Resource Allocation Strategy For Device To Device Communications In Cellular Networks

The fifth generation(5G)communications is featured by ultra-low latency,massive device interconnection,ultra-high traffic density,and high mobility.In response to these challenges,we investigate Device-to-Device(D2D)communications in this thesis.From the perspective of improving the space utilization of resources,the D2 D communication in cellular networks enables users to directly communicate with each other by reusing cellular resources.D2 D communications can effectively reduce the transmission power,improve the resource utilization,and increase the system capacity.Besides,without being transmitted by the base station(BS),D2 D communications can also reduce the end-to-end transmission delay and reduce the load of the BS.Moreover,under the management of cellular network,D2 D communications in interference-controllable environments can establish more reliable transmission links than traditional short distance communication technologies.Therefore,the D2 D communication in the cellular network can not only enhance the performance of the conventional cellular network,but also can serve the emerging applications such as massive machine type communications.It is a promising candidate technology for 5G.In order to break through the restriction of channel conditions between D2 D users and interference constraints of cellular communications,relays are introduced in D2 D communication to implement cooperative transmission,which can further improve the performance and extend the application scenarios of D2 D communications.In addition,D2 D communications with low latency and high reliability can provide effective technical support for intelligent transportation systems and mobile Internet.Therefore,D2 D communications in mobile environment also attract increasing attentions.In the above scenarios,allowing D2 D communications to reuse cellular licensed spectrum resources can improve the utilization of network resources,but the accompanying co-channel interference brings new challenges to achieve the technical advantages of D2 D communication and guarantee the performance of conventional cellular communications.In order to meet the urgent need for significantly increasing spectral efficiency(SE)and energy efficiency(EE)in 5G,we focus on resource allocation techniques for both relay D2 D communications and mobile D2 D communications.The main contributions obtained in this thesis are summarized as follows.1?EE oriented resource allocation schemes for relay D2 D communications are investigated.The introduction of the relay increases the SE and transmission reliability of D2 D communications while it also brings additional transmit power and circuit power consumptions.The future 5G not only pursue high transmission rates,but also require high EE to truly realize green communications.For this purpose,we conduct theoretical analysis of the EE performance of relay D2 D communications and propose resource allocation schemes that maximize the EE of D2 D communications.· A power control scheme maximizing EE is proposed.Firstly,we theoretically analyze the average EE of relay D2 D communications with co-channel interference and obtain the closed-form expression.Meanwhile,for practical communication systems,the SE of users is determined by the channel quality identity(CQI)level which is the quantification of the signal-to-interference-plus-noise ratio(SINR).When the transmission power is increased and the CQI level stays the same,the extra power consumption will not increase the SE but will reduce EE of system.Therefore,based on theoretical analysis,we propose a CQI-based transmit power control scheme for the relay user to maximize the EE of D2 D communications while guaranteeing the interference power constraint imposed by cellular communications.In the proposed scheme,according to different CQI levels,we discretize continuous transmit power to discrete values and then conduct power control.It can avoid unnecessary power adjustment and invalid transmission power consumption.· In addition to power control,channel selection and communication mode selection are also effective means to improve the EE of D2 D communications and guarantee the performance of cellular communications.Therefore,a power-channel-mode jointly optimized resource allocation strategy is proposed in this thesis.In the proposed strategy,the EE of D2 D communications is maximized while guaranteeing the quality of service of cellular users.Utilizing the uniqueness of user transmission mode and the orthogonality of resources between D2 D users,the three-dimensional optimization problem is decomposed,and the corresponding power iterative solution algorithm and the overall solution flow are designed.2? Power allocation schemes for the EE-SE trade-off in relay D2 D communications are studied.Both the EE and SE are two key performance metric for 5G.Whereas,maximizing EE and maximizing SE are two objectives that contradict each other.The overall performance cannot be optimized relying on single performance metric.Therefore,the trade-off between EE and SE in relay D2 D communications is investigated in this thesis.In order to characterize the effect of resource reusing on cellular communications,firstly,we theoreti-cal analyze the outage performance of cellular communications with co-channel interference and derive a closed-form expression for the outage probability(OP).Based on the theoretical analysis,with the OP as a constraint,the transmit power of the relay user is optimized by maximizing the SE of D2 D communications while the EE requirement of D2 D communications requirement is guaranteed.Secondly,to further improve the performance of the relay D2 D communication,the transmit powers of both source and relay users are jointly optimized.The optimal transmit powers of both source and relay users are obtained by maximizing the SE of D2 D communications while the interference power constraints imposed by cellular communications and the EE requirement of D2 D communications are guaranteed,and the impact of the EE threshold on the optimal transmit power is also analyzed.In addition,although the cellular network can accommodate more users on the same resources when the resources are reused by D2 D communications,the resulting co-channel interference will degrade the performance of conventional cellular communications.To evaluate the overall performance of network with D2 D communications,we introduce a network SE variation function and analyze the impact of interference threshold of cellular communications on the performance trade-off between D2 D and cellular communications.3? For mobile D2 D communications,we investigate the optimization of EE and SE performance,the trade-off of which is also analyzed from the perspective of economic efficiency(ECE).To accurately characterize effects of the mobility of D2 D users and the vehicle traffic density(VTD)in a realistic vehicular environment on the characteristics of wireless propagation channel,a three-dimensional(3D)vehicle-to-vehicle(V2V)channel model is adopted to investigate the interrelationship of EE and SE in mobile D2 D communications.We observe that a small degradation in EE around its peak value can significantly increase the SE for high VTD scenarios,while a marginal degradation in SE results in a considerable gain in EE for low VTD scenarios.Thus,two VTD scenario related power control scheme are proposed in this thesis.In high VTD scenarios we maximize SE while in low VTD scenario we maximize EE.Moreover,to further optimize the EE/SE thresholds of the obtained optimal transmit power,ECE is employed as a evaluation criterion to quantitatively evaluate the EE-SE trade-off from the perspective of benefit and cost.Simulation results are provided to demonstrate that ECE can serve as a general metric for EE-SE trade-off in different communication conditions.