Broadband Trials to Fixed Users from Aerial Platforms
The purpose of this part of the project is to demonstrate the different broadband services and applications and undertake several measurement campaigns.
Both mm-wave and free space optical technologies will be examined.
We first of all describe the communications equipment under development and this is followed by a description of the System Testbed, which will provide a unified test and measurement methodology in which to carry out the communication trials with several types of aerial platform.
Wireless EquipmentThe mm-wave trial equipment will be based around 28/29 GHz transmission frequencies. The first trial will use a tethered aerostat with stabilised antennas with appropriate pointing acquisition and tracking (PAT) technology used on the aerial platforms.
Equipment on the platforms will be kept relatively simple, RF signals at the intermediate frequency fed from a modem on the ground over a fibre radio link. The aerostat tether contains the optical fibre and power feeds to the platform. Routing and networking functions carried out on the ground.
The second trial utilising a free flying stratospheric balloon will implement a simpler payload due to stringent payload mass and weight constraints. The application server will be flown on the platform and the services delivered to the CPE over a mm-wave link.
The performance will be demonstrated over a wide range of connection lengths and a range of data rates.
The payload will implement a single beam to cover the whole of the ground footprint. This will be achieved using a dielectric lens antenna with embedded feed designed to either produce a constant field strength on the ground or constant gain over the whole coverage area.
The mm-wave link will use the IEEE 802.11b standard by utilising commercial wireless base unit equipment as the modem with an intermediate frequency input and output at 2.45 GHz feeding the mm-wave transceiver.
On the ground the CPE will use a larger than standard dish to compensate for the reduced effective radiated power on the HAP due to the single rather than multicell architecture.
This dish will be steered to point at the HAP using a pointing and tracking controller.
The mm-wave radio will again interface to a commercial remote bridge and onwards to the customer equipment.
The modem can switch seamlessly between data rates between 1 and 11 Mbit/s depending on received signal strength demonstrating adaptive modulation and coding on the link.
Free-Space Optical CommunicationsAs we have discussed above, reliable pointing and tracking systems, as well as understanding the impact of the atmosphere on optical wave propagation, are the most important requirements in the design of reliable optical terminals for HAP applications.
This equipment will then be used to conduct the first known experimental optical link from a HAP in the stratosphere and the measurement data delivered from this experiment will serve to evaluate models to design future optical inter-platform links where similar technology is used.
Beside the demonstration of a broadband optical downlink, research on index of refraction effects are one of the main interests.
Temperature inhomogeneities of the air create regions with different local densities. Each of these inhomogeneities is actually a region of high or low refractive index and is unstable in size and time.
The thermal energy of air leads to a turbulent mixing process of these local regions with different refractive index (eddies). This spatial redistribution of irradiance due to the atmospheric turbulence along the path of an optical beam propagation leads to a speckle pattern and a distorted wavefront at the receiver.
These effects and their causes will be measured during the experiment with two instruments. The turbulence profile is depicted by the refractive-index structure constant Cn2(h) which is a measure of the strength of turbulence over the height h.
The first instrument, the single-source turbulence profiler deduces the turbulence profile Cn2(h) along the propagation path during the trial from the spatial structure of scintillation which will be recorded at the optical ground station.
The second instrument, the DIMM instrument (Differential Image Motion Monitor), derives the spatial statistics of phase fluctuations within the pupil plane of the receiver.
These parameters are important to assess the atmospheric impacts on free-space optical communication systems, evaluate different modulation formats and compare with simulation results.
System TestbedThe "System Testbed" will be used primarily to test several possible broadband applications and services selected. It will build on previous work developed as part of the HeliNet project, and to provide vital measurement data that will be used elsewhere, and assess the viability of the optical communications aspects.
Three different aerial platform technologies are envisaged for supporting the aerial segment of System Testbed through a complete test campaign. To support this a common test/measurement methodology will be developed in order to maximise the coordination and use of common equipment and maximise the applicability of the different measurement data.
The aerial platforms envisaged are:
Trial 1 - a set of tests (over several weeks) with a 300 m altitude tethered aerostat (Pershore, UK)