Abstract
The appetite for high data rate and ubiquitous terrestrial connectivity has stimulated huge investment in low Earth orbit (LEO) constellation systems. Low orbit significantly reduces launch and equipment costs, cuts latency by 20× compared with GEO links, and manages bandwidth and users more efficiently. LEO satcom services promise worldwide coverage on the ground, at sea, and in-flight, reaching rural and remote regions, as well as supporting disaster zones where ground-based networks might be disabled. For this vision to be realized, electronically steered arrays at both ends of the link become essential to support the continuous, independent, fast scanning of multiple beams and the regular hand-offs as satellites come in and out of view. The payload is particularly challenging because it operates in a power-limited environment when DC power dissipation is critical. In addition, a high level of functionality such as multibeam capability, beamhopping, beam memory, and the ability to scale the number of beams to support any mission requirement. Analog Devices has designed and productized a family of multibeam beamforming ICs that support payload beam- forming applications with low power, high level of functionality and can scale with mission requirements.
Introduction
Most common satellite communications were originally based on geostationary satellites (GEO) where only three satellites were required for global coverage. These are large satellites (>1000 kg) where typically a single satellite would be on a launch vehicle. While this type of deployment was beneficial for broadcast applications such as television and radio, there were limitations. One is the latency involved in communications simply by the great distance between the user and the satellite. Typical GEO orbits are near 36,000 km, which has an approximate end-to-end latency of 400 ms, approximately 10× higher than point-to-point fiber optics connections in the United States.1 Secondly, while the GEO satellite covers much of the Earth, it cannot effectively cover the northern or southern poles of the Earth. As an example, Inmarsat’s Global Xpress GEO satellites cover approximately ±75° off the equator.2
To create true global coverage, smaller (<500 kg) low Earth orbiting (LEO) satellites are being deployed with inclined orbits to cover both major and rural population areas and polar orbits to cover the poles. These constellations vary from several hundred to several thousand satellites. All of these require beamforming antennas since they are traveling around the Earth at 27,000 km/h at 600 km to 1200 km orbit altitudes. This translates to the end-to-end latency reducing to approximately 50 ms. Many satellites are in a single launch vehicle for each deployment, so the size and weight of these LEO satellites are critical. In addition, these satellites were developed to provide high speed data to the user so using the proper frequency plan is important. Traditionally, Ku band (10.7 GHz to 12.7 GHz downlink/13.75 GHz to 14.5 GHz uplink) had been used. However, there is a drive to higher frequencies that have wider bandwidths and can support higher data rates. K/Ka band is being actively leveraged (17.7 GHz to 21.2 GHz downlink/27.5 GHz to 31.5 GHz uplink) and many are investigating Q/V bands as the next frequency band (37.5 GHz to 42.5 GHz downlink/47.2 GHz to 51.4 GHz uplink).
These higher frequencies pose new challenges in the design and realization of a payload phased array antenna. As the frequency increases, the lattice pitch of the antenna elements decreases, minimizing the available board space. Traditionally, a discrete approach may have been possible using transmission lines for time delays, beam steering using phase shifters/digital step attenuators or vector modulators, and embedding the Wilkinson splitters/combiners in the printed circuit board (PCB) itself. However, at these higher frequency bands, the PCB area is a significant challenge, which is driving the need for higher integration for ease of design and manufactur- ability. Additionally, the need for multibeam arrays drives additional complexity.
Beamforming ICs define electronically steerable phased arrays (ESA) and serve as the most critical building block. Interwoven in between the beamforming ICs are a power combiner and splitters, which distribute the signals to every beamforming IC. It is the combination of the beamforming IC, power combiner/splitter, and the fabric that weaves these two components together in a PCB design that determines the perfor- mance of the ESA.
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