Software Defined Radio
It is common for time to divided into time slots with transmissions from different radios happening in their own assigned time slot. This VI demonstrates Time Division Multiplexing (TDM) using two NI USRP-2920. One full second is divided into 10 equal time slots for illustration purposes. A single tone is transmitted in the time slot when two conditions are true: 1. There is data in the data FIFO and 2. The current time is the assigned time slot. In this case time is shared between the two radios using the MIMO cable.
Setup:
2 NI USRP-2920
3 antennas (2 TX and 1 RX)
1 MIMO Cable
The attached TDM Transmit VI is pictured below.
Press the Enqueue USRP buttons repeatedly to enqueue packets of data to be sent out in the next time slot or next few time slots.
This is the resulting spectrum. Notice that each time slot is 500ms and the two signals at the receiver occupy exactly 1 time slot. Zero's are being passed to the USRP VI during other time slots allowing the transmit VI to run continuously.
Figure 2: Time Domain Graph shows single tone transmitted by two USRP on their own unique time slot
Zooming in you can see that the settling time is very small (within sub-sample alignment at 1MS/s IQ Rate). Note that the RX is being done on the RX2 port of the same USRP being used for transmission of the first time slot.
Using the USRP MIMO Cable:
Figure 3: Zooming into the transition between two time slots shows the precision timing of the USRP using the MIMO Cable
Zooming into the transition point...
Figure 4: Zooming in further the transition point is sub-sample, providing very tight time syncronization with the MIMO cable
Using an external Reference and PPS Clock:
Figure 5: Zooming into the transition between two time slots shows the precision timing of the USRP using an external Ref In and PPS
Zooming into the transition point...
Figure 6: Zooming in further the transition point is sub-sample, providing very tight time syncronization using an external Ref In and PPS
Thanks to Tony for contributing this excellent example code!!
