[Discuss-gnuradio] Re: Discuss-gnuradio Digest, Vol 91, Issue 29

[Tuan Ta]

Hello all,

I am working in the same team with Shalabh. We have been trying to figure out this problem for a week now and so far we found nothing concrete. We have been looking in every corner of the archive. We found some people who had similar problem with us (some dated back to 2008!) but no one had a clear explanation. 

We found in some post that if the sampling rate of our signal is too small, which leads to small bandwidth, our signal might get lost in the filtering stage due to some frequency offset of the filter. Increasing the sampling rate (e.g. 500k -> 1M) indeed improved the reception ratio at the receiver. 

We managed to successfully send a whole file (small) using gmsk, sampling rate 1.5M, payload size 3 bytes. However, if we increased the payload size (say 100 bytes), then we still got packet lost even at 1.5M. This leads to another question: how does the size of the payload impact the reception at the receiver?

Since all packet lost occurred at the end of transmission (always last packets that got lost), we suspected some kind of buffer overflow. But frankly we don't know how to check that. We printed out the size of the message buffer at the receiver USRP after every rx_callback but nothing seemed to be out of the ordinary.

This problem has been really bugging us. Since we are new to both the hardware and software, we might miss some crucial details. We will be very thankful if anyone can provide some pointers / suggestions / explanations that help us clarify this issue.

Thank you,

Tuan

Message: 13
Date: Mon, 28 Jun 2010 19:25:01 -0400
From: Shalabh Jain <address@hidden>
Subject: [Discuss-gnuradio] benchmark_tx/rx with RFX 2400
To: address@hidden
Message-ID:
       <address@hidden>
Content-Type: text/plain; charset="iso-8859-1"

Hello,

I am new to the USRP/gnu-radio world, trying to understand the basic
working. I have been trying the following 2 experiments which I've not been
able to successfully debug or find documentation for. Can somebody help me
out to explain what is going wrong or point to some useful references.

The hardware I am using is the RFX2400 daughterboard with USRP and a
2.4-2.48G antenna. I'm using two separate boards in receive and transmit
configuration.

1. I tried the digital/benchmark_tx/rx.py provided with the examples (3.2.2
tarball).  I tried to read from a simple ascii file containing a sequence of
numbers repeated 1000 times. I've tried different options modulation schemes
but the problems persist. Two main problems in this example
    a) It seems that using the default gmsk modulation, although 179
packets are sent, only 147 are received. Essentially, the last packet is
always received in error and the end 15% of the file is always lost.
    b) The transmission seems to be very sensitive to the bit rate
specified. i.e. there are several combination of big-rates at which the
receiver receives nothing.

2. I tried so send a simple tone across by hooking a sine wave generator to
the USRP using the grc. Even that seems the just end up in noise reception.
I'm attaching a small snippet of the code generated.

I appreciate any help.

Thanks
Shalabh

-------------------------------tx-------------------------------------------
               self.gr_float_to_complex_0 = gr.float_to_complex(1)
               self.gr_sig_source_x_0 = gr.sig_source_f(500000,
gr.GR_COS_WAVE, 100000, .5, 0)
               self.gr_sig_source_x_1 = gr.sig_source_f(500000,
gr.GR_SIN_WAVE, 100000, .5, 0)
               self.usrp_simple_sink_x_0 = grc_usrp.simple_sink_c(which=0,
side="A")
               self.usrp_simple_sink_x_0.set_interp_rate(256)
               self.usrp_simple_sink_x_0.set_frequency(2.44e9,
verbose=True)
               self.usrp_simple_sink_x_0.set_gain(80)
               self.usrp_simple_sink_x_0.set_enable(True)

               self.connect((self.gr_float_to_complex_0, 0),
(self.usrp_simple_sink_x_0, 0))
               self.connect((self.gr_sig_source_x_0, 0),
(self.gr_float_to_complex_0, 0))
               self.connect((self.gr_sig_source_x_1, 0),
(self.gr_float_to_complex_0, 1))
------------------------------rx-------------------------------------------
               self.usrp_simple_source_x_0 =
grc_usrp.simple_source_c(which=0, side="A", rx_ant="TX/RX")
               self.usrp_simple_source_x_0.set_decim_rate(256)
               self.usrp_simple_source_x_0.set_frequency(2.44e9,
verbose=True)
               self.usrp_simple_source_x_0.set_gain(80)
               self.wxgui_fftsink2_2 = fftsink2.fft_sink_c(
                       self.GetWin(),
                       baseband_freq=0,
                       y_per_div=10,
                       y_divs=10,
                       ref_level=50,
                       sample_rate=500000,
                       fft_size=1024,
                       fft_rate=30,
                       average=False,
                       avg_alpha=None,
                       title="FFT Plot",
                       peak_hold=False,
               )
               self.Add(self.wxgui_fftsink2_2.win)
               self.connect((self.usrp_simple_source_x_0, 0),
(self.wxgui_fftsink2_2, 0))

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