Ka-Band HTS Satellites: A transformational shift?

Transcription

1 Ka-Band HTS Satellites: A transformational shift? Jan Hetland Director Datacomms Systems Telenor Satellite Broadcasting AS June 2013

2 Introduction Over the past few years, several Ka-band satellites have launched and many more are planned for launch over the coming years. In terms of Gigabit throughput, these satellites are undeniably the highest performing commercial communication satellites in existence. The commercial introduction of a new frequency band has prompted lengthy discussions on which band is better for a certain service type. It is sometimes difficult for those less familiar with satellite signal propagation theory to differentiate between truth and myth. In this white paper, we substantiate that Ka-band HTS satellite systems can deliver services that parallel most Ku-band services. The analysis covers various aspects relevant to any satellite communications system, such as Satellite signal propagation theory The regulatory environment Spot-beam satellite design System design considerations Still, we remain convinced that both Ku- and Ka-band frequencies have a role to play in the future and that choice between one and the other will be largely dependent upon availability and applications. Satellite Signal Propagation Theory A satellite signal typically travels between 36,000 and 42,000 kilometres between a point on the Earth and a geosynchronous satellite. Throughout most of this journey, the satellite signal travels through empty space which contributes only fixed-path loss. higher directivity of Ka-band antennas, hence the net difference between Ku-band and Ka-band is very small. More interesting things happen as the satellite signal travels through the atmosphere, a layer which wraps itself around the earth and is about 1,000 kilometres thick. Within the atmosphere, there are two layers of particular interest: Troposphere: 0 15 km above sea level Ionosphere: 50 1,000 km above sea level Together these two layers expose the satellite signal to the following perturbations: Rain attenuation Scintillation Gaseous absorption Cloud attenuation The ITU and various other national and international bodies have created sophisticated mathematical and statistical models for predicting the effects of each of these perturbations. These show that rain attenuation is by far the most dominant perturbation to which a satellite signal is subjected. Throughout Europe the effect of the last three factors scintillation, gaseous absorption and cloud attenuation is roughly db for a 30 GHz uplink signal, assuming the availability target of The corresponding figure for a 14 GHz uplink signal meeting the same availability target is roughly db. Hence, the net difference is around 1 db. In the rest of this paper we focus on the rain attenuation effects as these dominate the overall attenuation picture. To visualize the effects of rain on a satellite signal, it is useful to look at the relative attenuation between Ka-band and Ku-band satellite signals, as many of us are familiar with how it works for the latter. This path loss can be expressed by the following formula: Free Space Path Loss (db) = 20*log10 (d) + 20*log10 (f) Eq. (1) Where d is the distance in kilometres and f is the frequency in GHz. Although the loss formula is dependent on frequency, it is dwarfed by the distance parameter d. To put this into perspective, the free-space path loss of a Ku-band uplink signal from a teleport in northern Europe towards a geosynchronous satellite almost due south is on the order of 207 db. The corresponding free-space path loss for a Ka-band uplink signal transmitted from the same teleport to the same geosynchronous satellite would be roughly 213 db. The difference in path loss is typically compensated for by the 2

3 Delta attenuation Ka (20.0 GHz) and Ku (11.0 GHz) at 0.3 % of yearly time. Satellite at W Delta attenuation Ka (30.0 GHz) and Ku (14.5 GHz) at 0.3 % of yearly time. Satellite at W Figure 1: Delta attenuation contours for 20/11 GHz downlink signals at 99.7% availability. Figure 2: Delta attenuation contours for 30/14.5 GHz uplink signals at 99.7% availability. The figure above shows the delta attenuation between a Ka-band downlink signal at 20 GHz and a Ku-band downlink signal at 11 GHz, assuming a yearly availability target of 99.7%. The difference in attenuation is typically around 2-3 db and is quite uniform across Europe and all surrounding waterways, including the Mediterranean Sea and the North Sea. As the frequency increases, the effect of rain attenuation becomes more severe. In figure 2, the difference in attenuation is shown between a Ka-band uplink signal at 30 GHz and a Ku-band uplink signal at 14 GHz. The difference, still rather uniform across the entire region, is now in the range of 5-6 db. This corresponds roughly to a doubling compared to the downlink scenario. For very low elevation angles, typically below 5 degrees, the delta attenuation increases noticeably as the satellite signal travels a greater distance through the atmosphere. The effect is present in both bands but affects Ka-band more severely. When designing teleport infrastructure, it is useful to also use local measurements, if for no other purpose than to verify ITU models. Figure 3, below, is an example of real attenuation data extracted from satellite telemetry obtained from one of Telenor Satellite Broadcasting s (TSBc) THOR satellites during the entire year of The horizontal axis show the attenuation level recorded, while the vertical axis indicates the corresponding link availability. Yearly and worst-month fading data were recorded for an uplink signal close to 19 GHz and shown as the solid red and blue curves. 3

