Optical Feeder Research Articles

Pre-distortion adaptive optics for optical feeder links: simulations and performance analyses

Optical feeder links offer immense utility in meeting future communication demands—however, atmospheric turbulence limits their performance. This work targets this challenge through analyses of a bidirectional free-space optical communication (FSOC) link that incorporates pre-distortion adaptive optics (AO) between the next-generation optical ground station at the German Aerospace Center (DLR) Oberpfaffenhofen and the laser communications terminal on Alphasat—a satellite in geostationary orbit (GEO). The analyses are performed via end-to-end Monte Carlo simulations that provide realistic performance estimates of the bidirectional FSOC link for a GEO feeder link scenario. We find that applying pre-distortion AO reduces the total uplink losses of the bidirectional FSOC link by up to 10 dB and lessens the scintillation at the GEO satellite by an order of magnitude. Moreover, applying pre-distortion AO eases the link budget requirements needed for maintaining 99.9% link uptime by as much as 20-40 dB, while its use with a laser guide star shows an additional performance improvement of up to 8 dB. These findings demonstrate the desirability and feasibility of utilizing pre-distortion AO for the realization of optical feeder links.

Link budget analysis of bi-directional LEO and GEO optical feeder links advancing the beam wander model’s accuracy

The telecommunications of the future rely on the concept of a three-dimensional architecture able to integrate terrestrial and non-terrestrial networks with the goal to ensure a reliable and high-speed connectivity to users located anywhere. In this context, free space optical communications constitute a candidate technology for feeder links, thanks to their advantages in terms of bandwidth and achievable data rates. Nonetheless, due to the propagation impediments encountered by an optical beam travelling through atmosphere, flexible and accurate instruments able to support the design of optical feeder links are needed. Therefore, in this paper a link budget numerical tool able to meet these requirements is presented and the link budget analysis for real optical feeder links is performed demonstrating its prediction accuracy by means of the comparison with experimental results for both low Earth orbit and geostationary Earth orbit based configurations. Finally, the limits of the conventional beam wander model are analyzed and overcome.

Free-space optical (FSO) feeder link planning in space-ground integrated optical networks (SGIONs): trade off throughput and dynamics.

The low-orbit satellite communication can provide users with low-delay and ultra-wideband communication services worldwide. By the wide available bandwidth and immunity to interference, free-space optical (FSO) feeder links are presented as an ideal alternative to radio frequency (RF) feeder links for satellite-to-ground backhaul, and the space-ground integrated optical network (SGION) is gradually formed by using FSO feeder links to integrate the low-orbit satellites and the terrestrial optical network (TON) for backhaul services. The propagation channel characteristics of FSO feeder links vary significantly during low-orbit satellite passes, and atmospheric turbulence causes serious scintillation in FSO feeder links at low elevations, increasing link budget and restricting link capacity. Limiting the observation range of optical ground stations (OGSs) to the high elevation area can establish high-capacity feeder links to improve SGION's throughput, but inevitably increase the network dynamics and reduce the satellite visibility. This paper trades off SGION's throughput and dynamics by planning FSO feeder link handover and capacity adjustment. Two baseline schemes and the feeder link handover and capacity adjustment scheme based on non-dominated sorting genetic algorithm (NSGA-FLHCA) are proposed. By finding the Pareto edge of the multi-objective optimization problem, NSGA-FLPCA is more effective in improving network throughput and reducing network dynamics compared with two baselines.

On the Use of NB-IoT over GEO Satellite Systems with Time-Packed Optical Feeder Links for Over-the-Air Firmware/Software Updates of Machine-Type Terminals.

The verticals of 5G, such as the automotive, smart grid, and smart cities sectors, will bring new sensors and IoT devices requiring Internet connectivity. Most of these machine-type terminals will be sparsely distributed, covering a very large geographical area and, from time to time, will have to update their software, firmware, and/or other relevant data. Given this situation, one viable solution to implement the “Over-the-Air” update of these IoT terminals can be done with the aid of GEO satellite systems. However, due to the ultra-dense radio frequency reuse factor that contemporary High-Throughput Satellite (HTS) systems implement in the access link to serve the IoT terminals, the use of a time-packed Free Space Optical (FSO) link represents a practical solution to avoid the bottleneck that the satellite gateway experiences in the feeder link. The performance of both Detect-and-Forward and Decode-and-Forward relaying strategies are studied, assuming that the single-carrier M-PAM symbols that are transmitted on the optical feeder link are mapped into M-QAM symbols that modulate the multiple sub-carriers of the OFDM-based radio access link. In addition, the benefits of encapsulating the NB-IoT frames into DVB-S2(X) satellite frames is also analyzed in detail. The effects of the impairments introduced in both the optical feeder and radio access links are characterized in detail, and the end-to-end error correction capabilities of the Modulation and Coding Schemes (MCS) defined in the contemporary releases of the NB-IoT and DVB-S2(X) standards are studied for different working regimes.

