Re: [Discuss-gnuradio] Basic Bandwidth math

[Eric Blossom]

On Thu, Sep 06, 2007 at 11:56:30AM -0400, Jeffrey Karrels wrote:
> Ok. I am having a world of problems today, the main one being
> determining if it is my issues causing the headache or the headache
> causing the issues. ;)
> 
> I have a basic bandwidth limit that I am hitting and for some reason I
> cannot do basic math today.
> 
> I am trying to see how many carriers I can transmit on. I am mixing a
> single gmsk modulated source with LOs in software and seeing how many
> times I can do that. A single channel is 250kHz wide and the channels
> are 400kHz apart center to center. In other words my LOs are 400e3,
> 800e3, ...
> 
> How many carriers would you expect that I could transmit on?
> 
> 
> Thanks
> Jeff
> P.S. Sorry in advance, I am nuts today.

In theory, 20.  In reality probably about 16, unless you run out of
CPU first.

You can get 8Mz of IF bandwidth across the USB.
This is 8MS/s * 2 * 2 = 32MB/s (16-bit I & Q).

8e6/4e5 = 20 "bins"  (ignoring roll off)

Allow for roll off because of CIC interpolator in FPGA.  Say that 80%
is usable (depends on your app, whether you preemph signal etc).

20 * .8 = 16 bins.

There's a fence post problem here ;)

So, 1 carrier needs 250kHz

3 carriers needs 2 * 400 + 2 * 125

5 carriers needs 4 * 400 + 2 * 125

N carriers needs (N-1) * 400 + 250 (for odd N)

  15 -> 5.85MHz
  17 -> 6.65MHz
  19 -> 7.45MHz

Put the middle carrier at 0 Hz,
then arrange the others symmetrically about it.

Eric