Why Does Synchronization in 5G Matter?
Timing and synchronization is everything in 5G
For a mobile network operator, timing is everything. Not just determining when to upgrade the network to bring new services to market, but in the literal sense as well. If the radio clock loses synchronization accuracy in a radio access network (RAN), or the radios are out of synchronization, interference between cells is likely. The less accurate the clock source, the higher the probability for time shifts. Ultimately there will be performance challenges. This issue wasn’t a significant concern in legacy networks, but as we transition to 5G it becomes a really big deal.
To use available spectrum as efficiently as possible, 5G technology introduces a time division duplex (TDD) environment. Here both uplink (UL) and downlink (DL) use the same radio channel. Consider this: operators need large amounts of spectrum to deliver on the enhanced mobile broadband (eMBB) use case of 5G, amounts much greater than the 5 to 20MHz that is generally available for LTE networks. Further, most of the available wideband 5G spectrum is either in the C-Band or mmWave, which only supports TDD. This means that TDD is a key factor in enabling eMBB services.
Complexities in terms of synchronization
Because a lack of synchronization in UL/DL frames further exacerbates interference problems, industry standards introduce stringent restrictions on LTE and 5G new radio (NR) TDD transmission. While the absolute time synchronization margin in a frequency division duplex (FDD) LTE environment is in the magnitude of 10µs, in the TDD radio environment it is restricted to just 1.5µs.
In addition to the absolute time error margin, another consideration is management of over-the-air synchronization requirements for advanced radio features. These include MIMO, eCIC, COMP, and location-based services. In 5G, we are moving away from a synchronized fronthaul CPRI to a packet-based fronthaul. While this approach offers a number of advantages, packet-based fronthaul introduces complexities for synchronization. Providers need different approaches depending on the topology and configuration of their networks. In most cases, we expect to see precision timing protocol (PTP) for distributing time of day (ToD), and Synchronized Ethernet (SyncE) for distributing frequency. This means that radio units (RU) will be synchronized over Ethernet.
A test of time, synchronization requirements
Providers can implement various methods to meet these stringent phase and time synchronization requirements. The intent is to ensure synchronization of all nodes to the primary reference time clock (PRTC) source. However, the location of the source may vary depending on the network topology, cost, and application. By using a grand master clock synced to a satellite source and a combination of boundary clock and slave clocks, network nodes can be aligned to a common time and phase. For networks that cannot adhere to full timing support, such as networks that are not PTP aware, there are other options. For example, network operators can implement assisted partial timing support with appropriate consideration for the network topology and cost.
Lastly, it’s important to consider the use cases for frame and slot synchronization. 5G 3GPP standards defined 56 slot formats, each of which is a predefined pattern of downlink/flexible/uplink symbols during one slot. These formats allow flexibility in terms of the application supported on a 5G node B (gNB). Yet, this also creates a challenge if two networks offering different types of service are located next to each other. Interference can result even if they are synchronized in time, but their slot formats are not synchronized. Essentially, when operating a 5G or 4G LTE network in a TDD environment, we not only need frequency and phase synchronization, but also frame and slot synchronization. This avoids inter-network interference.
Synchronization is fundamental to the performance of a cellular network and the services it offers. Both 3G and 4G cellular technology required frequency synchronization, primarily to prevent interference when cells overlap. But with the introduction of 5G technology, we’ve reached a new level in terms of TDD phase and frame synchronization. Validation testing is essential to meet stricter synchronization requirements and to ensure quality of service.
Essential Resource: 5G Timing and Synchronization Handbook for TDD Deployment