Time-critical IoT Communication with 5G NR: A Technical Overview

 

5G New Radio (NR) is equipped to fulfill time-critical communication needs, resulting in several emerging use cases in the areas of real-time media, mobility automation, and remote control, as well as, industrial automation IoT. 5G is known to serve a wide range of use-cases and devices that go well beyond smartphones and mobile broadband.

New real-time applications are enabled that require guarantees of low latency with high reliability, i.e., ultra-reliable low latency communication (URLLC) [1]. In addition, 5G provides the functions and interfaces to become integrated with advanced industrial communication standards, such as IEEE Time Sensitive Networking (TSN). In the following, we explain the features introduced to the 5G NR standard that keep its latency within guaranteed bounds, hence enabling Industrial IoT use cases.

Time-critical IoT Communication Use-cases and Requirements

Many industries can benefit from integrating devices with time-critical requirements wirelessly into their IoT systems. Several examples can be envisaged [2], as shown in Figure 1:

Figure 1: Time-critical communication use-cases [2].

• Mobile AR devices with off-loaded rendering and processing from the Augmented Reality (AR) device itself to the edge cloud. A 5G connection with bounded latency makes this possible.
• Similarly, real-time media or online gaming devices wirelessly connected via 5G can be enhanced with additional information and processing in the edge cloud and provide richer interactivity with other users.
• Vehicles and machines can be remote-controlled, based on video or AR overlay, with a robust wireless connection. The vehicles may even be integrated into a mobility automation system controlled by the edge cloud itself.
• In smart manufacturing environments, simplicity and flexibility for any reconfiguration of the factory are dramatically increased by replacing cables with dependable wireless connections. Sensors and machines, like robots, may then be operated by a centralized controller.

These use cases all depend on a fast, dynamic response to changes in the environment, and therefore rely on the interaction between the wirelessly connected entities with short round-trip times, which for the use cases above, lie in the tens of millisecond to the one-millisecond range. It’s important to reach these latencies with a very high probability (reliability) for the systems to operate properly. These reliabilities, expressed in success rates of packets reaching the receiver within a latency bound, are in the order of 99 to 99.999 percent.  

Bounded-latency Scheduling with 5G NR

In wireless communication, the reliability bottleneck was traditionally the radio interface due to limited available transmission bandwidth, the signal-to-noise ratio at the receiver, and the time to transmit. This is, in particular, the case for the real-time use-cases requiring a strict latency bound. In cellular systems, spectrum resources are scarce and are shared among all connected devices in the cell. Hence, to improve reliability, the cellular system has the optimization target to organize itself so that the right amount of spectrum is used by the right device at the right time. Quality of service (QoS) and bounded low latency are achieved by centralized admission control and scheduling of the wireless frequency resources, which are typically licensed frequency bands assigned to a network operator. The scheduler can choose from a variety of features to achieve QoS in terms of latency and reliability for the user.

For a certain segment of the spectrum, the NR base station (gNB) scheduler can choose the spectral characteristics of a signal, which also includes the duration of a schedulable transmission slot, i.e., the time granularity. For a typical NR mid-band spectrum around 3.5 GHz, the slot duration is 0.5 ms; and for mmWave spectrum, it is even shorter, 0.125ms. Furthermore, processing times also need to be accounted for. In NR, the encoding and decoding of the transmissions can be as fast as a fraction of the actual slot duration. Another latency component is the alignment delay, i.e., the time from when data is provided to the 5G network until the next transmission slot starts. The NR standard also allows sub-dividing a slot further into sub-slots. With seven sub-slots, the duration would be shortened from 0.5ms to ~0.071ms for mid-band, or from 0.125 ms to ~0.02 ms for mmWave [3]. Latency-critical application data would, when using these techniques, wait for less until the next transmission opportunity and, as a result, the data is transmitted faster over the air. In addition, the round-trip time until retransmission occurs scales down – if it’s the case that the initial transmission did not succeed.

For extra robust transmissions, NR specifies modes for increased reliability, for both data and control radio channels. Reliability is further improved by various techniques, such as multi-antenna transmission, the use of multiple carriers, and packet duplication over independent radio links. NR also provides full mobility support, which is an important reliability aspect, not only for devices that are moving but also for stationary devices in a changing environment.