4 Signal attenuation Nittedal % 99% 99.9% 100% Figure 3: Availability for 20 GHz and 30 GHz signals from Nittedal Teleport in The attenuation curves for 30 GHz were obtained by frequency scaling the results for 20 GHz using ITU-R models, and are shown respectively as the dotted red and blue lines. There appears to be reasonably good correlation between actual measurements and the ITU model, shown as the solid blue and black lines, respectively. Later on, we will use these results to demonstrate that added attenuation associated with Ka-band can be dealt with quite comfortably through proper teleport infrastructure and system design. 4

5 Regulatory Environment In several ways, the current regulatory regime encourages the use of Ka-band, and it seems inevitable that more and more satellites will include Ka-band payloads. Still, the regulatory landscape is somewhat fragmented and can vary between regions and countries. To give an accurate and detailed account of the current situation for Ku-band and Ka-band is beyond the scope of this paper. We do, however, seek to point out some of the key areas where Ku-band and Ka-band face different challenges from a regulatory perspective. The most prominent are: There is more Ka-band spectrum available than Ku-band spectrum It is increasingly hard to find new orbital positions for Ku-band satellites Ka-band potentially allows satellites to be spaced closer together Satellite downlink EIRP is generally higher for Ka-band than for Ku-band VSAT uplink EIRP is generally higher for Ku-band than for Ka-band The FSS part of Ku-band offers some 750 MHz of bandwidth in two polarizations ( GHz uplink band) while Ka-band offers 2.5 GHz in two polarizations ( GHz). With more than three times the amount of spectrum available, Ka-band has the potential to deliver more bandwidth than Ku-band to a specific area, whether via a single spot beam or through an aggregate coverage area consisting of multiple spot beams. In densely populated regions it is increasingly difficult to obtain new GSO orbital positions for satellites that use the Ku-band. Most of the satellites that are being launched to provide coverage in these regions are therefore replacement satellites. A replacement Ku-band satellite need not be an exact replica of the satellite it replaces, and may incorporate HTS design elements to significantly boost its throughput. This, however, assumes the satellite operator is able to coordinate usage of a high-powered Ku-band satellite payload, which is not a trivial issue. The combination of these factors is typically what leads satellite operators to consider Kaband. Not only are there more satellite positions available, but the spectrum coordination is also more straightforward because there are fewer satellite neighbours to worry about. In the case of Ku-band, the typical power spectral density coordination limit of the satellite downlink is around -20 dbw/hz. If referenced to the same 36MHz transponder, this would correspond to a 55dBW downlink EIRP. For Ka-band the comparable coordination limit is typically around -12 to -15 dbw/hz. If referenced to the same 36 MHz transponder, this would correspond to a transponder downlink EIRP of dbw. Thus it can be seen that, from a regulatory perspective, the current regime favours Ka-band for the satellite downlink. But for remote terminals, the situation is reversed because regulatory limits for a Ka-band remote are more stringent than for a Ku-band remote, as demonstrated in Table 1, below, which shows the off-axis density limits for antennas transmitting in Ku-band and Ka-band. Freq. Band Off-axis angle ITU/ EIRP density recommendations Ku 2.5º Φ 7º -7 25*log10(Φ) dbw/hz Ka 2.0º Φ 7º *log10(Φ) dbw/hz Table 1: Ku- and Ka-band off axis spectral density limits according to ITU-R S At first glance, the difference appears to be huge. But if we account for antenna directivity and the de-facto coordination limits in place in many parts of the world, the comparative advantage for Ku-band is typically much less. In ITU Region 1, for example, the part of Ka-band reserved for Fixed Satellite Service (FSS) in the band GHz has super primary allocation. In this band, hard-power flux density limits do not exist and successful coordination is easier to achieve. Overall, the justification for more stringent regulatory limits for Ka-band seems questionable and they may be challenged in the future. With higher frequencies comes increased antenna directivity for a fixed antenna size. Thus, a Ka-band antenna picks up less interference from adjacent satellites than would be the case with a Ku-band antenna. Ka band satellites could potentially be spaced more densely than Ku band satellites - thus enabling more satellites to target a specific area of the Earth. 5

