MIMO Technology specific question

[joaorob]

Hey all,

I have a question regarding MIMO technology. I could find a huge amount of information but found nothing that specifies how (for example) a system 2x2 (2 antennas input and 2 antennas output) can differenciate between the 2 streams working at the same frequency and at the same time. Is it by coding?

I would appreciate if anyone can explane me or just indicate a website that could help me in this specific question.

Thanks for your attention and sorry my english

João

 

[RadioRon]

It is a good question, but not easy to answer in a short form. I know of at least two methods and I am sure there are a few more that allow different signals to be sent over the same radio spectrum at the same time. One way is using spatial multiplexing. This relies on the fact that a radio signal will arrive at different antennas with different amplitudes and phases. If you arrange to send two radio signals from two different antennas to two different receive antennas, it is possible to separate the tx signals by taking advantage of those differences in phase and amplitude. In a simple example, imagine that each of the receivers was able to cancel out one of the transmit signals while still receiving the other transmit signal by adjusting the relative phase of the tx signals back at the transmitter. If both of the receivers was able to do that, you can have independent signals going to each receiver at the same time.

Another way is to use coding to create diversity. One way to do this is when you use coding to split a data stream into two that when transmitted over the air, are orthogonal to each other. When you receive two signals that are orthogonal, it is possible to separate them. Existing CDMA systems use a method like this to allow many spread spectrum phone signals to occupy the same spectrum at the same time and all can get through. The key is coding to create orthogonality.

One reference that might be helpful is:
"Channels, Propagation and Antennas for Mobile Communication" by Rodney Vaughan and Jorgen Andersen, IET, 2006