Why can't we use MIMO antennas in a cubesat?

Question

I understand that there are no scatterers in communication between ground station and satellite but does it mean it cannot be used or is it just inefficient ?

Answer 1

Of course we can. You can probably see MIMO antennas on your WiFi router already, so with some modification they could be fit onto a cube. By the way: there is no such thing as a "MIMO antenna" - it is a name of a technology that integrates multiple antennas, multiple radios and heavy signal processing in a single system, but I will use the term for simplicity.

The main question is: why would you use it in the first place? The benefit of MIMO is that you can effectively get more throughput from the same RF bandwidth using many physical antennas.

An oversimplified example: you have two tablets at home in different places and a WiFi router in the middle. Without MIMO there is basically a single channel for both, so the tablets can't transmit at the same time, (otherwise the router will "hear" a mix-up). This is solved by a medium access mechanism. With MIMO the router can "make" virtual antennas that will "look" in different directions allowing both tablets to transmit at the same time on the same channel (and rotate them around when you move the tablets). You need more physical antennas and radios to implement MIMO.

MIMO is not limited to point-multipoint network topology. A single tablet can also use MIMO to make its own "virtual channels" to communicate faster with the router, like it was two separate devices.

Of course real life is not that simple, but it is enough to say that with MIMO you will simply get more throughput, than without it (using the same RF bandwidth). It is not that simple as "two antennas, twice the throughput".

Now back to the cubesat. The cubesat is limited by the dimensions. You would have to devote more precious space for antennas, that you could to spend for example on sensors. Available power is also limited (MIMO needs more processing power and more radios). You also get much more complex system that is harder to develop, validate and make reliable. Why would you need very high bandwidth for such a small device?

However I am almost certain that SpaceX will use MIMO for its Internet satellite constellation, because it will allow them to service more user terminals using the same amount of (expensive) bandwidth. Of course the satellites will be larger than a cubesat.