A PHY Layer Security of a Jamming-Based Underlay Cognitive Satellite-Terrestrial Network

In this work, we investigate the physical layer security of a jamming-based underlay cognitive satellite-terrestrial network consisting of a radio frequency link at the first hop and an optical feeder at the second hop. Particularly, one secondary user (SU) is transmitting data to an end-user optical ground station through the aid of a relay satellite, in the presence of an active eavesdropper at each hop. Moreover, another SU located in the first hop is acting as a friendly jammer and continuously broadcasting an artificial noise that cannot be decoded by the wiretapper so as to impinge positively on the system’s secrecy. Owing to the underlying strategy, the SUs are permanently adjusting their transmit powers in order to avoid causing harmful interference to primary users. The RF channels undergo shadowed-Rician and Rayleigh fading models, while the optical link is subject to Gamma-Gamma turbulence with pointing error. Closed-form and asymptotic expressions for the intercept probability (IP) are derived considering two different scenarios regardless of the channel’s conditions, namely (i) absence and (ii) presence of a friendly jammer. The effect of various key parameters on IP, e.g., sources’ transmit power, artificial noise, maximum tolerated interference power, and fading severity parameters are examined. Precisely, we aim to answer the following question: could a friendly jammer further enhance the security of such a system even in a low SNR regime? All the derived results are corroborated by Monte Carlo simulations and new insights into the considered system’s secrecy are gained.

Outage Performance for Optical Feeder Link in Satellite Communications With Diversity Combining

This letter investigates the outage performance of an optical feeder link in satellite communications, where the gateway is equipped with multiple apertures and employs diversity combining schemes to mitigate the effect of atmosphere turbulence. In particular, by assuming that the optical link undergoes the Málaga fading, we first derive a closed-form expression for the outage probability (OP) of the considered system with selection combining (SC). Subsequently, when equal gain combining (EGC) is utilized, we present an approximate yet accurate probability density function expression for the sum of independent and identically distributed Málaga random variables. Next, we derive an OP expression for the considered system with EGC. To gain more insights, we further obtain diversity order for both SC and EGC schemes. It is found that the diversity order for both schemes is related to the number of receive apertures and the effective number of large-scale cells of the scattering process.

Ground‐to‐GEO optical feeder links for very high throughput satellite networks: Accent on diversity techniques

SummaryThis paper studies the use of optical feeder links in very high throughput satellites (VHTS) networks with emphasis on gateway diversity techniques to mitigate the inherent propagation losses in optical frequencies. Focusing on a GEO scenario, the paper considers a system‐wide approach investigating various challenges of optical feeder links. These include transmission schemes amenable for transparent on‐board processing, optical channel models taking into account blockage by clouds and fading caused by atmospheric turbulence in addition to complexity of on‐board and on‐ground processing. The channel models are then used to dimension the ground segment towards ensuring a given availability percentage (e.g., 99.9%). The channel model and payload complexity further influence the choice of link layer techniques used for counteracting fading due to atmospheric turbulence in the absence of blockage. An elaborate end‐to‐end simulator incorporating the proposed channel models capturing the nuances of various processing blocks like optical‐electrical conversion is developed. The system performance results provide interesting insights and a framework for assessing the feasibility and advantages of optical feeder links in VHTS systems.

Performance evaluation of a DVB‐S2 system with a digital optical feeder link

SummaryWe investigate the physical layer performance of a DVB‐S2 system with a digital optical feeder link. We consider three scenarios depending on how the DVB‐S2 modulator functionalities are distributed between the optical gateway and the satellite. Our main objective is to specify the residual bit error rate of the optical channel error correction scheme in order to ensure a quasierror free link to the users. To that end, we provide theoretical models and/or simulations results. In two scenarios, our results show that a residual bit error rate of 10−4 (assuming equally distributed errors) gives end‐to‐end physical layer performance compliant with the DVB‐S2 standard. This specification is much higher than typical values in practical systems.

