2016 - 2020 High-Throughput Satellite systems on the right track

Erwan Corbel, Bernard Charrat, Mathieu Dervin, Cédric Baudoin, Laurent Combelles, Benoit Garnier, Jean-Michel Mérour Thales Alenia Space {first_name.last_name}@thalesaleniaspace.com 26, avenue J.-F. Champollion, BP 1187, 31037 Toulouse France

Abstract Recent studies have identified the key technologies at payload level to be developed for HTS to reach the expectations in terms of capacity / cost ratio. In particular, emphasis has been placed on the exploitation of both exclusive and non-exclusive Ka bands and the decrease of the user beam size below 0.3°. While near-term HTS will benefit from a sharing of non-exclusive Ka bands between the feeder and the user links, it is envisaged, in most capacitive scenarios, to dedicate all available Ka- band spectrum to the user downlink, namely the band 17.3 – 20.2 GHz. This feature leads to consider alternative bands for the feeder link, in particular the Q (40 GHz) and V (50 GHz) bands.

The paper aims at providing a status of the roadmap of Thales Alenia Space towards such HTS systems, addressing the development of key payload subsystems, but also the availability of enhanced user terminals and the development of the Q/V-band gateway. It is shown that a step-by- step deployment of the next-generation HTS systems is on the right track.

1. Introduction Although not limited to the consumer residential market, the next generation of High-Throughput Satellite (HTS) systems will be able to deliver high-data rates to households in underserved and unserved areas. To meet a deep market penetration, HTS systems shall pursue the reduction of cost per transmitted bits with respect to the current in-orbit HTS. Even if initiatives to promote HTS using Ku band or using several frequency bands exist (see Intelsat EPIC), most of future HTS will likely rely on Ka band on the user link.

Thales Alenia Space (TAS) has recently been involved in several multi-partners system-level studies on next-generation HTS, namely : “MultiMedia 2nd Generation” (CNES - French Space Agency), “Terabit/s satellite : a system study” (ESA), “Broadband Access via Integrated Terrestrial & Satellite Systems - BATS” (EU). According to these studies, the common features of next-generation HTS to come in the 2016-2020 time frame are : · High spot density with user beam size smaller than 0.3°. · Allocation of large spectrum chunks thanks to the exploitation of non-exclusive Ka bands. In Europe (ITU region 1), exclusive Ka bands offer 500 MHz of spectrum on two polarizations for both uplink and downlink, while non-exclusive bands represents more than 2 GHz in both directions. · Use of mitigation techniques on ground to cope with intra-system interference sources, most preferably located in gateways to limit the cost of the user terminal.

Although all these features contribute to increase the system capacity with respect to the current generation, the trend towards very high throughputs is mainly supported by the enhanced user beam density (typically multiplied by a factor 3 to 4) and by the enlarged user bandwidth (typically multiplied by 2 to 5.8).

Figure 1 presents the frequency plan under consideration in this paper, with no explicit reference to any color pattern. The full downlink Ka band suitable for civilian applications is allocated to the user link. The ratio of bandwidth between the forward and the return links is 3:1. The feeder link relies on V (50 GHz) and Q (40 GHz) bands.

The design of a HTS system with the aforementioned features shall meet several technical challenges, which have been divided into three categories : user link design and associated waveforms (addressed in section 2), feeder link and network backbone sizing (in section 3) and payload equipment developments (in section 4). The following sections present these challenges and the related solutions that are currently under development. They are suitable for a wide range of coverage, from a regional coverage (30 – 60 spots) to a continental coverage (150 – 300 spots).

Figure 1 : reference frequency plan.

2. User link design and associated waveforms 2.1 Regulatory context in Ka band in Europe Services in Ka band for civilian satellite communications rely on worldwide ITU allocations to the Fixed Satellite Service (FSS) in the bands 27.5 - 30 GHz and 17.3 - 20.2 GHz. The band 19.7 – 20.2 GHz and 29.5 – 30 GHz is dedicated to the exclusive primary use by FSS and it usually hosts the user segment of current Ka-band systems. In contrast, the vast majority of the Ka spectrum, i.e. 27.5 - 29.5 GHz and 17.3 - 19.7 GHz, is allocated to other primary services. Each country is free to determine the use of shared bands, either by giving priority to some of the services, or by establishing technical and regulatory co-existence conditions.

