Evaluation of communication performance for adaptive optics corrected geo-to-ground laser links

Abstract

I. INTRODUCTION By using existing single mode components developed for fiber technologies (optical detectors and amplifiers, MUX/DEMUX...), the very high throughput of future satellite-to-ground optical communication links might be achievable at a reasonable cost. However, atmospheric turbulence induced perturbations compromise the injection efficiency of the wave into single mode components. The resulting channel impairments can be significantly reduced by the use of appropriate adaptive optics (AO) compensation at the expense of potentially complex and expensive systems if very high stability of the injection is required. Feasibility demonstrations of AO correction were recently reported in the US [1] and France [2] with Low Earth Orbit (LEO) satellites, and in Germany with GEO satellites [3]. To reduce the loss in useful information and to relax by the same way the specifications and cost of AO systems, numerical techniques such as error-correcting codes have to be considered as well. These codes introduce a degree of data redundancy, which allows a transmitted codeword to be correctly interpreted despite the loss of a significant fraction of the individual codeword elements. However, they are usually adapted to combat randomly distributed errors rather than bursty errors. Therefore, over the free-space optical channel, a fading event that is much longer than the codeword duration will usually result in an unrecoverable loss of information. To overcome this non-uniform distribution of errors that occur in slow fading channels, symbol interleaving is mandatory. Such numerical interleavers after disassembling each codeword will space the individual symbols with a temporal interval comparable to a characteristic duration of the channel fades. At the receiver, the symbols are reassembled in the proper order but the errors are redistributed such that there is a higher probability that each codeword is correctly decoded and reconstructed. In addition to the drawback induced by the latency introduced by the symbol spreading, this interleaving process could lead to unmanageable buffer size. Extensive work on numerical approaches to mitigate channel impairments were conducted in the US [4], Germany [5] and Japan [6]. Whereas the AO correction will impact the temporal characteristics of the channel and can therefore, in turn, diminish the drawbacks related to the interleaving process, none of the reported work considered the influence of the AO residuals on the characteristics of the channel. Consequently, the need for a joint optimization of the performances of the AO system and the interleaving process, with respect to power constraints and coding rate, appears necessary. In this paper, appropriate telecom metrics such as the channel capacity and outage probabilities are computed using a simplified AO corrected channel model to assess the influence of AO residuals on the required interleaver size. The specific case of a GEO to ground link with on-off keying (OOK) modulation is addressed. The outline of this paper is ...

Statistical properties of single-mode fiber coupling of satellite-to-ground laser links partially corrected by adaptive optics

“…(24) are given, respectively, by Eqs. (30) and (31), in which σ 2 I is the aperture-averaged logirradiance variance (approximated by the scintillation index in the weak fluctuation regime) and μ I μ exp2χ Ap 1 (see Subsection 3.B). These analytical PDFs and CDFs of the instantaneous coupled flux attenuation are, per se, useful results in deriving the channel capacity and probability of outages of freespace optical (FSO) transmissions [31].…”