As mentioned, since the NR over-the-air transmissions in both UL and DL are centrally scheduled by the gNB, it can ensure radio resource efficiency, fairness of resource usage, and differentiated QoS treatment among applications and users. While in dynamic DL scheduling, transmission can be initiated immediately when DL data becomes available in the gNB, for dynamic UL scheduling, it is more complicated. If UL data arrives but no UL resources are yet assigned, the user equipment (UE) indicates the need for UL resources to the gNB via a scheduling request (SR) message and is subsequently assigned the needed resources for transmission. To avoid the latency introduced in the scheduling request loop, UL radio resources can also be pre-scheduled. In particular for periodical traffic patterns, as one would find in the critical communication use cases mentioned above, the pre-scheduling can rely on the UL configured grant (CG) feature. With this feature, periodically recurring UL resources can be preassigned for a device. Many of these configurations are supported in parallel, to serve multiple parallel UL traffic flows on the same device. An example is an industrial robot with multiple servo engines, sensors, and a camera connected via the same 5G IoT device to the system. In this case, besides time-critical data, other non-critical data (for example video or updates) needs to be transmitted too, from time to time.

Industrial Automation IoT Support

5G includes features to support Industrial IoT use cases, for which requirements have been collected by 5G-ACIA in [4]. For example, configured as a non-public network (NPN) deployment 5G can provide network services to a defined organization and its premises, such as a factory deployment. By this isolation, quality of service requirements, as well as security requirements can be achieved. Integration with a public network, if required, is however possible. Furthermore, 5G supports the integration with TSN. The main objective of TSN is to provide guaranteed data delivery within a guaranteed time window, i.e. bounded low latency.  IEEE 802.1 TSN [5] is a set of open standards that provide features to enable deterministic communication on standard IEEE 802.3 Ethernet. TSN standards can be seen as a toolbox for traffic shaping, resource management, time synchronization, and reliability – for which 5G supports the necessary supporting mechanisms. The basic idea of TSN integration is that the 5G system acts as a virtual bridge and transports the TSN Ethernet frames, adapts itself to the network settings of the TSN network, and implements required interfaces (e.g. application function (AF)) towards the TSN controller functions such as the centralized network configuration (CNC). Device-side and network-side TSN translator functions (DS-TT and NW-TT) are defined to convert the traffic and its requirements between TSN and 5G protocols. An illustration of this integration can be seen in Figure 2.

Figure 2: 5G TSN integration.

One interesting 5G feature is hereby the support for time synchronization, which is an essential part of TSN where a common absolute time reference is shared by all TSN network entities. NR supports accurate reference time synchronization in 1us accuracy level. Since NR is a scheduled system, an NR UE and a gNB are tightly synchronized to their OFDM symbol structures. A 5G internal reference time can be provided to the UE via broadcast or unicast signaling, associating a known orthogonal frequency division multiplex (OFDM) symbol to this reference clock. The 5G internal reference time can be shared within the 5G network, i.e., radio and core network components. This enables the support of TSN-based time synchronization with IEEE 802.1AS generalized precision time protocol (gPTP) [6], which relies upon that the 5G system at UE and network side are internally synchronized.  

Concluding Remarks

5G NR is equipped to reach the challenging requirements of time-critical IoT communication use cases from the areas of real-time media and AR applications, remote driving, and advanced industrial control systems.

References

1. 3^rd Generation Partnership Project, 3GPP TS 38.300 v16.5.0, NR and NG-RAN overall description, March 2021, [link]
2. F. Alriksson, L. Boström, J. Sachs, Y.-P. E. Wang, A. Zaidi, Critical IoT connectivity: Ideal for time-critical communications, Ericsson technology review, June 2020, [link]
3. T. Dudda, A. Shapin, A technical overview of time-critical communication with 5G NR, Ericsson blog, February 2021, [link]
4. 5G Alliance for Connected Industries and Automation, 5G for Connected Industries and Automation (Second Edition), Whitepaper, March 2019, [link]
5. 5G Alliance for Connected Industries and Automation, Integration of 5G with Time-Sensitive Networking for Industrial Communications, Whitepaper, April 2021, [link]
6. I. Godor et al., "A Look Inside 5G Standards to Support Time Synchronization for Smart Manufacturing," in IEEE Communications Standards Magazine, vol. 4, no. 3, pp. 14-21, September 2020.

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Torsten Dudda is a Master Researcher at Ericsson located in Aachen, Germany. He contributes to the radio architecture and protocol design of 5G NR, working closely with 3GPP standardization teams. His current research focuses on evolving the 5G NR support for critical communication use-cases such as Industrial IoT. Torsten Dudda joined Ericsson in 2012. He graduated with a diploma in electrical engineering and information technology from RWTH-Aachen University, Germany.