A Measurement Study of the Impact of Adjacent Channel Interference between C-band and CBRS

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, under strict authorization of the Spectrum Access System (SAS). However, adjacent channel interference may reduce the performance of GAA and C-band systems due to lack of guard bands and no Time Division Duplexing (TDD) synchronization, i.e., uplink/downlink configurations are not synchronized. In this paper, we quantify the effect of this mutual interference by performing experiments with a real-world deployment. Significant downlink throughput reductions on both systems are observed when two devices are in close proximity to each other, with 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 CBRS base-station (i.e., 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.