The newly introduced ultra-reliable low latency communication service class in 5G New Radio depends on innovative low latency radio resource management solutions that can guarantee high reliability. Grant-free random access, where channel resources are accessed without undergoing assignment through a handshake process, is proposed in 5G New Radio as an important latency reducing solution. However, this comes at an increased likelihood of collisions resulting from uncontrolled channel access, when the same resources are preallocated to a group of users. Novel reliability enhancement techniques are therefore needed. This article provides an overview of grant-free random access in 5G New Radio focusing on the ultra-reliable low latency communication service class, and presents two reliability-enhancing solutions. The first proposes retransmissions over shared resources, whereas the second proposal incorporates grant-free transmission with non-orthogonal multiple access with overlapping transmissions being resolved through the use of advanced receivers. Both proposed solutions result in significant performance gains, in terms of reliability as well as resource efficiency. For example, the proposed non-orthogonal multiple access scheme can support a normalized load of more than 1.5 users/slot at packet loss rates of ∼ 10 −5 − a significant improvement over the maximum supported load with conventional grant-free schemes like slotted-ALOHA. Index TermsURLLC, Grant-free random access, 5G NR, NOMA.
In the context of 5'th Generation (5G) New Radio (NR), new transmission procedures are currently studied for supporting the challenging requirements of Ultra-Reliable Low-Latency Communication (URLLC) use cases. In particular, grant free (GF) transmissions have the potential of reducing the latency with respect to traditional grant-based (GB) approaches as adopted in Long Term Evolution (LTE) radio standard. However, in case a shared channel is assigned to multiple users for GF transmissions, the occurrence of collisions may jeopardize the GF potential. In this paper, we perform a system analysis in a large urban macro network of several transmission procedures for uplink GF transmission presented in recent literature. Specifically, we study K-Repetitions and Proactive schemes along with the conventional HARQ scheme referred to as Reactive. We evaluated their performance against the baseline GB transmission as a function of the load using extensive and detailed system level simulations. Our findings show that GF procedures are capable of providing significant lower latency than GB at the reliability level of 1 − 10 −5 , even at considerable network loads. In particular, the GF Reactive scheme is shown to achieve the latency target while supporting at least 400 packets per second per cell.
The continuous proliferation of applications requiring wireless connectivity will eventually result in latency and reliability requirements beyond what is achievable with current technologies. Such applications can for example include industrial control at the sensor-actuator level, intra-vehicle communication, fast closed loop control in intra-body networks and intra-avionics communication. In this article, we present the design of short range Wireless Isochronous Real Time (WIRT) in-X subnetworks aimed at life-critical applications with communication cycles shorter than 0.1 ms and outage probability below 10 −6. Such targets are clearly beyond what is supported by the 5th Generation (5G) radio technology, and position WIRT as a possible 6th Generation (6G) system. WIRT subnetworks are envisioned to be deployed for instance in industrial production modules, robots, or inside vehicles. We identify technology components as well as spectrum bands for WIRT subnetworks and present major design aspects including frame structure and transmission techniques. The performance evaluation considering a dense scenario with up to 2 devices per m 2 reveal that a multi-GHz spectrum may be required for ensuring high spatial service availability. The possibility of running WIRT as an ultra-wideband underlay system in the centimeter-wave spectrum region is also discussed. Aspects related to design of techniques for the control plane as well as enhancements to the presented design is the focus of our ongoing research.
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