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.

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Abstract-Ultra-reliable and low latency communication (URLLC) presents the most challenging use cases for fifth generation (5G) mobile networks. Traditionally the focus for mobile broadband has been to optimize the system throughput for high speed data traffic. However the optimization criteria for URLLC should focus on achieving small packets transmissions under strict targets such as 99.999% reliability within 1 ms. Power control is one candidate technology component for improving reliability and latency. In this work we investigate the power control for grant-free URLLC transmissions through extensive system level simulations in a urban outdoor scenario. We initially compare different settings for open loop power control (OLPC) with full and with fractional path loss compensation. Then we evaluate whether power boosting the retransmission can reduce the probability of packets delays under the 1 ms constraint. We also discuss the practical implication of applying power boosting. With full path loss compensation and boosting retransmissions, we show that a URLLC load such as 1200 small packets per second per cell can be achieved in the considered scenario.

Time-sensitive communications (TSC) in wireless networks is an emerging paradigm that gains research momentum as an enabler of the industrial Internet of Things. As compared to ultra-reliable low-latency communications (URLLC) in fifth-generation cellular networks, TSC has stricter requirements in terms of latency and reliability, and also demands absolute time-synchronization and on-time delivery of packets for deterministic and isochronous real-time applications. In this regard, a key question is how to schedule TSC traffic flows effectively. This paper presents a radio resource allocation strategy for deterministic downlink TSC flows leveraging traffic pattern knowledge. Taking physical layer control channel effects into account, a comparison of semi-persistent and dynamic packet scheduling methods is presented as well as necessary enhancements to link adaptation and interference coordination procedures. With the proposed methods, the network capacity in terms of number of supported TSC flows can be more than doubled compared to traditional dynamic scheduling methods as commonly assumed for URLLC applications.