The first ever truly embedded SDR

Feb 11, 2017

Project update 7 of 35

MIMO and Independent Receive/transmit

We often get asked how many receive and transmit chains does XTRX have and whether you can tune them to different frequencies, so I decided to clarify this once and for all.

XTRX is a 2×2 MIMO transceiver, which means that:

  1. XTRX has 2 receive channels and 2 transmit channels, so you can see “Rx1”, “Rx2”, “Tx1” and “Tx2” antenna connectors on the board.
  2. Both receive channels share a common local oscillator (LO) and thus always receive the same frequency. Same for transmit – both channels share a common LO and always transmit at the same frequency.
  3. At the same time receive and transmit have independent LOs, so you can transmit on one frequency while receiving on another – a mode commonly referred as Frequency Division Duplex (FDD). For communication systems where transmitter should work on the same frequency as a receiver (also known as Time Division Duplex, TDD) LO can be shared between transmit and receive sides of XTRX.

If you want to receive or transmit at two frequencies located close to each other, you still can do this with XTRX by digitally shifting one of the channel frequencies relative to the central frequency. This is called baseband shifting or mixing and can be done either at the time you generate your signal or later by XTRX itself. How far you can move your transmit/receive frequency from the central frequency is limited by the configured sample rate. E.g. for a sampling frequency of 100 MHz you can shift your transmit/receive frequency by 50 MHz above or below the central frequency.

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