September 2003
Aircraft are one of the few places where mobile phones cannot be used. Sandra Gilligan and Nick Johnson believe that this is about to change.
Most members of the public take it for granted that mobile-phone use on aircraft should be prohibited for safety reasons. This has been backed up by a report published by the UK’s Civil Aviation Authority (CAA), which reiterates the dangers of mobile phones on aircraft.
Published earlier this year, the report cites studies that demonstrate that a mobile phone operating at a maximum transmission power of 2 W and within 30 cm of some avionics equipment or associated wiring can upset VHF omni-range (VOR) displays, induce audio interference in aircrew headsets and have other deleterious effects. As a result, the CAA will continue its call for aircrews to restrict mobile-phone usage on the flight deck and to remind passengers to switch off their handsets.
However, what is not widely reported is that over the past 6 and a half years, only 35 aircraft-safety incident reports cited mobile phones as a contributing factor. But surprisingly, this figure is not a result of passengers turning off their phones – the German airline Lufthansa admitted recently that, on average, at least one mobile is left switched on during every one of its flights. This information, combined with the difficulty of confirming that an incident is phone-related, has led to many in the aviation and telecoms industries – including pilots – to question whether a genuine problem exists. Indeed, if there is a serious risk, it would be appropriate for agencies like the CAA to suggest that airlines install live-handset detectors in aeroplanes.
Given the scant evidence of real – life problems, airlines have become more relaxed about interferences issues. Both American Airlines and Continental Airlines allow passengers to use mobile phones shortly after landing and before the aeroplane has reached the terminal. Scandinavian Airlines is looking for a technology to allow their business-class travellers to use their mobile phones while airborne, and Virgin and other airlines are investigating the deployment of in-flight base stations. With airlines looking for new revenue opportunities, it is only a matter of time, technology and a change in legislation before using mobile phones onboard aircraft will be commonplace.
Any in-flight solution must give the passenger the convenience of using their own mobile phone, with it’s personalized features and address book. An analysis of existing onboard telecoms services reveals little take-up because most are seen as expensive and cumbersome to use. As well as being user-friendly, in-flight GSM services would preclude the need for expensive seat-back phone systems.
The realization of onboard GSM networks could result in the emergence of a new breed of operators. These could be spawned by existing satellite or cellular operators, or by the airlines themselves. Whoever provides the service, it must appear seamless to the passenger. A method of signing up for "sky-mobile" before travel would enable the network to authenticate the user when they try to access the service onboard. The sky-mobile service could be accessed either by manual selection once onboard the aircraft or through a dual subscriber identification module, which would see the onboard service as a preferred network and connect to it.
There are two issues to resolve before in-flight mobile-phone services can be launched: how to send and receive signals to mobile devices without interfering with avionics equipment; and how to connect with earthbound telecoms infrastructure.
Handsets transmit pulsed radio-frequency (RF) signals at low power levels while in close proximity to a base station. The metal body of an aircraft acts as a Faraday cage, which attenuates the transmission between the phone and an external base station. Inside an aeroplane a handset must increase its transmission power to maintain a link with the outside world. During take-off – the most crucial time in terms of safety- the mobile phone will be transmitting at its highest level because it is moving away from the earth bound base stations. To make matters worse, base-stations antennas are tilted downwards, which further reduces an airborne phone’s ability to communicate with the ground.
The best way to minimize interference is to have an onboard base station that instructs all GSM handsets to reduce their power. However, not just any base station will do because size and weight are all-important parameters for avionics equipment. UK-based ip.access has developed a compact and lightweight system that combines Internet-protocol network equipment with handset RF technology. The result is GSM picocellular base station that is the size of a notebook computer and can operate at output power levels as low as 1mW. This power level is adequate for providing cellular coverage in the passenger and crew compartments of an aeroplane, and the base station is able to ensure that emissions from GSM mobile phones are kept at a minimal level.
It is important to note that this system is concerned primarily with the safe provision of GSM mobile-phones service on-board aircraft. If, for safety reasons, the power levels of phones based on other cellular standards had to be controlled, this could be done using commercial jamming equipment. Wireless local-area network and Bluetooth devices do not need to be controlled because they operate at much lower power levels and are not believed to cause interference.
Once an on-board network is installed, it must communicate with the outside world. For aircraft that fly overland in known flight paths, antennas could be placed at appropriate locations on the ground, pointing up at the aircraft. Existing base-station sites could be used where possible to reduce costs. Over water, of course, a satellite connection would be required.
Once safety and backhaul issues have been addressed, the precise nature of the network architecture – circuit – switched versus packet-switched – must be considered. An in-flight circuit-switched architecture would be based on a standard GSM base transceiver station (BTS), base-station controller (BSC) and mobile switching centre (MSC). These infrastructure components are large, both in terms of physical size and the number of callers that they can handle, and must therefore be scaled down in size for aircraft use.
The BTS would handle the aircraft interface to the mobile phone and the BSC would control one or more BTSs on the aircraft. The MSC would be connected to a satellite or a ground station and would also perform tasks such as authenticating the phone.
Although a circuit-switched system would be perfectly functional, the advent of pay-by-the-bit satellite networks make a packet-switched architecture a more attractive option (see figure). A key feature of this implementation is that the BSC and MSC need not be on the aircraft. This confers significant cost, weight and power benefits. Only the BTS and equipment that performs mobility management and authentication proxy functions would be onboard. An additional benefit is that services could be provided to a large number of aircraft using the same ground-based infrastructure.
Another important issue facing on-board operators is RF distribution within the passenger and crew compartments. There are two approaches to this. One involves distribution from the BTS at the RF level using leaky-feeders or indoor antennas fed by coaxial cables. The second is to use CAT5 UTP cable to create Abis connections to remote BTS located throughout the cabin. An Abis distribution scheme benefits from having a lower component count, cost and weight than an RF distributor scheme when multi base-station transceivers are required to cope with higher user demand.
In studies of in-building propagation. Ip.access’s BTS was able to provide coverage to a 50 x 15 m space, even at the lowest power setting of 1 mW. The entire main deck od a boeing 747 aircraft can therefore be covered easily by a single transceiver in a suitable position.
Beyond the safety issues, any in-flight system must be economically viable. A circuit-switched architecture involves the management of a complete GSM network onboard each aeroplane, which is a costly proposition. This suggest that a packet-switched system, with distributed picocellular base stations coupled with packet satellite services, is a more cost-effective solution.
There maybe a disgruntled few who are quite happy not hearing the irritating ring tones and shouts of "I’m on a plane" throughout a long haul flight. However, airlines believe that there is a need for an in-flight telecoms system that is more cost-effective and less cumbersome than existing solutions.
Sandra Gilligan is Marketing Manager at ip.access and Nick Johnson is Chief Technology officer at the UK-based company.