D2D(终端直通)技术以非正交复用资源的方式引入LTE(长期演进)系统会带来干扰问题，即导致系统中蜂窝用户和D2D用户间存在互相干扰。在D2D系统中，跨层的资源优化能应对蜂窝用户因干扰影响瞬时变差的信道情况，从而提高蜂窝用户的平均频谱效率，提高系统容量。文章研究了基于链路自适应的跨层优化方案，并结合了物理层的自适应调制编码技术和数据链路层的自动选择重传技术，旨在降低D2D技术引入的干扰。仿真结果表明，在4个蜂窝用户、4对D2D用户的单小区场景中，采用基于链路自适应的跨层优化方案后，D2D通信系统的总容量提升了约5%。

The introduction of Device-to-Device (D2D) technology into the LTE systems in a non-orthogonal way of reusing resources may bring about interferences, i.e. mutual interferences between the cellular users and the D2D users. In the D2D systems, cross-layer resource optimization can cope with instantaneous channel worsening arising from the interferences for the cellular users, thus improving their average spectral efficiency and increasing the system capacity. This thesis studies a link-adaptive cross-layer optimization scheme for the D2D system, which combines the adaptive modulation and coding technology in the physical layer with the auto repeat request technology in the data link layer, aiming at lowering the interferences arising from the introduction of the D2D technology. Simulation results indicate that in a single cell scenario with 4 cellular users and 4 pair of D2D users, the total capacity of the D2D communication system is increased by about 5% when the link-adaptive cross-layer optimization scheme is adopted.