Pass It on: Advanced Relaying Concepts and Challenges for Networks Beyond 4G

Relay nodes (RNs) will be a key feature of future wireless networks. RNs can extend coverage, increase network capacity, and provide more uniform quality-of-service (QoS) across the cell area in a cost-effective manner. Therefore, not surprisingly, relaying techniques have attracted a significant amount of attention from the wireless industry and standards. The Third-Generation Partnership Project (3GPP) release 10 has considered RNs that act as base stations (BSs), known as type-1 RNs, aiming only for coverage extension. However, RNs can be employed in different ways, and several challenges have to be addressed to attain the theoretical gains. This article presents an overview of the relaying concepts related to the 3GPP long-term evolution (LTE) road map, i.e., concepts related to type-1, type-2, and moving RNs. The implementation challenges are outlined, and a number of promising solutions for each RN type are discussed. More specifically, for type-1 RNs, this article focuses on the allocation of resources to the backhaul and access links. For type-2 RNs, the focus is on designing distributed hybrid automatic repeat request (HARQ) protocols that involve RNs. Moving RNs are presented as an efficient solution to the ever-growing demand for wireless broadband by vehicleborne users. Overall, the presented relaying concepts and solutions can significantly improve the user experience and can play an important role in the future.