The introduction of new, innovative payload baseband Modem ASICs (such as SatixFy’s SX 3000 series) enables low-power, low-weight scalable payload design. Coupled with the use of unlicensed ISM 24 GHz frequencies allows unique LEO payload designs with weight in the 100Kg range and power within less than 1KW DC.
The renewed interest in LEO systems is motivated by the need to provide global coverage for internet services in remote and underserved areas. In this post, we describe the architecture of an On-Board Processing (OBP) system based upon baseband ASIC, for a multibeam LEO satellite. The architecture is low power, flexible and provides an excellent connectivity through a single gateway.
Since one of the major barriers for satellite services is frequency spectrum. We propose to use the 24 GHz unlicensed band, following the successful utilization of unlicensed bands for terrestrial wireless data services. This band offers an excellent opportunity for such services. We also analyze and present the expected performance of such links in that frequency bands.
Non-geostationary (NGEO) satellite communication systems offer an array of advantages over their terrestrial and geostationary counterparts. They are seen as an integral part of next-generation ubiquitous communication systems.
A Low Earth Orbit (LEO) is positioned at altitudes varying between 160km-2000 km above mean sea level. At 850 km, the velocity of satellites placed on LEO orbits is approximately 7500 m/s whereas their orbital period is 90min.
Another interesting advantage of LEO satellites is the relevant small propagation delay. On the other hand, a major disadvantage of LEO satellites is the limited lifetime of the spacecraft due to the increased atmosphere friction effects at the lower altitudes, and the need for tracking antennas on the ground.
The inherent advantages of the satellite communications, as well as the development of the DVB-S2X and DVB-RCS2 pair of standards, allows SatCom systems to provide reliable and efficient Broadcast, Fixed and Mobile interactive satellite services.
There is significant frequency allocation constraints. New global satellite services such as OneWeb, SpaceX and others are trying to get allocations in the crowded Ku and Ka bands – and almost no available spectrum segments were left to be used by others.
Multibeam satellites with on-board processing capabilities can play a key role. Multibeam satellites employ large on-board multibeam antennas. The resulting high gain offered by these antennas compensates for the performance degradation exhibited by low-cost user terminals, at the same time minimizing the satellite transmit power. Moreover, multibeam antennas with On-Board Processing (OBP) capabilities can provide the necessary flexibility by offering frequency reuse and beam traffic reconfiguration responding to the varying traffic demand over the coverage area of the satellite. This is due to the capability of multibeam antennas to implement multispot coverage similar to the terrestrial cellular application.
To preserve a balanced capacity distribution to the system users under non-uniform gateways (GW) availability and different weather non-uniform across the planet, can be achieved using dedicated inter-satellite communication links (ISL), which allows each satellite in the constellation to share its load/free capacity with its neighbouring satellites to form a most optimum traffic distribution between all system nodes.
The architecture of an On-Board Processing (OBP) system based on SX-3000 ASIC, for a multibeam LEO satellite. The architecture is based on a low power air interface. It supports links to user terminals (Customer Furnished Equipment, CPE), a gateway (GW) and inter-satellite links (ISL), which makes it flexible and provides excellent connectivity through a single gateway. In addition to the presented architecture, the LEO system coverage and link budget analysis for operation in the unlicensed band of 24GHz.
The use of new and innovative on-board-processing concepts enables substantial advantages over ordinary bent-pipe payloads. Key examples:
- Multi-beam approach as used in High Throughput Satellites, enabling any number of beams (in our design 19 beams), with the frequency reuse of 4 colors (frequency can be used 19/4 times), with improved link budget compared to bent-pipe. Multibeam approach supports higher antenna gain in the payload and enables significantly higher throughput in the user link beams.
- On-board-processing enables over 6 Gbps per LEO throughput over a coverage area of 8,000,000 Km2, to 60 cm user link antennas on the CPE side.
- Improved link budget, like performance, is only dependent on the uplink because of the On-Board-Demodulation. Hence – even if the CPE antennas or GW antennas are suboptimal – user link throughput can be maintained.
- Single Gateway link – the complete decoupling of the user link and GW link, enable single GW beam per satellite.
- Better utilization of the gateway link, thanks to the ability to stack much more throughput in the gateway link. Much higher MODCODs are available on the GW link. Links could be designed with no inter-beam interference and hence high SNRs (and hence bits/Hz) are possible.
- Supports inter-satellite communication (through a dedicated payload), allowing the reduction of the amount of implemented GWs. Could not be achieved by bent pipe payloads.
