Paving the way for a wireless time sensitive networking future

It is no secret that the introduction of 5G has led to an explosion of new applications and use cases, many of which require some form of coordinated or simultaneous action. Consider, for example, an industrial robot that aims to move its two arms in a coordinated manner. To achieve and control this parallel execution of tasks, clock synchronization across the robot’s simultaneous actions is needed. Other use cases where clock synchronization over 5G is important can be found in the domains of factory automation, smart grids, telesurgery and high data rate video streaming.

Enabling synchronized wireless TSN through 5G integration

Traditionally, these TSN use cases have been supported by a wired ethernet-based technology for converged networks of Industry 4.0. Realizing the vision of wireless TSN, however, requires a new approach and the interworking of 5G with TSN is now being pursued where 5G wireless technology is expected to provide the equivalent performance of the wired TSN.

Among the verticals listed above, factory automation and smart grids have the most demanding synchronicity requirements, as shown in table 1 below. For example, in a factory deployment of mobile robots, a task often requires precise coordination between multiple robot units. To ensure the correct sequence and timing of events, each unit must act according to a synchronized clock. Another example is the smart electrical grid where a synchronized clock is fundamental for not only accurate state monitoring but coordinated adjustments of distributed power sub-stations and other units in the electrical grid as well.

Table 1 below shows that the 5G system (5GS) clock synchronicity accuracy needs to reach at least 900 nanoseconds (ns) to meet the requirements of a wireless TSN system.

To fulfill the requirement of wireless TSN, it is important to ensure accurate clock synchronization over the wireless part of the 5G network. This means that 5GS acts as a virtual, transparent TSN bridge between a master clock and end-stations, allowing smooth integration of the 5GS into TSN. This ensures stringent clock synchronization among independent, distributed devices in the network such as sensors, actuators and controllers distributed in both wireless, as well as the wired part, of the connected networks.

Clock synchronization in 5G: a practical example

In reality, time synchronization is an inherent part of the wireless communication system. Let’s explore an example where time synchronization is foundational to the wireless communication:

When user equipment (UE) moves to the coverage area of a base station, the first step of the UE is to listen to the periodical broadcast signal from the base station to ensure the UE can synchronize to the base station's frame timing. Once done, and should the UE wish to communicate with the base station, preparation work is done between the UE and the base station to achieve time alignment between downlink and uplink frames, in the sense that the UE would advance the transmission time of the uplink frame relative to the arrival time of downlink frame. The amount of advancement depends on the distance between the base station and an individual UE, to ensure that uplink frames from all UEs arrive at the same time at the base station and the uplink and downlink frame are time-aligned at the base station.

Time synchronization mechanisms like the above are defined to support data communications where only relative timing requirements are necessary (for example, align downlink and uplink frames). In TSN delivery of absolute timing (i.e., the time of day is important), this is a new component for 5G. Figure 1 below illustrates the clock distribution from a 5G base station to a UE integrated in a smart grid.

Figure 1. Smart grid use case where the sync master resides at the base station.

The timing accuracy to support data communication only needs to be adequate to support the time resolution of the 5G frame structure with some accuracy. Each frame comprises a number of OFDM symbols which typically carry data. The accuracy requirement is typically a function of the OFDM symbol duration which, in many cases, is more relaxed than the aforementioned TSN clock synchronicity requirement of 900 ns.

The TSN synchronicity requirement (for example, 900 ns) of verticals is the total timing error budget permitted across the full 5G system. Thus, all error sources in the 5G system need to be accounted for, including network interfaces, air interface and device internal timing error. In contrast, for data communication, the synchronization requirement focuses on the RAN air interface only.

Clock synchronization and propagation delay: key considerations

To support the delivery of absolute time, 3GPP in Release 16 (Rel-16) defines new RRC messages that support the delivery of GPS time from the 5G network to the devices it serves. After receiving this information, the UE sets its internal clock to match the received GPS time. Afterwards, the UE utilizes its internal oscillator to progress its internal clock, very much like the minute or second hand of a wristwatch. The UE's internal oscillator may run slightly slower or faster, causing the UE clock to drift over time. To compensate for this, the RRC message may be periodically transmitted with updated information to allow the UE to reset and correct its clock.

In a wireless system, the over-the-air propagation delay between a base station and UE creates a time error in the UE. Let’s consider a base station that transmits its RRC time message to a UE at 12 AM sharp. A naive UE receiving the message 1 ms later will believe that the clock is still at 12 AM, missing that it should compensate for the 1 ms of propagation delay by setting the time to (12 AM + 1 ms).

In a dense deployment with small cell radius, the propagation delay between the base station and a UE is small. Thus, it may be adequate for the UE to receive the broadcast reference time and use it without compensation as illustrated in Figure 2. For example, for a small cell deployment with an inter-site distance smaller than 200 meters, the propagation time from the base station to the UE is no more than 333 ns. Therefore, it is acceptable for the UE to directly use the reference time received from the base station when the clock synchronization messages traverse the air interface.

