This letter proposes a new user cooperative offloading protocol called user reciprocity in backscatter communication (BackCom)-aided mobile edge computing systems with efficient computation, whose quintessence is that each user can switch alternately between the active or the BackCom mode in different slots, and one user works in the active mode and the other user works in the BackCom mode in each time slot. In particular, the user in the BackCom mode can always use the signal transmitted by the user in the active mode for more data transmission in a spectrum-sharing manner. To evaluate the proposed protocol, a computation efficiency (CE) maximization-based optimization problem is formulated by jointly power control, time scheduling, reflection coefficient adjustment, and computing frequency allocation, while satisfying various physical constraints on the maximum energy budget, the computing frequency threshold, the minimum computed bits, and harvested energy threshold. To solve this non-convex problem, Dinkelbach's method and quadratic transform are first employed to transform the complex fractional forms into linear ones. Then, an iterative algorithm is designed by decomposing the resulting problem to obtain the suboptimal solution. The closed-form solutions for the transmit power, the RC, and the local computing frequency are provided for more insights. Besides, the analytical performance gain with the reciprocal mode is also derived. Simulation results demonstrate that the proposed scheme outperforms benchmark schemes regarding the CE.

翻译：本文提出一种面向背向通信(BackCom)辅助移动边缘计算系统的新型用户协同卸载协议——用户互惠协议，其核心思想在于每个用户可在不同时隙交替工作于主动模式或BackCom模式，且每个时隙内一个用户处于主动模式而另一用户处于BackCom模式。特别地，处于BackCom模式的用户始终可重用主动模式用户发射的信号，以频谱共享方式实现额外数据传输。为评估所提协议，通过联合功率控制、时隙调度、反射系数调整与计算频率分配，构建基于计算效率(CE)最大化的优化问题，同时满足最大能量预算、计算频率阈值、最小计算比特数及能量采集阈值等多重物理约束。针对该非凸问题，首先采用Dinkelbach方法与二次变换将复杂分式形式转化为线性形式，进而通过分解转化后的问题设计迭代算法获取次优解。给出发射功率、反射系数及本地计算频率的闭式解以提供更深入的理论洞察，同时推导了互惠模式的解析性能增益。仿真结果表明，所提方案在计算效率方面优于基准方案。