Currently the USRP I am using for my work can be equipped with 2 RF daughter boards so that you can connect 2 antennas. How can I extend this to more antennas?
Can I use the technique of antenna selection and use a RF switch between the antennas and daughter boards?
It depends on the USRP version you are using. For the USRP N2xx series, you can connect two USRPs together to share the same FPGA clock, so you can have a 2x2 MIMO.
Look at this page, there is (a bit) more info:
http://files.ettus.com/uhd_docs/manual/html/usrp2.html#using-the-mimo-cable
If you planning to implement MIMO system, the sample at each antenna must be synchronized. To do this with multiple usrp, you must lock the local oscillators.
In this link you will find how to lock local oscillators;,
http://gnuradio.org/redmine/projects/gnuradio/wiki/MultiUsrp
There are two possibilities. One is to buy additional USRPs with RF daughterboards for multichannel USRP setup that can be scaled to your desired MIMO order and other is to use multiantenna array at transmitter and receiver and fast RF switch depending on the MIMO order you want.
In terms of SDRs there are new SDR devices being launhed with more number of channels such as XTRX multichannel SDR.
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