The band 17.3 - 17.7 GHz is used by feeder uplink stations for satellite broadcasting systems. As per regulation, Broadcast Satellite Service (BSS) feeder stations have priority over other uses of the spectrum, and HTS terminals cannot claim protection if interference occurs. Discussions in CEPT has however shown that BSS feeder stations are limited in number, their location is known and the interference area is about a few tenth kilometers around BSS stations. Therefore, it is possible to implement a database of those stations in Europe so that a HTS system is able to avoid allocation of shared frequencies in the vicinity of these stations.

The 17.7 - 19.7 GHz band is shared with mobile network operators for Fixed Service (FS) to support a significant part of the backbone of the public mobile network (2G/3G/4G). The number of fixed links in Europe is nearing 100,000 in 2014 in this band. Here again, a HTS user terminal is not entitled to claim for protection from FS. Countermeasures are preferably based on the dynamic awareness of interference levels in the vicinity of the terminal. They may rely on satellite Cognitive Radio techniques, which currently benefits from a large R&D effort, in particular in the frame of the CORASAT project (funded by EU) [2], which Thales Alenia Space is participating to. Because fixed links are highly directional, and use a moderate bandwidth (from a few MHz to about 100 MHz), it can be shown that the amount of spectrum locally usable for interference-free FSS reception is sufficient to ensure the service, provided that the carrier bandwidth is not too large. This principle is know as Dynamic Channel Assignment (DCA), and is recognized in the applicable regulatory framework in Europe. Note that CEPT is currently examining possible enhancements of this framework to facilitate the use of uncoordinated FSS Earth stations.

In the band 27.5 - 29.5 GHz, CEPT has adopted a band segmentation approach between FSS uncoordinated Earth stations and fixed links. Under this regime, about 880 MHz would be available for use by HTS user terminals, in addition to the 500 MHz of the exclusive band 29.5 - 30 GHz. In particular the band 28.4445 – 29.9485 GHz is part of this segmentation.

To summarize, solutions to ensure a peaceful sharing of the Ka band with FS are identified, both for the downlink and the uplink. In all cases, they rely on the preservation of the exclusive bands 19.7 - 20.2 GHz and 29.5 – 30 GHz for FSS, which are critical to guarantee the HTS system QoS.

2.2 Interference management Despite next-generation HTS systems will rely on the use of large spectrum chunks, these bands are used as intensively as possible, both through tight carrier packing, and through frequency reuse among the user beams. The price to pay is a degradation of the signal to interference ratio at the receive side. · Non linear Interference coming from inter-modulation products: In the payload design reported in [3], the output section is composed of 1 TWTA for 2 spots, which clearly benefits to the payload mass efficiency. The amplified bandwidth is thus 2.9 GHz. Since the carrier bandwidth may not exceed 200 to 400 Mbauds, this leads to consider a strong multicarrier operation of the TWTA, with typically 6 to 12 carriers per TWTA. The subsequent increase of the relative power of intermodulation products with respect to the current generation could be made even worse by the possible imperfection of TWTA linearization over such a large bandwidth.

· Inter-Symbol Interference (ISI) and Adjacent Channel Interference (ACI): Considering the current trend towards tighter roll-offs in the waveform shaping (5% is now proposed in DVB- S2x) or closer carrier spacing, the transmitted carriers are more sensitive to the in-band distortions and to the non-ideal out-of-band rejection induced by the satellite channel filters.

· Interference coming from the frequency re-use between user beams :as the user beams are getting closer (0.3° or lower with respect to 0.5° – 0.7° as of today), larger satellite antenna reflectors are required to provide sufficient discrimination between spots using the same color. Nevertheless accommodation and manufacturing issues limit the reflector size to 3.5m, as far as solid reflector are considered, which results in a degradation of the antenna beam-to-beam isolation. The situation is worsened by the satellite instability which becomes quite significant with respect to spot size, despite possible improvement of the tracking system.

