How to network clock synchronization in wireless networks?

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This article is reprinted from the WeChat public account "Wireless Deep Sea", written by Fei Caicai. Please contact the Wireless Deep Sea public account for reprinting this article.

PART 1

Synchronization is a basic requirement

Clock synchronization is crucial for wireless networks. From 2G to 5G, different wireless access technologies have different requirements for the accuracy of frequency synchronization and phase synchronization. For details, please see my previous article: Why do wireless communication networks need synchronization?

The basic principle of synchronization is similar to that of table alignment.

Each base station has its own independent clock module: crystal oscillator, which is in a free oscillation state when there is no external clock source.

As you can imagine, the clocks of the various base stations in a free oscillation state are not synchronized, and each base station is just an isolated island that can only operate independently and cannot work together. Only by synchronizing such a "clock operation" through a reference clock can they keep pace with each other and thus form a close connection to form a network.

For the reference clock, it first needs to determine a reference source, and then the synchronization relationship between different nodes. In a communication system, generally speaking, nodes with lower accuracy obtain reference clock synchronization from nodes with higher accuracy.

PART 2

Clock grade and accuracy

ITU-T defines four types of clock accuracy, generally referred to as Class 1, Class 2, Class 3 and Class 4. Their accuracy requirements decrease as the level increases.

The first-level clock is the highest-level clock, so it is also called PRC (Primary Reference Clock). It requires very high accuracy, that is, the frequency accuracy under any circumstances is ±1x10^-11, that is, the frequency error is one hundred billionth.

The best primary clock is a reference clock composed of cesium atoms, which uses the electromagnetic waves radiated when the electrons inside the cesium atoms transition between two energy levels as a reference to control the accuracy of the clock. Each different atom has its own unique vibration frequency. The most common phenomenon is the yellow light emitted by sodium atoms when salt is sprinkled on a flame.

The principle of cesium atomic clock

The stability of cesium atomic clocks is very high, with a deviation of only one second in 5 million years.

However, its cost is too high, and only the state can afford to build such an expensive system. For communication systems, GNSS (Global Navigation Satellite System) represented by GPS plus rubidium clock can be used as the primary clock source to become LPR (Local Primary Reference).

The characteristics of secondary and tertiary clocks are that they need to select a synchronization link from the outside to obtain the clock signal, attenuate the jitter and offset, and then forward the clock to other devices. Therefore, they are called SSU (Synchronization Supply Unit), the secondary clock is also called SSU-A, and the tertiary clock is also called SSU-B.

Accuracy requirements for different clock levels

The accuracy of the secondary clock needs to reach 0.016ppm within a year, and the accuracy of the tertiary clock needs to reach 4.6ppm within a year.

The fourth-level clock is also called SEC (SDH Equipment Clock). As the lowest level clock, its accuracy is no more than 4.6ppm.

By comparing with the clock synchronization accuracy of wireless communication, we can find that the frequency synchronization accuracy requirement of all wireless communication standards is 0.05ppm. The third-level clock and the fourth-level clock cannot meet the requirement and generally need to reach the level of the second-level clock. In other words, the base station needs to be synchronized with the first-level clock or the second-level clock to work properly.

Synchronization requirements for different wireless access technologies (2/3/4G)

PART 3

Clock Synchronization Networking

With a clock source, you also need a network to distribute the clock. Depending on the network configuration, there are two types: centralized and distributed.

Representatives of centralized clock synchronization are technologies such as Synchronous Ethernet (SyncE) based on packet networks and 1588v2. The general networking is shown in the figure below.

Centralized Synchronous Distribution Network

The master clock PRC, as the highest node, transmits the clock layer by layer through a pyramid-like transmission network. The SSU in the figure is the secondary clock, which can be considered as the base station in wireless communication.

In this mode, Synchronous Ethernet (SyncE) can only achieve frequency synchronization, while 1588v2 can not only achieve frequency synchronization but also support higher-precision phase synchronization.

Synchronous Ethernet + 1588v2 Synchronous Distribution Network

However, to achieve phase synchronization in 1588v2, each transmission node needs to support the PTP (Precision Time Protocol) protocol, and the latency of the uplink and downlink links must be completely consistent, which is very difficult to implement in existing networks.

The representative technology of distributed clock synchronization is the United States' GPS, as well as China's Beidou, Russia's GLONASS, and Europe's Galileo and other GNSS systems.

Distributed Synchronous Distribution Network Based on GPS

As shown in the figure above, the GPS as the primary clock is not directly physically connected to the lower-level SSU, but broadcasts clock information through a wireless interface. All SSUs are synchronized directly with the master clock through GPS receivers.

Undoubtedly, wireless communication base stations also belong to SSUs. High-precision synchronization can be achieved by simply installing a GPS receiver on the base station. GPS synchronization is currently the most widely used synchronization method in the world.

Since 5G requires higher-precision phase synchronization, the accuracy of centralized network synchronization represented by 1588v2 is difficult to guarantee, so the importance of GNSS systems represented by GPS is further enhanced.

Well, that’s all for this issue. These are just my personal opinions, and I hope they will be helpful to you.