Toward Practical Temporal Diversity Coherent Combining Based Variable Data-Rate Free Space Optical Communication Systems

Free-space optical communication (FSOC) systems with variable data-rates can support different missions with different transmission distances and ensure reliable transmission in periods of unfavorable weather conditions. The temporal diversity coherent combining (TDCC) technique can adjust the data-rate in a very large range using a software-defined manner by taking advantage of the digital coherent receivers able to recover the signal field. In this paper, we investigate the relationships between the computational complexity, optical phase alignment error, combining loss (CL) and data-rate for the TDCC-based variable data-rate coherent optical receiver. We also propose a method to minimize the computational complexity while achieving the expected output optical signal-to-noise ratio (OSNR) and the highest data-rate for an arbitrary input OSNR. Numerical simulations and experiments are carried out to validate the analytical expressions and proposed methods. The results provide an efficient tool and useful guidelines for the design of low computation complexity TDCC-based variable data-rate FSOC systems.