Digital modulation and coding for satellite optical feeder links

Abstract

In this paper, a packet-level forward error correction (FEC) coding technique and pre-distortion adaptive optics (AO) technology are applied to a digital transmission scheme for optical feeder links in a geostationary Earth orbit (GEO) satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency (RF) front-end and the optical front-end, as well as the digital signal processor. The system is designed to cater for Terabit/s highthroughput satellite (HTS) applications. The performance of the digital transmission scheme is evaluated in the forward and return links. The turbulent atmospheric optical channel is modeled for different optical ground station (OGS) altitudes. It is shown that fade mitigation techniques such as packet-level FEC coding and pre-distortion AO in the forward link, as well as large-aperture OGS telescope in the return link, are essential to close the link budget of a Terabit/s satellite communication system. Prepared using satauth.cls [Version: 2010/05/13 v2.00] 2 S. DIMITROV ET AL.Index-of-refraction turbulence (IRT) [3] is known to degrade the achievable data rates of FSO communication links [4]. For uplinks above 30 • elevation, the IRT falls within the weak turbulence regime [5], where the probability density function (PDF) of the irradiance at the receiver plane can be modeled by a log-normal distribution [6]. Moreover, the correlated fading events represent a lowpass process defined by the Greenwood frequency [7]. Regarding site selection and deployment, an OGS at sea level may suffer from strong influence of the turbulence, since the atmosphere is denser at lower altitudes. Astronomical sites are generally located on mountain tops, but these locations are usually isolated, and Terabit/s infrastructure needs to be developed. In this paper, 300 m, 900 m and 1500 m are chosen as examples of possible OGS altitudes.Adaptive optics (AO) is a technology used for correction of the incoming optical wavefront phase which is distorted by the turbulent atmosphere [8]. In ground-to-GEO FSO links, the scale size of the phase distortions is much larger than the feasible satellite telescope aperture size [3], and therefore, the spatial phase at the satellite receiving aperture is rather flat. Nevertheless, in an uplink scenario the traveling wavefront is initially perturbed by the atmosphere, and then it propagates through free space up to the GEO satellite. Thus, at the satellite plane the received wavefront profile differs from that of a pure diffracted laser beam. These distortions can be mitigated by applying pre-distortion AO (PAO) techniques to the outgoing optical wave at the ground station, using a beacon reference that senses the distortions introduced by the turbulent atmosphere.The simplest PAO correction method is tilt compensation of the first three spatial modes of the wavefront phase information [9]. It uses pointing by tracking, where a downlink beacon from the satellite is used a...

Exploiting Diversity in Future Generation Satellite Systems with Optical Feeder Links

“…Since the limited spectrum available in Ku-Ka bands may not be sufficient to ensure Terabit/s communications by means of efficient frequency reuse alone, most feeder link studies focus on moving to higher frequency bands (Radio Frequency (RF) bands like Q/V (40/50 GHz), [3][4][5][6] W (70/80GHz) 7 or even Optical. [8][9][10] Free Space Optics (FSO) communications represent an interesting alternative due to the following advantages over RF:…”

Optical feeder links for high throughput satellites

“…Some error correcting codes, like low-density parity-check, are able to correct random errors. To deal with fading events, one can either use interleavers which allow spreading burst errors induced by fading into small length burst which can be corrected by the random error correcting codes or erasure correcting codes which are proposed in [4].…”

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“…This approach enables in theory the use of a single active optical link per satellite thanks to a very large available bandwidth (BW) and a high telescope gain. However, many challenges have to be addressed to consolidate the outline of an optical feeder link including the high level of availability required for feeder links (addressed in [3]), the transmission through atmospheric turbulences (addressed in [4]) and the choice of the modulation of the optical carrier.…”

Forward error correcting code for high data rate LEO satellite optical downlinks

“…The high complexity refers to the HDR proposition at CCSDS with a receiver based on an erbium doped fiber amplifier (EDFA) with a complex adaptive optic system to inject into its single mode fiber [1]. Since several years, the free space optical downlink community has investigated many different protection schemes: physical layer error correcting code combined with an interleaver [2] [3], packet level erasure code [4] [5], combination of all of them [6]. In the meantime, various FEC code families have been investigated: graph based codes (serial turbo code, or polar code [7], or sparse code such as Low Density Generator Matrix [3], Raptor [6], LT, Low-Density Parity-Check [9]), algebraic (MDS like Reed Salomon, BCH), and trellis based convolutional codes.…”