Re: [Discuss-gnuradio] Question on the Packet Decoder

[Tom Rondeau]

On Tue, Jan 22, 2013 at 11:07 AM, Zhiwen He <address@hidden> wrote:

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

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