It should be noted that the penalty induced by these sources of interference on the link budget is all the more significant as higher order modulations, with higher SNIR requirements, are considered.

To mitigate these sources of interference, several techniques are investigated for future HTS systems. Linear and non-linear interference respectively induced by the satellite channel filters and the high- power amplification can be separately or jointly mitigated through ground processing. Focusing on the forward link, where the spectral efficiency is more critical, data pre-distortion in the feeder stations is an attractive approach, as the processing is concentrated in the gateways, with no impact on the user terminal cost. It should be highlighted here that a challenge here is to design algorithms with reasonable complexity. Another challenge is to provide a sufficient pre-distortion gain in multi-carrier scenarios. In this respect, recent studies [4] show very promising results.

Regarding the Frequency Re-use Interference, conventional interference cancellation techniques are not well suited to a multi-gateway topology. Indeed, since all the beams at a given color may not be handled by the same gateway, huge data rates (up to several terabits/s) should be exchanged on the network backbone to make the gateways be mutually informed of the interfering signals.

2.3 Consumer and SoHo terminals The main driver of the terminal design is its cost. To that regard, pressure on the design of the terminal to comply with next-generation HTS systems is rather high since : · The tuner shall be able to receive any carrier in the band 17.3 – 20.2 GHz on the forward link, and in the non-adjacent bands 28.4445 – 28.9485 GHz + 29.5 – 30 GHz on the return link. · The terminal shall be able to receive any carrier sizing 200 to 400 Mbauds, and transmit using a carrier sizing 10 Mbauds or more. · The terminal size is typically 75 cm. · The terminal power amplifier is typically 2W or more. · The terminal shall include the aforementioned interference management techniques : possible spectrum sensing to support DCA, equalization...

As far as the consumer and Soho market is targeted, DVB-S2 (forward link,) and DVB-RCS2 (return link) are well suited to the HTS system needs, since they both include : · Efficient channel coder and decoder, featuring coding gain close to the Shannon limit. · A set of modulation and coding scheme spanning over the range of operational Signal-to- Noise Ratio in HTS systems, typically 3 to 14 dB, including the effect of rain attenuation. · Adaptability of the physical layer to the state of the RF link (VCM, ACM, power control). · Compliance with very efficient data link layer protocols such as GSE and RLE.

The newly published DVB-S2x introduces two features that will improve the capacity of future HTS systems : · Low roll-off factors (5%,10% and 15% in addition to 20%, 25% and 35% in DVB-S2). · A finer granularity of modulation and coding schemes, especially in the range 5 to 10 dB of C/N, which is of particular interest for HTS system .

On the contrary DVB-S2x also extends the range of modulation and coding schemes beyond C/N = 15 dB and below -3 dB, with no benefit for the consumer FSS market.

Thales Alenia Space is developing DVB-S2/RCS2 terminals and hubs for delivery in 2016, while DVB-S2x/RCS2 terminal will come in 2017, in coherence with the availability of a DVB-S2 compliant chipset.

3. Feeder link and network backbone sizing 3.1 GW sizing including diversity Since a gateway of a HTS system shall handle thousand of user terminals, the targeted availability of the feeder is high, typically 99.9%. Meanwhile the degradation of the gateway capacity under rain until this threshold shall be limited with respect to the capacity in clear sky (typically between 0% and - 20%). In V band, the atmospheric attenuation is typically 15 dB higher than at 30 GHz in Europe.

Because of manufacturing issues, the antenna reflector of the first generation of Q/V-band gateway will not exceed typically 5 m (instead of 7 to 9 m in Ka band). Similarly the power of on-ground V-band HPA will be limited to 250 – 500 W before 2020 to handle 700 MHz. The reason lies in the availability of V-band products until now, but also because of the possible issues with the accommodation and the dissipation considering 8 or 12 HPAs in a single gateway. These facts lead to resort to gateway spatial diversity to face to deep fading events, instead of relying on the EIRP sizing of each gateway. The design of a gateway spatial diversity architecture results from a trade-off between : 1/ the sizing of the station (and thus its cost/feasibility), the cost of the whole feeder segment (minimization of the number of gateways) but also the costs related to the connection to the network backbone (the more redundant gateways, the higher the costs, see §3.2).

