Comments on the devlopment of DRM (research paper)
 

Ultra-long-distance tests:

Europe/ Canada / Caribbean to Australia / New Zealand

In April and May 2002, a new series of (ultra) long-distance tests was
carried out, this time aimed at receiving DRM signals over even longer
distances of 23,000 km. For this purpose, reception locations in Australia
and one in New Zealand were chosen. The transmitter sites were the same as
the ones used in the earlier Europe/Canada to Madagascar tests, but with the
addition of a 4th transmitter site in the Netherland Antilles in the
Caribbean.



The Sydney reception site offered the possibility of testing the
effectiveness of different receive antennas: a 1-metre whip, an 8-metre
long-wire as well as a 20-metre long-wire. The whip performed best, with the
highest SNR and comparatively shallow fading, compared with the long-wire
antennas. It was observed at the reception locations that switched-mode
power supplies (e.g. for the PC laptops) raised the electrical noise floor
considerably.

Magnetic chokes were fitted to the laptop power supplies and to the coaxial
cable connecting the antenna to the receivers. This reduced the interference
to reasonable levels.

It was possible to receive DRM signals over distances of more than 23,000
km. The signals originating in Jülich and Sackville were strong enough to
allow for data bitrates of 17 kbit/s and above. However, the majority of the
Bonaire (Netherland Antilles) transmissions could only be received at lower
bitrates (those associated with a 16-QAM constellation) and most of the
Sines transmissions were too weak to be received even when using the most
robust modulator settings. Co-channel interference and adjacent-channel
interference made reception consistently impossible on some circuits from
Bonaire.

High values of delay spread could be identified as a major problem for at
least two circuits from Bonaire. An impulse response of 8 - 9 ms made it
impossible even for the receivers. implementation of the .long-delay.
robustness mode D to decode the signal. The most likely explanation of the
phenomenon is linked to antipodal focussing. The focusing effect leads to
the main and side lobes of the transmission signal reuniting at the
transmitter.s antipodal point.

During the trip, recordings of the I/Q baseband signals were made. These
recordings proved to be useful in recreating observed receiver anomalies in
the laboratory. Anomalies identified on both the BBC and Fraunhofer
receivers were subsequently examined and corrected, resulting in performance
improvements. This experience showed that future generations of DRM
receivers are likely to require special attention with respect to the
development of certain algorithms. The areas of mode detection,
synchronization and channel estimation proved to be especially critical.

Reception locations for part II of the long distance tests

Name Symbol Location Host organization

Melbourne, Australia MEL 37.7S 144.9E WinRadio

Wellington, New Zealand WEL 41.3S 174.8E Radio New Zealand International

Sydney, Australia SYD 33.9S 151.2E Philip Collins & Associates

============================

Conclusions

The DRM field tests have been a very worthwhile exercise. The tests
uncovered problems and allowed

improvements to be addressed before the final system specification was set.

Receiver design evolved throughout the trials, often as a result of
shortcomings observed during the tests. The

developments and performance improvements made to the test receivers can be
directly applied to optimizing

the performance of future consumer receivers.

The field trials also made a significant contribution to the standardization
of DRM. In just five years, the system

has rapidly moved from concept to IEC, ETSI and ITU standards.

Following the launch of inaugural DRM transmissions in June 2003, we now
look forward to the next step: the

appearance of the first consumer receivers.

Acknowledgement

The author would like to thank BBC R&D and T-Systems for their contribution
of diagrams to this article. He

also acknowledges the support (financial and otherwise) of the European
Commission.s IST programme.

References

[1] J.H. Stott: DRM . key technical features

EBU Technical Review No. 286, March 2001.

Web sites

http://www.drm.org

http://www.drmrx.org

http://www.ist-qosam.com

http://www.ist-radiate.com

==========================

James Briggs studied Chemistry at Exeter University, graduating with honours
in 1988. He then joined the BBC Transmission Department, qualifying in
Transmission Engineering at the BBC training college at Wood Norton,
Evesham, in May 1991. He then worked for BBC Transmission throughout the
Southeast region of England and was involved in the installation of
prototype DAB equipment in 1994.

When the BBC Transmission Department was privatised in 1997, Mr Briggs
transferred to the newly formed Merlin Communications. In January 2000, he
started working full-time on the DRM project, and was appointed Coordinator
of the DRM Radiate field trials in July 2001. He is the vice chairman of the
DRM Systems Evaluation group.