Re: [Discuss-gnuradio] complex multiplication question

[John Andrews]

Hi Bob,

Thanks for your reply. I am trying to lock the system show in the figure found on this link.

http://img19.imageshack.us/img19/826/dsssrxr.jpg

If S(n) = RxD signal which is modulated by a PN Sequence

1st step --> The input to Carrier Loop = S(n)*P(n), which the product of the RxD signal and the PN Sequence assuming P(n) matches in phase.

2nd step --> The carrier tracked poptr(n) is the multiplied with RxD signal S(n) to remove the carrier from it.

i.e. Input to Code Loop = S(n) * complex_conjugate(poptr(n))

Is this how you see the system? 

Thanks

John

On Wed, Aug 4, 2010 at 3:07 AM, Bob McGwier <address@hidden> wrote:

The SECOND ORDER Costas loop produces foptr(n) and poptr(n) which is the frequency and phase estimate for the carrier.

sin(poptr(n)) is the estimated carrier.

If S(n)*sin(poptr(n) is the spreading code modulating the estimated carrier,  then

S(n)*P(n) * complex_conjugate(S(n)*sin(poptr(n)) should be approximately P(n) up to error in


a) your estimate of the carrier

BUT ALSO

b) the clock of the transmit system and its initial phase offset for the complex spreading code MUST ALSO be estimated to close this system and track.  My little equation above ASSUMES perfect knowledge of S(n) which is NEVER the case in a real system.

Bob

On 8/3/2010 9:45 PM, John Andrews wrote:

Hi,
can someone guide me a little here please. I have a complex signal S(n)
that I multiply with a sequence P(n) of length N (the sequence consists
of {-1,1} ). I pass the product into a Costas Loop to track the carrier.
Btw, the complex_input signal a spread signal spread using the sequence
P(n) and BPSK modulated.

S(n)*P(n) ---> Costas Loop

Then I want to remove the carrier from the original signal which can be
done by multiplying the frequency output of the costas loop which is
foptr[i] with S(n). Is that right? I want to do this in GRC. Can someone
guide me a little here and tell me if I am understanding it right.

Will be eagerly waiting for a reply. I am not good at Communications
stuff as this is not my major but I am trying hard. :) A little help
will greatly appreciated.

Thanks
John

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