Performance analysis for near-field atmospheric optical communications

Most atmospheric optical links are set up to operate in the far-field power transfer regime, in which diffraction spread is the dominant effect on the beam, resulting in very weak power coupling between the transmitter and the receiver. However, it is also possible to establish geometries such that the link operates in the near-field regime, where, in the absence of turbulence, it is possible to focus the beam onto the receiver with almost perfect power coupling. Work on the performance of near-field atmospheric optical communication systems is scarce in existing literature, perhaps due to increased complexity in prescribed models. We analyze error probability bounds for optical communication links operating in the nearfield regime, utilizing on-off keying (OOK) and a coherent or direct detection receiver. In addition, we obtain bounds on the ergodic capacity of a near-field atmospheric optical link with local oscillator shot-noise dominated coherent detection, or shot-noise limited direct detection.