Cross-Layer Network Codes for Completion Time Minimization in Device-to-Device Networks

In this paper, we jointly optimize cross-layer network coding (CLNC) and device-to-device (D2D) communications to facilitate the recovery of popular common files that are missed at multiple user devices (UDs). In the envisioned system, a group of near-by UDs cooperate with each other and use CLNC to combine their received files, such that the UDs can recover their missing files using D2D communications. The proposed CLNC mechanism enables the cloud base-station (CBS) to select the coding decisions, adapt the rate allocation, and adjust the transmit power of the UDs to reduce the interference. Specifically, such a CLNC configuration brings a new trade-off among selecting the transmitting UDs and their coding decisions, and scheduling the transmission rate and power. To this end, we formulate an optimization problem to minimize the completion time required for recovering the missing files at the UDs. The proposed optimization problem is shown to be intractable because an optimal solution depends on the future coding decisions and UDs’ heterogeneous rates. To overcome such intractability, we propose an online solution that is updated at each transmission slot. In particular, we first design a graph called herein the D2D rate-aware instantly decodable NC (RA-IDNC), where its vertices have weights that judiciously balance between the rate of the transmitting UDs and the number of their scheduled UDs. Subsequently, we propose an innovative and efficient CLNC solution that iteratively selects a set of transmitting UDs and optimizes their powers using a function evaluation (FE) method. In each iteration, a new transmitting UD is selected provided that the resultant interference does not significantly degrade the completion time performance. Simulation results show that the proposed CLNC can significantly reduce the completion time compared to the benchmark schemes.