Validation Software for Rec. ITU-R S.1503
A personal view, by John Pahl
The Battles of WRC 97
Joint Task Group 4-9-11 was born out of the arguments and deal-making at WRC 97. In the basement of the Geneva CICG conference centre discussions went on during evenings and weekends for almost 4 weeks to shape the satellite systems of the future.
The driving force was the boom in satellite communications: after the voice and narrow-band data of MSS systems the next stage was for high data rates, broadband services - the so called "Internet in the Sky". There was excitement at the idea of using non-GSO rather than GSO orbits to provide global coverage and get shorter delay times. However the most useful frequency bands were also heavily used by GSO systems and the Radio Regulations said that non-GSO systems would have to protect the GSO and not cause "unacceptable interference". This phrase was considered problematic as "acceptable" levels were not defined but would have to be agreed during negotiations with the many parties involved.
Europe and the US came to WRC 97 with two very different visions about how to proceed. The US wanted to build on the previous conference, WRC 95, where 400 MHz in Ka band had been allocated to be available for both non-GSO and GSO systems on the basis that the first to apply for use, to file, gets priority over the next. The first to file was the US non-GSO system, Teledesic, and they wanted to increase the allocation to 500 MHz and make it retrospective to WRC 95. On the other hand Europe proposed to open up the lower Ku band to non-GSO system by quantifying what "unacceptable interference" meant by specifying limits in terms of the EPFD, the Equivalent Power Flux Density. Any non-GSO system that produced less than this interference would have met its obligations under the Radio Regulations.
Neither were happy with the others view. Europe didn't like allocating more spectrum to non-GSO systems in a way which would make it very hard for GSO systems to operate in the same band. The US objected that the values of the hard limits proposed by Europe hadn't been properly studied and that GSO use of the Ku band was too important to take any rash steps. The stage was set for a classic WRC battle.
After many formal debates in committees and less formal corridor discussions, a deal was hammered out for both sides to get what they want. The US got its additional 100 MHz in Ka band, while Europe got its hard limits in Ku band, though on a provisional basis, with the ITU-R studying the values themselves before the next Conference. The forum for this would be a new group, a Joint Task Group, the JTG. It would look at how to protect a range of services from the non-GSO FSS, namely the GSO Fixed Satellite Service (the remit of ITU-R Study Group 4), Terrestrial Fixed Service (Study Group 9), Broadcast Satellite Service (Study Group 11), and others including Space Science and Radiolocation. From the numbers of the relevant Study Groups came its full title: Joint Task Group 4-9-11.
So the JTG was started, and a chairman agreed, John Leary - a Brit. working for Japan's SCC. Soon we had terms of reference, web sites, email distribution lists, and papers - lots and lots of papers.In total there were:
- 4 meetings
- 414 input documents - maybe 5,000 pages - including 25 produced using our software program Visualyse.
- 111 output documents
- Around 250 delegates, usually 150 per meeting
The JTG formally opened its first meeting on 9th March 1998 in Geneva. Following meetings were a great circuit of Toulouse (July 98), Long Beach (January 1999), before finally finishing back in Geneva in May 1999. There were other ITU meetings too - WP 4A, WP 4-9S and JWP 10-11S in between these which contributed much to the work done. Also in this small world there were many informal meetings, where the main players could negotiate and discuss behind closed doors.
It wasn't all work: in Toulouse we were entertained by the French delegation to dinner at a beautiful Chateau, with entertainers and fireworks. In Long Beach we were invited by the US GSO coalition to the Queen Mary moored across the bay from the conference centre.
But during the meetings it was very hard work indeed. Often drafting groups and technical discussions went on late at night - not just for one day, but day after day. The final session of WP 4A ended at 5 in the morning.
The work of JTG 4-9-11
There were so many questions to look at, so many different ideas to try, assumptions to check, methodologies to develop. We built up a new set of acronyms, code words for our little world, where we knew why the limits were for EPFD down / up / is, the difference between Methodologies A to D, what was 1323, 1325, 1328, in-jokes (such as Francois Rancy, head of the French delegation, correcting the English of the UK delegation). There were changes as people moved on, changed jobs, got married, had children, retired. Representatives from 20 to 30 countries plus international organisations had a job to do, a task that could affect business and communities all over the world.
