Tussle over 28 GHz spectrum

With 5G on the way, Indian telecom companies are demanding spectrum in the mmWave bands, which is also sought by satellite companies. This conundrum has drawn a line of battle between the telcos and satellite companies over the 28 GHz band.

The DoT is expected to put 50 percent of the airwaves in the 28 GHz spectrum band, which will range from 27.5 GHz to 29.5 GHz for 5G services. However, the satcom industry stands its ground against the fractional allocation of the band.

Tussle over 28 GHz. The satellite communication providers and the telcos are divided over whether 50 percent of the 28GHz band be put up for bids in the 5G spectrum auction to be held in India.

Hughes Network Systems, Inmarsat, and Viasat have requested DoT that half of 28 GHz spectrum not be put up for auction to mobile operators for 5G services as proposed. On the other hand, OneWeb, partly owned by Bharti Airtel group, is not with the global satellite communication providers on this and has no issue with regard to sharing the 28 GHz band with the telcos.

The Indian telecom operators, including Reliance Jio, Airtel, and Vodafone Idea, however, are demanding the 28GHz spectrum be put up for sale in the forthcoming 5G auctions. 5G trials might now get delayed as Airtel, Reliance Jio, and Vodafone-Idea insist that the government sell millimeter wave bands in the auction. All telecom operators state that they cannot spend time and effort to develop trials around on these bands unless the government approves these bands for the auction.

Besides, telecom operators want the DoT to release the revised National Frequency Allocation Plan (NFAP). If mmWave is not included in the upcoming revised policy, then it might increase the cost of overall deployment of the 5G network. In addition, the 5G services might become unaffordable for consumers in the country.

The government stands to gain additional revenue if this spectrum is auctioned. The satellite companies recently met DoT officials, and discussions are ongoing, with no resolution so far. The Department of Telecommunications (DoT) recently approved 5G trials, and a 5G spectrum auction is likely to be held in 2022. It is not clear whether any mmWave spectrum is up for sale.

Satellite operators’ take. Implications are huge for Hughes Networks’ project to build a satellite exclusively for India, and Elon Musk’s SpaceX and Jeff Bezos’ Amazon, who are likely to use this band for running both their satellite gateways and terminals to deliver high-speed broadband.

If this satellite spectrum is sliced off, it would rapidly reduce the serving capacity of satellite systems and the number of people that can be offered quality broadband services in India.

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Viasat has urged TRAI to disregard any attempts by the terrestrial 5G proponents to seek identification of the 28 GHz band in India for terrestrial 5G and to instead preserve access to the 28 GHz band for satellite broadband services and identify the 26 GHz and other mmWave and low-and mid-bands for terrestrial IMT/5G.

The Satcom Industry Association (SIA), India, has asked the DoT to separate the frequencies in the 28 GHz band for satcom operations. According to SIA, the investments made by satcom companies would be left impaired if the telecom department decides to avail the frequencies in the 28 MHz band to the operators for 5G. Thus, the satcom body has urged the DoT to leave the 28 MHz band alone for the satellite companies and give frequencies in the 24.25 GHz – 27.5 GHz band to the telcos for rolling out 5G.

Over 120 GSO fixed satellite service (FSS) Ka band satellites, including those being built and deployed by ISRO for use in India are now in orbit around the world, providing a wide range of services to individuals, businesses, and governments. Many more Ka band GSO satellites are under construction to meet the growing demand for service and need to use the 28 GHz band to meet this demand. Hence, the 28GHz (27.5-29.5 GHz) band is key for domestic gateways for (but not only) FSS with payloads in Ka-band. On account of capacity requirements, the entire band is normally needed for gateway use.

In addition to the wide variety of conventional fixed uses of the band, the use of Earth Stations in Motion (ESIM) is growing rapidly. ESIM operation will protect terrestrial systems in the 28GHz band. All of this helps to explain why the 28 GHz band was not identified for possible use by IMT under WRC-19 Agenda item 1.13.

Overall, the 28GHz band is a poor candidate for global harmonization/economy of scale. Use by 5G on a national basis will disrupt global harmonization for satellite use. Satellite operators argue that the 26GHz (24.25-27.5 GHz) band is an excellent alternative to the 28 GHz one for 5G, with much better chances of global harmonization. Also, there is ample other spectrum being studied for 5G under WRC-19 Agenda Item 1.13.

