Chapter 14 Applications of Free Space Optical Communications in Hap Scenarios

Chapter 14 Applications of Free Space Optical Communications in Hap Scenarios

Sub-Editor:M Knapek

Abstract: Relevant application scenarios for laser communication in HAP scenarios are shown. Main applications are backbone scenarios, where the high data-rates of optical communication can fully develop their advantages. Laser links would be used to interconnect networks of HAPs. Additional connections to LEO/GEO satellites would provide the connection to international telecom networks. Links between HAPs and to satellites are not influenced by clouds, as they are operated above the cloud layer. Therefore full availability can be provided. Laser downlinks from HAPs to ground stations suffer from a limited availability due to clouds and ground fog. Improvement schemes like ground station diversity, i.e. flexible links to several ground stations without cloud blockage, or hybrid microwave/laser connections are discussed.

A special application of optical communication is quantum key distribution from HAPs. Quantum key distribution provides a method for absolutely secure key exchange. HAPs could provide the ideal platform to distribute security keys to separated regions as between cities or neighbouring countries.

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List of Authors:

M. Knapek / German Aerospace Center, Germany
/ 14.1, 14.2, 14.3
S. Arnon / Ben Gurion University, Israel
/ 14.4
D. Kedar
T. Sheidl
R. Ursin

Table of Contents

Chapter 14 Applications of Free Space Optical Communications in Hap ScenariosFormel-Kapitel 1 Abschnitt 1

14.Applications of Free-Space Optical Communications in HAP Scenarios

14.1.Introduction

14.2.Networks above the Clouds – Inter-HAP Links

14.3.HAPs as Relay Stations

14.4.Hybrid RF & Optical Communication Systems for HAP-to-Ground Links

14.5.Quantum Key Distribution from HAPs

14.5.1.Motivation

14.5.2.Introduction

14.5.3.Technical Concepts

BB84 Protocol with Weak Coherent Laser Pulses

Decoy State Protocol with Weak Coherent Laser Pulses

Entanglement Based BB84 Protocol

14.5.4.State-of-the-Art Realizations

Weak Coherent Laser Pulse Based Systems

Entanglement Based Systems

14.Applications of Free-Space Optical Communications in HAP Scenarios

M. Knapek, S. Arnon, D. Kedar, T. Sheidl, R. Ursin

14.1.Introduction

High data-rates in the range of Gbps are the strongest motivation for the use of free-space optical communication systems in comparison to microwave communication. Optical communication technology offers small terminal sizes and low weight, very small aperture sizes and low power consumption. The small divergence angle of optical communication systems excludes in addition the danger of eavesdroppers. However optical beams are blocked by clouds, which have to be taken into consideration in the planning of such communication systems. Fig. 14.1 shows possible scenarios involving HAP optical communication links:

Inter-HAP optical connections to form a network of HAPs
HAP to satellite optical links to relay data via satellite to the ground
Satellite-to-HAP optical links to relay data via HAP to the ground
HAP-to-ground links by microwave and/or optical means. Hybrid schemes are conceivable.

Fig. 14.1HAP-scenarios involving optical communication links: Basis is an inter-platform network indicated by three HAPs. The HAPs are connected by optical and RF downlinks to the ground. In addition links to GEO and LEO satellites are shown.

14.2.Networks above the Clouds – Inter-HAP Links

Inter-HAP connections form the basis of this network. They allow the exchange of date between HAPs over large distances respectively between regions. Cloud blockage or atmospheric attenuationare not limiting factors for inter-platform and platform-to-satellite links, as they operate above the cloud ceiling. HAP networks would allow large scale diversity schemes to transmit data at cloud-free locations by optical means to the ground. The network configuration would be dynamically adapted to optimize the throughput. As the HAPs are interconnected by optical links, there is no necessity of a terrestrial network.The optical links provide almost unlimited bandwidth to the network without the limiting aspects of spectrum availability and spectrum regulations.

HAP networks would be installed to provide communication services with one platform covering an extended area. These communication services could be required for urban areas, where the communication infrastructure is not fully developed, or in order to provide services for large rural, thinly populated, areas. HAPs show the advantages of satellite services with a large coverage area, but they avoid the link-budget problems of the larger satellite-to-ground distances. In this way HAPs combine properties of terrestrial and satellite networks.

Fig. 14.2Artists impression of a HAP network over the southern part of Africa with optical interconnection links. Five HAPs with microwave payloads are shown with the indicated footprint (gray).

Optical and microwave links interact with the atmosphere in different ways. It is therefore important to investigate the effects of atmospheric structure, attenuation, and turbulence on the propagation separately for an optical beam. In addition, geometrical concerns and background light levels will also affect the design of an optical HAP-HAP link.

Stratospheric platforms operate at low air pressure due to the altitude. In the consequence atmospheric attenuation effects (absorption, scattering) and index-of-refraction effects are rather weak. Index-of-refraction effects are explained in the next Chapter.