6 High Throughput Satellites (HTS) High Throughput Satellites (HTS) differ from conventional C-band or Ku-band satellites in one important aspect the number of Gigabits/second of user traffic they can carry. The definition is not precise, but generally everything above 2-3 Gigabits/second is considered to be an HTS satellite. Today, the highest performing HTS satellites can carry well in excess of 100 Gigabits/second. A key question that many ask is whether Ku-band HTS satellites can meet or even exceed the performance of recent Ka-band HTS satellites. This question has gained significant interest recently in light of Intelsat s EPIC plans and Inmarsat s soon-to-launch GX system. It is relatively straightforward to show that frequency bands in themselves do not represent the key issue. A satellite s total throughput, irrespective of frequency band being used, is largely determined by the following factors: Frequency re-use The size of spot beams The amount of spectrum available within the chosen band The frequency re-use concept is illustrated below, where each unique colour represents the same frequency being reused. Regardless of whether you favour Ku-band or Ka-band, the key to achieving the highest possible throughput if that is the sole measure of success is to use small spot beams and reuse the spectrum as many times as possible, within practical limits. An added advantage of Ka-band results from the fact that there is more Ka-band spectrum available than Ku-band spectrum. Consequently, Ka-band allows more bandwidth to be commissioned within each and every spot beam. From this perspective, one can justifiably argue that a Ka-band HTS satellite has the potential to provide a higher aggregate throughput than a Ku-band HTS satellite for a given service area. One should be aware that gateway beams can be located both within and outside the end-user coverage area. The latter will increase the throughput of the satellite even further, but may not always be possible to achieve. Often, gateway beams have distinctively different performance parameters from those of the user beams, as they need to carry the aggregate bandwidth of the associated user beams. Since, in this model, the gateway beams and teleport infrastructure are closely linked to the satellite design, it is typically the satellite operator which will be in charge of constructing and operating the required teleport infrastructure. Thus, as more HTS satellites are launched, there will be a greater degree of centralisation and homogeneity as the infrastructure owned by the satellite operator increases. This will not replace the situation where teleport infrastructure is more distributed and diverse, which will continue to suit some system set ups and applications. Figure 4: The concept of frequency reuse and small spot beams vs. larger spot beams. Frequencies are reduced, but not in contiguous cells. Smaller Radius cells, compared to larger radius cells serving the same geographic area, are more efficient and serve more users. 6

7 System Design Considerations Previously we looked at the difference in rain attenuation experienced by Ka-band signals compared to Ku-band signals. We can now elaborate further on this and break up the two-way satellite link into its four distinct parts as illustrated below. Each part will be analysed to show that it is indeed possible to create Ka-band services with availability targets that match current Ku-band services usually in the 99.5% 99.8% range. ALC exploits the fact that satellite link budgets are not symmetrical in the uplink and downlink. Because of the large gateway antennas that are being used on the teleport side, the uplink C/N is typically much higher than the downlink C/N, which is seen by a much smaller remote antenna. This concept is illustrated in Figure 6. below. Figure 5: The 4 parts of a two-way satellite link Figure 6: ALC exploits difference in uplink and downlink C/N. C/N up C/N down C/N up >> C/N down (1) Up-linking all carriers from a single teleport into a gateway beam allows a satellite feature called Automatic Level Control (ALC) to be used. Today this feature is used mostly on broadcast satellites that provide Direct-to-Home (DTH) services with very high availability requirements. ALC compensates for reduction in uplink EIRP caused by rain fade by increasing the transponder gain. The net effect is nearconstant downlink EIRP over a wide fading range. ALC may be used only when all carriers are transmitted from a single teleport location as it works on the basis of received composite power rather than individual carriers. When the teleport experiences heavy rainfall, the uplink C/N is downgraded. But as long as the uplink C/N is still higher than the downlink C/N, the overall C/N of the satellite link is only marginally reduced. In a typical Ka-band scenario, the uplink C/N is easily 25 db or more, while the downlink C/N towards a small antenna might be in the region of 10 db. This means that a fade of 15 db on the teleport side will translate into merely a 3 db reduction in the overall C/N. When ALC is combined with a DVB-S2 ACM outbound carrier. The resulting uplink fade margin can exceed 25 db. This corresponds to a yearly availability for the 30 GHz uplink signal of 99.99%. 7