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Telcos’ take. Much recently, the telcos have asked the DoT for updating the NFAP and include additional spectrum in the 26 GHz and 28 GHz bands for 5G plans. The telcos are of the opinion that it would allow them to roll out next-generation connectivity technology for a much affordable cost.

According to Vodafone Idea, to ensure optimum utilization of precious and scarce natural resource, spectrum to be used for satellite-based services should be put to auction route only before allocations. Any other administrative allocations would cause a huge loss to the national exchequer. The spectrum availability should not be blocked for any niche service like satellite-based IoT services.

India’s satellite space is witnessing heightened activity with several international players such as Elon Musk’s SpaceX and Amazon’s Project Kuiper likely to provide services in the coming year.

The DoT is yet to release the updated NFAP policy, and this might force the department into restricting telcos from getting airwaves in the 28 MHz band. Only time will tell which way DoT’s pendulum will swing.

The telcos’ choice of the 28 GHz band is motivated by several reasons. First, there is extensive licensed but underutilized mmWave spectrum around 28 GHz bands that have been shown to support cellular communications in the range of 500 meters. Second, 28GHz band is still useful to create multipath environments compared to higher frequencies and can be used for non-line-of-sight communications. Moreover, an additional important advantage of exploiting 28 GHz band for wireless backhauling is the possibility of reuse 3GPP LTE functionalities.

For instance, 3GPP LTE allows to reuse the LTE physical layer, originally designed to operate at carrier frequencies around 2 GHz, and applied it to higher frequencies up to 40GHz for small cell backhauling. This requires small modification of the numerology to increase the subcarrier spacing. This results in a cost-effective adaptation of existing technology to accommodate the new 5G requirements as well as the opportunity of a quick launch of new features for wireless backhaul links.

The C-band debate. Currently, hot debates in India are raging around the C-band spectrum and the warring camps involved are the incumbent satellite operators, pitted against the new and upcoming 5G technology operators.

The C-Band (3.4-4.2 GHz) is a cornerstone for many satellite services including fixed satellite service (FSS), in particular above 3.6 GHz. The large geographic coverage of C-band satellite beams represents a cost-effective communication solution, while its robustness to weather impairments makes C-band the most suitable band to guarantee high service availability.

While C-band spectrum has been traditionally reserved exclusively for satellite use, the regulatory bodies are in favor of allocating a portion of C-band to terrestrial broadband operators for the upcoming deployment of 5G. In India, one portion of the band – from 3.7 GHz to 4.2 GHz – has traditionally been, and is still being, utilized by broadcasters for providing cable and satellite services for over two decades in conformance with the government’s spectrum allocation policy, the National Frequency Allocation Plan 2018 (NFAP) – which itself is aligned to the global body, the International Telecommunication Union’s Radio Regulations. However, part of the C-band, the globally coveted mid-band spectrum, is earmarked by the NFAP for allocation to 5G operators.

The potential assignment of C-band to the 5G cellular systems is seen as a threat by the satellite operators, who are concerned about the inevitable out-of-band emission (OBE) interference that the 5G-cellular system may cause to their services, potentially leading to service interruption and causing a serious economic impact. Since 5G signals would be far more powerful and all-pervasive as compared to satellite signals, they maintain that a separation of 100 MHz is essential between 5G services and satellite services in order to ensure continuous satisfactory signals.

Hence, the protection of the existing satellite systems operating in the C-band is crucial while allocating C-band frequencies to the upcoming 5G deployment, and to accomplish this, suitable spectrum sharing mechanisms need to be investigated in the scenarios where satellite systems receive harmful interference from the terrestrial systems. In general, there is a clear agreement on the fact that the C-band should be carefully assigned to new 5G systems so as to ensure the continuity of vital satellite communication services. It is also important to note that other solutions are available for the mobile industry, without needing to wipe out critical and widespread C-band satellite services.

