13 April 2010
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How a commercial 3G system has been adapted for in flight communications.
With air traffic across Europe at an all time high, the aviation industry is running short of communications spectrum and onboard security has presented additional challenges.
To help find solutions, the European Commission and Eurocontrol, the organisation in charge of European air navigation, have explored the potential of using 3G wideband technology for secure communications. In particular, Eurocontrol has been looking at 3G as a potential solution for Air Traffic Management (ATM) security.
One option was to provide a high capacity air-ground downlink to support the transmission of encrypted voice, flight data and onboard video. This could be transmitted from the cockpit during a security alert, providing ground based decision makers with a clearer picture of the situation onboard the plane.
System overview
The European Aviation Security based on 3G technology (EAS-3G) project is centred on a C-band air-ground link operating at around 5GHz.
The concept is the 'traditional' node B and UE elements of the UMTS TDD system are replaced by ground stations (reconfigured node Bs) and air stations (reconfigured 3G PCMCIA modems). A data link is established and maintained between air and ground stations, with the system performing handovers across cell boundaries. In effect, 3G UMTS TDD technology provides an IP bit-pipe between the ground segment and the air station.
Triteq became involved in the project in 2006, following initial concept trials by Eurocontrol. The requirement was to develop a working test system based on a commercially available 3G modem, enclose this in an avionics box and conduct flight trials. Triteq provided electronics design support to the project, which meant overcoming several technical challenges. A key aspect was to adapt a commercially available modem and ensure the system would operate at aircraft speeds.
The ground station was based on industry standard UMTS-TDD equipment working at 1.9GHz, with a conversion to the 5GHz aeronautical band. The converter was a separate development carried out by Triteq in parallel with the avionics circuit board manufacture.
Challenges
One of the main technical challenges was implementing the avionics equipment, particularly because of size, power and weight constraints.
* Air side configuration
The avionics system was conceived as a PCMCIA 3G modem with the receive and transmission signals being converted between 2GHz and 5GHz for the air interface link. A PC/104 module provides control functions, including compensation for Doppler shift and the correct timing advance for random access channel (RACH) transmissions.
* Modifications
A commercially available 3G modem minimised development costs, but had to be modified to gain access to specific signals and to split the transmit and receive signals. In addition, the extended timing advance mechanism within the modem had to be controlled. Range limitation (due to the RACH configuration) was solved by sacrificing a timeslot and by modifying the RACH burst type and mapping it to a normal burst.
* Doppler shift and correction
UMTS is not designed to cater for aircraft velocity – 250km/hr is about the maximum possible. But, because planes can cruise at ground speeds in excess of 1000km/hr, the Doppler shift is well outside of a 3G system's normal operating tolerance.
Triteq had to enable the modem to compensate for both Doppler shift and reference frequency tolerance using an automatic frequency control (AFC) methodology. AFC was determined from the decoded received signal and used to control the reference crystal oscillator. This, in turn, was used to provide the clock for digital processing, to phase lock the local oscillator for both the received and transmitted signals and to correct the frequency error of the reference oscillator.
To maximise performance, the mobile modem needed to appear nearly stationary with respect to the fixed ground station. Hence, the Doppler shift compensation applied to the transmitted signal had to be equal and opposite to that of the received signal. So the transmit (Ftx) and receive (Frx) frequencies for a nominal channel frequency (Fchannel) were corrected by the Doppler frequency (Fdoppler), such that:
Frx = Fchannel + Fdoppler
Ftx = Fchannel – Fdoppler
The Doppler frequency was estimated based on the position and velocity of the aircraft relative to the base station – the information being provided by the aircraft's navigation system via an ARINC interface.
The frequency error introduced by Doppler Shift depends on the speed of travel and operating frequency. The worst case was assumed to be an operating frequency of 5.15GHz and a speed of 1225km/hr. This produced a Doppler shift of ±5.8kHz. The frequency synthesiser therefore had to allow for this range of correction.
* Frequency drift
The initial frequency error between modem and base station needed to be corrected. The modem's AFC mechanism was designed to correct for the frequency error introduced by a crystal oscillator in the modem. A higher specification part was substituted in place of the modem oscillator and this improvement had to be sufficient to allow for the error introduced by the frequency translation.
* Phase noise and spurs
The system does not have to meet 3GPP standards because it is not connected directly to a commercial network. It is also subject to cochannel and adjacent channel interference, so it is not possible to fully specify the phase noise and spur requirements.
Good rf filtering was provided so out of band spurs did not need to be suppressed. An initial target of non harmonic spurs of less than -60dBc was achieved.
Phase noise performance similar to a typical 3G system was also specified –
less than 100dBc at 100kHz and less than -130dBc/Hz at 10MHz
* Switching time and frequency control
A special allocation of time slots was required in the avionics TDD system to allow for the potentially large range between the aircraft and each base station. One time slot was allocated to switch between transmit and receive channels. This gave a maximum 'window' of 600µs to carry out the switching on the airside.
For the initial prototype, the lowest risk option was followed. Separate tx and rx vctcxos were used to drive separate tx and rx synthesisers, allowing rapid switching between transmit and receive without the need to vary vcxo frequency or to allow the pll to settle.
The tx and rx vcxos were both of nominal 10MHz frequency. At turn on, a calibration of each vctcxo's tuning slope provided a coarse guide to the control voltage required to achieve a particular output frequency. During normal operation, the vctcxo's output frequency was measured by the control system and the control voltage adjusted to achieve the correct frequency.
PC/104 control was also implemented. This platform was used to develop software for modem, system and Doppler control. It also allowed interface to other wireless systems, including 802.11a/g.
Test and future
The prototype system has been tested successfully by Eurocontrol, with high speed live data and video transmission. The project demonstrated that working with commercially available products can reduce system development costs significantly. Triteq has since been working with Eurocontrol on the evolution of system, including improved spectrum utilisation and frequency synthesis/control.
Author
Steve Lane, commercial director at Triteq
Supporting Information
Downloads
24260\P21-22.pdf
Websites
http://www.triteq.com
Companies
Triteq Ltd
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