On Temporal and Spatial Behaviors of CBRS SAS

The recently established Citizens Broadband Radio Service (CBRS) has attracted attention because it accommodates incumbents, auctioned license users, and unlicensed users. Incumbents retain unconstrained transmission rights, but when inactive, the spectrum is shared between the Primary Access License (PAL) users and the General Authorized Access (GAA) users. A cloud-based centralized administrator controls the spectrum sharing, Spectrum Access System (SAS), using an environmental sensing capability network to establish transmission rights for PAL and GAA users without hindering the incumbents. This paper reports findings from GAA CBRS Devices’ (CBSDs’) deployments in California’s San Diego County, which has a large incumbent presence and characterizes temporal and spatial behaviors of CBRS SAS based on measured temporal data. We propose a one-step time-homogeneous Markov chain-based model that effectively captures the hidden state machine of the SAS-CBRS authentication steps from the measured logs. We then estimate the stationary distributions of existing process states and expected values of hitting and return times for CBRS spectrum availability. Further, by studying the CBRS spectrum availability at various locations in San Diego County, we found evidence of undisclosed obfuscation in reporting the spectrum availability by the SAS administrator. Finally, two novel optimal spectrum obfuscation strategies are proposed to maximize the entropy of the Markov Chain’s hidden states and provide more significant obfuscation than the undisclosed obfuscation strategy of the SAS.