WP3 Communications Nodes

This Workpackage has two facets:
  • Development of communications equipment configurations for the aerial platform and customer premises / ground nodes. These may be deployed in the testbed, and used in the BFWA tests and performance measurements.
  • Development of advanced techniques critical to HAPs broadband communications. These include smart antennas, the associated signal processing for beam forming, and optical links.

Sub-workpackage 3.1 is concerned primarily with the integration of existing equipment supplied by BT, SKYLINC and CSEM within the testbed.

In WP 3.1 and WP3.2 particular emphasis is placed on the mm-wave equipment and antennas to further the aerial platform communications payload concepts developed within the HeliNET project.

Firstly, the work on HAP aperture antenna payloads will be extended to consider real antenna radiation patterns, and their statistical variation, on the co-channel carrier to interference ratios.

Secondly, key advanced technologies will be investigated in detail to determine the appeal for aerial platform applications. These include smart antennas, where the key challenges include wide-angle beam scanning, beamforming algorithms, and the associated signal processing burden, which will be tackled in WP 3.3.

Free space optical links for the backhaul will be developed by DLR and CSAG in WP3.4. The performance of these will be assessed using a testbed.

Trial Equipment (non-optical)

The mm-wave trial equipment will be based around 28GHz transmission frequencies. Stabilised antennas with appropriate pointing acquisition and tracking (PAT) technology will be used on the aerial platforms.

Equipment on the platforms will be kept relatively simple, with routing and networking functions carried out on the ground.
System level diagrams are shown below.
The mm-wave transceiver for the CPE will be interfaced with the customer terminal equipment(s) and a commercial dish antenna and positioner. There is also the option of interfacing with other terminal equipment, for example a DBS set top box for video services or existing 802.11 or 802.16 equipment.

system level diagram (1)
system level diagram (1).
(click to enlarge).
system level diagram (2)
system level diagram (2).
(click to enlarge).

Separate equipment will be used for the propagation measurement campaign. The ground equipment will comprise a number of relatively broad beamwidth antennas with individual mm-wave down converters which will interface to the measurement equipment so simultaneous measurements can be made for each antenna.

Steerable Antenna Technologies

HeliNet developed an outline system design for the HAP payload based on aperture antennas. Detailed co-channel interference analysis was carried out using simple models for antenna beam patterns. This will be extended to include real antenna patterns and the implications quantified.

Aggregate data rates will be calculated based on the Carrier-to-Interference Ratio offered by a payload of aperture antennas, and this will serve as a benchmark estimate of performance.

Compared to the aperture antenna solution, enhancements will be investigated for future HAP applications based on smart antenna concepts, in order to improve efficiency, flexibility, and cost effectiveness. The ability to control beam shape, direction and other characteristics would be of considerable benefit when using a HAP, owing to its relatively loose station-keeping characteristics.

Smart antenna techniques will be considered including array-based technologies. The radiating elements in such an array will be critical, and the pros and cons of different configurations, such as series-fed (resonant) arrays and corporate-fed arrays, will be assessed for their suitability for HAPs communications. Transmission line configurations e.g. waveguide and microstrip will likewise be assessed.
Array spacing, the elemental radiation pattern and the resulting field of view for beam scanning must all be considered.

Another key issues to address will be RF phase shifter performance and the associated costs.

These are all very active fields of research worldwide, and a survey of developments will be conducted. This will lead to recommendations for the best combination of technologies which should be applied to HAPs communications. It is anticipated that a combination of electronic and mechatronic steering will be required for the high-speed train antenna. Key RF components will be designed and their electromagnetic properties modelled, prior to fabrication and assessement, including radiation pattern measurement.

Advanced beamforming algorithms and the associated signal processing burden will be considered, particularly from a point of view of efficient algorithms which can be implemented in hardware.

A specific output will be to develop methods that will cope with high aggregate data rates, be computationally efficient, and minimise power consumption.

The antenna technology for the high-speed vehicle will also be challenging and will need to sweep more rapidly and over a wider angle than the HAP antennas.

Again appropriate mm-wave equipment will be selected and suitable beamforming algorithms and signal processing techniques will be selected.

