Low-latency Radio-interface Perspectives for Small-cell 5G Networks

The exponential growth projections for mobile data traffic have initiated a significant research effort on 5G wireless access. The 5G solutions should provide at least a tenfold reduction in latency, a tenfold improvement in peak data rates, a hundredfold improvement in area capacity, and a thousandfold improvement in energy efficiency over 4G solutions. Several innovations and new concepts of network layers, protocol layers, and hardware are required to achieve these targets. In this chapter, we consider one option for a new centimeter-wave radio interface for 5G, which will support dense small-cell networks and ultra-low-latency communications. We start by describing the expected channel environments for below-6-GHz carriers and for 38-GHz and 60-GHz carriers. Then we briefly discuss the traffic expectations and ways of modeling the traffic. Based on these, we define requirements for a new frame design and discuss our new physical layer numerology and frame design entitled 5GETLA. We provide a detailed description of how sub-millisecond round-trip-times are achieved with our design and explain how reduced latency leads to improved energy and spectral efficiency. We also discuss the multiple-input multiple-output reference symbol layout and show that significant overhead savings can be achieved when compared to LTE-A. Finally, we extend the reference frame design to millimeter-wave communications and describe two designs: one for line-of-sight and one for non-line-of-sight communications. These designs provide ultra-low latency and ultra-dense spatial reuse wireless access with multi-gigabit data rates for end users. In the conclusion, we wrap-up the discussion related to the new low-latency radio interfaces and indicate the most important open topics for research in the area of low-latency 5G physical layer design for ultra-dense small-cell communications.