Multi-hop relaying in 5G: From research to systems, standards, and applications

In recent years, the demands of high traffic transmission motivate the rapid development of wireless access techniques, and it becomes promising to design the fifth generation (5G) wireless networks. Essential requirements for 5G involve higher traffic volume, indoor or hotspot traffic, and spectrum, energy, and cost efficiency. So compared with the previous four generations, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities and unprecedented numbers of antennas. For this next generation networks, it will meet the new and challenging requirements that today's wireless access networks could not meet. Especially, multi-hop relaying in 5G wireless networking is foreseen to be a revolution in the way of connections with or without the direct intervention of the infrastructure. In traditional centralized schemes of connectivity, users connect exclusively through their nearest Base Station, or at most from a fixed relay station. However, 5G networks will use multi-hop relaying connection, i.e., besides the above traditional centralized schemes, they will also apply the new paradigm of Device-to-Device (D2D) networking, in which users are allowed to communicate peer-to-peer through direct links that do not necessarily involve the intervention of a Base Station. This new relaying approach can improve spectrum efficiency, better fairness in resource allocation, energy efficient usage, and etc. Meanwhile, this novel approach in 5G will face the challenge of distributed management of communications, subject to mobility, in which end-user terminals will play a more active role in the self-configuration of the network.