Satellite - Interference Management
Regulators need to continuously define coordination rules to balance the equity between competing players - not possible if the spectrum is auctioned.
In the past, I had pointed to many reasons for the assignment of spectrum for satellite services administratively and not by auctions. Today, I plan to take you through the complex process of interference management for the satellite spectrum. After going through this note you will appreciate why auctioning of satellite spectrum will lead to a disastrous situation, thereby preventing the regulator from intervening and dampening the incentive for the satellite players to coordinate, and make investments in improving the capabilities of their technology to drive spectrum efficiencies, etc. Hence, without wasting any more time let’s begin.
Sharing is Mandatory
Sharing of spectrum is a mandatory requirement for all Satellite players. Why? Unlike terrestrial players, they (satellite players) can’t harness even a fraction of the capacity of the assignment spectrum due to their limited “spectrum reuse” capabilities. “Spectrum reuse” is nothing but the capability of the network to be able to use the same block of the spectrum repeatedly throughout the geography. Greater the reuse capability, the higher the spectrum capacity that can get harnessed. Terrestrial networks (due to their much smaller cell size compared to the satellite) can harness significantly greater capacity from the same spectrum compared to their satellite counterparts. Now, sharing can be of two types - a) Sharing with satellites that are in geosynchronous orbits (GSO) with respect to the earth; b) Sharing with satellites that are in non-geosynchronous orbits (NGSO) with respect to the earth.
NGSO sharing with GSO
GSO satellites are located at a distance of 36,000 Km from the surface of the earth. At this radius of the orbit, the rotational speed of the satellite is the same as that of the earth - making the satellite look stationary from the surface of the earth. On the contrary, the NGSO satellites are at a much lower height (500 to 1200 Km from the surface of the earth). They are deliberately positioned so as to reduce the round-trip delay of signals (latency) - critical for supporting many important services. Now due to the much lower height, the NGSO satellites compared to the GSOs - a)have a much smaller footprint; b) appear in continuous motion from the surface of the earth. The following figure explains.
Now this larger footprint of GSOs (compared to NGSOs), makes the GSOs vulnerable to interference from the NGSOs. In the figure above, the RF signals from the GSO satellite are designed to hit the receivers on the ground (let’s say DTH antenna). Hence, when the NGSO crosses the GSO footprint, its signals are likely to traverse the same path and cause interference with the DTH receivers, thereby blurring the TV reception.
India Situation
The chart captures the situation in India as regards GSO operations in the key bands which are likely to get opened up for satellite (NGSO) BB services. Each of these bands is mapped to the satellite operator’s band of interest. Note, that the column tagged as “India Usage” lists the existing operations in the respective bands of interest mapped to the satellite operators. Color Code - Orange indicated the bands which are “occupied” and those marked as Green are “free” of encumbrances.
Note - GSO usages are marked as DTH D/L and ISRO. Also, note that heavy GSO DTH satellite usages exist in India in the core band of interest - which is from 10.7 to 12.7 GHz (Total Bandwidth = 2 GHz). GSO usages in this block of airwaves can be further broken down as - a) 10.7-10.95 (Appendix 30B - ISRO); b) 10.95 - 11.2 (ISRO-DTH); c) 11.2 - 11.425 (Appendix 30B - ISRO); 11.425 - 11.7 (DTH); d) 11.7-12.2 (Appendix 30A - Foreign Sat DTH); e) 12.2 - 12.75 (Foreign DTH).
The frequency block - 10.7 to 12.7 GHz is the Achilles heals, as unless this interference (GSO vs NGSO) is resolved the rest of the bands (User downlink, feeder Uplink & Downlink) have no meaning, as they are all tightly held together for the purpose of enabling service.
Limiting Interference into GSO
ITU has clearly laid out guidelines for the NGSOs so that the GSOs operating in the same bands can continue to do so without any problems. This is laid down in Resolution 76 at World Radiocommunication Conference (Geneva, 2015). The whole idea of the resolution is to set limits on the aggregated power flux density to be met by all the NGSOs operating in the GSO bands. These limits are listed in Article 22 of the radio regulations.
Now, why this is a problem and will require continuous supervision by the in-country administrations? This is due to the following reasons.
The responsibility of compliance lies with the Administration
See extracts below from Res 76.
“… that administration operating or planning to operate non-GSO FSS systems, for which coordination or notification information, as appropriate, was received after 21 November 1997, in the frequency bands referred to in above, individually or in collaboration, shall take all possible steps, including, if necessary, by means of appropriate modifications to their systems, to ensure that the aggregate interference into GSO FSS and GSO BSS networks caused by such systems operating co-frequency in these frequency bands does not cause the aggregate power levels given in Tables 1A to 1D to be exceeded (see No. 22.5K);”
Methodology of measuring aggregate interference in an evolving subject
See extracts below from Res 76.
“…to continue its studies and to develop, as appropriate, a suitable methodology for calculating the aggregate epfd produced by all non-GSO FSS systems operating or planning to operate co-frequency in the frequency bands referred to above into GSO FSS and GSO BSS networks, which may be used to determine whether the systems are in compliance with the aggregate power levels given in Tables 1A to 1D;”
Given this background, if the spectrum for satellite services is auctioned then it will be difficult for the administrations to intervene and make course corrections in order to protect the operators running DTH/other services in this band using GSO. Why? Rules of interference management have to be clearly laid out in the NIA (Notice Inviting Applications for Auctions) and these can’t be changed later. We see this problem in action in the US already.
