USRP Software Radio
ASK demodulation questions in building a RFID reader with USRP
Hello everybody,
Recently I was making an RFID reader with labview and USRP. The tag to be read is a passive tag, which response the activation signal by modulating the tag data with ASK scheme at symbol rate 80k and reflecting the modulated signal back to the reader. The data in the tag consists of two parts: 496 bits user data and 16 bits frame marker. The user data is encoded to avoid being same with the frame marker, as shown in the following figure.
To send and receive the signal via one attena in the USRP, I use a circulator and directional attena. The circulator is used to isolate the activation signal sent from TX/RX and the reflected signal sent to RX2, as shown in the following figure.
Then I write two applications: sender and receiver. The sender generates and sends a 915MHz signal to the tag via the TX/RX port, circulator and the directional attena. The receiver receives the reflected signal via the directional attenal, circulator and RX2 port, and then demodulates the signal with the ASK demodulator in the modulation toolkit.
After setting up the platform, I run the applications. Firstly I set the center frequency of the sender and the receiver to to 915MHz, and run them on the same laptop simultaneously. From the received signal, I can see that the ASK waveform is very good. But I find that the ASK demodulation module can not output correct bitstream(actually I can see the output bitstream from the ASK signal directly). The received waveform is in the 915MHz-0-deviation.dat attachment.
To find which part is incorrect, I do two following experiments.
1. I set the center frequency of the sender to 915MHz, but set the receiver to 914.999999MHz, and run them on the same laptop simultaneously. From the received signal, I can see that there is a frequency deviation. If I room in the waveform, I can still see ASK waveform on the signal. The demodulation module shows that there is a 1Hz frequency deviation(via the measurement output), but it still can not output correct bitstream(actually I can see the output bitstream from the ASK signal directly).
The received waveform is in the 915MHz-1Hz-deviation.dat attachment.
2. I set the center frequency of the sender to 915MHz, but set the receiver to 914.999965MHz, and run them on the same laptop simultaneously. From the received signal, I can see that there is a bigger frequency deviation ,and I can still see ASK waveform on the signal if I roon in. The demodulation module shows that there is a 35Hz frequency deviation(via the measurement output), But no data are output at all(actually I can still see the output bitstream from the ASK signal directly).
The received waveform is in the 915MHz-35Hz-deviation.dat attachment.
My question is: why my ASK demodulation module does not work correctly?
To demostrate the problem, I write a application which reads the received waveform file, demodulates the waveform and shows the demodulated signal.
Can anyone please help me to solve the problem?
Thanks in advance!
