Special Report Aerospace

With air traffic across Europe at

an all time high, the aviation

industry is running short of

communications spectrum.

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.

In this concept, 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

transmit 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

13 April 2010 21www.newelectronics.co.uk

Corenetwork

Integratednetwork

controller

Airside

Groundstation

IP bit pipe

Groundstation

Groundstation

The EAS-3G system

concept links ground

stations, air stations

and integrated network

controllers to provide

3G wideband

communications for

aerospace applications

Let’s talkHow a commercial 3G system has been adapted to provide a high capacity air to ground link.By Steve Lane.

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Special Report Aerospace

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. The transmit (Ftx)

and receive (Frx) frequencies for a

nominal channel frequency (Fchannel)

were corrected by the Doppler frequency

(Fdoppler), such that:

Frx = Fchannel + FdopplerFtx = Fchannel – FdopplerThe Doppler frequency was estimated

based on the position and velocity of the

aircraft relative to the base station –

with information provided by the

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 in the

avionics TDD system allowed for the

potentially large range between the

aircraft and each base station. One time

slot was allocated to switch between

transmit and receive channels, which

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

commercially available products can

reduce system development costs

significantly.

Triteq has since been working with

Eurocontrol on the evolution of the

system, including improved spectrum

utilisation and frequency synthesis/

control.

Author profile:

Steve Lane is Triteq’s

commercial director

(www.triteq.com).

An industry standard

ARINC 4MCU enclosure

allowed the system to

be certified for

airworthiness and flight

trials to be conducted

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.

• 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.

13 April 201022 www.newelectronics.co.uk

Fig 1: The aircraft side system

3G pcmiamodem

1·9GHzPower

amplifier

5GHz

tx/rxswitch

Low noiseamplifier

Fig 2: Top level view of the system

GFC AFCNode BINCIP core

networkcomponents

3G modem(PCMIA)

Applicationsprocessor

Aircraft position information

Ground network

5·09 to5·15GHz

air interfaceGround station

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