The earth's atmosphere consists of several distinct layers with the troposphere at the bottom. Usually all weather phenomena (and thus cloud coverage) happens inside the troposphere which has a negative temperature gradient on average – typically 1 degree cooler per 100m rise in altitude. The tropopause is defined by the reversing of the temperature coefficient which then is positive inside the stratosphere. The lower bound of the stratosphere varies from below 8km at the poles, up to 18km at the equator.

Giggenbach et al. [[1]] studied the maximal distance between two HAPs taking into consideration the maximum height of clouds (cloud ceiling). In moderate latitudes clouds can be found up to 13km altitude. This altitude increases to about 16km closer to the equator. Only in rare weather conditions like severe thunderstorms might clouds rise above the tropopause.

Fig. 14.3 shows the link geometry between two HAPs, where the curvature of the earth and the cloud altitude limit the link distance. If the grazing height of the link drops below the upper limit of the clouds, the availability drops below 100%. Fig. 14.4 shows the maximum inter-platform distance in relation to the height of the cloud ceiling. Three different HAP altitudes are indicated. Assuming a typical upper limit of the clouds of 13km at moderate latitudes, link distances of about 600km to 800km are possible with full availability.

Fig. 14.3Illustration of the influence of the curvature of the earth on the link distance. The cloud altitude influences the maximum distance for inter-platform links avoiding cloud blockage.

Fig. 14.4Dependence of maximum link distance on the cloud top height (CTH) under the assumption that both HAPs have the same height above ground.

14.3.HAPs as Relay Stations

HAPs could be additionally connected to GEO or LEO satellites. These links could be used to relay data via satellite to ground.The relay of data via GEO satellites would allow real-time data access from/to distant regions, which might be interesting for reconnaissance scenarios and for operations in emergency situations.

In the other direction HAPs might be used as relay stations above the clouds for optical downlinks from Earth Observation (EO) satellites [[2]][[3]]. In this scenario the link from the satellite to the HAP would be achieved by optical means. For the last 20-60km to the ground, a microwave or a hybrid optical/microwave scheme would be used. Optical links could be only deployed during favorable weather conditions but with higher data throughput. The availability of the optical link would strongly depend on the location of the HAP operation (see section 15.5).

EO satellite missions produce a large amount of data for example using high-resolution optical or radar sensors. During the last decades the amount of data has steadily increased due to improved sensor technologies with increased temporal resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps overcome the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability through the atmosphere due to cloud blockage above the ground station. Apart from diversity concepts with several ground stations or satellite networks, stratospheric HAPs could act as a relay station to forward the optical communication beam over the last 20km through the atmosphere to the ground station, where short-range, high data-rate microwave systems are feasible or hybrid concepts could be implemented. Decisive for such a scenario is the link capacity in comparison to direct satellite-to-ground microwave links.

The link capacity results from the number of satellite-HAP contacts and the duration of the contacts. The number of contacts and therefore the link duration per day of a LEO satellite to a HAP strongly depends on the geographic latitude of the HAP location and the orbit of the satellite. Fig. 14.5shows link durationsfor TerraSAR-X as an example of a typical EO LEO satellite. TerraSAR-X has a nearly polar orbit with 97.4° inclination and 508km orbit height. The difference in the link duration is mainly caused by the contact number satellite-to-HAP. For a nearly polar orbit there are about 3 contacts per day for a HAP position at low geographic latitudes and up to 15 close to the earth’s poles.

The link duration from a LEO satellite to the HAP or ground station also depends on the minimum elevation angle under which the satellite can be seen and a communication link is possible. Larger elevation angles (5-10°) are required to establish a communication link for stations on the ground due to a limited horizontal line-of-sight and atmospheric turbulence/attenuation effects. For links to HAPs elevation angles even below the horizon (-2°) are possible due to the elevated position of the platforms. The maximum link duration per pass is about 8minutes at a minimum angle of 10° and 13 minutes for -2° elevation.

Fig. 14.5Link duration from an earth-observation satellite (TerraSAR-X) to ground stations at various latitudes. Link durations for minimum elevation angles of zero and 5 degrees as a constraint are shown. Link durations slightly change for a LEO-HAP link with the HAP at 20km altitude.

The link capacity (e.g bits per day) from the LEO satellite to the ground depends on several factors:

Effective downlink bitrate (of the segment with the lowest capacity) in bits per second. Modern optical systems like the Laser Communication Terminal (LCT) on TerraSAR-X have bitrates of 5.6Gbps over several 1000km and 2.8Gbps for GEO distances (40000km). Microwave links operate at several 100Mbps. Wavelength Division Multiplex (WDM) methods are envisioned for optical links, which would significantly increase link capacities into the 100Gbps region.
Cumulative link duration per day (line of sight). The link duration for links from LEO satellites to HAPs or ground stations mainly depends on the number of satellite passes over a station and the minimum elevation angle.
Cloud-free time is a crucial issue for optical links to the ground. Minimum values of cloud coverage appear at about 20 degrees latitude north and south (Chapter15).

