Abstract
Digital coherent combining (DCC) technique can increase the free space optical signal collection area by combining the signals received by an array of small apertures in a coherent manner. To realize DCC the different versions of signals must be aligned in phase by the digital phase alignment algorithm (PAA). Low computation complexity is imperative for the PAA because the main obstacle to implement the PAA and DCC in a real-time manner is the availability of digital signal processing (DSP) circuits offering very high gate density and processing speed. In this paper we investigate the relationship between the computation complexity, optical phase offset estimation error and the combining loss for the equal gain combining technique. Analytical expressions are deduced allowing easy minimization of the computation complexity at an arbitrary input OSNR and acceptable combining loss. Extensive numerical simulations are carried out to validate the analytical expressions.
Highlights
Low computation complexity is imperative for the phase alignment algorithm (PAA) because the main obstacle to implement the PAA and Digital coherent combining (DCC) in a real-time manner is the availability of digital signal processing (DSP) circuits offering very high gate density and processing speed
In this paper we investigate the relationship between the computation complexity, optical phase offset estimation error and the combining loss for the equal gain combining technique
In this paper we investigate the relationship between the computation complexity of the PAA, the optical phase offset (OPO) estimation error and the combining loss for the equal gain combining (EGC) based DCC technique
Summary
Free space optical communication can exploit the unregulated and nearly unlimited bandwidth in the near-infrared band and provide higher data rate and lower size, weight and power (SWaP) profile lasercom terminals compared to microwave communication. [1]–[5] For the satellite to ground downlink optical communication systems it is desirous to reduce the power-aperture product of the space-borne terminals by developing ground terminals with a large collection area. [6], [7] But large diameter telescopes are difficult to build and have focal plane thermal heating problem when operating in the day or pointing near the sun. [7], [10] the light collected by the large diameter telescopes is difficult to efficiently deliver to a small area detector or single-mode fiber due to the atmospheric turbulence induced wave front distortion unless complex adaptive optics is employed. [11], [12] These problems can be solved by the coherent combining techniques. Proposed recently [7]–[9] They can increase the collection area by combining signals received by an array of small apertures in a coherent manner. The first kind of methods relies on analog approaches [13]–[17] They need to realize optical path length matching between different branches down to a small fraction of a wavelength utilizing complex optical phase-locked loops (OPLL) [13]–[16] or fiber variable phase delays in combination with feed-back optical phase locking techniques [17]. The digital coherent combining (DCC) technique is appealing because instead of using the complex optics to stabilize the phase, it uses more robust and easier to implement digital time and phase alignment techniques.
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