  The 3.7 - 3.98 GHz frequency band (also known as C-band) was recently
allocated in the US for the deployment of 5G cellular services. Prior to this,
the lower adjacent band, 3.55 - 3.7 GHz, had been allocated to Citizens
Broadband Radio Service (CBRS), where the entire 150 MHz can be used for free
by Tier 3 General Authorized Access (GAA) users, but access to the spectrum
needs to be authorized by the Spectrum Access System (SAS). GAA users are
allowed on a channel only when there are no Tier 1 Incumbents (Navy radars) or
Tier 2 Priority Access License (PAL) users in the area. However, since there
are no guard bands between GAA and C-band, and both systems employ Time
Division Duplexing (TDD) where the uplink/downlink configurations are not
synchronized, adjacent channel interference can potentially reduce the
performance of both systems. In this paper, we quantify the effect of this
mutual interference by performing experiments with a real-world deployment. We
observe significant downlink throughput reductions on both systems when two
devices are in close proximity to each other, and one is transmitting uplink
while the other is transmitting downlink: 60% for 4G CBRS and 43% for 5G
C-band. We believe that this is the first paper to demonstrate this in a real
deployment. This throughput degradation was reduced when the CBSD changed its
channel and operated 20 MHz away from C-band, essentially creating a guard band
between the channels. We also demonstrate the improvement in latency under
adjacent channel interference by implementing MicroSlicing at the CBSD. Our
results indicate that addressing adjacent channel interference due to the lack
of guard bands and TDD configuration mismatch is crucial to improving the
performance of both CBRS and C-band systems.