- The overall system reduced cost. – Payload now is no more than LNAs, HPAs and a dedicated PCB board that runs all the electronics. Smaller, lower weight, much easier to manufacture, scalable and substantially lower cost.
- Low power design – Use of OBP and Baseband chips in payload enables significant power reduction – every beam needs to operate only when there is actual traffic involved. Dummy frames are not transmitted. Average battery life and weight (which is a major contributor to weight and lifetime of the satellite) is significantly enhanced.
Use of unlicensed ISM Bands
The industrial, scientific and medical (ISM) bands are radio bands reserved internationally for the use of radio frequency (RF) energy are defined by the ITU-R in 5.138, 5.150, and 5.280 of the Radio Regulations.
Because communication devices using the ISM bands must tolerate any interference from ISM equipment, unlicensed operations are typically permitted to use these bands, since unlicensed operation typically needs to be tolerant of interference from other devices anyway. The ISM bands share allocations with unlicensed and licensed operations; however, due to the high likelihood of harmful interference, licensed use of the bands is typically low. In the United States, uses of the ISM bands are governed by Part 18 of the Federal Communications Commission (FCC) rules, while Part 15 contains the rules for unlicensed communication devices, even those that share ISM frequencies. In Europe, the ETSI is responsible for governing ISM bands. For satellite communications, only microwave frequencies (above 2GHz) are relevant and are listed in Table 1 below. Of those we focus on the 24GHz band, which is accepted worldwide, contains wide available spectrum, relatively free of atmospheric attenuation and yet underutilized.
The use of 24 GHz ISM band is best suitable for the Satellite-CPE link. It can also be suitable for the GW-Satellite link with limited capacity. To overcome the capacity constraints ISM frequencies in the E and V bands also seems viable (~60 GHz and ~75 GHz respectively), especially for ISL, due to lower atmospheric fade.
ITU-R ISM bands relevant to Satellite Communication
OBP System Payload
The Satellite is used to host and serve the communication payload for its primary mission. The satellite has a unique location within the constellation and orbits earth in one of the planes. The satellite is aimed to provide the payload with the needed electrical power for proper operation.
Figure 1 depicts the satellite OBP payload design used to accommodate the GW and user links from one hand, as well as four Inter-Satellite Links (ISL). The following section describes the various payload components and their connectivity in more details.
The main processing units are based on the SX-3000 chips , which perform the modulation and demodulation for each link. The chip also includes packet processing, Ethernet port and optionally antenna control capability.
10Gbps LAN switch
A high throughput Ethernet backbone is required to aggregate, switch and distribute traffic between SX-3000 chips (different links) based on the traffic destination. When a GW link is available to the satellite, the GW-Satellite-CPE link becomes the primary data path. In case a GW link is not available the satellite aggregates/distributes the traffic to/from the CPEs (users-links) to/from the four ISLs links.
In order to support beam switching, L3 (or tunneled MPLS) switches shall be used to switch the Ethernet frames (containing IP packets) between other SX-3000 BB chips and the corresponding beams according to beam switching state. A proprietary switching/routing process is implemented where the GW labels the packets, and the packets are routed accordingly at the Satellite payload.
Traffic Switching/Routing at the payload
The overall links BW management at the payload is implemented at layers 1 and 2.
At layer 1 time-slice feature of DVB-S2 defines and provides different time-slice numbers (TSN) to differentiate services or user groups. In the proposed OBP architecture DVB-S2 TSN shall be used to segment the overall forward link capacity from the GW to the users (CPEs) and with other links with other satellites.
At layer 2 several existing techniques can be used. The implementation of “Explicit Load Balancing” (ELB) scheme with other L2 schemes i.e VLAN-tagging (802.1q) and VLAN-priority (802.1p), bringing more flexibility to differentiate traffic classes and to better manage traffic distribution across the system.
On one hand, implementing Quality of Service (QoS) ensures that users will get their required service level and their committed bandwidth. On the other hand, the system should provide sufficient bandwidth to the ISLs.
The satellite primary Telemetry Tracking and Command (TT&C) is carried on a separate link outside of this system. TT&C redundant links can be implemented as part of the GW-Satellite link.
The SX-3000 transmit signal should be upconverted and RF modulated over the transmitted frequency (direct frequency conversion). The chip Receive (Rx) should be down- converted to produce demodulated I/Q baseband waveform.
