error in mt offset qpsk shipping example

[nolsqn]

hi i was using the mt offset qpsk transciver example for a project. what i dun understand is that why at the demodulate psk.vi the output bitstream is  having the sync bits present at the start..shdnt the vi automatically remove the bits and send us the message bits futher when i checked the array size for both only the transmited(message not inculding sync and guard)  bits and the size of the received bit they were also different.i.e. for a message symbol of 1000 the received bits comes to be 1984(that is the array size) also as i mentioned the starting bit of the received bit array are the sync bit that are 20 so actually the received bits are 1964(array size ) so my question is that why is there loss of data ?? furthur more plz see the attached vi . In this vi i have used the example mt offsetqpsk mod/demod.vi  the only change i have done is , instead of giving the bitstearm from the mt generate bit.vi i have given it a user based bits and still is is giving errors...................................and i have checked the generated bitstream it is correct ...

thnx

[Paul_C.]

Hello nolsqn,

I believe you might be referring to a different example than the one discussed regarding the sync and guard bits.  Is that the case?  The MT Offset QPSK Mod-Demod.vi example doesn't add guard and sync bits.  There is a great PSK example which demonstrates how to add sync and guard bits.  This is called MT PSK Transceiver(with FFT Spectrum).vi and can be found in the example finder by going to Toolkits and Modules » Modulation » Simulation.  In addition, there will be a difference between the input bitstream size and the output bitstream size.  This is due to the pulse shaping filter.  The important thing to note is that your input and output bitstreams are out of sync.  This is causing the BER to be high. This can be manually fixed by manually adjusting the input and output bitstreams to place them in sync.  For your added example, I simply removed the first two bits from the input bitstream and that fixed the syncing problem.  Here is the picture of the change:

 

Regards,

Paul C.

Message Edited by Paul C. on 03-02-2009 04:30 PM

[nolsqn]

hi .....okay i got it now .........thnx...