The transferable mean amount of data per day D for an optical HAP-to-ground link is calculated from the effective downlink bitrate feff,the cumulative link duration T per day in dependence of the geographic latitude λgeo and the minimum elevation angle α, and the probability of cloud blockage pcloud, which depends mainly on the latitude λ and the time of the year t.

/ (14.1)

Possible scenarios could be links from the LEO satellite via HAP to the ground, but also links from the LEO satellite via a GEO satellite and a HAP to the ground. For the link through the atmosphere a high capacity microwave link is assumed with a bandwidth of 800Mbps, since the link distance is only a few 10’s of km. As the data rate to the ground is smaller than the rate from the satellite to the HAP, data buffering on the HAP might be necessary. Data transmission would continue from the buffer when the satellite would be already out of sight. Data transfer to the ground would be delayed but the full optical link capacity could be used. Data delay would be acceptable for EO missions, however not for real-time communication.

Fig. 14.6 shows the mean transmitted data volume from a LEO satellite for various transmission schemes. Example geographic locations (Tenerife, Calar Alto, Weilheim, etc.) were taken to obtain mean cloud cover values, which resulted from a study with satellite EO images [[4]]. A typical microwave X-Band LEO-to-ground (LEO-GND) downlink (DL) as on TerraSAR-X offers a data rate of 300Mbps (Curve 1). This results in a downlink capacity of about 0.45Tb to 2.25Tb per day (1500s-7500s link duration per day in dependence of the latitude at a minimum elevation of 5deg). In comparison the link via a HAP relay (LEO-HAP-GND) increases the downlink rate to 1.9-8.0Tb per day (unbuffered) at 800Mbps (Curve 2), and 13.3-55.8Tb per day (buffered) at 5.6Gbps (Curve 4), both with full availability due to the microwave link on the HAP-to-ground link (800Mbps). Typical link durations for LEO-to-HAP links are 2400-10000s at a minimum elevation angle of zero degrees elevation. Using an optical HAP-to-ground link at 5.6Gbps provide mean daily transfer rates of 7.3-14.0Tb (Curve 3), however the link might be blocked for longer periods due to clouds. The increase of data volume due to the larger contact time at higher latitudes is almost defeated by the increased cloud coverage at these latitudes.

A future network of HAPs interconnected with optical links would eliminate the cloud blockage problem of optical links by providing optical HAP-to-ground links at different geographic locations. HAP constellations in favourable locations achieve an overall link availability of over 95%. In addition the HAP network would multiply the link duration LEO-to-HAP by the number of HAPs and therefore the transferable data volume. Optical HAP-to-ground links appear to be sensible for a HAP network of three or more stations, where the overall availability is close to 100%. Details on diversity schemes are given in the next chapter. Also hybrid systems with an optical and in parallel a microwave communication system provide full availability and higher data rates.

Fig. 14.6Mean daily transmitted data volume from a LEO satellite to the ground over the elevation angle of the ground stations. Mean cloud coverage at the ground stations were used.

Optical data transmission from LEO satellites via GEO relay satellites has the advantage of higher link durations as they cover nearly half of the LEO orbit, however, the overall system complexity significantly increases with the requirement of an additional GEO satellite. Due to the larger link distances the terminal size, power consumption and weight of optical LEO-to-GEO link terminals are higher, which beyond that prevents the use of these terminals on small LEO satellites.

14.4.Hybrid RF & Optical Communication Systems for HAP-to-Ground Links

The high data-rates have made free-space optical (FSO) communication systems the preferred technology for inter-satellite links, where the impediments of atmospheric propagation are absent. However, the high outage probabilities associated with FSO links in an atmospheric channel have hindered their widespread terrestrial implementation. Only recently a growing interest in optical links in the terrestrial sphere is emerging as a method to boost traffic throughput where radio frequency infrastructure exists and to provide easily deployable and flexible enhancements to wireless communication systems. These are hybrid systems where optical and radio frequency systems work side-by-side and overall system performance can be increased by the diversity offered.

Weather conditions such as fog and haze threaten the availability of an optic link, while rainstorms can reduce power reception in a millimetre wave link to prohibitively low levels. The presence of a hybrid optical/millimetre link offers the possibility of reduced downtime by switching from one communication modality to the other as weather conditions change. When the propagation allows for FSO links the high data rates offered by the optical method can be transmitted, increasing the throughput. When only lower data-rate (radio) transmissions are possible due to fog or haze, traffic can be prioritised with data transmissions that allow long latency (email, file downloads) buffered and sent later, while real-time transmissions and high priority services (voice, high QoS transmissions) would be sent at lower data rates via the radio link.

The addition of networking between HAPs and ground stations further promotes the implementation of hybrid RF-optical systems. A HAP-to-ground station link that is temporarily unavailable for high data-rate optical transmissions can be circumvented by transmitting the signal via inter-platform links to another HAP and then sent to a nearby ground station, from which it can be transferred by an optical fiber link to the original destination. This simple scenario is illustrated in Fig. 14.7.