Some of the questions were looked at included:
- What are the characteristics of GSO systems?
- How can we derive a methodology to calculate the impact of EPFD limits on these GSO systems?
- How many non-GSO systems can share simultaneously?
- What would happen to GSO systems that are in slightly inclined orbits?
- What receiving gain pattern of the Earth Stations (ES) for GSO FSS and BSS systems should be used?
- How should interference into the GSO satellite be measured?
- How could the Radio Regulations be updated to include the ideas generated by the JTG?
- What special methods should be used to protect earth stations with exceptionally large dishes?
- What operational methods would allow GSO and non-GSO systems to share better?
- What EIRP limits should there be on GSO and non-GSO ES?
- How can non-GSO ES share with GSO ES and FS stations?
- How should limits be extrapolated between specific values in the Radio Regulations?
- What should be done about Quasi Geo-stationary Systems?
- What special allowances should be made for telecommand carriers?
- At what point do GSO carriers lose synchronisation?
- How could one measure actual interference
There was too much to follow each in finest detail - we each had our own set of pet subjects of interest, and left the rest to others.
Recommendation ITU-R S.1503
Our special interest at the JTG was related to developing a software specification. EPFD is not just interference at a particular moment, it is a distribution, a range of interferences associated with how often those levels are exceeded. This is needed as satellites in non-GSO constellations move with respect to Earth and so give interference levels that vary in time. So calculating the EPFD that a particular non-GSO system would generate is not simple, but requires such extensive calculation that software must be written.
The approach proposed was to use worst case assumptions to generate a PFD mask on the satellite and then use that in the simulations.
The "PFD Mask" approach meant that all the key parameters of the non-GSO system would have to be available to generate the mask itself, but as some worst case assumptions were made there would be no need to reveal some specific aspects such as detailed resource algorithms. Additionally minor operational changes to the system could be made providing that they did not result in the levels in the PFD mask being exceeded. Finally the PFD generated by each non-GSO satellite could, in theory, be measured operationally.
So this compromise was accepted as the way forward. However writing a detailed specification for such software was clearly a task that required more time than was available just at these meetings. So John Leary set up the Software Group of the JTG, and over the next 6 months it worked by email, gradually putting together the various parts of the software specification, which became Rec. ITU-R S.1503.
The Debate Concludes
The JTG's most important question was "What power limits should there be to protect the GSO FSS / BSS and the FS?"
To the last hour of the last day the JTG debated the hardest of these, the EPFDdown limits for the GSO FSS. There were two views: the gap between them had been reduced to a few dB but not closed. The room was tense as almost 18 months of work built up to a final debate. But it turned out we weren't able to walk away then with all issues closed.
However the list of agreements is long, too long for this document. Those who are interested can read the Chairman's reports and the text to go in the Conference Preparatory meeting (CPM) report - they can be found on the ITU web site. A massive amount of work was done and the JTG achieved much.
There are those that say there were too many meetings, not enough time for study and analysis, to review the work done by others. That may be true, especially as it seems hard that there should be a whole year between the end of the work of the JTG and the next WRC.
But one of the keys to the whole process is getting people together: the corridor discussions, unofficial hints about possible areas of softening, explaining complex issues over a white board, and sharing data from laptops.
Since the JTG closed we had CPM 99 and finally WRC 2000. Here under blue skies and on airy terraces, in smart new conference rooms, at parties in palaces beside the Golden Horn, and on boats floating under the Bosphorus Bridge we met for the final meetings.
In the years afterwards the software specification in Rec. S.1503 was reviewed and as we implemented its algorithm to build Visualyse EPFD we inevitably found minor gaps, ambiguities, and a few errors. In addition, there was testing and verification against a set of test cases, and discussions with the ITU about integration with their systems.
But the work has shown that we are ready: we have a Recommendation and Software that could be used to verify that non-GSO FSS networks meet the EPFD limits in Article 22.2 of the Radio Regulations.