The Study Of Relay Assisted Device-to-Device Communications Based On Maximum Link Capacity

Device-to-Device technology is the direct communication between mobile user equipments which are relatively close to each other, on the licenced spectrum without going through base station. Numerous researches have shown that the introduction of D2 D technology in cellular network could effectively improve the system capacity and spectral efficiency. Since D2 D is a kind of short-distance and low-power direct communication, the introduction of relay is considered to extend communication distance and enhance quality of D2 D communication.A relay selection and resource allocation joint algorithm based on maximum capacity is proposed to select optimal relay node in D2 D relay communication that reuses the cellular uplink resource. The algorithm takes relay selection and resource allocation into consideration simultaneously. And it determines the best relay node based on maximum capacity of D2 D relay link while allocating spectral resource for two-hop D2 D links. Simulation results show that the proposed algorithm could absolutely fulfill the QoS requirement of the partner CU who shares spectral resource with D2 D user, and significantly promote the capacity of D2 D relay link and system spectral efficiency.In order to increase the throughput of D2 D relay link, this thesis proposes a cross layer design combining AMC in physical layer with HARQ in data link layer. First, the probability density function of D2 D received SINR is derived. Then, AMC strategy is applied in physical layer to alleviate the effect of time-varying channel and HARQ technology which allows packet retransmition is introduced to data link layer to reduce the burden of controlling BER in physical layer. In the end, simulation results show that adopting AMC with HARQ scheme can improve spectral efficiency of D2 D communication and increase the throughput of D2 D relay link.A cross layer optimization combining of adaptive modulation in physical layer with adaptive packet length in data link layer is proposed over D2 D link to promote link adaptation and system spectral efficiency of D2 D communication. This scheme optimizes the packet length corresponding to channel state and modulation mode in physical layer. The threshold of SINR intervals and the optimal packet length are obtained from target function of system spectral efficiency. Simulation results validate that this strategy could improve system spectral efficiency in some extent.