8 (2) For Ka-band, the frequency of the downlink from the satellite to the VSAT is around 20 GHz. If we assume the outbound carrier uses DVB-S2 and ACM, and further that the link uses MODCODs (i.e. combination of modulation and coding) ranging from QPSK rate 2/3 up to 16-APSK rate 5/6, we have in effect implemented a fading margin of db. This corresponds to a yearly availability of the 20 GHz downlink signal of at least 99.95% (3) For the 30 GHz uplink from the VSAT to the satellite, there are two mechanisms available to boost signal availability uplink power control and carrier adaptivity. Uplink power control is really meaningful only if the VSAT is limited under nominal regulatory conditions. When experiencing a fade, the VSAT can then boost the transmit power back up to the regulatory limit. As the transmitter cost increases almost exponentially with transmit power, the built-in margin for uplink power control tends to be relatively modest and usually no more than 3-5dB. Significantly greater margins can be added by using carrier adaptivity in the return direction, either DVB-S2 combined with ACM or adaptive TDMA techniques, where the return carrier modulation order and coding rate is dynamically changed when the VSAT undergoes a fade. As an example, with inbound carriers ranging in modulation rate from 256 ksymbols/second up to 1 Msymbols/second and code rates ranging from 1/2 to 6/7, a fade margin of around 10 db can be implemented. For the 30 GHz uplink signal, this translates to a yearly availability of 99.9%. (4) In the downlink from the satellite to the teleport, antenna site diversity can be used to boost satellite link availability. The benefits of antenna site diversity increase with the separation of the antennas. Through a detailed meteorological study, we also discovered that there was good correlation between measurements and ITU models with respect to antenna diversity gain for a specific choice of teleport location. If the main antenna is designed to provide a yearly availability of 99.9% for the 20 GHz downlink signal (top red dot) and we add another identical antenna separated by 30 km from the first antenna, we should expect the combined yearly availability to be better than 99.99%, which is a significant improvement. Further separation of antennas would provide even better results. Perecentage of time Predicted Lognor mod d = 5 km d = 10 km d = 20 km d = 30 km d = 50 km d = 100 km Attenuation (db) Figure 7: Availability improvement for 20 GHz downlink signal using antenna diversity. 8

9 Total yearly link availability can now be found by multiplying the uplink and downlink availability figures found above for the outbound and inbound, respectively. Yearly availability Ka-band achievable Ku-band typical Outbound * % 99.5% % Inbound 99.9 * % 99.5% % Table 2: Achievable yearly availability for Ka-band systems. Hence, when properly designed a Ka-band system is capable of delivering the same sort of service availabilities that are typically provided by current Ku-band services. Generally, a Ka-band system lends itself better to exploit the upsides of adaptive carrier techniques such as DVB-S2 ACM and Adaptive TDMA. The reason is that Ka-band systems are typically designed to have higher clear-sky margins to deal with the effects of rain fade than Ku-band systems. If this extra margin is fully exploited it means a Ka-band system will have a higher average throughput compared to a Ku-band system if both are designed to achieve the same yearly service availability. Another characteristic of HTS satellite systems where all carriers are uplinked through a small number of gateway beam is that they become more broadcast-like in nature. We saw that by up-linking from a single antenna ALC could be activated to significantly boost the uplink signal availability. If, in addition, each satellite tube amplifier (TWTA) transmits only one or two carriers, the amplifier can be operated with less output back-off (OBO), resulting in higher carrier downlink EIRP. This is one of the reasons why most HTS satellites systems often use a single platform to generate a single shared TDM carrier in each outbound transponder. One of the key differences between most Ku-band and Ka-band systems is that Ku-band uses linearly polarized signals, while Ka-band uses circularly polarized signals. This is a characteristic that Ka-band shares with most C-band satellite systems, though the reasons for choosing circular polarization are different. While the choice of circular polarization for C-band was mostly because of effects produced by the Earth s magnetic field (Faraday rotation), its choice for Ka-band is mostly driven by the desire to simplify the installation and line-up of VSAT terminal equipment. For circular polarized signals there is no need to adjust the antenna s polarization plane to match the satellite, one just selects either right-hand or left-hand polarization, according to the design of the antenna. In theory, this should eliminate many of the problems with cross-polarization interference that are typically caused by poor VSAT installations. For stabilized antenna platforms there is an added advantage, as the motor and control system responsible for tracking polarization skew can be eliminated altogether. The net result is an antenna with fewer moving parts, which often translates as reduced weight and increased reliability. 9

10 Summary Ka-band does present added challenges related to rain fade. We have demonstrated that, through careful system design. Ka-band satellites are capable of delivering service availability that matches current Ku-band satellites. This is achieved using satellite ALC, antenna site diversity and carrier adaptivity. Overall throughput of a satellite system is dependent mainly on three factors: available spectrum, spot beam size and frequency re-use. These factors apply equally to Ku-band and Ka-band, and suggest that spot-beam satellites will play an increasingly important role in the future because they have the potential to significantly reduce the cost per bit delivered over a satellite link. With more spectrum available for Ka-band than for Ku-band, the highest overall satellite throughput is realized using Ka-band. We believe that Ka-band is not a replacement for Ku-band. The two will complement each other and service providers will learn to exploit the unique characteristics of each to maximize the value for end-users. 10

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