Forum: GNU Radio Question on the Packet Decoder

Hello all,

i'm working on the digital modultaion, and i build a system like this:

random source -> packet encoder -> qam mod -> (adder and noise source) 
->
qam demod -> packet decoder ->file sink

I found that,  when the amplitude of the noise is low, there will be 
some
data in file sink, but when the amplitude of noise is high, the packet
decoder doesn't output any more. Who can tell me why? does the noise
destroy the structure of the packet so that i can not be decoded any 
more?

I also tried psk mod/demod and gmsk mod/demod, it comes the same 
problem.


thanks,

Zhiwen

On Tue, Jan 22, 2013 at 11:07 AM, Zhiwen He <heyutu@gmail.com> wrote:

> destroy the structure of the packet so that i can not be decoded any more?
>
> I also tried psk mod/demod and gmsk mod/demod, it comes the same problem.
>
>
> thanks,
>
> Zhiwen
>

Yes, you are correct. The packet decoder looks for an access code. If 
the
access code is valid to some number of bits, it will pull out a frame. 
If
there are too many garbles in the access code, the packet decoder will
never see it as a valid frame. By default, the access code is 64 bits 
and,
I think, there can be up to 12 bit errors in it (this is the threshold
value).

Tom

On Wed, Jan 23, 2013 at 4:11 PM, Zhiwen He <heyutu@gmail.com> wrote:

On Wed, Jan 23, 2013 at 10:20 AM, Zhiwen He <heyutu@gmail.com> wrote:

>
>
> On Wed, Jan 23, 2013 at 4:11 PM, Zhiwen He <heyutu@gmail.com> wrote:
>
>> you mean that i need to add an access code for the system, or just leave
>> blank?
>
>
There is a default 64-bit access code that is used when you leave this
field blank.


>
It's possible that you are calculating your SNR incorrectly.I know that
under standard channel conditions (that is, AWGN) the modulators work
pretty close to their theoretical performance. So when you say SNR < 20,
which modulation are you trying to use? GMSK, DBPSK, and DQPSK should 
work
under these conditions.

Tom

I tried 4 QAM and 16 QAM.

I used a FFT Sink to see the Amplitude in dB after the kanal, which 
include
the signal and the noise, I think :
SNR = Amplitude of Singal - Amp. of Noise,  is this right?