IIoT Features in Rel-16 and Rel-17

In Rel-16, further enhancements for IIoT are being standardized in two work items. The first work item addresses physical layer enhancements for URLLC and includes the following items:⁵

• Control channel enhancements - Control format with configurable size to improve reliability, increased control channel monitoring capabilities to reduce scheduling delay, transmission of more than one acknowledgment within a slot and support for two simultaneous HARQ codebooks.
• Data channel enhancements - Mini-slot level hopping.
• Scheduling enhancements - Support for out-of-order scheduling and HARQ feedback, multiple active grant-free configurations.
• Uplink transmission prioritization and multiplexing among users - Interference cancellation to support uplink pre-emption and enhanced power control.

The second work item addresses support for new time-sensitive network use cases such as factory automation and electrical power distribution. These use cases have tighter requirements than URLLC (e.g., reliability of up to 99.999999 percent) and may also require strict time synchronization for packet delivery (e.g., deterministic services with bounded latency and extremely low variation). The work item includes the following enhancements:⁶

• TSC - Specify accurate reference timing delivery, address support for TSC messages, hard guarantees for QoS characteristics such as packet loss and reliability, latency bounds and synchronization down to the nanosecond level.
• Support PDCP duplication with higher reliability and better efficiency.
• Address resource conflicts and collisions between multiple transmissions from the same user.

Figure 3 Comparison of NR-Light to other NR features.

In addition to the above work items, positioning is also an important aspect for IIoT deployment. In Rel-16, 3GPP is working to specify positioning support for NR.⁷ They include:

• Define interfaces, signaling and procedures.
• Extension of LTE Positioning Protocol (LPP) and NR Positioning Protocol “a” (NRPPa) for NR positioning.
• Specifying reference signals to support various positioning techniques.
• Define UE and gNB measurements and requirements applicable for positioning.
• Study the feasibility of RAN-based location management.

The positioning requirements for commercial use cases are:⁸

• Indoor deployment scenarios - Horizontal positioning error less than 3 m and vertical positioning error less than 3 m for 80 percent of UEs.
• Outdoor deployment scenarios - Horizontal positioning error less than 10 m and vertical positioning error less than 3 m for 80 percent of UEs.
• End to end latency less than 1 second.

Further, there is a need to address use cases that cannot be met by NR eMBB, URLLC, eMTC and NB-IoT. As such, a version of NR known as NR-Light will be introduced in the 3GPP Rel-17/18 time frame. With this new feature, operators can migrate their spectrum to NR which can support both URLLC and NR-Light on the same carrier as well as deploy eMTC/NB-IoT either in-band or in guard-band. The NR-Light comparison to other IoT related technologies with respect to data rates, latency and reliability are shown in Figure 3.

System Performance for a Typical Indoor Factory

In this section, we present simulation results for a typical indoor factory using ray tracing. It may be noted that, various real-time and ray tracing measurements in an industrial environment were done by multiple entities and used to derive IIoT channel model in 3GPP standards body.^9-10

We chose to study performance using an indoor factory scenario with a 400 MHz bandwidth mmWave carrier at a 28 GHz carrier frequency. The high carrier bandwidths available in mmWave spectrum is very attractive from a capacity point-of-view, and the indoor factory setting inherently provides good isolation against the external environment which allows the possibility of using both licensed and unlicensed high bandwidth mmWave spectrum.

In this study we created a fictitious factory of dimensions 120 m x 50 m x 10 m with 12 single sector gNBs mounted on the ceiling and pointed downward. This is very similar to the indoor office scenario in the 3GPP TR 38.901, “Study on Channel Model for Frequencies from 0.5 to 100 GHz,” with the ceiling height increased from 3 to 10 m to be more appropriate of a factory setting.¹¹ As for the propagation modeling, 3D ray tracing was utilized with the WinProp tool from Altair. Random rectangular metal objects were placed in the factory to be representative of production lines, heavy duty machinery, tool containers, etc. The walls and roof material were chosen to be concrete and metal support beams for the ceiling were added. This is depicted in Figure 4.

We chose to study the TSC use case as it is more demanding than looking at URLLC only. On top of the high-reliability with low latency (i.e., 99.999 percent reliability at less than 1 ms latency) as with URLLC only, TSC now also enforces that the delay between the transmitter and receiver is deterministic rather than variable. As such, a de-jitter buffer is added at the receiver to account for the variability in delay that occurs over the 5G NR air interface from things such as HARQ or variable scheduling delay that is seen with dynamic packet schedulers.

