Understanding the Role of Satellite Synchronization in the 5G Network
With the rollout of 5G, synchronization in carrier networks is becoming even more critical. Mobile cellular networks need to be synchronized to avoid base stations with shared coverage interfering and disrupting each other, leading to declining network quality for customers. Throughout the Third Generation (3G) cellular communications, the NodeBs relied on satellite communications for synchronization, which is also the same case with the Baseband Units (BBUs) in 4G cellular communications. Until now, most mobile network operators (MNOs) have relied on the Global Positioning System (GPS) satellite constellation for synchronization, as satellite antennas have high speed and accurately timed pulses.
Updated Synchronization Requirements for the 5G Network
With the 5G network coming into the picture, the speed requirements will increase substantially, so radios will need to be placed closer to the users. More antennas will need to be installed so that signals can overlap and more interference can be detected. The latency in exchanging data and messages across the network decreases, which would pave the way for newer technologies such as autonomous vehicles. This is called the Ultra-Reliable Low Latency Communications (URLLC).
Lastly, the network needs to support an exponential number of devices, especially for the Internet of Things (IoT) to occur. Massive Machine Type Communications (mMTC) must be supported by the 5G network. The direct implication of this is that more sub-frequencies will need to be used, as many devices will communicate systematically and continuously. However, more sub-frequencies will increase the risk of signal interference, which is why the timing and synchronization techniques used in the cellular network will need to evolve to adapt to the new requirements of the 5G network.
The predecessors of the 5G network, the 3G and 4G networks, embedded satellite receivers in NodeBs and BBUs. These controllers take the time of day messages and propagate them over the air to UEs. To keep all cell towers’ frequency synchronized, they also take the accurately-timed pulse received every second (1PPS). Both the 3G and 4G networks need a line of sight to only one satellite to frequency synchronize.
With the 5G network, the satellites are used slightly differently. The time of day messages will still be received and sent over the air to UEs and the Distributed Units (DUs), the controllers used in 5G networks. To stay frequency synchronized, the DUs will also use the 1PPS received from the satellite. However, there is a second use to the time of day messages: to keep overlapping cells phase synchronized to avoid interference. A line of sight to multiple satellites will therefore be required to achieve this type of synchronization.
Time Synchronization is Crucial to Phase Synchronization
To phase synchronize overlapping cells, all equipment needs to have the same concept of a shared global time to phase align their transmissions. To fully accomplish this, the delay between the time when the satellite sends the time of day message and when that message arrives at the satellite receiver needs to be calculated and factored into the time synchronization process. Satellites are always moving in orbit. Therefore, their Ephemeris – which are the mathematical descriptions of orbits -needs to be calculated. Four other variables are required for this calculation – longitude, latitude, altitude, and time. After calculating the accurate position of the satellites, you can then compute the delay between the satellites and the satellite receiver to “correct” the time of day it was received.
Avoiding Synchronization Related Interference in 5G Networks
If existing satellites used for the 3G and 4G networks are shared with 5G networks, then it would only be reasonable to ensure that at least four satellites are in continuous view over 12 hours to accommodate different vendor equipment requirements.
Why 12 hours? The typical orbital time of a GPS satellite is a little over 11 hours and 57 minutes. If you can continuously see four (4) satellites for 12 hours, then you know you will be able to see that many all the time. This would guarantee that a satellite receiver embedded in any vendor equipment will be able to run a Satellite Survey however long is needed until its requirements are met.
The role of satellite synchronization plays an even more critical role in accommodating the increased requirements of the 5G network. With a need for four satellites, time and phase synchronization are two crucial factors to fulfill to ensure good network quality. If you’re keen to learn more about the role of satellite synchronization in the 5G network, please visit this VIAVI whitepaper.
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