On-Off Keying Communication over Optical Channels with Crosstalk

We investigate the fundamental limits of communication over optical on-off-keying channels with crosstalk, where a light pulse may span over multiple time slots or spatial pixels, and the receiver is equipped with single-photon detectors. First, we analyze achievable rates of communication over such channels, and observe that increasing transmission power (expected number of photons emitted per slot or pixel) does not necessarily lead to higher rates. Under simple but reasonable models, the highest rates are often achieved in a low-photon regime, with an average of 3 to 7 photons received in each slot or pixel. We further characterize the tradeoff between information rate and photon efficiency (in terms of the expected number of bits transmitted per photon) in the presence of crosstalk. Finally, we develop guidelines for slot length and pixel size selection for different application scenarios. Our analysis reveals that optimum optical-communication systems do not minimize the level of crosstalk.