There's only 64 antennas but 256 SIMs, so there are either 64 separate quad-SIM modems (AFAIK only Mediatek made those, and in 2G only) or it's just one big FPG

There's only 64 antennas but 256 SIMs, so there are either 64 separate quad-SIM modems (AFAIK only Mediatek made those, and in 2G only) or it's just one big FPGA, managed by a small CPU and connected to a bunch of RF ICs, like the inverse of a base station.