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The significance of network synchronization in CBRS

Citizens Broadband Radio Service (CBRS) is a shared spectrum band that is opening up new possibilities for wireless networks. However, there is one crucial technical aspect to consider when using CBRS: network synchronization. In this blog, we will discuss different ways to mitigate interference and maximize performance.

Customer Solutions Sales Manager

The significance of network synchronization in CBRS

Customer Solutions Sales Manager

Customer Solutions Sales Manager

It has never been easier to own and operate your own cellular wireless network. It's all possible due to CBRS. Since Q2 2021, the number of Citizens Broadband Radio Service Devices (CBSDs) deployed has been growing at a robust 12 percent quarter-over-quarter (Source: NTIA May 2023). You can get CBRS cellular service from traditional communications services providers, and it’s even possible to set up your own cellular wireless network if desired.

CBRS employs time division duplexing (TDD), meaning the same frequency is used for both uplink (UL) and downlink (DL) transmissions. This necessitates precise synchronization of all CBRS networks in a given area to ensure that UL and DL transmissions don’t interfere with each other.

The 3GPP standard specifies that the absolute time error requirement for TDD cell phase sync in CBRS is 3us, (microseconds), between antennas or 1.5us from a common primary reference . This means that all CBRS network transmitters and receivers must be synchronized to within 3 µs of each other. There are various methods to achieve this level of synchronization. One common approach is to use GPS receivers to provide a precise time reference. However, GPS signals can be blocked or unreliable in some areas, so it’s important to have a backup synchronization method in place.

Another approach is to use a wireline network supporting the IEEE 1588v2 Precision Time Protocol (PTP). No matter which approach is chosen, it’s essential to ensure that all CBRS networks in a given area are synchronized to within 3µs of each other. This is critical for the reliable operation of CBRS networks and to prevent interference between different networks. Here are some benefits of network synchronization in CBRS:

  • Improved performance: Synchronized networks operate more efficiently and deliver better performance to users.
  • Reduced interference: Synchronized networks are less likely to interfere with each other, enhancing the overall capacity of the CBRS band.
  • Increased reliability: Reliable synchronization is crucial to avoid outages.
  • Enhanced security: PTP Synchronized networks can be more secure, as they are less vulnerable to attacks exploiting GPS timing vulnerabilities.

Use of shared spectrum requires network-wide synchronization

CBRS uses a novel scheme to flexibly and safely share spectrum. As our blog, titled Is CBRS for Everybody? – Growing Pains and Progress Towards a Practical Solution explains, the spectrum allocation service (SAS) proactively manages shared spectrum so that priority and interference is controlled. That said, cellular networks that use CBRS depend on all participants to follow the TDD standard. This brings us to the importance of very good network timing and synchronization, of which more details can be found in our blog, titled Finding the Right Synchronization Solution for 5G Transport: Hard or Easy. It is important to note that 4G LTE TDD needs the same level of network synchronization as for 5G TDD.

The shared spectrum implies that every site must be time-synchronized to prevent interference with neighboring sites. 3GPP standardizes these requirements in TS 38.410 requiring that all networks and clocks be precisely aligned in time. (It is also important that all cells use same TDD configuration/pattern to avoid interference).

Figure 1: Example of interference situation due to misalignment

Figure 1: Example of interference situation due to misalignment

Technical note: Cell 1 is out of time alignment and interferes when Cell 2 listen for UL transmissions from devices. Likewise Cell 2 DL transmissions will interfere with Cell1 UL. SS denotes the Special Subframe that is inserted between the DL and UL transmissions.

In this way, inter-network interference is minimized, as all radios are time and phase synchronized. Non-isolated networks, as in the case of CBRS, allow anyone to set up their network as long as it complies with FCC rules.

To achieve proper synchronization, the traditional method involves adding a GPS receiver and associated antenna per cell site location. Another viable approach is to use terrestrial-based network timing and synchronization distribution. This can be accomplished with the ITU-T G.8275.1 profile in the network nodes between the network clock and the CBRS node. Ericsson’s CBRS network solutions offer support for GPS and IEEE 1588 PTPv2 with ITU-T G8275.1 through Router 6000 or MINI-LINK 6600 right out of the box.

Summary:

CBRS will introduce exciting new uses cases for various sectors, including enterprise and industry applications, by enabling spectrum sharing among networks. Achieving precise synchronization, especially crucial for TDD spectrum usage, can be accomplished using GPS or PTPv2 1588 ITU profile G.8275.1 as the primary reference. The Ericsson Router 6000 series and EP5G offer robust support for both of these, ensuring reliable network synchronization.

If you’re planning to deploy a CBRS network, it’s vital to acknowledge the challenges of network synchronization. By ensuring that your network is synchronized to within 3 µs of other CBRS networks in the area, you can help ensure that your network operates reliably and efficiently without interfering with other networks.

 

For more details about the importance of 5G network timing and synchronization requirements and solutions, read our Ericsson Technology Review article.

More information about CBRS.

More information about Ericsson Router.

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