A ‘N+P’ diversity scheme, with N nominal gateways and P redundant gateways, has emerged [5]. It relies on the anticipated detection, supported by weather conditions monitoring and forecasts, of a deep fading event for a given feeder link. The network operation controller decides and prepares the switching of the corresponding feeder link to a redundant gateway. The latter should be located in another feeder spot beam, typically more than 400 km away from the nominal gateway. The switching back to the nominal gateway is performed as soon as possible in order to leave the redundant gateway free for another switching. The switching shall be transparent from the terminal point of view, at least at link layer. Note that this switching process requires an on-board switching between feeder beams. A ‘N+P’ diversity scheme is rather ‘efficient’ since 1 to 2 redundant gateways are typically enough to back 20 nominal gateways.

On the contrary a diversity scheme with no redundant gateways has been proposed. It assumes that every spot is handled by M gateways (instead of one gateway). In case of deep fading, the remaining gateways ensures a minimum of traffic on every spot. Note that this solution offers a M:1 scalability. Nevertheless, on top of the degradation of the system capacity, this scheme has two drawbacks : 1/ the cost of network backbone, and 2/ the induced complexity at payload level.

3.2 Network backbone design The main purpose of the network backbone is to interconnect the set of gateways to the various Internet Service Providers (ISP), who benefit from the satellite capacity. This interconnection will be based on the European backbone, through the Internet eXchange Points (IXP). Since the number of network nodes (the gateways) as well as their capacity significantly increase in next-generation HTS systems, the weight of the terrestrial backbone in the economy of the satcom system will automatically increase. That is why a particular attention must be paid to the network backbone design.

Firstly, depending on the role model, ISPs can be national or regional, and operators can be virtual or not, with different constraints on the overall network topology. In addition, the ability to define hybrid access system mixing different access technologies such as satellite, LTE and xDSL will bring another level of complexity to the interconnection issue.

Then the position of gateways under the satellite coverage is mainly driven by the necessity to maximize the distance between the stations, so as to maximize the angular spacing from the satellite point of view, and subsequently minimize the interference between the Q/V-band satellite beams. Nevertheless this situation leads to increase the length of the network backbone and thus its cost. In addition a given gateway likely handles some spots that serve users connected to various ISPs, which increases the number of connections and the cost of the backbone.

Lastly, the diversity scheme itself shall be taken into account, with the interconnection of every redundant gateway to 1/ every ISP and 2/ every nominal gateway, in order to limit the impact of the hand-over on the user traffic. The diversity scheme may represent a significant part of the overall cost of the backbone.

Considering the previous constraints, the aim of the backbone architecture design for the feeder segment is to establish a mapping between the gateways, the Points of Presences of the ISPs and the spot beams. The mapping shall complies with the satellite sub-system requirements and with the ISP connectivity requirements, while striving to reduce as much as possible the monthly cost of the terrestrial backbone infrastructure. To solve this extremely complex optimization problem, we have developed an innovative algorithm that jointly optimizes the routes in the backbone, based on a derivative of the Shortest-Path routing optimization, and the beam-to-gateway mapping using simulated annealing algorithm. This algorithm includes a dedicated cost model, which takes into account both access (gateway to IXP) and transit (IXP to IXP) costs, as well as the traffic demand requirements. In the frame of BATS, we demonstrated [6] that this joint optimization allows significant cost reduction for the feeder segment including the interconnection to the network backbone.

Another important issue introduced by the spatial diversity is related to the impact of gateway hand- over on network operation. This gathers several issues, from the possible internal rerouting in the diversity backbone to the impact on TCP performances. To alleviate the internal rerouting issue or to avoid the transfer of the Performance Enhancement Proxy (PEP) context from one satellite hub to another one, we defined a reference network architecture based on a centralized TCP PEP, which is now located in the ISP network. The advantage is three-fold : 1/ this network architecture is close to a xDSL architecture, 2/ it reduces the cost of the feeder segment thanks to the simplification of the gateways and 3/ it shortens the hand-over process between a nominal gateway and a redundant gateway.

