|Coordination For Satellite Networks (http://www.satcom.co.uk/article.asp?article=18)|
|The pages which follow are the slides of a lecture presented
by Mark C J Posen of RPC Telecommunications Ltd.
The slides were created using Microsoft PowerPoint and converted to HTML. During conversion the slides were combined into 7 sections.
You can navigate between these sections at will from the left hand side. However, bear in mind that for the slides to make the most sense, it is suggested that you read through them in order.
Please note that this presentation contains the lecturers notes, and so in some places are not complete. If you have any questions regarding these lectures please contact the author here.
Satellite Coordination Background
|The physical problem:
The paper problem:
In this introductory lecture I hope to show you…
All radio-based services can cause and receive interference but satellite networks are generally trans-border because they have global or regional service areas so can cause and receive interference across wide regions. They are also interference constrained in a crowded orbit environment. Coordination is thus critical.
Trans Border Networks
What Is Coordination?
|Before an “administration” allows an operator to
commence operation of a new network it must in principle undergo coordination. This involves
the following steps:
Coordination thus ensures:
|Digression: ITU Priority|
|Coordination is closely bound to "date of protection" or "priority", defined by the date on which complete coordination data is received by the ITU. New planned networks must coordinate with all networks with an earlier “date of protection” but are protected against all networks with a later "date of protection". Planned (but not implemented) networks acquire status under this procedure but time limits ensure that protection is not for ever if networks are not implemented.|
|Interference Paths For Satellite Coordination|
|Frequencies can be used for both satellite and terrestrial
networks. The same frequency can (in some cases) be used for satellite uplinks and downlinks.
Therefore there are 6 paths by which a satellite network transmitter can cause interference to
a satellite or a terrestrial receiver.
This lecture will focus only on paths 1 - 4.
|Coordination And Interference|
|Coordination is thus concerned with:
|Why Is Satellite Coordination Important?|
|Coordination may seem like a rather arcane and specialist activity, and maybe not the most important aspect of network planning but for satellite networks, frequency coordination is critically important. Over-filing can mean that coordination constraints make a network unusable so coordination tends to mean if and not how. Satellite projects are increasingly driven not by financial/marketing/construction etc. but by coordination!|
|Some Key Issues...
|Until the end of the 1980’s, almost all filings were made after the design and planning of the satellite. At the start of the 1990s Tonga began filing for orbital filings without specific projects. This was the start of the revolution in filing that has significantly altered the ITU process. Now many administrations file for many more slots than they will or can ever use.|
|The increase in "paper" satellites has led to a massive over-filing in the ITU process. Most of these filings will never be implemented (they are technically incompatible!). Electronic filing has only made things worse. It is now very easy to make several hundred filings at a single time, if this is desired. The ITU has no ability or mandate to "judge" if a filing is serious. All filings must be processed by the ITU and coordinated by administrations. This is imposing a serious burden on all concerned.|
|A key effect of over-filing is a significant increase in the ITU backlog. Ten years ago the time from ITU receipt to publication was 4 to 6 months, today it is 2 to 3 years! The time for completion of coordination has been reduced; before 1997 it was effectively 9 years. Now it is 5 years with a limited 2 year extension. Thus it is now almost impossible to complete coordination within the time limits. This is a serious burden on administrations and operators that are planning real networks.|
|To try and improve the situation, WRC-97 adopted an "Administrative Due Diligence" procedure. Information about the manufacture and launch contracts must be provided for all networks within the regulatory time limit but this is having little impact on the situation. The information is very general and the ITU has no policing function. Many administrations are supplying this data "creatively".|
|Some administrations have been suggesting that deposits and fees should be payable to make satellite filings e.g. US$ 1M to make a filing, returnable if the network is launched. This has not found acceptance in the ITU membership as it is seen as penalising the poor and small countries. “Cost recovery” of processing and publication has been adopted however none of this impacts the backlog!|
|Reform Of The ITU Process|
|Some attempts have been made to reform the ITU process and deal with the backlog but there is no consensus in the ITU as to how this should be done. How to be fair to all administrations? How to change things without penalising the networks in the backlog? How to stop the massive over-filing?|
|Possible "implosion" of the satellite coordination process.|
|Clearly the system cannot go on as it is. The backlog is increasing, not diminishing. Soon the coordination requests will be published after the networks have expired! ITU will address this again at Plenipotentiary Conference in 2002 and WRC-2003 but as of yet there is still no emerging consensus as to how to change things in a way that can be accepted by a majority of members.|
|Frequencies are allocated by the ITU at "World Radiocommunication Conferences". Allocations are made on a regional basis and are made for different services. First space allocations were made in WARC-63 and have been updated regularly since.|
|ITU Radio Regulations|
|The Radio Regulations define the "law" relating to
the use of spectrum. International frequency allocations are defined in article S5 and all
administrations must adhere to these allocations. The only exception is use on a
Article S5: Regions
|Allocation For Different Services|
|Fixed Satellite Service
"A radiocommunication service between earth stations at given positions, when one or more satellites are used; the given position may be a specified fixed point or any fixed point within specified areas; in some cases this service includes satellite-to-satellite links, which may also be operated in the inter-satellite service; the fixed-satellite service may also include feeder links for other space radiocommunication services"
Broadcasting Satellite Service
Mobile Satellite Service
Space Operations Service
Article S5: Detail (example)
|Main FSS Allocations|
|Almost all these bands are shared with other services. Inter-service sharing issues are thus very significant and much time is spent ensuring that the different services are compatible - detailed studies are made before a band is allocated at at WRC. Small-dish satellite systems are especially limited by sharing with terrestrial services. In Europe and North America the bands marked * in the table above are generally kept free of terrestrial systems to assist small-dish satellite operations.|
|Satellite - Terrestrial Coordination|
Almost all FSS allocations are shared with terrestrial fixed and mobile services. 3 – 30 GHz (SHF) FSS bands share with point-to-point radio relay chains. Transportable line-of-sight (mobile) links above 40 GHz, FSS bands will share with HAPS (High Altitude Platform Stations).