On the Next-Generation High Throughput Satellite Systems With Optical Feeder Links

Next-generation multibeam high throughput satellite (HTS) systems aim at achieving Terabits/s throughputs with thousands of spot beams. Conventional radio frequency (RF) Ku-/Ka- band feeder links, between gateways and satellites, are unable to achieve such throughputs due to limited spectrum availability in these bands. Free space optical (FSO) feeder links present a promising alternative to RF feeder links and have gained increased attention lately in the satellite community. This article presents performance analysis of multibeam HTS systems where the feeder link operates in the FSO band and the user link, between the satellite and the user terminal, is in the RF Ka-band. We consider that the FSO feeder link is impaired by the scintillation fading due to atmospheric turbulence and the RF Ka-band user link is attenuated by the rain fading. In multibeam HTS systems, frequency reuse in the user link creates interbeam interference (IBI) due to overlapping side lobes of the multibeam antenna radiation pattern. Considering a forward link with a fixed gain transparent satellite payload, we use the zero-forcing precoding technique to mitigate IBI in the user link. We derive exact and approximate statistics of the end-to-end signal-to-interference-and-noise ratio. Also, by capitalizing on these results, we analyze information-theoretic metrics including the outage probability and ergodic capacity. Our results provide various important insights, e.g., 1) outage performance and ergodic capacity is typically restricted by rain fading in the RF link, and 2) outage diversity of rain fading in the RF link is 0, hence outage performance only increases marginally with power at high signal-to-noise ratio.

Fading loss for Earth-to-space lasercom affected by scintillation and beam wander composite channel

When a link budget for Earth-to-space connectivity needs to be prepared for a scenario in which optical technology is used, in terms of atmospheric turbulence random degradation, the most important losses to account for are those due to scintillation and beam wander fading processes. Currently, the standard approach is to include the margins for scintillation and beam wander separately in the link budget, treating the statistical distribution of both processes independently. In essence, this approach is incorrect as the actual statistics for the received irradiance at the spacecraft are governed by the composite channel arising from the joint interaction between these two phenomena. A method to calculate a fading loss for the composite channel is developed. The calculation is performed by inverting the cumulative distribution function of the governing statistical model for a given outage probability p that the received irradiance is below a certain threshold value. A tractable approximation solution is found, and an accuracy analysis is conducted to give the limits of applicability of such an expression. Finally, the derived expression is used in a practical example framed in an Earth-to-space optical geosynchronous orbit feeder link scenario in which the fading loss approximation derived for the composite channel is found to be below 1 dB error from the exact solution for most of the range of interest, and well below 0.1 dB error near the optimum transmitter size.

Long Term Irradiance Statistics for Optical GEO Satellite Feeder Links: Validation Against Experimental Data

In this paper, optical feeder links to support high throuput satellite communication systems are studied. A methodology for the generation of the received irradiance/power time series for GEO optical feeder links (ground-to-GEO), that reproduces the long term irradiance/power statistics, is presented. In the proposed synthesizer, the atmospheric phenomena under clear sky that degrade the optical signal and the weak turbulence effects, i.e. scintillation, beam wandering and beam spreading, are considered. The proposed methodology takes advantage of the use of stochastic differential equations driven by fractional Brownian motion, for the time series synthesis. The methodology refers to the transmission of a single collimated beam. For the modelling of turbulence effects, Kolmogorov turbulence is assumed and the model is based on Rytov theory under weak fluctuations. Additionally, the proposed model is validated in terms of first order statistics with experimental data from the bi-directional optical GEO ARTEMIS satellite link experiment showing good agreement. Finally, the synthesizer is employed for the generation of fist order statistics for different transmission scenarios.

