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[Discuss-gnuradio] On Off Keying

From: sri ram
Subject: [Discuss-gnuradio] On Off Keying
Date: Mon, 29 Sep 2008 14:52:20 -0400

    I am trying to send a stream of bits using On Off keying and am having some issues.

At this stage, I just want to check if 1's and 0's are getting received with a high and low amplitude respectively. I have modified dbpsk.py setting the constellation to 0+0i and 1+0i in psk.py and invoke the tx/rx as in benchmark_tx,benchmark_rx ..

 My flowgraph is
Bytes2symbols ->RRCFilter->USRP


I have a Bytes2symbols file which just writes the complex symbols for a given set of bytes as in gr_chunks2symbols_bc.cc.
 I have also checked that the complex symbols entering the USRP at the transmitter are as expected.

However, at the receiver (USRP baseband samples without any processing) when I measure the power, I do not see the power going low for the 0 bits. Specifically, when I send a 101010... bit stream of 128 samples (just these bits without any headers/trailers).  The transmitted baseband complex symbols are as expected with the real part going between 1 and 0 alternatively. At the receiver, the received power stays almost the same high value throughout the packet duration, whereas I would have expected it to alternatively go high and low.

Adding or removing the RRC filter doesn't affect the observation. The following observations are true for the power and the real part of the baseband samples.
1. For a tx. stream of all 1's, i can see the beat frequency or the frequency offset for the duration of the packet (as expected).
2. For a tx. stream of all 0's , i see a low received value. (almost close to the noise levels) as expected.
3. However, for a tx. stream of 1's and 0's mixed, I still see the received amplitude (real part) showing the beat frequency continuously and not going to 0 for the 0 bits.
I am using the latest stable version i.e gnuradio-3.1.3 on Ubuntu laptops.
Could this be Inter Symbol Interference or a setting which makes the (carrier) power coming out of the USRP constant for the packet duration irrespective of the tx.data?

Any help is greatly appreciated.


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