Receiving garbage in USRP QPSK example

I have two USRP N210 modules and I want to do simple QPSK transmission from one module and reception using the other. I took the Simulink examples sdruqpsktx and sdruqpskrx, copy pasted them into a single simulink model, entered the correct ip addresses and center frequency of the USRP modules and ran the model. I did not change any of the parameters in any parts of the models. I receive the transmitted data for some time but then I start receiving garbage. What can I be doing wrong?

A few things I must mention: 1. I have connected both the USRP modules and my Computer to a Gigabit Ethernet switch. The network card in my Computer does not support Gigabit Ethernet so I cannot connect the USRP modules directly to my Computer. 2. I have already used the frequency calibration examples (sdrufreqcalibtx and sdrufreqcalibrx) and the frequency offset is shown to be -588.3. However adding this to the receiver center frequency does not solve the problem, the received data still turns to garbage after a few seconds. 3. I am using the correct RF ports for TX/RX and the RF daughter card is working fine. 4. I am using MATLAB R2014a.

Answer 1

[Mir Lodro]

You should reduce the gain of the Tx USRP or increase the distance between two USRPs. You could also use ethernet to USB converter and connect two USRPs with the same PC. I had very similar experience in the beginning but later it was fine. I had extended the very same QPSK model to 2x2 MIMO for BER measurements. I had two X310 USRPs connected to same host one with Ethernet cable and another with ethernet-to-USB cable. It's true the QPSK receiver has high model complexity.

HTH!

Mir Lodro

Answer 2

[Ethem]

Do you see any overflows (O) or underflows (U)? The QPSK models have high complexity and require high processing power. Running both Rx and Tx on a single Simulink model may limit your ability to run these models in real-time and you may start seeing overflow or underflow.

Answer 3

[Munish Raj]

Use an attenuator between the Tx and Rx.

During applications with high gain, some data might get clipped off.

This leads to partial data getting transmitted over the channel, which looks like garbage data.