Synchronization: From 0 to 5G.

If you've seen old (and not so old) movies where a group of people makes a plan for a robbery, it should be familiar that they gather before starting to synchronize their watches. If, in addition, you’ve ever had a non-smartwatch, you should be aware that this synchronization is necessary for three main reasons: each watch is adjusted manually, probably using different references, and most importantly, each one introduces a different lag that accumulates over time, eventually leading to delays of several seconds or even minutes over the course of a month.

This is a problem of the past. Today, if I ask myself what time it is, I check my cell phone or smartwatch, and I have no doubt that it’s the exact time. This was achieved by addressing the three issues mentioned: watches no longer require manual adjustments, different references are no longer used, and the natural delays of each system are now constantly corrected.

However, despite these adjustments and the precision we demand as users when checking the time, there are applications that require even greater accuracy. What are these applications? How have the various synchronization problems been solved? And how does the arrival of 5G impact this?

Basic precision for end users.

The solution to the first problem (manual adjustments) is clear: the adjustment is made automatically through the network, eliminating human error when synchronizing. But where does the network get the time from? This is answered with the solution to the second and third problems: the standardization of protocols that use a reference clock to distribute the time through the packet network.

In the case of our devices, this standardization takes the form of NTP (Network Time Protocol). NTP uses primary servers to receive the reference time for synchronization. This reference can be obtained from a cesium clock, another NTP system, or a global navigation satellite system (GNSS); the most common is GPS, owned by the United States. Some of its counterparts are GLONASS (Russia) and Galileo (European Union). In this way, the issue of using a single reference is resolved, while the problem of delays in each device is addressed by regularly checking the time and adjusting the difference.

The primary NTP servers send the signal to secondary servers through the telecommunications network, which then distribute it to the clients (the devices). Each hop between servers and nodes to reach the client adds a small delay to the clock signal, caused by both the propagation time of the clock signal (delay) and the difference between clocks (offset). These delays cause NTP clients to receive the time with a precision that can differ by up to a few milliseconds.

This level of precision is more than enough to know if I'm on time for a meeting or if I need to take the food out of the oven, so the NTP solution is suitable for use in end devices. However, we cannot say the same for all applications.

Need for precision in telecommunications networks.

When we talk about telecommunications networks, there are functionalities that require much higher precision than milliseconds. Let’s explore an example: wireless communication using the TDD (Time Division Duplex) technique, present in some LTE systems and in 5G.

To understand it, let's first talk about its alternative: FDD (Frequency Division Duplex). The underlying requirement is that, in the data exchange between your device and the radio station of the internet provider (the base station), there needs to be a way to download data (from the base station to the device) and another way to upload data (in the reverse direction). Using FDD, a downlink channel is reserved at one frequency and an uplink channel at a different frequency, allowing both communications to occur simultaneously.

What TDD does to separate the uplink and downlink is divide time into small windows and assign some of them to the uplink and others to the downlink. This improves the efficiency of the frequency spectrum, as only a single channel is used, and it also allows for dynamic allocation of resources to uplink or downlink based on demand. Its main challenge lies in the accuracy with which the radio bases mark the beginning of those windows.

Cellular coverage in an area is achieved by deploying radio bases separated into smaller regions. Since the transmission powers of the radio bases are high to reach all devices in their region, it is expected that the transmission from one radio base will be received by a neighboring radio base.

What would happen if one of the radio bases had its clock misaligned, for example, during an entire TDD window? What one base interprets as downlink could be received by its neighbor as uplink, causing significant interference in the network.

How do we make the clocks more precise?

To remedy clock misalignments when higher precision is required, the PTP (Precision Time Protocol) is standardized and begins to be implemented for LTE systems. PTP allows the synchronization of clocks in distributed systems with high precision using a master-slave approach, where a master clock sends time signals to the slave clocks over the network. The slave clocks adjust their local clocks to keep them synchronized with the master clock.

The master, similar to the primary server of NTP, takes the reference signal to synchronize the time from a GNSS receiver. From this, the question may arise: Why use PTP to distribute time over the network if I can receive a GPS signal from any radio base? The question is not only valid, but it also leads to an existing synchronization solution in the market, called GNSS Everywhere.

Although it has the advantage of not losing precision since it doesn't have to propagate, its main disadvantage is the possibility of losing the GNSS satellite signal due to bad weather or limited line of sight between the base and the satellites. If a radio base loses the signal, it can start interfering with all adjacent bases, so it must be taken offline.

On the other hand, PTP clocks have a high-precision internal oscillator that allows them to maintain the clock signal for hours or even days after they stop receiving the GNSS signal, making them robust to circumstantial reception issues. Additionally, GNSS reception is centralized at specific points, which are then distributed across the network, eliminating the need for receivers at each node and thus reducing costs.

What changes with 5G?

In NTP, we talk about a precision of milliseconds. In 3G, the precision requirements set a maximum error of 10 microseconds (an order of magnitude 1,000 times smaller). In 4G or LTE, it's 5 microseconds.In 5G, the maximum error that radio bases can have to avoid interference is 1.5 microseconds from the clock signal to the transmission of the radio base, including all intermediate hops.

The conditions are more restrictive than ever due to the new technologies enabled by the 5G paradigm. The substantial reduction in the time it takes for each communication in the network (latency), the use of transmission techniques to each mobile user with high directivity (beamforming), and the increase in frequency resources to improve the bandwidth of a communication (carrier aggregation) are examples that raise the bar in terms of synchronization and translate into a better user experience and, in general, a great advancement in telecommunications.

The precision requirements are increasingly higher, making synchronization systems a fundamental part of the operation of networks, both telecommunications and other systems (including utilities). This is why it is necessary to turn to synchronization solutions that meet the requirements to ensure smooth and reliable connectivity. Additionally, the vulnerabilities generated by the dependency on GNSS signals must be addressed, but that is a topic for another article.

By: Pablo Bertrand, Support Engineer Analyst de Telcos.

Pablo is an Electrical Engineer with a specialization in telecommunications and a Master's student in Electrical Engineering at the University of the Republic. He has over 7 years of experience in the telecommunications field and works in pre-sales, implementation, and project support.