ITU methods intend to ensure that interference to and from space and terrestrial systems does not produce more than a small fraction of the total allowable noise / BER in the end-to-end connection. Reference circuits define hypothetical links and allowable levels of noise due to interference.
Terrestrial Interference Modes
|Earth Station <-> Terrestrial|
|For any given earth-station, it is possible to determine the likely interference to and from the earth-station and those terrestrial stations in overlapping frequency bands and close enough to be likely to be able to cause and receive interference. Then it is possible to find means to reduce the interference to acceptable levels. This is, of course, frequency coordination as already discussed - determining compatible assignments which may be recorded in the MIFR.|
|Satellite <-> Terrestrial|
|Because of wide coverage, one satellite footprint is likely to include very large numbers of terrestrial stations. Coordination is thus impractical (except in some specific cases). Inter-service sharing must thus be ensured by imposing "envelope" technical conditions in the shared bands determined by empirical and analytical study and defining limits on the power of satellite and terrestrial emissions.|
|Limits On Satellite Emissions
In bands shared with terrestrial services satellite emissions are constrained to a power flux density at the surface of the Earth which may not be exceeded. This is defined in a measurement bandwidth and elevation angle dependant - higher limits at higher elevation angles because terrestrial fixed stations have higher gain at lower elevation angles. Limits are mandatory and are in S21 of the Radio Regulations.
Limits On Terrestrial Station Emissions
S21 sets mandatory limits for terrestrial stations. The EIRP limited to 55 dBW. Power to the antenna input also limited to 13 dBW below 10 GHz and 10 dBW above 10 GHz.
The transmit beam is to be directed away from the GSO in the following way:
|Earth Station <-> Terrestrial Coordination|
|This is a terrestrial sharing path. In addition to coordination there are some sharing constraints - limits on EIRP towards the horizon and earth-stations not to transmit below 3º elevation. The selection of an FSS transmitting earth-station site requires careful evaluation, to avoid the possible coordination problems. An increased number of FSS stations makes this more difficult|
|Identification Of Affected Stations|
|At "Stage 2" the administration responsible for the
new earth-station prepares coordination maps of the coordination area based on pessimistic
assumptions. Terrestrial stations in the contour could be affected.
If the coordination area includes the territory of a foreign administration the maps and technical data about the station must be sent to those administrations. Maps are prepared using the methodology of Radio Regulation Appendix S7.
|Appendix S7 Procedure|
|WRC-2000 approved an updated methodology.
The permissible level of interference is first defined and propagation for different modes is taken into account:
(mode 1): attenuation of signals due to tropospheric propagation (ducting) via
|Satellite - Satellite Coordination|
|Satellite Coordination Requirements
The satellite coordination procedure is described in Article S9 of the Radio Regulations. Before bringing an assignment in a satellite network into use an administration must undertake the S9 coordination procedure:
These are in parallel to the satellite-terrestrial coordination already described.
|Between 2 and 5 years before bringing into use an administration planning a new satellite network must initiate the Advance Publication. For networks subject to coordination the advance publication is very general and contains little information. For networks not subject to coordination the advance publication is very detailed and contains specific information about the network implementation. The API is mainly for the purposes of informing all administrations of developments in the use of space radiocommunications.|
|For the cases requiring detailed coordination the administration must follow the API with the coordination request. This is a detailed description of the network parameters to sufficient detail to allow interference calculations to be made. This must be submitted not earlier than 6 months after the receipt of the API data. This defines the “priority” (date of protection) of the network. Administrations must object within 4 months of publication or lose the right to future objection.|
|The Need To Coordinate|
|The right of coordination with a new network depends mainly on
|Coordination is required in the bands:
|DeltaT/T - Appendix S8 Procedure|
|This procedure estimates the likely interference caused by an
"interfering" network by estimating the increase in system noise temperature that
this network might cause to a "wanted" network.
Since the noise temperature of the "wanted" network is known then the value of DeltaT/T is easily calculated. If the value of DeltaT/T exceeds 6% then coordination is required. If the value of DeltaT/T is 6% or less then coordination is not required. In some cases the uplink and downlink can be treated separately.
If no coordination is indicated then the new network can proceed directly to notification, otherwise detailed bilateral coordination must be undertaken with all affected administrations. Only when the agreement of all affected administrations has been obtained can the network proceed to notification. Status and protection is only finally achieved following successful notification and entry onto the MIFR.
The procedures adopted in coordination meetings are for bilateral agreement between administrations, normally based on ITU-R Recommendations but administrations may agree other procedures and techniques if they wish. Procedures (currently) only exist for GSO-GSO coordination. DeltaT/T is not normally used in coordination meetings and interference is usually analysed on the basis of carrier-to-interference ratio assessed against a protection ratio.
During coordination interference can be mitigated by:
Following the completion of all required coordinations the responsible administration can submit details of all coordinated assignments to the ITU. The ITU will undertake a number of regulatory and technical checks. If these are successful then the assignments will be entered into the MIFR and gain international recognition and protection. Procedures exist to deal with cases where agreements can’t be achieved.
|Coordination is critically important for satellite networks. Some interference is managed by sharing constraints but others require detailed bilateral coordination. The procedures of Appendices S7 and S8 are used to determine need to coordinate. Bilateral coordination will require agreements between administrations before putting networks into use.|
Copyright 2002 Satcom Online (http://www.satcom.co.uk)