Low Noise Amplifier (LNA) & Power Amplifier (PA)
These RF components are used to amplify and match transmitted (PA) and received (LNA) RF levels. The amplifiers are operating in the respective receive and transmit bands (24 GHz, Ku, K, Ka and E band).
Multibeam CPE link
The multibeam approach is used in High Throughput Satellites (HTS), enabling any number of beams to reuse frequencies. In our design with 19 beams in 1+6+12 center-mid-outer arrangement (see figure 3), 4 color frequency reuse pattern is used (frequency can be used 19/4 times), with improved link budget compared to regular single-beam bent-pipe approach. The design implies multiple beams in forward and return links to support the designed coverage area. Each beam is using a dedicated Rx/Tx chains. This link could be operated at 24 GHz ISM band.
The gain levels of the antennas are designed to achieve the required coverage area. Thus, different antennas are used for the center beam, beams in the mid ring, and beams in the outer ring.
The GW demands high capacity and throughput. In this example, this link is designed to operate at the 24 GHz ISM band, but another alternative higher ISM frequencies may be used or high-speed laser-based communication technology.
Inter–Satellite link (ISL)
Each satellite establishes persistent communication links with its neighboring satellites.
As previously stated, the inter-satellite links are used as redundancy links, in the case of poor GW link and in the case of no available GW link in the satellite coverage area. In this case, all the traffic arriving from the CPEs will be distributed among the four ISL links to reach another GW link (via a neighboring satellite). On the other direction, all the traffic from the ISLs aimed to the CPEs will be aggregated from the ISL links by the LAN switch and will be distributed to the CPEs via the CPE link.
The ISL links are notated according to the link general cardinal directions of the north (N), east (E), south (S), and west (W).
All four ISL links (two on same-plane and two on adjacent planes) are identical in their structure, performance and links capacity. These links are designed as point-to-point SCPC links based on DVB-S2X ACM mode of operation, with a maximum of 3Gbps expected throughput on each direction. The total of 12 Gbps capacity available from these links can easily overcome GW availability issues with minimum effect on the guaranteed service to the CPEs.
The table below summarizes the satellite links general characteristics:
Low Power and Air-Interface Architecture
One of the key elements in the design is low power architecture that is intended to power on every beam and the payload itself only when there is actual traffic. This can be achieved using highly integrated modem SoC with low power design such as our next-gen low-power ASIC.
The main principles for the air interface are as follows:
- Forward and return link TDMA transmissions would be centrally controlled by the gateway scheduler.
- The gateway scheduler has the information of terminals and LEO constellation satellites locations, and antenna directions for all forward and return link antennas.
- The waveform is based on tight synchronous time frame, denoted as the fixed grid.
- Terminals are partitioned into groups, subgroups, sub-subgroups etc, such that addressing can be made to a given set of terminals, as required.
- Dummy frames will not be transmitted.
- Overall reduced power consumption by switching off major portions of TX chains
LEO Satellite Coverage at 850 Km
The Image below illustrates LEO satellite constellation at 850 Km altitude. The constellation is based on 6 planes (30° apart) in 85° orbit plane inclination. On each plane 12 satellites are placed, a total of 72 satellites.
The image below illustrates a single multibeam LEO satellite footprint coverage. A single satellite projects 19 beams (in 1+6+12 center-mid-outer arrangement) over the earth surface, covering about 600,000 (km)2 each. The global coverage of a single satellite is about 8,000,000 (km)2. The global coverage of the whole constellation (72 satellites) is about 576,000,000 (km)2, namely a complete plane coverage.
The image below illustrates Inter-Satellite Communication links of LEO satellite with two neighboring satellite on the adjacent plane and two neighboring satellites on the same plane. Each satellite in the constellation communicates with four other satellites simultaneously.
Many new LEO system operators are coming up with LEO system design. Their main problem is payload weight, size, power, complexity, and performance – as well as the availability of spectrum. We presented a payload architecture, based on a new, innovative ASICs coupled with the use of 24GHz unlicensed frequency in the user (CPE) link which opens the door for smaller, lower weight (100Kg), lower power (less than 1000W even before applying low-power techniques), less expensive ($500K or less per payload) solution. A preliminary link budget, using typical value has demonstrated that such a system can provide more than 6 Gbps of user link throughput to a low cost 60 cm tracking antennas on user premises, enabling up to 200,000 users per satellite.
* This paper was previously published by: Published: Yoel Gat and Yoram Ben-Ami, at the Ka conference, Bologna, Italy, 2015 (http://www.kaconf.org/2015/)