# BPSK demodulation

Started by November 10, 2005
```Hi,everyone

I'm kind of a newbie in everything relatated to modulation/demodulation
and as I am now working with it on Matlab, I'm just learning it by
myself.

With the amplitude modulation I get by,
but I now have to demodulate a BPSK modukated signal and I don't really
know how to do it.

What I have is a 847KHz signal (actually it's the side band signal)
modulated with what I think is the simplest BPSK modulation there is,
which is: if the binary logical state changes, the signal's phase shifts
180&#2013266096;.
And this side band signal modulates the amplitude of a 13.56MHZ carrier
signal.

Can anybody help me on how to demodulate this signal?
I tried the easy way: Matlab's signal processing toolbox demodulation
functions but they don't really work with this signals.
I'm guessing the solution has something to do with IQ demodulation, but
what i've done so far didn't work.

Thiago

```
```Thiago wrote:
> Hi,everyone
>
> I'm kind of a newbie in everything relatated to modulation/demodulation
> and as I am now working with it on Matlab, I'm just learning it by
> myself.
>
> With the amplitude modulation I get by,
> but I now have to demodulate a BPSK modukated signal and I don't really
> know how to do it.
>
> What I have is a 847KHz signal (actually it's the side band signal)
> modulated with what I think is the simplest BPSK modulation there is,
> which is: if the binary logical state changes, the signal's phase shifts
> 180&#2013266096;.
> And this side band signal modulates the amplitude of a 13.56MHZ carrier
> signal.
>
> Can anybody help me on how to demodulate this signal?
> I tried the easy way: Matlab's signal processing toolbox demodulation
> functions but they don't really work with this signals.
> I'm guessing the solution has something to do with IQ demodulation, but
> what i've done so far didn't work.
>
>
> Thiago

You should really refer to a communications textbook for this.  Most
good ones have plenty of information on practical demodulation of BPSK etc.

What you want to do is a matched filter.  This means that after
multiplying the input signal with the basis function (ie a cosine at the
passband frequency) you want to integrate the output over a complete bit
period (hence you'll need some kind of synchronization) and then just
read out the output.  If it's > 0 then the bit is a 1 (or whatever
you've defined it as) if it's < 0 then it's a 0.  If it's equal to 0
then just guess as to whether it's a 1 or a 0.

If you're trying to do BPSK to encode an analog signal, just read the
textbooks.
```
```>
>You should really refer to a communications textbook for this.  Most
>good ones have plenty of information on practical demodulation of BPSK
etc.
>
>What you want to do is a matched filter.  This means that after
>multiplying the input signal with the basis function (ie a cosine at the

>passband frequency) you want to integrate the output over a complete bit

>period (hence you'll need some kind of synchronization) and then just
>read out the output.  If it's > 0 then the bit is a 1 (or whatever
>you've defined it as) if it's < 0 then it's a 0.  If it's equal to 0
>then just guess as to whether it's a 1 or a 0.
>
>If you're trying to do BPSK to encode an analog signal, just read the
>textbooks.
>

Thank's a lot for the help.
Actually I just need to demodulate the BPSK encoded signal, not modulate
it. So I'm hoping that just with what you said I'll get by.

I'm just afraid that because the encoded signal is
modulated over a carrier signal of higher frequency, i'll have some
trouble multiplying it with the basis function.

Anyway, if it doesn't work, then it's to the library!

Thanks,
Thiago
```