A PDM-based 128-Gb/s PAM4 fibre-FSO convergent system with OBPFs for polarisation de-multiplexing

A polarisation-division-multiplexing (PDM)-based four-level pulse amplitude modulation (PAM4) fibre-free-space optical (FSO) convergent system with optical band-pass filters (OBPFs) for polarisation de-multiplexing is feasibly demonstrated for the first time. In a PDM scenario with PAM4 modulation, the transmission capacity of fibre-FSO convergent systems is enhanced four times with an aggregate channel capacity of 128 Gb/s (64 Gb/s PAM4/polarisation × 2 polarisations). With an OBPF, polarisation-tracking free de-multiplexing is attained by eliminating other optical carrier with orthogonal polarisation. An OBPF is a simple polarisation de-multiplexing scheme in which the polarisation-orthogonal carrier can be effectively de-multiplexed and the cross-polarisation interference can be nearly eliminated. Compared with traditional PDM-based fibre-FSO convergent systems with sophisticated polarisation-tracking mechanism and elaborate digital signal processing (DSP) approach, it reveals a noteworthy one with the advantage of simplicity. Through 25 km single-mode fibre transport and 500 m FSO link, sufficiently low bit error rate and qualified PAM4 eye diagrams are attained. This proposed polarisation-tracking free PDM-based fibre-FSO convergent system is notable because it not only incorporates the fibre backbone and optical wireless feeder, but it also simplifies the framework since complicated polarisation-tracking mechanism and DSP approach are not involved.

A PHY Layer Security Analysis of a Hybrid High Throughput Satellite With an Optical Feeder Link

Hybrid terrestrial-satellite (HTS) communication systems have gained a tremendous amount of interest recently due to the high demand for global high data rates. Conventional satellite communications operate in the conventional Ku (12 GHz) and Ka (26.5-40 GHz) radio-frequency bands for assessing the feeder link, between the ground gateway and the satellite. Nevertheless, with the aim to provide hundreds of Mbps of throughput per each user, free-space optical (FSO) feeder links have been proposed to fulfill these high data rates requirements. In this paper, we investigate the physical layer security performance for a hybrid very high throughput satellite communication system with an FSO feeder link. In particular, the satellite receives the incoming optical wave from an appropriate optical ground station, carrying the data symbols of N users through various optical apertures and combines them using the selection combining technique. Henceforth, the decoded and regenerated information signals of the N users are zero-forcing (ZF) precoded in order to cancel the interbeam interference at the end-users. The communication is performed under the presence of malicious eavesdroppers nodes at both hops. Statistical properties of the signal-to-noise ratio of the legitimate and wiretap links at each hop are derived, based on which the intercept probability metric is evaluated. The derived results show that above a certain number of optical apertures, the secrecy level is not improved further. Also, the system's secrecy is improved using ZF precoding compared to the no-precoding scenario for some specific nodes' positions. All the derived analytical expressions are validated through Monte Carlo simulations.

Transmission analysis for OFDM signals over hybrid RF-optical high-throughput satellite.

In this paper, a theoretical investigation of the performance of a communication scenario where a geostationary-orbit satellite provides radio-frequency broadband access to the users through orthogonal-frequency-division multiplexing technology and has an optical feeder link is presented. The interface between the radio frequency and the optical parts is achieved by using radio-on-fiber technology for optical-electro and electro-optical conversion onboard and no further signal processing is required. The proposed scheme has significant potential, but presents limitations related to the noise. The noise in both forward and reverse links is described, and the system performance for an example scenario with 1280 MHz bandwidth for QPSK, 16QAM, and 64QAM subcarrier modulation is estimated. The obtained results show that under certain conditions regarding link budget and components choice, the proposed solution is feasible.

Demonstration of intradyne BPSK optical free-space transmission in representative atmospheric turbulence conditions for geostationary uplink channel.

Binary phase-shift keying optical transmission in the C-band with coherent intradyne reception is demonstrated over a long-range (10.45km) link through the atmosphere. The link emulates representative channel conditions for geostationary optical feeder uplinks in satellite communications. The digital signal processing used in recovering the transmitted data and the performed measurements are described. Finally, the bit error rate results for 10Gbit/s, 20Gbit/s, and 30Gbit/s of the outdoor experiments are presented and compared with back-to-back measurements and theory.

