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Re: [Discuss-gnuradio] BPSK to QPSK

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] BPSK to QPSK
Date: Wed, 3 Mar 2010 10:34:22 -0800

On Wed, Mar 3, 2010 at 03:36, amarnath alapati
<address@hidden> wrote:

>        I am using the programs of DIGITAL-BERT folder to see the bit error
> rate pattern versus the SNR. The given programs are for BPSK scheme. I tried
> to change it for QPSK Scheme. But seems that I did not do it right. I am
> giving the modified code in the programs . Please help me out......Thanking
> in advance....

Two major issues:

1) On the receiver, you are not demapping your received constellation
into QPSK dibit symbols.  The current script simply discards the Q
channel and slices the I channel about zero to recover BPSK symbols;
you never changed any of that.

2) Even were you to do this, the phase ambiguity of QPSK demodulation
means you can end up in one of four states on the receiver, so I and Q
can be swapped or inverted.  In real communication systems this is
either handled with differential coding or known synchronization words
that allow the receiver to detect the rotated phase lock.  BPSK has
this problem as well, though limited to correct or inverted
demodulation.  The original BPSK-based BERT scripts use a
self-synchronizing differential bit scrambler and descrambler that is
okay with inverted demodulation.


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