Hello. I am working on a software demodulator and have a few questions. First off let me say that I am just getting into this DSP stuff. Anyways... I have a file of samples representing a bpsk modulated wave. The popular method of demodulating seems to be through I/Q channels so that the phase and magnitude can be detected. Is there a way to convert my non complex samples to an I/Q format? If not, is there another reasonable demodulation method? Once i get a grasp on this I plan on adding support for qpsk and different forms of qam. Thanks in advance.
demodulation problem
Started by ●July 14, 2004
Reply by ●July 14, 20042004-07-14
kalden <aldenk@rl.af.mil> wrote: kalden, I am not an expert in this area, but just the same, if I'm not mistaken, Rick Lyons 2nd ed. book ("Understaning Digital Signal Processing") may have some information on this topic. I don't have my copy handy at the moment but I think Rick updated the secion discussing I/Q. A very "user friendly" book. FWIW, Dave> Hello.> I am working on a software demodulator and have a few questions. > First off let me say that I am just getting into this DSP stuff.> Anyways... I have a file of samples representing a bpsk modulated > wave. The popular method of demodulating seems to be through I/Q > channels so that the phase and magnitude can be detected. Is there a > way to convert my non complex samples to an I/Q format? If not, is > there another reasonable demodulation method?> Once i get a grasp on this I plan on adding support for qpsk and > different forms of qam.> Thanks in advance.
Reply by ●July 15, 20042004-07-15
kalden wrote:> Hello. > > I am working on a software demodulator and have a few questions. > First off let me say that I am just getting into this DSP stuff. > > Anyways... I have a file of samples representing a bpsk modulated > wave. The popular method of demodulating seems to be through I/Q > channels so that the phase and magnitude can be detected. Is there a > way to convert my non complex samples to an I/Q format? If not, is > there another reasonable demodulation method? > > Once i get a grasp on this I plan on adding support for qpsk and > different forms of qam. > > Thanks in advance.To convert to I/Q format modulate your sample set by cos(w*t) for the inphase and sin(w*t) for quadrature, and lowpass. You'll get two real-valued numbers which you can then play with. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●July 15, 20042004-07-15
Hi Kalden, I just went through the exercise of post process bpsk demodulation from a file of samples just like yourself. It was a fun exercise and turned out really good in the end. I started down the I/Q path just like you are thinking. I tried a Costas Loop which worked really well but I was unhappy with the processing time due to the math on both I/Q channels. So what I did was to go to a digital PLL with narrowing loop filters. Works excellent and super quick. However the PLL approach might not be the best solution with the consideration of qam or qpsk looming in the future. I would go Costas for that. Are your samples RF? How big is the file? What type of S/N ratio are we looking at? Is there preamble to lock and snap down on? If yes how long is the preamble. I have done bpsk with and without preamble. Without is a little more tricky. I might be able to help if you give me a little more info. Like I said I just finished the exercise myself. Thomas "kalden" <aldenk@rl.af.mil> wrote in message news:57834864.0407141121.58c1fa26@posting.google.com...> Hello. > > I am working on a software demodulator and have a few questions. > First off let me say that I am just getting into this DSP stuff. > > Anyways... I have a file of samples representing a bpsk modulated > wave. The popular method of demodulating seems to be through I/Q > channels so that the phase and magnitude can be detected. Is there a > way to convert my non complex samples to an I/Q format? If not, is > there another reasonable demodulation method? > > Once i get a grasp on this I plan on adding support for qpsk and > different forms of qam. > > Thanks in advance.