Cambium CBRS Service Bulletin – Coexistence Implementation Update

As was previously announced in December 2020, Cambium officially released both PMP 450 Software (R20.0.1) and the December re-release of cnMaestro 2.4.2 (Cloud and On-Premises) for the entry of new CBRS Coexistence Parameters. These new parameters are critical to ALL CBRS GAA channel users to begin to populate ASAP, thereby providing SAS Admins (via CBSD heartbeats) crucial AP-to-SM association data. This also provides each operator’s Frequency Reuse plan needed for the beginning of more intelligent SAS GAA channel management functions when CBRS operators are serving the same market areas (i.e., helping to proactively avoid interference amongst all CBSDs). A detailed training video was created and posted to Cambium’s YouTube channel in support of this crucial feature introduction:

As part of our first Coex data integrity audit jointly conducted with the SAS Admins (initially Google), we can see that close to half of all CBSDs have provided these initial Coex parameters. We want to thank all those who have already started this process, and STRONGLY encourage those who have not to please begin the process (to everyone’s benefit). Please feel free to contact Cambium via CBRS community post or contact Cambium Support if you have any confusion or questions with how this process is done.

The two new parameters configured as part of the coexistence feature are the Sector ID and the Spectrum Reuse ID. The Sector ID indicates to the SAS which devices must operate on the same channel; the Spectrum Reuse ID indicates to the SAS which devices can manage their own interference and may operate on the same channel.
Currently, parameters are sent to the SAS only if the device is connected to the Google SAS. The other two SAS vendors supported by Cambium devices (Federated Wireless and CommScope) will support this feature in the upcoming months. cnMaestro stores the configured coexistence parameters regardless of the SAS the device is connected to, to ease the transfer of data when the SAS vendors start accepting these values.

Existing devices that upgrade to R20.0.1 or later will automatically report their Sector ID, which by default is the MAC address of the AP in the sector. The Spectrum Reuse ID must be manually configured by the operator. When registering new devices using the CBRS tool in cnMaestro, the user can now configure the coexistence parameters directly in the tool.

:point_right: Note that when using the Google SAS, if the user does not configure the coexistence parameters, the SAS will use default values, which may or may not be applicable to a specific deployment. It is recommended to configure your own parameters, to make sure the channels are assigned as intended by your frequency planning.

Some operators have already started configuring the coexistence parameters. The recommendation is not to change the default value of the Sector ID, unless the deployment has some unique property and the user fully understands the implications of changing it.

The Spectrum Reuse ID is just another way of keeping track of channels during frequency planning. When selecting sectors that can reuse the same channel, do not assign a specific frequency; assign just a common name for the Spectrum Reuse ID.

:point_right: Note that at the moment the SAS does not assign channels to the devices, but this can change in the future. Because more devices may be installed in the area, or channels become unavailable (permanently because of PALs or temporarily because of DPA activation), the SAS may eventually assign different channels to different devices. Keep this in mind when selecting names for the Spectrum Reuse groups.

Looking at the values some operators have chosen for this parameter, here are some recommendations of what NOT to do:

  • Do not use a MAC address as the Spectrum Reuse ID. The Spectrum Reuse ID will be common across multiple sectors; using the MAC address of one of the APs in the group may be confusing or misleading. If later the operator decides to move that AP to another group, then the name would be even more misleading.

  • Do not use the site or end-customer name or location in the group name. Again, the Spectrum Reuse ID will be common across a large number of sectors. You do not want a sector in a county to possibly be in a group with a name associated to another county.

  • Do not use the center frequency as part of the group name. Even if today you assign that center frequency to that group of devices, over time the SAS may assign different frequencies if the ecosystem around the deployment changes. Having devices operating on a channel but being associated to a group whose name includes another channel may be confusing as the network evolves over time.

  • Do not use a large number of groups. Try to identify the minimum number of distinct channels needed in the network to avoid self-interference and only use a limited number of Spectrum Reuse IDs. How many groups are defined in a single geographical area will affect how much spectrum each device will be able to operate on when the SAS starts enforcing channel assignments.

    In simple terms, the SAS looks at the available channels in an area and at the number of groups each operator has identified, and it divides the available channels by the total number of groups (the actual math is more involved, but at a high level this is what will happen). If each operator uses a large number of groups, each group will end up with a very small channel, or potentially with reduced EIRP, to accommodate all the different groups. If sectors can operate in the same channel without interfering with each other, communicating this information to the SAS benefits the whole ecosystem.

Above are some examples of what not to do. So what can you do? You can follow these simple steps when assigning Spectrum Reuse IDs to sectors:

  • Identify all sectors in your network that can operate on the same frequency and put them in the same group. For example, in a site with four APs facing four directions, the north and south AP can be in one group, and the east and west AP can be in another group.

  • Identify the minimum number of groups needed in the network. Most networks can operate with 2 to 4 channels without causing self-interference.

  • Assign names to each group. For example, use GroupA, GroupB, etc. or Group1, Group2, etc. without including the center frequency in the group name.

    You can configure the group name (Spectrum Reuse ID) directly in the AP GUI, or push a template from cnMaestro to all the APs that are part of the same group.

If you need to make a change to the Spectrum Reuse ID you have already configured, you can use one of two methods described in the CBRS Consolidated Procedures Guide Version 1.5.1, found here: (scroll down to the CBRS Supporting Documents and Tools section).

  1. Update the parameters in the AP GUI. Page 34 of the document describes this method in the section “Editing coexistence parameters directly on the radio”.
  2. Update the parameters in the cnMaestro CBRS tool. Page 36 of the document describes this method in the section “Editing Coex parameters on an existing sector”.