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Re: [Discuss-gnuradio] QPSK basic modulator

From: Karim El Defrawy
Subject: Re: [Discuss-gnuradio] QPSK basic modulator
Date: Wed, 08 Apr 2009 11:58:33 +0100
User-agent: Thunderbird (Windows/20090302)

Thanks a lot Eric.

That's very useful. Yes, you are right about the preamble.
At this stage I am just doing the transmitter, later I will worry about how to correctly receive it and will probably add a preamble.

Ed Criscuolo wrote:
karim wrote:
I need to implement a simple transmitter that uses QPSK as the demodulation mode. It should read data (characters) from a file and transmit them as they are without preamble or anything.

What's the simplest and fastest way to do it.
I checked the benchmark_tx.py it adds a preamble and a lot of data before the actual payload which I don't need.

Briefly, create a flowgraph as follows:

Start with your data stream as a stream of unsigned bytes, such as
reading from a file.

Run through gr_packed_to_unpacked_bb with two bits per chunk.

Run though gr_chunks_to_symbols_bc with the following symbol table:
        0,0 => -1-j
        0,1 => -1+j
        1,1 => +1+j
        1,0 => +1-j

Upsample to the desired sample rate with your favorite
interpolator/filter combination.

Multiply by around 2000 to get some amplitude.

Sink to the usrp

You realize, of course, that without any kind of preamble or
sync marker, it will be difficult to demodulate this correctly.
In addition, QPSK will have a 1-of-4 ambiguity in the received
phase constellation.  The usual solution to this is to use
some sort of sync marker that can be unambiguously recognized
irrespective of any phase offset.

Hope this helps.  Good luck!

@(^.^)@  Ed

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