Re: [Discuss-gnuradio] signal to noise ratio and multiply const

Thank you very much!


Am 30.10.2013 13:38, schrieb Martin Braun (CEL):

On Wed, Oct 30, 2013 at 01:18:53PM +0100, Baier wrote:  

Hi all,    I have simulated ofdm transmitter with 200kbps in grc. Without  function multiply const I got a very bad spectrum mask of the  transmiitter. If I insert the block multiply const with the 0.05 the  spectrum mask and SNR will be much better (see attached pictures).  How can I explain it? The dynamic range of the ofdm signal will be  reduced if I insert the multiply const? That could be better for the  amplifier (PAPR reducing)?  Thanks  

  Hi AB,    PAPR is part of the reason. Without such a multiplier, your signal  clips.    In gr-digital/examples/ofdm/tx_ofdm.grc you will find such a multiplier  and a scope sink (this example was recently updated, you should pull it  from current master). Use the scope sink to make sure your amplitude  stays smaller than 1.0 (lots smaller!).    This does mean you're not using all the bits of the D/A converter (I  guess that's what you mean with dynamic range reduction), but you don't  need them all to produce a clean OFDM signal. On the contrary, you want  to stay away from saturating the D/A range in order to avoid signal  distortions.    As for your comment on SNR: When clipping, you're increasing the  out-of-band emissions. Technically, that's a kind of SNR but it's not  what people usually think of.    MB    PS: If you have a copy of Kammeyer's book, he explains it quite well.    

_______________________________________________  Discuss-gnuradio mailing list  Discuss-gnuradio@gnu.org  https://lists.gnu.org/mailman/listinfo/discuss-gnuradio