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Experimental Work & Field Testing

Aegis BalloonMuch of the company's work is concerned with the mathematical modelling and simulation of radio systems, and the interaction (e.g. interference) between them. Generally, this simulation work can make use of established models and algorithms, such as those contained in the ITU-R Recommendations.

It is sometimes the case, however, that no suitable models exist, or that the existing models do not cover the required scenarios (frequency, environments, etc.). In such cases it may be necessary to gather new data as the basis for developing new models.

A recent example of such a measurement campaign was a study, funded under the British National Space Centre SATCOM programme, that sought to determine the degree to which radio signals to or from satellites are attenuated as they pass in or out of typical buildings. This information is important both for the designers of satellite radio systems that may be used indoors (e.g. satellite DAB or mobile communications) and in estimating the interference to satellite uplink receivers from terrestrial transmitters that share the same band (such as wireless LAN systems). To explore the dependence of such building penetration losses on the path elevation angle, a tethered helium balloon was used, and this was raised and lowered to explore the required geometries.

Measurements at 1.3 GHz, 2.4 GHz and 5.7 GHz were made using small transmitters carried by the balloon. The power received inside the buildings was sampled over a range of several wavelengths using a computer-controlled antenna positioner. Full details of the results of this study may be found in a paper given at ICAP.

Correlator Equipment   Correlator Output
Correlating Receiver   Typical channel impulse response (outdoor-to-indoor, 2.4 GHz, x-axis 0-60 ns)

A further project for BNSC investigated the wideband characteristics of the outdoor-indoor radio channel at 2.4 GHz. The sliding correlator constructed for these measurements made use of pseudo-noise code sequences, clocked at 100 MHz. These sequences have excellent autocorrelation characteristics, and offer an efficient approach to channel sounding. The reference sequence is clocked at a slightly lower rate than the transmitted sequence, and the two codes therefore drift past each other. The received signal is thus 'explored' by the reference sequence, and the impulse response of the channel determined. The sliding correlator architecture avoids the need for high-speed data capture, as the correlator output is dilated by the ratio of slip rate to clock rate. This correlator system was used to obtain statistics relevant to typical UK domestic building types, at high temporal resolution.

Channel Sounder  

More recently, on behalf of Ofcom, we have undertaken a campaign of outdoor measurements, for which a lower resolution is appropriate. This system, clocked at 10 MHz, operated at three frequencies, 237 MHz, 1477 MHz and 3430 MHz. Because of the lower clock rate, it is straightforward to sample the received signal directly at IF, and to perform the de-correlation in software. This dramatically reduces the time taken to sample each delay profile, allowing mobile measurements to be carried out.

DSP-based channel sounder showing receiver (above) and 3-band transmitter (below)  

When the power delay profiles are plotted, then discrete reflections can be seen, with relative delays that vary along the route. These delay values can be used to construct a series of ellipses, allowing the reflection points to be determined. In the example shown below, one of these reflection points is a large office block ("Brown & Root") to the north-west of the measurement area; a second reflection point could not be firmly identified, but appears to come from part of a hospital complex ("Ingleby House"); a reflection from a nearby gasometer is also apparent.

Reflections   Delay Ellipse
Power delay profile measured along 3.3 km route (1477 MHz)   Delay ellipses from mobile measurement

Other recent measurement work has involved the mobile recording of field strength at VHF, UHF and microwave frequencies to inform the development and verification of propagation models. For these measurements, position is logged by GPS, while samples are triggered at regular distance intervals by a transducer linked to the vehicle speedometer drive. A typical received data series is shown below, and compared with the prediction from a model currently under development.

Aerials   Delay Ellipse
Transmitting aerials for 3-band propagation measurements   Field strength comparison

We have a well equipped survey vehicle, capable of carrying out measurement and investigation work at frequencies from 30 MHz to 8 GHz. The facilities include an internally-mounted 10 m pump-up mast and a 16 m transportable mast. A variety of measuring receivers and spectrum analysers are available, interfaced to computer and GPS systems.

Land Rover   UHF Scan
Mobile survey ...   ... analogue (PAL) and digital (DVB-T) transmissions

To discuss how Aegis can help your organisation to plan and implement its measurement and field testing requirements, contact us at enquiry-2015@aegis-systems.co.uk.

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See also:

Interference Analysis | Propagation Modelling

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