Uplink Grant-Free Access Solutions for URLLC services in 5G New Radio

Nurul Huda Mahmood, Renato Abreu, Ronald Bohnke, Martin Schubert, Gilberto Berardinelli, Thomas H. Jacobsen
2019 2019 16th International Symposium on Wireless Communication Systems (ISWCS)  
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,
more » ... n 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 Terms URLLC, Grant-free random access, 5G NR, NOMA. I. INTRODUCTION Ultra-reliable low latency communication (URLLC) is a new service class introduced in Fifth-generation New Radio (5G NR) cellular standard [1]. The reliability and latency levels offered by URLLC improve those of earlier generations of cellular standards. Examples include, isochronous real-time communication for factory and process automation in Industry 4.0 scenarios, vehicular-to-anything (V2X) communication in Automotive sector and haptic communication for tactile Internet. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. 2 The key design challenge for URLLC is to ensure low latency and high reliability simultaneously. In the absence of a tight latency constraint, any desired level of reliability can be achieved by coding over larger blocklengths and introducing sufficient redundancy, including re-transmissions. The scheduling and the transmission delay are the two primary latency inducing components of a communication protocol at the lower layers that can be influenced by system design. The former is the time it takes from the point a packet arrives at the lower layer of a transmitter until it can access the channel, whereas the latter is the time it takes to successfully deliver the message. Other sources of latency include processing delays at the transmitter/receiver, propagation delay and queuing at higher layers. The minimum scheduling unit in Long Term Evolution (LTE) is, in general, limited to the transmission time interval (TTI) of one millisecond (ms). Whereas, 5G NR has introduced the concept of 'mini-slots' consisting of 1 − 13 orthogonal frequency division multiplexed (OFDM) symbols, along with support for a scalable numerology allowing the sub-carrier spacing (SCS) to be expanded up to 240 kHz. Collectively, this allows transmissions over shorter intervals. For example, a URLLC mini-slot of 2 OFDM symbols at 60 kHz SCS corresponds to a transmission time of only 35.7 micro seconds [1]. Access to the wireless channel is generally controlled by a grant based (GB) scheduling mechanism where users attempting to access the channel have to first obtain an access grant through a four-way handshake procedure. This ensures that the user has exclusive rights to the channel, thus avoiding any potential collisions, at the expense of large latency and signalling overhead [3] . Grant-free (GF) random access, where the grant acquisition by the user prior to transmission is skipped, is proposed as a solution to reduce the access latency [4]. With GF transmissions, a user with available traffic transmits the data (along with required control information) in the first transmission itself. GF transmissions can be preallocated over dedicated resources, or shared among multiple users through contention. The former is better suited for periodic traffic with a fixed pattern, whereas the latter is more resource utilization efficient and flexible, especially in case of sporadic traffic. GF transmissions over shared resources are subject to potential collision with other neighbouring users transmitting simultaneously, thus jeopardizing the transmission reliability. Techniques to improve the supported load with GF random access while ensuring high reliability and low latency is are therefore currently being discussed in academic research and in standardization bodies [5] . State of the art solutions include GF transmissions with K-repetition, where a pre-defined number of replicas are transmitted, and proactive repetition, where the transmission is proactively resent until an acknowledgment is received (also known as repetitions with early termination) [4]. This article discusses GF random access in the uplink as an enabler for URLLC in 5G NR. The main contribution is two-fold, namely: i) giving an overview of GF random access in 5G NR and discussing its shortcomings, and ii) presenting two advanced GF schemes that go beyond 5G NR. In particular, the first proposal presented in Section IV-A introduces a novel transmission scheme where dedicated resources are allocated for the initial transmission, whereas blind retransmissions occur over shared radio resources. The combination of non-orthogonal multiple access (NOMA) and GF access is considered next in Section IV-B. NOMA relaxes the paradigm of orthogonal transmissions by allowing different users to concurrently share the same physical resources in time, frequency, and space. More specifically, NOMA techniques are exploited at the transmitter end to improve the reliability and resource efficiency, while advanced receivers are used to resolve April 11, 2019 DRAFT Discussions on 5G-NR URLLC in Release-16 are grouped into three different study items. The first deals with Layer-1 enhancements, including potential control channel and processing timeline improvements. The second studies the potential benefits of uplink inter-UE transmission