These optimizations of the network architecture will be reflected in Thales Alenia Space products in 2017, as well as the Q/V-band gateway.

4. Payload equipment developments 4.1 Principle of HTS payload architecture The figure 2 shows the concept of the payload architecture well-suited to next-generation HTS systems for the forward repeater. High payload mass efficiency is reached thanks to the use of wideband products. In particular the V-to-Ka frequency converter shall handle 2.9 GHz. Similarly each TWT amplifies the band of two spots (2.9 GHz), while an ODMUX splits the band in two parts. The same principle stands for the return link. Depending on the antenna architecture (SFPB : Single Feed Per Beam vs MFPB : Multiple Feed Per Beam), the number of user antennas may vary.

Figure 2: typical forward link payload architecture for HTS.

Category Products To be developped V-band Input filter x V-band Low Noise Amplifier x V-to-Ka down-converter x Forward repeater Ka-band low power filtering Ka-band power module x Ka-band High Power Isolator x Ka band Output Demultiplexer x Ka-band input filter Ka-band Low Noise Amplifier Ka-to-Q up-converter x Return repeater Q-band low power multiplexer x Q-band power module x Q-band High Power Isolator x Q-band Output channel filter x Q-band feeds x Q/V-band antenna (feeder link) Q-band 2 - 2.5m class reflector x Ka-band feeds x Ka-band antenna (user link) Ka-band feed block assembly x Ka-band 3.5m class or higher reflector x Ka-band antenna tracking Antenna Pointing System Q-band beacon x Table 1 : products to be developed for next-generation HTS relying on Q/V and Ka bands.

4.2 Products to be developed The table 1 gives the main bricks at repeater and antenna levels of next-generation HTS based on Q/V and Ka bands. It shall be kept in mind that an intermediate generation of HTS will be available featuring wideband Ka technologies, before the Q/V bands technologies being ready.

4.3 THD-SAT programme and current developments In order to foster those developments, the French Government is supporting, via the programme “Economie Numérique” of the PIA (“Programme d’Investissement d’Avenir”), a programme of research and development, carried under the direction of the French Space Agency (CNES) named THD-SAT. The programme covers most of the aforementioned technologies. The objective of THD-SAT is to reach EQM-level equipment and technologies : · by 2015 for the Ka-band technologies (Ka-band power module, Ka-band 3.5 m antenna), · by 2016 for the Q/V-bands technologies (V-to-Ka and Ka-to-Q frequency converters, Q/V- band 2 - 2.5 m class reflector and feeds). Thales Alenia Space is leading all the on-going developments, with the support of key partners : · Thales Electron Devices (TED) for the Ka-band 170 W TWT and the Q-band 40 W TWT, · Airbus Defense and Space (ADS) for the Ka and Q/V reflectors.

The 3.5 m class Ka-band antenna & the 2 - 2.5 m class Q/V-band antenna Departing from a 2.6 m solid reflector, the design of the 3.5 m class antenna requires a significant reduction of the size of the feeds so as to keep a reasonable focal length with respect to the bus height. The new generation 4 ports RF chains (TAS patent) fits with a distance between feed axis around 36 mm. It accounts for significant gains in terms of mass (75%) and footprint (75%) with respect to the current generation. In addition, thanks to the THD-Sat programme, in combination with the ARTES 3-4 ESA KISS concepts, TAS develops and builds a SFPB EQM feed-block assembly and its associated structural support. TAS is now able to produce feeds in cluster assembly complying with a regular lattice of 30 mm. Furthermore TAS proposed an innovative approach to offer HTS coverage with only two user antennas thanks to a MFPB configuration. TAS patented concept allows to share feeds in the focal arrays without using orthogonal law constraints. The Tx and Rx laws are independent thus limiting the defocusing effect.

Taking into account Ka-band feed heritage, a new Q/V-band feed has been designed. An EM has already demonstrated the very good performances of the new design.

Finally a strong effort has been devoted to the 3.5 m reflector design so as to match the accommodation constraints due to the Ariane-5 launcher fairing diameter. The reflector backing structure has also been optimized to reduce the global reflector thickness. A detailed analysis demonstrated that those large reflectors can be accommodated on SPACEBUS and ALPHABUS platforms.

