Towards Ultra-Reliable Communications: A D2D-based Protocol with Leader Selection
- Khosravirad S.
- Liu L.
- Yu W.
As one indispensable use case for the 5G wireless systems on the roadmap, ultra-reliable and low latency communications (URLLC) is a crucial requirement for the critical applications such as industrial automation. The key performance indicators for URLLC are in sharp contrast to the current broadband communications, since latency and reliability are paramount but lower data rates can be tolerated. This paper thus aims at an early attempt to make such a paradigm shift from the conventional broadband communications to the emerging URLLC. Motivated by the geographic feature in industrial automation that in the factory different tasks are in general assigned to different groups of users who work in close proximity to each other and thus form clusters of reliable device-to-device (D2D) networks, this paper proposes a novel two-phase transmission protocol for achieving URLLC. Specifically, in the first phase within the latency requirement, the base station (BS) combines the messages of each group together and multicasts them to the corresponding groups, while in the second phase, the devices that decode the messages successfully, who are defined as the leaders, help relay the messages to the other devices in their groups. Due to the proximity and thus reliable channels between users in the same group, this paper points out that the number of groups with at least one leader, rather than that of total leaders in the whole system, should be maximized in the first phase via a proper multi-group multicast beamforming design at the BS. We formulate the leader-selection based beamforming problem from a sparse optimization perspective, and achieve the desired sparsity pattern, i.e., at least one leader in one group, by introducing a geometric-mean based penalty. Simulation results are provided to show that the proposed beamforming design leads to a fair leader assignment among all the groups and thus a better utilization of all the available D2D networks in the second phase. As a result, our proposed two-phase transmission protocol considerably improves the reliability of the whole system within the stringent latency requirement as compared to other existing schemes for URCCL such as Occupy Cow.