Re: [Discuss-gnuradio] Re: Mulitply PN sequence to real and convert output to complex - Question

[Peng Huo]

First of all thank you very much for replying and I will refrain from posing questions that way. 

I still have some doubts about this. I will be thankful to you if you clear these.

I want to de-spread a spread spectrum signal. I have a transmitter that transmits data spread using PN sequence. These can be any type such as m-sequence or gold sequence. I want to strip the PN sequence from the received spread spectrum signal. We get complex baseband signal from USRP_source. I want to write C++ blocks to do this.

a. How can I multiply this to this received complex baseband? Can I do it directly on this complex representation? 
b. The output of this needs to be input to the costas loop for carrier tracking and I want the result of the above step in complex.

Thanks,
Peng.

On Mon, Jun 15, 2009 at 6:01 PM, Bob McGwier <address@hidden> wrote:

Sir:

Welcome to GnuRadio and happy to have a newcomer.  Let me spend some time on netiquette here.  Asking the same question, many times in many different ways is a sure fired way to get LESS help and for it to be delayed by an irritation factor.

To convert a complex signal to real,  it depends on where you are doing it.  You are writing what looks like C/C++.   There are gr_complex types for GnuRadio in C++.   Let us assume you have a complex number X.   Then X.re() is the real part.

Your x(t) is not "making the signal real", it is modulating the complex signal Y at baseband by the complex carrier exp(j 2pi t fc).

This gives

Yc(t)Cos(2*pi*fc*t) - Yi(t)Sin(2*pi*fc*t)

as the REAL PART of that modulation process.  
Yc(t)Sin(2*pi*fc*t) + Yi(t)Cos(2*pi*fc*t)  is the imaginary part of that complex modulation process.



This is not taking the real part, or "making it real",  it is making a passband signal out of a baseband signal.

Your question is somewhat ill posed.  Assuming that you meant modulating the signal away from zero frequency by
the carrier frequency fc, and taking the real part to get a passband signal, then the answer to your question is yes, you did it right.

Bob




Peng Huo wrote:

sorry a correction,

   To convert this complex signal to a real, one has to do

   this according to the following formula, isn't it?

   x(t) = Yc(t)Cos(2*pi*fc*t) - Yi(t)Sin(2*pi*fc*t) where    Yc(t) -> Inphase component,Yi(t)-> Quadrature component, fc -> Carrier frequency, x(t) -> real signal.



   I looked at the block "complex_to_real" and it outputs the Inphase component
   as the real signal. Is it because, "fc" is 0Hz as the signal being at
   baseband? I think this sounds right.



   I want to mulitply this real signal with a PN sequence and then pass the result
   on to a costas loop which takes a complex input. I want to know how can we
   get a complex signal from a real one. Is there a block that does that?


   The book "Communication Systems" by Simon Haykin 2nd edition says,
   - The complex envelope g_complex(t) equals a frequency shifted version of the pre-envelope g+(t) as shown
                     g_complex(t) = g+(t)*exp(-j2*pi*fc*t), where fc-> carrier frequency.


     and the pre-envelop is defined as, g+(t) = g_real(t) + j*g_hilbert(t).

   I suppose this is what I need to do to get what I want. Is there a block that does it all?

   Thanks,
   Peng.


   P.S. - I am sorry but I thought to make sure before I go ahead so that I don't spend time doing things that may be unnecessary.

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