Re: [Discuss-gnuradio] Two Questions regarding the very beginning steps of implementing DSSS!!

[Johnathan Corgan]

On Sun, Feb 8, 2009 at 3:19 AM, Bishal Thapa <address@hidden> wrote:

> Q.1. TX side: I simply want to take a packet and XOR each bit of the packet
> with a PN-code (hint: final goal will be to implement a DSSS). Could you
> please hint me some pointers as to where catch the packet before it gets
> modulated so that I can XOR it with my code? I read and re-read
> benchmark_tx.py code and tried to trace the function send_pkt(). But I
> couldn't do a good job at it. Please give me some pointers. PS: My
> modulation choice will be DBPSK.

You will find the call to create the packet contents prior to modulation here:

http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/python/gnuradio/blks2impl/pkt.py#L86

After this function returns, the 'pkt' variable will contain an array
of bytes representing the full packet (header, payload, CRC).  You can
manipulate its contents from Python here.

If you only want to modify a portion of the packet, you can instead
add your code to the called function:

http://gnuradio.org/trac/browser/gnuradio/trunk/gnuradio-core/src/python/gnuradio/packet_utils.py#L104

> Q.2. RX side: I want to use usrp_rx_cfile.py to capture my sent data, then
> change it into matlab readable data format using "read_binary_complex.m" and
> finally will do XOR with my code in MATLAB. I have one small question and
> possibly a stupid one..that I am about to ask it hesistantly.. the binary
> file I get as an output of usrp_rx_cfile.py, say binary.dat, do I need to
> demodulate it at all (with dbpsk_demod) as the usrp_rx_cfile.py has no
> options for allowing me to select my demod_type.

Yes, you will need to demodulate the data.  The usrp_rx_cfile.py
program records complex, downconverted baseband samples directly from
the USRP.

Johnathan