Free-Space Optical Communications

Optical communications will considerably enhance backhaul and interplatform communications. HeliNet identified one of the main constraints of HAP communications systems is dealing with the aggregation of traffic (up to 14Gbit/s per platform). To cope with this amount of traffic will require 10-12 mm-wave backhaul ground stations over the coverage area. For the sake of cost/environmental impact the objective is to limit the number of backhaul ground stations.

Optical communication systems will allow data rates (622Mbit/s per link is proposed) in excess of that available using mm-wave bands based links. On backhaul links this can be used to augment, but not replace, the mm-wave bands to deliver non-time critical traffic (e.g. content distribution) in clear air conditions. The mm-wave bands can be reserved for the more time critical traffic.

Interplatform links can replace terrestrial (infrastructure or be used in areas where no infrastructure exists) and ensure that configurations of multiple platforms can realise their full potential in terms of capacity per sq km. Optical interplatform links will not suffer outages due to rain/cloud as they will be used well above cloud height (at 17-22km) compared with cloud heights of <5km altitude. Optical interplatform links will help to deliver spatial diversity and as such enhance backhaul availability. E.g. if it is raining at one platform site, traffic can be diverted to a clear site and backhauled into the wider network.

As well as developing the backhaul terminals, it will be the purpose of CAPANINA to validate the feasibility of using optical communications on HAPs.

The objectives of the first part of the free-space optical sub-workpackage are to design an optical terminal for stratospheric conditions, the necessary reception-hardware on the ground and perform measurements on the optical backhaul downlink channel. This will be carried out by DLR. This will be the first known experimental optical link form a HAP in the stratosphere and measurement data from this experiment will serve to evaluate models to design future optical HAP-HAP crosslinks where similar technology is used.

Channel measurements on the optical backhaul downlink from the HAP includes Bit Error Rate (BER) tests for data rates of several hundred Mbps, measurements of the atmospheric turbulent layers, measurement of the deterioration of the beam wavefront caused by atmospheric turbulence as well as measurements of the power scintillations etc. by use of scientific instruments on the ground (see WP3.4 for details)

For the optical HAP-terminal, technologies will be considered that can provide the required transmission power for the targeted bit rate (Direct modulated source or use of an optical amplifiers to meet the link budget requirements). A suitable pointing acquisition and tracking system (PAT), a data source (e.g. video and BRBS generator) for the transmission experiment, will be developed. Finally, the design of the optical transmitter terminal with minimised size and lightweight construction will be carried out.

An appropriate optical counter terminal for the ground station will be developed.

One critical aspect of optical links is the ability of them to cope with the HAP station-keeping characteristics, including HAP rotation and vibration. The potential of the high data rates for optical HAP-HAP crosslinks is strongly related to a reliable optical beam steering and tracking under the presence of the expected HAP environmental conditions.

In the second part of the free-space optical sub-workpackage Contraves Space is focusing on the development of a reliable optical beam steering and tracking for free space optical HAP-HAP crosslinks (specifically for 17-22km altitudes).

This involves both, a simulation phase about pointing, acquisition and tracking (PAT), the design of beam steering units for free space optical HAP-HAP crosslinks as well as a proof-of-concept hardware validation on breadboard level that verifies the critical items involved. Latter comprise the impact of HAP attitude motion and microvibration disturbances.

A rapid prototyping approach based on simulation results and involving a breadboard for the optical beam steering unit will be developed. This proof-of-concept hardware will include software modelling as well as the integration of complementary hardware items that have been realised in the frame of several ESA projects on optical intersatellite link terminal developments (SROIL, ISLFE).

A third aspect to the free-space optical communications sub-workpackage will be to examine the potential of non-mechanical beam steering. This will be carried out initially using numerical simulations, a selection of options will be compared assuming a selection of 2D phased arrays of lasers, optical amplifiers, or phase (electro-optical) modulators.

Additionally, the compatibility of non-mechanical beam steering with modulation at GHz rates will be assessed, allowing a PAT design recommendation to be made. Finally, the feasibility for the HAP application of using a quantum-cascade based infrared source operating in the mid-IR atmospheric window of 5-8 m will be assessed. A possible receiver construction to work with such a transmitter, and the applicability of non- mechanical PAT design in this wavelength range will be developed.