Robust and Practical 3-D RFI Nulling for 5G and Radio Astronomy Coexistence

5G network operators are constantly under pressure from regulatory agencies who restrict the deployment of base stations (gNBs) close to incumbent services, such as radio astronomy services (RAS), to avoid interfering with them. Recent works for coexistence with RAS employ limited channel modeling approaches and use explicit out-of-band communication between the gNB and RAS. However, the strict latency requirements of 5G make explicit techniques less desirable due to their communication overheads. Deploying gNBs close to RAS is also not yet supported. In this paper, we propose a proactive implicit beamforming and interference nullification technique in which a gNB nullifies its downlink signal at a nearby RAS telescope while beamforming its users (UEs). We estimate the gNB-RAS channel using raytracing on open-source terrain maps which also accounts for the height of the gNB, UE and RAS telescope. We formulate a max-min rate problem for UEs under the constraint of maximum allowable interference power at the RAS and show that the time complexity of the max-min rate solution depends cubically on the number of gNB antennas. Hence, we propose a heuristic solution that achieves 4 orders of magnitude faster latency and 100 dBW lower interference power than the max-min rate solution with similar sum rate on users. Our proposed solution consistently achieves less than -310 dBW interference power in simulations, even when the gNB-RAS distance is less than 1 km, while being robust against channel estimation errors at more than 20 dB estimation SNR. Through over-the-air experiments, we demonstrate that our proposed scheme reduces the interference power to less than -115 dBW at the cost of 1-5 dB SNR loss at a nearby UE.