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.

翻译：3.7-3.98 GHz频段（亦称C波段）近期被美国分配给5G蜂窝服务部署。在此之前，其较低相邻频段3.55-3.7 GHz已分配给公民宽带无线电服务（CBRS），其中全部150 MHz带宽可供三级通用授权接入（GAA）用户免费使用，但频谱接入需经频谱接入系统（SAS）授权。仅当区域内不存在一级现有用户（海军雷达）或二级优先接入许可（PAL）用户时，GAA用户才可在特定信道上使用。然而，由于GAA与C波段之间未设保护频带，且两者均采用时分双工（TDD）且上下行配置不同步，相邻信道干扰可能导致两个系统的性能下降。本文通过实际部署实验量化了这种互干扰效应。当两台设备彼此靠近且一台发送上行链路信号而另一台发送下行链路信号时，我们观察到两个系统的下行吞吐量均显著下降：4G CBRS下降60%，5G C波段下降43%。我们相信这是首次在实际部署中证实该现象的论文。当CBSD更换信道并移至距C波段20 MHz处运行时，该吞吐量退化得以缓解，本质上在两个信道间创建了保护频带。我们还通过在CBSD上实施微切片技术，展示了相邻信道干扰下延迟的改善。研究结果表明，因缺乏保护频带和TDD配置失配导致的相邻信道干扰问题，对提升CBRS与C波段系统的性能至关重要。