See Dish (a DTH player in the US) writing to FCC (6th Mar 2023) seeking to resolve the problem of Power Flux Density in its GSO used to provide TV services. Though FCC denied this request, however, the debate is on - requiring the regulator to keep evaluating such requests on a continuous basis, and this is not possible once spectrum is granted exclusively or non-exclusively through auctions. For any auctions to be successful the rules of the game have to be defined in advance and can’t be changed later. The problem in India will be much greater than in the US due to the heavy usage of the band by the DTH operators (See page 143 of RR). In the US, the Broadcasting Satellite is permitted in 12.2 to 12.7 (500 MHz of the band 10.7 to 12.7), whereas in India it is 11.7 - 12.2 & 12.5 - 12.75 (Total 750 MHz).
NGSO Sharing With NGSOs
One of the key responsibilities of the regulator is to continuously manage interference between competing NGSOs operating in the same block of spectrum. Ideally, the regulator will like all the NGSO players to have voluntary sharing agreements between them with the objective of preventing inefficiencies and improving the quality of services. Therefore, the whole idea of laying out optimal rules for sharing is to drive the players into voluntary agreements. Hence, the sharing rules need to be balanced and not tilted toward any of the NGSO players irrespective of their deployment status. This is why these rules of coordination can’t be spelled out upfront in the NIA, as they will change rapidly in time, with changes in technology, and the deployment status of each of the NGSO players.
Recently, FCC after a series of public consultations revised its sharing rules for NGSOs on 20th April 2023. The key principles of the sharing guidelines are enumerated below.
Default Spectrum-Splitting For Initial Systems (For Regulatory Stability)
In layman’s terms when different NGSO system operates in the same block of airwaves their unsynchronous movements (relative to each other) will make some of its satellites overlap on top of each other - causing direct interference in each other’s receivers. See the figure below.
In such as situation, the total spectrum band let’s say 500 MHz will get split into two halves 50% (250 MHz) each will be available to the individual operator till the time the interference persists. However, in the rest of the geography, each of the operators will have full access to 500 MHz of spectrum for serving its customers. Now FCC wants to limit this sharing strategy to only those players whose system got approved in the beginning. The idea is to provide greater regulatory stability and predictability to NGSO players as they deploy their initial constellations.
Protecting Earlier Round Systems From Later Rounds (Balancing Task)
FCC wants to protect the systems which are deployed initially for the purpose of greater regulatory stability to drive investments. However, FCC consciously refrains from using the band-splitting strategy or I/N limits to enable protection of the earlier round systems from the later rounds. The chosen protection mechanism is called “degraded throughput methodology”. This as per FCC is the best option for optimally protecting the earlier round systems without unnecessarily burdening the later round system. The whole idea of adopting an optimal protection strategy is to balance the equity between the players so that they can enter into voluntary sharing agreements between them, which will result in better usage of capacity.
However, the FCC instead of freezing the protecting methodology now, FCC has opened it for public consultation to be decided at a later date.
Sharing Information During Good-Faith Coordination (Will Decide Later)
As already stated, the whole purpose of defining sharing rules is to balance the equity between the sharing entries for driving them into entering into voluntary agreements - the best outcome for optimal usage of airwaves. However, voluntary agreements will need information sharing, and this needs to happen in “good faith”. FCC chose not to codify the specific information that the players need to share under “good faith”. It believes that default spectrum splitting will drive enough incentives for motivating the players to voluntarily enter into sharing agreement between themselves. Extract from the FCC rules dated 20th April 2023 is reproduced below.
“However, we will monitor the progress of NGSO FSS systems as they proceed in coordination and deployment and may revisit this issue in the future if ongoing coordination difficulties among operational systems suggest that more information sharing requirements are required”
Sunsetting of Inter-Round Protection Requirement (Spectrum-Splitting For all)
FCC has chosen a 10-year period from the first processing round to bring all players at the same level for the purpose of sharing spectrum, i.e. default spectrum-splitting. The whole idea is - by that time the NGSO system will become mature and therefore it is highly unlikely that later NGSO players will be able to function without entering into agreement with the earlier players. Also, any burden imposed by sunsetting the inter round protection is likely to get offset by the benefits to the later generation of their systems.
Conclusion
One can see that the quality of satellite services is highly dependent on the player’s motivation to coordinate with competing services operating in the same block of spectrum. If left alone, it can drive the system into a mess, as the motivation for coordination might not be the same for all players. Hence, the role of the regulator becomes very important here. It needs all the flexibility to not only keep monitoring the space to prevent the existing services (DTH, VSAT, IFMC) to get impaired by the NGSO operations but also to set rules such that the NGSO players are driven towards voluntary agreements for efficient usage of spectrum.
Note, this the regulator can’t do so if the spectrum is granted through the process of auctions, as it is impossible for these coordination rules to get defined in advance, and therefore can’t get embedded into the auction document (Notice Inviting Applications). Moreover, these rules are not static, they are continuously evolving with technology, the state of deployment, and the business cycle of the system in play. Hence, the best way of assigning spectrum for satellite services is through an administrative method. Doing so will empower the regulator to set rules to mitigate conflicts as and when required. And it will not be possible for the regulator to do so if the spectrum is assigned through auctions.