The proliferation of all types of mobile devices makes the traffic from wireless devices expected to exceed user traffic from wired access networks in the next few years. To meet this trend, 5G networks will be capable of delivering multigigabit throughput with extremely low latencies, even in highly dense locations. On the one hand, mobile users are looking forward to a much richer media experience as 5G quickly approaches. On the other hand, wireless researchers are facing unprecedented challenges to make this vision come true.

One of the most critical 5G research topics is how to improve the spectrum efficiency in radio access network (RAN). For example, heterogeneous cloud radio access network (H-CRAN) is a revolutionary architecture that takes full advantage of both cloud radio access networks (C-RANs) and heterogeneous networks (HetNets) to achieve high cooperative gains. H-CRAN allows seamless integration of multiple cutting-edge wireless technologies, including massive MIMO, small cells, mmWave, NOMA, etc. In this special issue, we have invited a few papers to address the emerging technologies for 5G radio access network.

The paper “Self-Organized Cell Outage Detection Architecture and Approach for 5G H-CRAN” introduces the general structure of H-CRAN and reveals that massive access nodes increase the risk of cell outages. To solve the problem, the authors suggest to employ the cell outage management (COM) function proposed in self-organized network (SON). The paper “Opportunistic NOMA-Based Massive MIMO Precoding for 5G New Radio” highlights 5G new radio (NR), which is being designed to significantly improve the performance, flexibility, scalability, and efficiency over the current 4G. In particular, the authors design a NOMA driven MIMO precoding scheme for hybrid unicast/multicast scenario. The paper “Robust Drones Formation Control in 5G Wireless Sensor Network Using mmWave” develops a network of unmanned aerial vehicles (UAVs) for the purpose of wireless sensing, and the specialty lies in the involvement of 5G mmWave communications. The paper “mmWave Measurement of RF Reflectors for 5G Green Communications” emphasizes the energy consumption and environment protection by means of passive RF reflection, where line of sight (LoS) blockage is a major reason to hinder the coverage. The paper “Rate-Adaptive Multiple Access for Uplink Grant-Free Transmission” simplifies the signaling procedure via uplink instant transmission in 5G networks and develops a promising multiple access protocol to address the massive connectivity and low latency requirements for future machine type communications. The paper “On the Secrecy Capacity of 5G New Radio Networks” proposes a ray tracing based 5G NR network channel model and investigates the secrecy capacity in mmWave band.

Acknowledgments

We would like to express our gratitude to all the authors for their generous submissions and all reviewers for their timely and professional reviews.

Bo Rong
Jiazhen Zhou
Michel Kadoch
Gui-Ling Sun