~ 36 mm

Figure 3: new generation of Tx/Rx dual Figure 4: Ka-band power module. polarization Ka-band feed

The high power Ka-band power module & the Q-band power module The power module is composed of a LDLA (Linearizer Driver Limiter Amplifier), a TWT and an EPC (Electronic Power Conditioner). LDLA balances the TWT gain compression and phase lag in order to improve the overall channel linearity. It handles the full Ka band (2.9 GHz) with good linearity performance. The Q-band linearizer is also being developed in the frame of THD-SAT. The TWT developed by TED in the frame of THD-SAT is a radiated cooled TWT providing 160 W over 2.9 GHz. The Q-band TWT under development aims at providing more than 40 W between 37.5 - 39.5 GHz. The EPC for Ka-and and Q-band TWTs is a minor adaptation of the existing TAS dual EPC. The V-to-Ka down-converter & Ka-to-Q up-converter Thales Alenia Space has a strong experience in the design of frequency converters for space telecom applications thanks to skills in the following topics: MMIC technology, passive functions such as filter, interconnections, packaging, gaskets, absorbers… The required converters, which are suitable for Q/V- and Ka-bands HTS, includes a RF slice for low- noise amplification, frequency conversion and amplification, as well as a LO slice with an external reference, a TMTC board and a DCDC converter. The other payload products The V-band LNA is under development by TAS-Italy and will be ready by 2015. The Ka-band LNA is already available, nevertheless TAS-Italy is working on the next generation which will provide enhanced performances and competitiveness. The required new filters are those listed here: · a Ka-band ODMUX (Output DEMUX) with two channels of 1400 MHz : the equipment is under development by TAS-France and will be ready by end of 2014. · a Q-band CMUX (low level MUX) with four channels of 500 MHz : the development is starting at TAS-France and will end by beginning of 2016. · a Q-band output filter with four channels of 500 MHz : the development is starting at TAS- France and will end by beginning of 2016.

Also development covering Q/V-band isolators, high power Ka-band isolators, Q-band coaxial cables, Q/V-band waveguides, and Q/V-band switches are starting being part of the 2nd phase of THD-SAT.

5. Roadmap of HTS systems in the 2016 - 2020 time frame The figure 5 gives the market availability of the aforementioned building bricks of future HTS systems, according to the roadmap of Thales Alenia Space. The gradual qualification of the products brings about a two-fold deployment of future HTS. Full Ka-band HTS systems using non-exclusive bands will enable to significantly enhance the capacity in orbit in 2016 – 2018, while the Ka/Q/V HTS systems will be fully ready to start operations in 2018 - 2020.

Figure 5: roadmap of Thales Alenia Space for next-generation HTS systems.

References 1 European Commission, “Digital Agenda Scoreboard 2014 - Broadband markets”, May 28, 2015, 2 CORASAT project , "http://www.ict-corasat.eu". 3 E. Corbel et al, “TERASAT : high-throughput satellite system by 2020”, proceedings of the 19th Ka and Broadcast Communications Navigation and Earth observation Conference, October 14-17, 2013, Florence, Italy. 4 T. Deleu et al, “Low complexity block pre-distortion of a multi-carrier non-linear satellite channel”, proceedings of IEEE International Communication Conference (ICC), Sydney, 10-14 June 2014. 5 P. Thompson et al, “Concepts and Technologies for a Terabit/s Satellite“, proc. Of the 3rd International Conference on Advances in Satellite and Space Communications (SPACOMM 2011), April 17 – 22, Budapest, Hungary. 6 J. Perez-Trufero et al, “High Throughput Satellite System with Q/V band Gateways and its Integration with Terrestrial Broadband Communication Networks”, AIAA International Communications Satellite Systems Conference (ICSSC 2014). 7 P. Bosshard et al, “Building the bricks of High Throughput Satellites”, proceedings of the 19th Ka and Broadcast Communications Navigation and Earth observation Conference, October 14-17, 2013, Florence, Italy.