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How the CBSD gets access to spectrum

  • CBSDs request a grant to reserve a portion of the spectrum for their use. If approved, the reservation has been made but the CBSD is not yet authorized to transmit using the grant.
  • CBSDs periodically send heartbeat requests for each of their approved grants in order to receive authorization to transmit.
  • When the CBSD no longer wishes to use a grant, it relinquishes the grant.
Request a grant

Once the CBSD has decided which frequency range it would like to operate on, it sends a grant request to the SAS.

The SAS approves or rejects the request as needed to protect incumbents,  and sends the result in a grant response.

If the request is approved, the SAS includes the grant ID in its response and gives the CBSD a grant. This means that a small portion of the available spectrum is reserved specifically for its use. However, it's not yet allowed to transmit.

If the grant request is rejected, the CBSD won’t get the grant. The CBSD may try to do another spectrum inquiry. Typically, a request is rejected because it had invalid parameters, or the CBSD requested spectrum that's unavailable.

Request size and power for a grant

A CBSD can request any grant size from 5 MHz to 150 MHz. The grant size must always be a multiple of 5 MHz with start and end frequencies divisible by 5.

For example: A grant request for 3640 MHz to 3655 MHz is correct, but a grant request for 3642 MHz to 3647 MHz is incorrect.

Spectrum inquiry response will always show spectrum availability in channels of 10 MHz.

If someone requests more than 10 MHz, the maximum approved Effective Isotropic Radiated Power (EIRP) will be the lowest from all channels within that range.

For example: The SAS indicates that 3550–3560 MHz can support an EIRP of 30 dBm/MHz and that 3560-3570 MHz can support an EIRP of 27 dBm/MHz. The device should conclude that 3550–3570 MHz is available with a maximum EIRP of 27 dBm/MHz. 27 dBm/MHz is the minimum value from the relevant channels.

Heartbeat to request authorization to transmit

Once the CBSD has a grant, it can send a heartbeat request for that grant. A CBSD may have multiple grants, but it must send a heartbeat request for each one.

The heartbeat request is used to ask the SAS if the CBSD can transmit with the parameters it sent in the grant request. 

If the SAS approves the heartbeat request in its heartbeat response, the CBSD is authorized to transmit with those parameters until the transmitExpireTime, typically 240 seconds after the heartbeat. The CBSD must send a heartbeat request to the SAS at least once before the transitExpireTime to be sure it can continue to transmit. To maximize uptime and reliability of CBSDs connected to Google SAS, the CBSD should heartbeat every 60 seconds, known as the heartbeatInterval.

In its heartbeat response, the SAS can either:

  • Authorize the grant, which means the CBSD can start or continue to transmit. 
  • Terminate the grant, which means the SAS can't allow the grant to transmit while also protecting incumbents.
  • Suspend the grant, which means the CBSD isn't allowed to transmit at that exact moment but may be able to transmit again soon.

If the CBSD doesn't receive a heartbeat response from the SAS after 30 seconds, it should send another heartbeat request, known as the retryInterval. Check with your CBSD manufacturer how to configure the heartbeatInterval to 60 seconds and the retryInterval to 30 seconds to maximize uptime of your CBRS network with Google SAS.

To learn more about reasons for grant suspension, you can read: 

Relinquish a grant
If a CBSD no longer needs a grant, it sends a relinquishment request to the SAS. The SAS responds with a relinquishment response, which almost always approves the request.
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