The technical objectives were to:
- Define the air-interface requirements for broadband access to a MEO satellite network in 2 selected reference scenarios: Scenario 1 with a dual-antenna user terminal located at the beam centre, and Scenario 2 with a single-antenna user terminal receiving signals from the sidelobes of different beams.
- Develop relevant transmission and reception schemes for these two scenarios, namely a seamless (i.e., without packet loss) handover technique (when two MEO satellites are in view, one rising and one setting) and diversity combining (when only one MEO satellite is in view). To this purpose, physical and higher layer handover techniques were considered, as well as various flavours of Maximum Ratio Combining, Equal Gain Combining, and Selection Combining
- Demonstrate achievable space link performance using an endto-end test-bed implemented in hardware including the transmit/receive techniques for the first selected scenario. To this purpose, a demod was prototyped.
The first challenges were in the investigation of the trade-off between performance and complexity of the different combining techniques and layer 2 handover. The design and prototyping of the seamless (i.e., without packet loss) handover and the chosen combining technique provided challenges from the hardware implementation point of view. Additionally, the performance evaluation of sidelobe communications in Scenario 2 provided theoretical and practical challenges.
The use of non-geostationary satellite for very high speed data services has been taking momentum in the past years. In particular, the launch of the first O3b MEO constellation satellites is a recent example of a new mission concept, taking advantage of a lower latency and a higher radiated power to cover under-served geographical areas compared to existing broadband coverage in GEO. While service provisioning by MEO satellites benefits from higher throughputs and lower latencies, the nature of the mobile infrastructure brings in novel aspects in architecture, network management, and ground segment. Multiple satellites and beams in conjunction with multiple receivers provide new scenarios characteristic to MEO constellations. These, in turn, offer new opportunities that demand new designs targeting an enhanced transmission capability with guaranteed availability, especially since some of the networks operate in the Ka-band. To this purpose, the prototype developed in the framework of this project provides a low-complexity solution for user terminals allowing reliable broadband communications through MEO satellites with improved capacity.
The prototype at first guarantees a seamless handover from the setting to the rising satellite, meaning not a single packet is lost and the throughput is not impacted in a noticeable way from retransmissions due to duplicate or reordered packets. Secondly, the prototype exploits the availability of the two antennas outside of the handover phase through diversity combining. While achieving the combining gain is relatively easy, achieving a low-complexity but robust implementation is more difficult. For example, most of the state-of-the art systems use resource-intensive correlations to align the streams or to estimate signal-to-noise ratios. Iterations to the software and back are required to compute combining coefficients, causing a delay in the adjustment of the coefficients which can cause a 3-dB degradation instead of a gain. This prototype is based on a low-complexity but robust implementation since correlation is not used. Furthermore, the combining process is completely autonomous without iterations with the software and by design never causes a degradation of the signal with respect to noncombining.
Scenario 1 - A user terminal is served by the MEO satellite network. The receiver employs two antennas and two receive chains, and signal combining is carried out at the baseband digital domain. The use of two receive chains is to ensure a make-before-break handover between two satellites (the setting and the rising one) in the visibility of the user terminal. Combining is active only when one satellite is visible, and the two receive chains receive two replicas of the same transmitted signal. The combining architecture includes the nature of combining per-se and its position.
Scenario 2 - A user terminal is served by the MEO satellite network but without steering a satellite beam directly to it. Its geographical location is in between multiple spot beams coming from the same or different satellites. As none of the satellite beams is pointing to the user terminal directly, useful power is received only through the sidelobes, which renders each individual received signal very weak. The planned architecture sustains low rate communications with terminals outside the normal coverage of a satellite beam by transmitting and receiving data on multiple beams (from the same or different satellites) in a region, and dynamically adjusting the proportion of data sent on a particular beam via ACM-type feedback.
Preparatory Phase: The initial phase of the activity dealt with consolidation of the scenarios, determining the scope of the prototype activity and deriving the requirements, configuring the air-interfaces, devising novel transmission and reception techniques, listing the performance metrics and preparation of a simulation plan.
Software Development Phase: A software simulator for assessing the performance of devised techniques using DVB-S2X air-interface was implemented in Matlab. The milestone was Preliminary Design Review.
Prototype Development: A significant effort of the project dealt with the design, development, and validation of the prototype to demonstrate the cost, functionality, and the performance of the selected techniq
The project is completed and the prototype was produced. Scenario 1 was selected to assess the performance of seamless handover and combining. Before the prototype design, a software simulation campaign was run to choose the most appropriate combining technique. The prototype was designed accordingly, and it underwent an extensive test campaign aiming at the assessment of both handover and combining. The prototype successfully passed all the tests and its features could be included in the next generation of modems by Newtec. Scenario 2 led to high-level capacity and outage investigations of sidelobe communications, performed by means of software simulations.