Figure 2. UE use reference time from base station without compensation.

While propagation delay is ignored in Figure 2, the over-the-air propagation delay between the base station and the UE becomes a prominent timing error component for other cases like deployments with a more sparse grid of base station sites.

Estimating and compensating for propagation delays: from 3GPP Rel-16 to Rel-17

Device internal timing error and timing error between the network interfaces are essentially related to equipment implementation errors. Instead, for RAN standardization work, a focus was placed on achieving the tight timing error budget over the standardized air interface.

Rel-16 studied the achievable time synchronization accuracy if a UE takes advantage of the received timing advance (TA) values to compensate for the propagation delay. The TA is used to advance the transmission time of the uplink frame relative to the arrival time of downlink frame at the UE. This ensures the uplink and downlink frame are time-aligned at the base station.

The granularity of the TA is, however, designed to cater for data communication and not TSN. As a result, it cannot satisfy the most demanding use case of control-to-control communication in factory automation. Furthermore, in Rel-16, propagation delay estimation and compensation are left up to UE implementation where nothing was explicitly specified. This presented two interrelated issues where the network:

• Has no way of knowing whether the UE performs the compensation or not.
• Cannot rely on the UE to achieve any performance target.

To address these issues, Rel-17 explicitly standardized methods to estimate and compensate for the propagation delay. As a result, the propagation delay over the air interface is compensated for in a predictable manner, regardless of the distance between the base station and the UE.

The round-trip-time (RTT) method for precise clock synchronization

Determining the amount of time for a message to traverse from the base station to the device can be done by combining one measurement performed by the UE with one performed by the gNB. Known as the round-trip-time (RTT)-based method, the work has its roots in UE positioning and was led by Ericsson.

The measurements used by the RTT-based method are UE Rx – Tx time difference measured by the UE, and gNB Rx – Tx time difference measured by the gNB. The propagation delay is calculated by: (UE Rx – Tx time difference + gNB Rx – Tx time difference)/2. Specifically, UE Rx – Tx time difference is a timing measurement by UE using downlink reference signals (TRS or PRS); gNB Rx – Tx time difference is a timing measurement made by base station using the uplink reference signals (SRS).

Figure 3. The round-trip-time (RTT)-based method.

It can be observed that two separate entities (UE and base station) perform the measurements for the RTT method, and the calculation uses both. Thus, one of the measurements must be sent to the side that performs the calculation and propagation delay compensation. Since the bandwidth of the reference signals measured for RTT are configurable, the achievable timing accuracy at the receiver is adjustable, where the timing detection accuracy is in reverse proportion to the reference signal bandwidth. If higher synchronization accuracy is required, the base station can configure larger bandwidth for the reference signal accordingly. Analyses show that the RTT-based method can achieve timing accuracy of ±255 ns and ±164 ns for subcarrier spacing of 15 kHz and 30 kHz respectively. Even better accuracy is achieved as the subcarrier spacing increases.

This means that the RTT-based method can satisfy the clock synchronicity requirement of the most demanding case, while the TA-based method of 15 kHz and 30 kHz can only satisfy the requirement of less demanding cases such as the smart grid use case illustrated in Figure 1. The achievable timing synchronization accuracy at the air interface is summarized in Figure 4 below.

Figure 4. Achievable timing synchronization accuracy at the air interface, as compared to the requirement of the smart grid use case and the control-to-control communication for factory automation.

The RTT-based method in action: an example

Using the RTT-based method as example, for base station pre-compensation, UE reports its measurement of UE Rx – Tx time difference to the base station. The base station estimates the propagation delay tpd, and compensates for it before sending the reference time to the UE, which can then be used directly by the UE. This is illustrated in Figure 5 (a).

For UE-based propagation delay compensation, the base station sends its measurement of gNB Rx – Tx time difference to the UE. The UE calculates how long it takes the message to go from base station to the UE (i.e., the propagation delay tpd) and adds the propagation delay to the reference time. This is illustrated in Figure 5(b).

Figure 5. Propagation delay compensation by the base station or by the UE.

As shown above, numerous options are defined in Rel-17 which can achieve a range of time synchronization accuracies and have different signaling and resource costs. The base station can configure the most efficient method for the UE to fulfill the clock synchronicity requirement of the desired use case.

In summary, the new standardized features in 5G are paving the way for high-accuracy clock synchronization for the interworking of both wired and wireless systems for time sensitive communications. This allows 5G systems to satisfy the demanding requirements of cyber-physical systems that are, and continue to become increasingly important for the needs of modern society.

Learn more

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Hear why the experts foresee the interworking of 5G and TSN will enable holistic communications for industrial automation.

Download the Ericsson Technology Review where we discuss 5G-TSN integration for industrial automation.

Read more about 5G synchronization requirements and solutions in this edition of the Ericsson Technology Review.