Dear Brain, Thank you very much for the reply. I didnt get this point.
> GMSK only rotates > ~pi/4 for every bit, so +/-(symbol_rate/2) ends up being your > bandwidth. Can you please explain it a little bit. I am sorry my DSP background is very less. I am still learning. Thanks for your help. -Anand PhD Candidate, Wireless Networking and Simulations Lab, Stony Brook University On Wed, Sep 3, 2008 at 5:27 PM, Brian Padalino <[EMAIL PROTECTED]> wrote: > On Wed, Sep 3, 2008 at 5:12 PM, Anand Prabhu Subramanian > <[EMAIL PROTECTED]> wrote: >> Folks, >> >> I am trying to change the channel width used for a transmission using >> the benchmark_tx.py program. I use the default modulation scheme >> (gmsk) and on the receive side, I am running the usrp_fft.py to see >> the spectrum and understand how wide the channel is used. >> >> If I use an interpolation rate of 128, I see a channel width of 500kHz >> showing high energy in the spectrum. Similarly, if I change it to 64, >> I see about 1MHz occupied. I read in one of the docs that the >> interpolation rate could be varied from [4, 512] and it should be a >> multiple of 4. I tried it but I was able to vary only from 16 to 512. >> I notice the channel width = 64MHz/interpolation rate. Please correct >> me if I am wrong. The bits/symbol always seems to be 1, so if I use a >> 500KHz channel, the bit rate is set to 500Kbps. >> >> On the receive side, if use the benchmark_rx.py, a decimation rate of >> 64 corresponds to 500Kbps bitrate while on the transmit side an >> interpolation rate of 128 corresponds to 500kbps. Also, if I fix the >> bitrate to 500kbps, I do not see the relation channel width = >> 64Mhz/interpolation rate to hold. Channel width seems to be 500kHz for >> different interpolation rates. >> >> It would be very helpful if someone can help me understand the >> relationship between channel width, bit rate and the >> interpolation/decimation rates. I want to be able tune the channel >> width for each transmission dynamically by playing with these >> parameters. Thanks a lot for your time and consideration. > > You're close. The USRP always transmits samples at a rate of 128Msps > (Mega-samples/second). The USRP samples at 64Msps. GMSK has a > spectral efficiency of 1 bit/sec/Hz. At 1 sample/symbol and an > interpolation rate of 128, approximately 1MHz worth of bandwidth is > used. If you are running at 2 samples/symbol, you are running at > 500kHz worth of bandwidth. > > Working through the math for TX, you come up with 128Msps / 128 > (interpolation rate) / 2 samples/symbol = 500ksps (kilosymbols/sec). > > On the receive side, just work in that samples are at 64Msps instead of 128. > > The actual bandwidth of your signal is determined by the modulation > type and the type of filtering done beforehand. GMSK only rotates > ~pi/4 for every bit, so +/-(symbol_rate/2) ends up being your > bandwidth. CPM is all about trajectory, but that's a little > off-topic. > > Hope this was helpful. > > Brian > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