Performance, cost analysis, and ground segment design of ultra high throughput multi‐spot beam satellite networks applying different capacity enhancing techniques

SummaryThere is an ever present demand for increasingly higher data rates in multi‐spot beam satellite networks. This can be enabled, by shifting carrier frequencies to higher bands, such as the Q/V‐band, where more bandwidth is available. Furthermore, the available bandwidth has to be used as efficiently as possible, which requires efficient capacity enhancing techniques. The present paper identifies and analyzes ground segment capacity enhancing techniques for ultra high throughput multi‐spot beam satellite networks operating in Q/V‐band in the feeder link and in Ka‐band in the user link. The impact of several capacity enhancing techniques on system performance is analyzed using a realistic time step‐based system simulator. Their impact on the ground segment costs is also analyzed. The examined capacity enhancing techniques are as follows: (i) reduced carrier spacing with adjacent channel interference cancellation; (ii) full frequency reuse with co‐channel interference cancellation and scheduling; (iii) four different smart gateway diversity techniques and; (iv) employing optical feeder links as an alternative to radio frequency feeder links. The paper shows that these techniques are capable of improving system performance at the expense of an increase in cost related to the complexity and maturity of the applied capacity‐increasing technique. Copyright © 2016 John Wiley & Sons, Ltd.

Digital modulation and coding for satellite optical feeder links with pre‐distortion adaptive optics

SummaryIn this paper, a packet‐level forward error correction coding technique and pre‐distortion adaptive optics technology are applied to a digital transmission scheme for optical feeder links in a geostationary Earth orbit satellite communication system. The architectures of the gateway and the satellite are defined, including the building blocks of the interface between the radio frequency front‐end and the optical front‐end, as well as the digital signal processor. The system is designed to cater for Terabit/s high‐throughput satellite 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 altitudes. It is shown that fade mitigation techniques such as packet‐level forward error correction coding and pre‐distortion adaptive optics in the forward link, as well as large‐aperture optical ground station telescope in the return link, are essential to close the link budget of a Terabit/s satellite communication system. Copyright © 2015 John Wiley & Sons, Ltd.

A minimal redundant shared OLT protection for hybrid WDM–TDM optical access networks

In this paper, a hybrid WDM---TDM-based optical access network architecture has been proposed that aims to provide optical line terminal (OLT) protection and partial feeder fiber protection without using any or minimal redundant OLT or any separate protection feeder fiber. The architecture is termed as minimal redundant shared OLT protection (MRSO) architecture. MRSO provides load shared OLT protection, where the ONUs of the failed OLT are supported by either another operational OLT that is lightly loaded in times of failure or by a redundant OLT located somewhere in the network. An appropriate control mechanism to enable communication among the mutually supporting OLTs has also been introduced. The control protocol enables distributed failure detection, updating of traffic load condition, and failure recovery. In MRSO, the remote node of the failed OLT can determine the node (OLT) that can take over the services along with the associated protection path depending on the current load condition. The paper further presents a new procedure for calculating availability of service for providing shared OLT protection. The availability of network service per end user subject to different failure scenarios within the proposed network architecture has been explored. Finally, our cost and scalability comparison shows that the proposed architecture provides significant cost and scalability benefit over previously proposed OLT protection schemes while achieving higher levels of network availability.

Ground Segment Design for Broadband Geostationary Satellite With Optical Feeder Link

High-data-rate space-to-ground optical links are anticipated to be an alternative to radio-frequency spectrum congestion for the next generation of high-throughput geostationary satellites. An affordable site diversity system based on several optical ground stations (OGSs) and an optical fiber network is necessary to overcome cloud obstruction of the feeder link. In this paper, we report three promising sets of OGSs in the vicinity of existing ingress and egress points of a selected pan-European optical fiber network. These OGS networks are optimized for reaching at least 99.9% feeder link availability while minimizing the overall cost of the system. For the first time, the optical fiber network between OGSs is optimized using existing high-data-rate fiber links to limit its expense. The resulting cost estimates for each OGS network highlight the need to define a new cost model considering optical feeder link specificity. In addition, the link availability is simulated using a 2 year cloud mask data bank, taking into consideration practical cloud blockage forecasting duration and assuming optical transmission through thin ice clouds.

Digitized Radio-Over-Fiber Technologies for Converged Optical Wireless Access Network

With the rapid deployment of optical access networks and the growing availability of mature and cost-effective opto-electronic system technologies, a unified optical feeder network could provide seamless integration of both broadband optical and wireless access networks. Radio-over-fiber transmission has been studied extensively as a way of achieving such high levels of network integration as well as simplifying wireless base-stations. Digitized radio-over-fiber has recently been reported as a possible cost-effective pathway for achieving this by taking advantage of parallel developments in electronic sampling systems and signal processing. In this paper, we present a review of recent progress in key systems and network technologies that can support the distribution of multi-band wireless signals in a converged optical wireless access network.