in most cases except for the flooding scheme in random i.i.d. mobility model. Since this thesis considers the goodput loss and decoding loss caused by network coding, which is usually ignored in large-scale network, the proposed network coding configurations are of higher application value.In the study of impact of users’ behavior for out of base-station coverage scenario, this thesis firstly focuses on the impact of the selection of destinations. According to the statistical results of the existing social networks, the rank-based network model is considered. A two-layer network model is proposed based on the network model, according to which this thesis analyses the probability density function(PDF) of destinations’ ranks and positions. Moreover, the PDF of destinations’ positions is verified by simulation. Furthermore, this thesis calculates the upper-bound of the per-user capacity according to the PDF, and also proposes a routing and scheduling mechanism to achieve this bound in most cases. Since the multicast scenario for the rank-based model is not studied in previous work, our study provides a certain theory support on this field.In addition, this thesis also studies the influence of user movement for out of base-station coverage scenario. The well-known restricted mobility model is introduced, which can be utilized to show the fact that users are more likely to move close to their home-points. The per-user capacity upper-bound and delay lower-bound are considered for both unicast and multicast cases,and the bound-achieving schemes are proposed in this thesis. The analysis results show that high moving speed improves the per-user capacity but reduces the multicast gain, i.e., the distinction between unicast and multicast. Moreover, this thesis further analyzes the main factors influencing the multicast gain, which form a framework of multicast gain study. Since the multicast scenario for restricted mobility model is not studied in previous work, the study in this thesis will be helpful for the performance analysis of restricted mobility model and the research of multicast gain.Finally, a summary of this thesis is given, and the main research points as well as the corresponding points of innovation are also concluded. Moreover, some research directions of the D2D technology are pointed out for further study.