The question of coexistence
IEEE proposed a method to confirm the possibility of coexistence between the existing satellite services and potential 5G cellular services in the mmWave band, according to the frequency-designation agenda of IMT-2020 for 5G. To evaluate the accumulated interference power of numerous 5G systems distributed globally at a satellite receiver, IEEE extended the satellite’s interference reception area to the entire coverage area, from which only the land area is extracted using the geospatial terrain data of Earth in three dimensions.

The simulation showed that the FSS satellite receives up to 7.9dB more interference than that obtained from the existing method. Although this is a substantial difference, the protection criterion is still satisfied. However, all EESS passive sensors do not meet the protection criteria in most scenarios, and additional frequency separation or interference mitigation techniques are required to protect these sensors. The proposed method is also applicable to the analysis of non-terrestrial network interference from airships, balloons, unmanned aerial vehicles, etc.

In addition, the organization performed interference analysis and presented the coexistence possibility of IMT-2020 and satellite services when IMT-2020 is operated in the candidate bands using the proposed methodology. The simulation results showed that the FSS satellite receives interference from many IMT-2020s because it is located at a high altitude. However, due to the high propagation loss and protection criteria, the operation of the FSS satellite is not affected, even when it operates in the same channel as the IMT-2020 in both the 27–27.5 and 81–86 GHz bands.

However, the EESS passive sensor is so sensitive that the low protection standards render it difficult to operate, even if the IMT-2020 is serviced in adjacent bands. In particular, when the main beam of the sensor is directed toward the urban area, the interference of most passive sensors exceeds the protection standard, which is a critical problem for the EESS passive service operations.

Therefore, to ensure the coexistence of the two services, consideration should be given to reducing the interference power of the IMT-2020, such as additional frequency separation or interference mitigation techniques.

Coexistence between IMT and FSS at 3.5 GHz was the subject of a 2019 Transfinite study for the GSMA. It considered adjacent band compatibility between 5G and FSS earth stations in the 3.4-3.8 GHz band. The study considered a number of different IMT deployments (macro and small cell), 5G emissions masks (based on 3GPP limits), FSS links (with different elevation angles) and FSS earth station receiver masks.

The results of the study indicate that, for 5G macro deployment and all combinations of spectrum masks and FSS links considered in the study, a guard band of 18 MHz would allow an I/N = -10 dB FSS protection criterion to be satisfied (some administrations stipulate a less conservative figure than this including in the US which uses -6 dB). For 5G small cell deployment, a guard band of 0 MHz would allow this.

Wherever deployed, 5G signals from large cell base stations can be millions to billions of times stronger (W/m²) than incoming satellite signals received at C-band earth stations. IMT transmitters generate unwanted emissions into adjacent bands. No filter can perfectly pass the IMT signals within a given frequency band and completely cut them just outside it.

Unwanted IMT emissions into the near adjacent satellite band will still be more powerful than the wanted satellite signals, yet cannot be filtered out at the satellite earth stations, blocking satellite signal reception. Satellite receivers cannot block strong unwanted signals on adjacent frequencies and still successfully receive the much weaker, wanted, satellite signals within the satellite band.

The performance of FSS earth station receivers will be very important in determining their resilience to interference from other services in adjacent bands. In event of interference to an FSS earth station, increasing the guard band will in many cases have little impact and will not be the best way of resolving interference cases, with other mitigation measures: site shielding or improved FSS receiver filtering being more effective.

Hence, the global satellite industry insists that a blanket approach to sharing C-band with terrestrial mobile IMT services will cause immense interference to satellite services and disruption to users. Considering its hundreds of millions of users worldwide, the global satellite industry remains fully committed to preserving the C-band frequency spectrum for satellite services.

Way forward
It is an indisputable fact that India needs to expeditiously roll out good quality 5G networks for customer benefits as well as for accelerating overall economic development. Hence, the availability of adequate mid-band-spectrum is a very critical need for the nation. In the end, a sustainable policy resolution would have to be based on the correct balancing of aspects of equity and respect for rights of existing players with the need for progress through adoption of new technologies. There need not, and should not, be any disruptions or disturbance of existing legal occupants and their customers. Peaceful coexistence of all is the goal and there could be healthy discussions to determine an acceptable way forward through provision of suitable incentives.