carrier synchronization for CPFSK demodulation
in [DSP]
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From: gobruins on 22 Mar 2010 16:47 I was wondering how to do carrier synchronization for CPFSK signals? I want to decode the CPFSK signal by computing the phase difference. The shape of the phase difference is similar to the following plot |---| |---| fc+fd/2 - - -|- -| - | - | - - - fc ---| |---| |--- fc-fd/2 where fc is the carrier, fd is the peak frequency deviation. Before send this demodulated data into the soft slicer, I have to build the sample clock and recover the carrier frequency. I plan to use the Gardner (or similar) detector to do the timing synchronization. The input to the TED is just the phase difference as shown above. But Ｉ ａｍ ｎｏｔ ｓｕｒｅ ｈｏｗ ｔｏ ｄｏ ｔｈｅ ｃａｒｒｉｅｒ ｓｙｎｃｈｒｏｎｉｃａｔｉｏｎ． Can anyone point me a method how to do this? Thanks a lot!
From: gobruins on 22 Mar 2010 17:05 >Search for "costas loop" if you must do coherent demodulation. If non- >coherent (i.e., no carrier recovery needed) demodulation is adequate, >search for "differential detector." > The reason I choose coherent demodulation is because I thought it has improved SNR performance than noncoherent demodulator, am I right on this? For my project the minimum SNR for detection is 3dB, does this limit my decision on choosing demodulator? any recommendation? If I use a Costas loop for the carrier recovery, what is the error signal feed into the loop? If it is the phase， the phase is ２*ｐｉ*fc*t+integral(m(t)), how do I remove integral(m(t))? That is the part confuses me, I definitely need some advise on that. Thank.
From: John on 22 Mar 2010 20:04 On Mar 22, 5:05 pm, "gobruins" <chunmei.kang(a)n_o_s_p_a_m.gmail.com> wrote: > >Search for "costas loop" if you must do coherent demodulation. If non- > >coherent (i.e., no carrier recovery needed) demodulation is adequate, > >search for "differential detector." > > The reason I choose coherent demodulation is because I thought it has > improved SNR performance than noncoherent demodulator, am I right on this? > > For my project the minimum SNR for detection is 3dB, does this limit my > decision on choosing demodulator? any recommendation? > > If I use a Costas loop for the carrier recovery, what is the error signal > feed into the loop?ãIf it is the phaseï¼ãthe phase > isãï¼*ï½ï½*fc*t+integral(m(t)), how do I remove integral(m(t))? That > is the part confuses me, I definitely need some advise on that. > > Thank. What BER do you expect at 3 dB SNR? There are a number of options, including multi-bit (sequence) detection. Look for this paper: Coherent and Noncoherent Detection of CPFSK, Osborne & Luntz Also the book Digital Phase Modulation by Sundberg is good, pretty much the bible on CPFSK. John
From: gobruins on 22 Mar 2010 20:38 >What BER do you expect at 3 dB SNR? > >There are a number of options, including multi-bit (sequence) >detection. Look for this paper: > >Coherent and Noncoherent Detection of CPFSK, Osborne & Luntz > >Also the book Digital Phase Modulation by Sundberg is good, pretty >much the bible on CPFSK. > >John > I do not know right now what BER I am expected, but I guess we want to approach as close as possible to the theoretical curve. I will take a look at the book you advised. I just tried some simulations with Matlab/Simulink, I am using discriminator method, w/o noise, it performs great, even with large carrier offset and timing offset. However, when I add noise, the performance degrades rapidly, and especially when modulation index h is small, say h=0.5. Can I say the discriminator method has good performance for passband demodulation with large frequency separation? In my case, I need to deal with also small h. My carrier fc=1KHz, Baud rate=50Hz, h=0.5 and other possible values, what demodulation technique should I use? I kind of still prefer using Costas loop, since it is going to be a nice fit to the PSK demodulator we developed earlier. I searched around this afternoon, but I am still not sure how I should use a PLL to recover the carrier. What should be the phase error? Once locked is my VCO output the demodulated signal or the carrier?
From: John on 23 Mar 2010 06:09
On Mar 22, 8:38 pm, "gobruins" <chunmei.kang(a)n_o_s_p_a_m.gmail.com> wrote: > >What BER do you expect at 3 dB SNR? > > >There are a number of options, including multi-bit (sequence) > >detection. Look for this paper: > > >Coherent and Noncoherent Detection of CPFSK, Osborne & Luntz > > >Also the book Digital Phase Modulation by Sundberg is good, pretty > >much the bible on CPFSK. > > >John > > I do not know right now what BER I am expected, but I guess we want to > approach as close as possible to the theoretical curve. > > I will take a look at the book you advised. > > I just tried some simulations with Matlab/Simulink, I am using > discriminator method, w/o noise, it performs great, even with large carrier > offset and timing offset. However, when I add noise, the performance > degrades rapidly, and especially when modulation index h is small, say > h=0.5. > > Can I say the discriminator method has good performance for passband > demodulation with large frequency separation? > > In my case, I need to deal with also small h. My carrier fc=1KHz, Baud > rate=50Hz, h=0.5 and other possible values, what demodulation technique > should I use? > > I kind of still prefer using Costas loop, since it is going to be a nice > fit to the PSK demodulator we developed earlier. > > I searched around this afternoon, but I am still not sure how I should use > a PLL to recover the carrier. What should be the phase error? Once locked > is my VCO output the demodulated signal or the carrier? The phase error detector has to take into account the phase change over a bit period. For h = 0.5, the phase changes 90 degrees over a bit period. If you establish timing recovery first, then look at the phase change from the start of a bit to the middle of a bit. It should be +45 for positive frequency shift or -45 for negative frequency shift. By adding or subtracting 45, you can get a phase error for your PLL. John |