Page 25 - EngineerIT December 2022

SPACE SCIENCES

How does a LEO satellite system
work?
LEO satcom systems are made up of
three major components, as shown in
Figure 1.

User terminals/user equipment
(UE)
These are the direct link between the
user and the satellite and tend to be low
cost, easy to set up terminals located
at homes, but also can be mobile
terminals (for example, maritime,
satcom on the move, tactical manpack
radios). User terminals leverage high Figure 1: An example of a ground-space scenario for LEO satcom.
levels of IC integration to simplify the
bill of materials (BOM), lower cost, and link speed to the ground station. Traditionally, satellites use bent pipe, which means the
maintain a small form factor. satellite must always find a link path to the Earth or some other means (aircraft) to serve
as a hop back to another satellite in space, which could then be in range of a ground
Ground stations/gateway station. A new technique is through intersatellite links using optical or V and E band
These are the ground connections to connections in space to link satellites.
the servers (data centres for internet
connection) typically over fibre, and they Advances in user terminal up/downconverters
link the satellite to the ground. They are User terminals are driving significant levels of IC integration and Analog Devices is
deployed at fixed locations across the responding to this demand by leveraging the performance and integration capability of
Earth. silicon process technology. These solutions require the highest level of IC integration
to enable the smallest form factor radio terminal, while maintaining the lowest power
Satellites consumption and a strict adherence to the optimal cost per radio.
Groups of satellites are called Up/downconverters (UDCs) are a foundational product in user terminals, and they
constellations and these orbit the Earth interface the modem IF or baseband information directly to Ku band or Ka band.
providing simultaneous links to connect
both terminals and gateways. The frequency coverage goals of RFIC UDCs are:
LEO satellites move across space and • Ku band: ~10.7 GHz to ~14.5 GHz
typically a single satellite will orbit the - Downlink (satellite to ground): 10.7 GHz to 12.7 GHz
Earth in a period of 90 mins to 110 mins, - Uplink (ground to satellite): 14 GHz to 14.5 GHz
referred to as the orbital period. Because • Ka band: ~18 GHz to ~31 GHz
of this, a user connecting to the satellite - Downlink (satellite to ground): 17.7 GHz to 21 GHz
will only be in range of that satellite for - Uplink (ground to satellite): 27 GHz to 31 GHz
a short period of time (up to 20 mins).
So, the average user will be connecting Downlink and uplink are separated in frequency so the communication from the satellite
to multiple satellites during normal to the user terminal is using two separate frequency bands. Therefore, RFIC companies
operation. Therefore, users of the system must design each user terminal up and down converter for separate bands.
must be handed off to other satellites Depending on uplink vs. downlink, user terminal links typically cover channel
that come into range, in a similar manner bandwidths (BW) of 125 MHz to 250 MHz and gateways cover between 250 MHz and
to a person using a cell phone in a 500 MHz. However, some deployments have a shared bandwidth capability between the
moving car and one base station in the user and gateway links, so the channel bandwidth is reconfigurable in the frequencies
cellular network handing off to another. they operate.
This places strict requirements on how to LEO satellites are moving constantly as shown in Figure 1. Thus, the up/downconverter
steer the beams to maintain the best link frequency synthesiser within the terminal must achieve fast lock times for uninterrupted
to the most appropriate satellite. connection. Synthesisers are used to assist in the frequency up-conversion and down-
Another interesting evolution is how conversion. They play a vital role in enabling the terminal to connect and reconnect to
a satellite system maintains operation different satellites during operation, as the frequency over the air changes constantly
when it is out of range of a ground within the operational bands (that is, Ka and Ku bands) from one satellite to another.
station. In Figure 1, we are showing some ADI has developed a family of Ku and Ka band UDCs targeting user terminals to
adverse weather that may affect the address the size, weight, area, power and cost (SWaP-C) problem. These UDCs contain

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