A Study of Timing Constraints and SAS Overload of SAS-CBSD Protocol in the CBRS Band

In the Citizens Broadband Radio Service (CBRS) band, the Federal Communications Commission (FCC) has set stringent timing constraints for the lower tier users to vacate the channel on which an incumbent shipborne radar appears. The standards body formulating various specifications for the CBRS operation has taken these timing constraints into consideration in the Spectrum Access System (SAS) - CBRS Device (CBSD) protocol. A transmitting CBSD continually heartbeats with its SAS. When required, the SAS sends commands to vacate a channel through these heartbeat messages. In this paper, we study the impact of the heartbeat interval on the CBRS system in terms of meeting the FCC timing constraints. We also study how the heartbeat interval can overload a SAS and how it can be used to determine the number of CBSDs a SAS can serve without causing unnecessary suspension of CBSD transmissions. We show the tradeoff between using a short heartbeat interval to meet the timing constraint early and the number of CBSDs that can be served by a SAS without causing unnecessary suspension of CBSD transmissions.