Point-to-point communication using IR transmitter and receiver - just as any regular remote controller.
Apply spread spectrum signalling to allow below noise floor reception.
Apply Error Correction Coding to squeeze the last few dB's out of the system
Use old camcorder-lens as receiver “antenna” to improve receiver bit-energy-to-noise ratio.
Use laser or high directional IR diodes as transmitter. Hmm, does it have to be eye-safe?
Look into types of hyper-sensitive IR receiver components - maybe combined with liquid nitrogen cooling to suppress noise?
Apply “front-end” with controllable baseband filter and Variable-Gain Amplifier(s) to increase the dynamic range.
Apply noise reduction techniques in PCB design.
Open-questions; sampling, chip and data rates, spreading code types, baseband signal processing (FPGA?), signal acquisition