14 Apr 2010
Satellite backhaul for In-Flight connectivity for Wi-Fi and Cellphones
Our editor, Ian Poole finds out about how Thales has succeeded in providing in flight connectivity for passengers as part of their integrated in-flight entertainment (IFE) system.
With aircraft passengers increasingly wanting to use their laptops on board aircraft, they need direct internet connectivity, as well as many wanting cellphone connectivity while the aircraft is in flight.
For this to be possible, it is necessary to locate both Wi-Fi access points and cellphone picocell base-stations on board the aircraft. The backhaul can then be provided by a satellite link.
The solution may appear simple at first sight, but requires careful design and many problems to be overcome. However the concept is now well proven, flight trials have taken place and deployment is at advanced stages on many aircraft with some having already launched services.
In-flight connectivity system
The basic Thales "TopConnect" system used to provide in-flight connectivity is centred around a satellite transceiver-modem or "Satellite Data Unit". This uses the Inmarsat SwiftBroadband service to provide the satellite link. This uses L-Band frequencies around 1500 MHz. This is ideal because it allows low profile electronically steerable antennas to be used, thereby reducing drag and fuel costs. Alternative satellite systems using KA or Ku Band require mechanically steerable antennas that have a much higher profile.
On board the aircraft there is also a server. This passes data to and from the satellite data unit and distributes it between the cellphone picocell, the wi-fi access point, or any other user of the data.
On the RF side, data is transmitted to and from the Inmarsat satellite and routed down to one of their ground stations. The Inmarsat system uses three satellites to provide near global coverage - like all geostationary satellites, coverage is limited towards the poles because of the geometry of the scheme.
From the satellites the signal is taken to the ground stations located at suitable points around the globe. In the ground stations, the data is linked in to the relevant public service network - a partnering telecommunications provider is used to then link the data into the public network. This enables Wi-Fi to be connected back to the Internet, or cellphones to be connected accordingly.
Figure 1 Thales TopConnect System Architecture
Satellite data unit
The Thales satellite data unit has been developed to specifically to provide communications via the Inmarsat satellite system. The satellite data unit is compliant with ARINC 781 and provides all the facilities required to interface IP data with the other units within the aircraft and then interface with the satellite.
With the Inmarsat SwiftBroadband system being based around the protocol stack that is very similar to the 3G UMTS system, protocol interchanges need to be managed by the SDU.
Figure 2 Thales SDU used for the satellite backhaul for the IFE connectivity
Facilities including accurate power control, logging on, handover from one spotbeam to another and all the protocol exchanges needed to carry the data are all managed within the SDU.
The Inmarsat SwiftBroadband system allows for data throughput rates up to around 432 kbps per channel in the uplink and downlink, dependent upon the signal levels and activity within the given geographical area. However the Thales SDU is able to provide higher overall data rates by aggregating further channels. Currently there is a maximum number of two, although there are plans in the future for additional capacity if airlines require this.
In order to manage the data requirements within the aircraft itself an external server is used. Not only does this manage the data routing, but it also provides additional compression of the data being transmitted. With bandwidth being limited, it is necessary to make the maximum use of the available resource. To this end, the system uses a compression algorithm that is able to compress the voice data from the mobile phones without any noticeable degradation in voice quality - quality is equivalent to that of the UMTS AMR codec. The use of the compression algorithm gives a significant reduction in the data rate while maintaining the user experience.
One of the interesting elements of the overall system is the cellular telecommunications picocell. This is a specialised unit for airborne applications. When able to do so, users turn on their cellphones and they connect to the picocell. Like all cellular base-stations this controls the output power from the cellphone which it will throttle back as much as possible to prevent the cellphone radiating any more power than is necessary. This will assist in preventing any interference with onboard systems, and also prevent the cellphone from connecting to any terrestrial base stations. Additional circuitry is available within the picocell to bar any unauthorised frequencies.
Additional IFE connectivity
Not only does the system provide for direct access for users, but it also enables updates to be provided for the in-flight entertainment system. Small news updates can be accessed over the satellite link and used to ensure that while the aircraft is flying, news broadcasts can be update. This can be achieved as a background activity during periods when the data from other applications falls. It is not anticipated that full video broadcasts would be updated in this way as a result of the levels of bandwidth required, although small "Youtube" style videos may be updated in this fashion.
A further facility is that the satellite system can be used for is to send SMS and emails from the seat back screens. With SMS messages and most emails not taking large amounts of data, this too can be done without adding a significant load to the backhaul.
In addition to this, the IFE system can offer the ability to connect wirelessly to the gate at the airport. Using traditional methods, updates to the on-board videos, including news which need to be updated before each flight, can be updates via the gate without the need to manually load a VCR style tape. Using either WiMAX, Wi-FI or even cellular systems, the data can be uploaded cleanly and effectively without the need for manual intervention.
Although many of the applications may appeal to the large airliners, the overall system has been designed to be scaleable. In this way it can not only be fitted onto a large 747 or A380, but it can be tailored for the growing bizjet market. In this way executives are able to operate.
The TopConnect Wi-Fi solution for business and regional jets offers Wi-Fi or wired broadband, enabling web browsing, connection to Virtual Private Networks (VPN), email, video conferencing, Voice of IP (VoIP) telephony and Unlicensed Mobile Access (UMA) for GSM telephony and SMS through passengers own smart phones.
The Thales TopConnect system provides connectivity using the Inmarsat SwiftBroadband system for air travellers from the small bizjet arena right up to the large 747s and A380s. Although many aircraft do not have these facilities yet, most of the large airlines are planning to incorporate this type of technology as they buy new aircraft or update their existing ones. Although many may not like the idea of people using their mobile phones in the next seat, this is likely to be the norm in the coming years as the traveller expects to have connectivity while airborne.
One major hurdle to acceptance is cost, especially with people remembering the costs of using satphones in seat arms in previous years. For this system users are billed via their normal cellphone bill and charges comparable to international roaming rates. Another major requirement is to have a system that works effectively - this seems to have been successfully achieved with trails proving the overall system.
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About the author
Ian Poole is the editor of Radio-Electronics.com. Having studied at University College London to gain his degree he went on to undertake a career in electronic development working for companies including Racal. He became the hardware development manager at Racal Instruments where he was in charge of the hardware development activities within the company. Later moving in to freelance work as a consultant he also developed Radio-Electronics.com to become one of the leading publications for professional electronics engineers. He is also a Fellow of the Institution of Engineering and Technology and is an author of over 20 books.
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