  The Federal Communications Commission (FCC) in the U.S. has made the Citizens
Broadband Radio Service (CBRS) band (3.55 - 3.7 GHz) available for commercial
wireless usage under a shared approach using a three-tier hierarchical
architecture, where the federal incumbent is the highest priority Tier 1 user,
Priority Access License (PAL) holders, who have paid for licenses, are Tier 2
users and Tier 3 users operate under General Authorized Access (GAA), without
license fees or protection from higher priority users. The Spectrum Access
System (SAS) ensures that higher priority users are protected from interference
from lower priority users. However, the lowest priority GAA users are not given
any protection from each other by the SAS and are expected to not cause any
harmful interference to Tier 1 and Tier 2 users. As the deployments of GAA
devices grow, the potential for secondary interference between GAA users
increases, especially since the SAS architecture does not allow dynamic channel
switching when faced with interference. In this paper, we present a
first-of-its-kind extensive measurement campaign of a commercial CBRS network
deployed in the city of South Bend, IN, that quantifies both co-channel
interference (CCI) and adjacent channel interference (ACI) caused by competing
GAA devices and C-band 5G, respectively. We (i) identify a particular CCI
scenario and improve performance by changing the frequency allocation based on
our study of other allocations in the vicinity and (ii) quantify ACI from 5G in
C-band (3.7 GHz) on CBRS throughput. We conclude that (i) CCI and ACI for GAA
users is not handled well by the SAS, (ii) proper frequency allocation for GAA
requires additional analysis of interference from other GAA users followed by
dynamical channel selection, and (iii) utilization of immediate adjacent
channels by high power 5G deployments limits the performance of CBRS.