Figure 4 Fictitious factory floor which was ray traced for system simulations with 12 single gNBs placed on the ceiling in two rows with a spacing of 20 m between each gNB, horizontally and vertically.

The subcarrier spacing of 120 kHz used in the mmWave frequency band provides a short slot length of 0.125 ms. However, with a 1 ms latency constraint, this means only 1/0.125 = 8 slots are available for scheduling, and this must be split between uplink and downlink as mmWave spectrum is time-division duplex (TDD). For the symmetric traffic expected for the TSC use case, this means four slots are available per link direction, and this means only four analog beams could be configured to serve the entire gNB coverage area. This is typically an insufficient beamforming gain for mmWave spectrum, so instead it is advantageous to configure mini slots even for the 120 kHz subcarrier spacing. By choosing a 2-symbol mini-slot, which has a duration of just 17.84 μs, we now have 56 mini-slots in a 1 ms interval, leaving 28 mini-slots available per link direction when considering an equal uplink/downlink split when considering the symmetric nature of TSC traffic. Hence up to 28 analog beams can be configured in the system and sweeping over 28 beams in both uplink and downlink can be done in less than 1 ms.

The full set of system simulation assumptions are given in Table 2. The complementary cumulative distribution function of packet delay (CCDF) is given in Figure 5. In this case there were no reported packet failures. It is seen that the target of 99.999 percent reliability is achieved with less than 1 ms latency for both uplink and downlink. In Figure 6 we focus on just the downlink and increase the total number of active users in the factory. We look at 120, 300 and 600 active users, corresponding to an average of 10, 25 and 50 users per gNB, respectively. We see that the target 99.999 percent reliability level with less than 1 ms latency can be met even at these higher traffic loading, thanks to the wide bandwidth available in mmWave spectrum.

Figure 5 CCDF of packet delay for both uplink and downlink.

Figure 6 CCDF of packet delay in the downlink, with UE density of 10, 25 and 50 users per gNB corresponding to 120, 300 and 600 active users on the factory floor.

Conclusion

Industry 4.0 powered by 5G networks will offer significant revenue expansion through new digital service provider markets. 5G NR and its evolutions offer features like URLLC and its enhancements, TSC and its enhancements, network slicing and positioning, which are essential for IIoT deployments in both public and private networks. Simulation studies for a typical factory environment shows strict latency and reliability bounds can be met using 5G NR URLLC and TSC features.

Acknowledgments

The authors would like to thank Eugene Visotsky, Peter Merz and Andreas Maeder for their extensive help in preparing the paper.

References

1. A. Ghosh et. al., “5G Evolution: A View on 5G Cellular Technology Beyond 3GPP Release 15,” IEEE Access, September 2019.
2. 3GPP, “RP-190831; Key Directions for Release 17,” RAN#84, June 2019.
3. D. Greenfield, “Nokia Employs 5G in its Own Factory,” AutomationWorld, August 12, 2019, www.automationworld.com/article/industry-type/discrete-manufacturing/nokia-employs-5g-its-own-factory.
4. 3GPP TR 38.912, “Study on Scenarios and Requirements for Next Generation Access Technologies,” V14.3.0, June 2017.
5. 3GPP, “RP-191584; Revised WID: Physical Layer Enhancements for NR Ultra-Reliable and Low Latency Communication (URLLC),” RAN#84, June 2019.
6. 3GPP, “RP-191561; Revised WID: Support of NR Industrial Internet of Things (IoT),” RAN#84, June 2019.
7. 3GPP, “RP-191156; Revised WID: NR Positioning Support,” RAN#84, June 2019.
8. 3GPP TR 38.855, “Study on NR Positioning Support,” V16.0.0, March 2019.
9. 3GPP, “R1-1909706; List of Measurements Used to Derive IIoT Channel Model,” RAN1#98, August 2019.
10. 3GPP, “R1-1909807; Addition of Industrial IoT Model to 38.901,” RAN1#98, August 2019.
11. 3GPP TR 38.901, “Study on Channel Model for Frequencies from 0.5 to 100 GHz,” V14.3.0, December 2017.