通常链路自适应技术均是基于分层的思想来设计的, 往往使得局部性能最优化, 但系统的整体性能却未达到最优。文章在LTE(长期演进)网络架构基础上, 针对物理层的AMC (自适应调制编码)和MAC (介质访问控制)层的HARQ (混合自动请求重传)两种自适应技术提出一种跨层设计方案。分析了跨层耦合参数与系统性能之间的关系, 并给出了详细的推导过程和具体的表达式。然后给出了平均时延和平均误包率约束条件下的跨层优化问题。仿真结果表明, 该跨层设计能进一步提高系统的平均频谱效率；在给定业务QoS (服务质量)需求下, 可以根据信道质量来选择最佳的最大重传次数和AMC方式, 使得系统的平均频谱效率最大化。

The adaptive techniques for links are usually performed within the layers, often resulting in the optimization of local performance, but not of the entire system. This paper first presents a cross-layer design scheme for LTE systems by using two adaptive transmission techniques, i.e. Adaptive Modulation and Coding (AMC) at the physical layer and Hybrid Automatic Retransmission Request (HARQ) at the Medium Access Control (MAC) sublayer. Then, it analyzes the relationship between the cross-layer coupling parameters and the system performance and gives a detailed derivation process and a concrete expression. Finally, it formulates the cross-layer optimization under the constraints of the average time delay and packet error rate. Simulation results indicate that the proposed cross-layer design can further improve the average spectral efficiency and select the optimum maximum number of HARQ and way of AMC in line with the channel quality so as to maximize the average spectral efficiency under the specified Quality-of-Service (QoS) requirements.