prioritization and multiplexing. GF transmission, in particular, is enabled in Release-15 by so-called "Configured Grant" operations [1]. A study item is focused on enhancing such operations, including methods for explicit hybrid automatic repeat request acknowledgement (HARQ-ACK), to ensure K-repetitions, and mini-slot repetitions. URLLC studies in Release-17 will mainly focus on use cases and end-to-end performance of different applications, such as i) audio-visual service production requiring tight synchronization and lowlatency, ii) communication services for critical medical applications including robotic aided surgery, and iii) support for unmanned aerial systems connectivity, identification, and tracking. The interested reader is referred to [6] for further details. III. THE BASICS OF GRANT-FREE RANDOM ACCESS The conventional GB scheduling procedure in LTE networks involves exchanging multiple messages between nodes to facilitate exclusive channel access. Due to the tight latency requirement and the associated signaling overhead, such GB schemes are not suitable for URLLC applications. GF schemes using semi-static configurations are an option to remove the signaling overhead caused by the request-followed by-grant procedure and to reduce the latency. April 11, 2019 DRAFT control is given by: P [dBm] = min{P max , P 0 + 10 log 10 (M ) + αP L + g(k)}, where P max is the maximum transmit power, P 0 is the target receive power per resource block, M is the number of resource blocks, α is the fractional path loss compensation factor, P L is the path loss and the function g(k) gives a power boosting step for the k th transmission. B. Performance Evaluation Methodology The simulation assumptions and parameters used for this study are in line with the guidelines for NR performance evaluations presented in [2] . A total of 21 macro cells are simulated with an inter-site distance of 500 meters, and uniformly distributed outdoor UEs. Linear minimum-mean square error with interference rejection combining (MMSE-IRC) and multi-user detection receiver is assumed in this case. A 10 MHz band within the 4 GHz carrier is considered. Open loop power control is used by the UE to compensate the coupling loss. The MCS is pre-configured as very conservative (QPSK with coding rate 1 / 8 ), which permits the UE April 11, 2019 DRAFT the best slots for decoding as in CRA does not make use of all available information at the receiver and fails to meet the strict reliability requirements of URLLC, even for small system loads. On the other hand, Chase combining repeated packets yields a significant performance improvement. For the considered example, the target packet loss rate of 10 −5 is just missed with d = 4 repetitions. The additional coding gain of the third method based on low-rate channel coding enables URLLC with GF access even for highly overloaded systems with two packets/slot, which means that on average eight users are transmitting simultaneously in each slot for d = 4. Note that the curves exhibit a threshold behavior, which was also observed for the fundamental limits in [12] : the packet loss rate remains almost constant up to a certain load and then sharply increases, which means that the interference can be completely removed with high probability up to this point. Increasing d further reduces the error floor, but also the load threshold. The results clearly indicate the benefits of sparse NOMA compared to simple CRA schemes with slot-wise decoding, albeit at the expense of increased processing complexity at the receiver. Further gains are possible, e.g., by using more advanced joint multi-user detection algorithms and multiple receive antennas to separate the superimposed signals. Note that user activity detection and channel estimation are required before the data detection, but this may also be done jointly [13] . For block-fading channels, there is further a tradeoff between the diversity achieved by using multiple slots and the required channel estimation overhead [14] . These practical issues are facilitated by the sparse resource allocation. April 11, 2019 DRAFT 10 Fig. 5. Comparison of grant-free access using d slots for each packet and SIC with selection combining (dotted), Chase combining (dashed), and low-rate channel coding (solid). C. Key Take-away Message The two schemes are motivated by the fact that different URLLC use cases can have very different traffic profiles. The first scheme discussed in Section IV-A is particularly well suited for periodic URLLC messages, as in Industry 4.0 scenarios [15] . Orthogonal resources can be allocated in a semi-persistent manner for the first transmission, followed by retransmission over shared resources to ensure very high reliability. On the other hand, the GF-NOMA scheme with advanced receivers proposed in Section IV-B rather targets random packet arrivals, e.g. emergency messages in V2X scenarios. The key take-away message in both cases is that packet collisions over shared resources are not bad per se. Resolving them with advanced receiver design instead of treating the resulting interference as noise leads to higher reliability, lower latency and improved resource efficiency.
doi:10.1109/iswcs.2019.8877253 dblp:conf/iswcs/MahmoodABSBJ19 fatcat:3ai5uh4smzeeljrykimtmmtmzy