How to demodulate composed signals of different modulation schemes?

[0711]

Hi everyone,

     These days I was trying to receive and decode signals composed of different modulation schemes. The frame of the frame is shown in the following image:

   

   The frame is composed of two parts. The first part uses the 2GFSK modulation, and the second part uses 2GFSK or 4GFSK according to the PSF field in the first part: when PSF=0, PSDU is modulated by 2GFSK, and when PSF=2, PSDU is modulated by 4GFSK, and in PSDU, there are no guard bits or sync bits any more.

    When PSF=0, I can receive and decode the frame correctly because I can demodulate and decode all the data in the frame with one demodulation scheme--2GFSK.

   The question is when PSF=2, the PSDU part is modulated with 4GFSK, and how can I retrieve the bitstream of PSDU correctly ?

   Firstly, I thought that maybe I can get the precise offset of PSDU in the sample sequence, and begin to do 4GFSK demodulation from the offset. I tried to  calculate the offset of PSDU by calculating samples/symbol * 128(here samples/symbol is 4), and demodulate the following samples according to value of the PLW field( when demodulating the PSDU part, the reset parameter of the FSK demodulator is set to false because I think it may need some phase information obtained in demodulating PLCP header). However, I found that  even when PSF=0, the demodulated PSDU bitstream is not correct.

   Then I doubt that the sample length of PLCP may be not 128*samples/symbol(here i.e. 512), and I do some experiments to see how many samples do I need to demodulate a correct PLCL header. I find that I need at least 574 samples. Here another question occurs: the bit length of PLCP header is 128, why do I need 574 samples to demodulate PLCP header?

   Can any one give me some good advices? Thanks in advanced!

[0711]

Hi everyone,

     These days I was trying to receive and decode signals composed of different modulation schemes. The frame of the frame is shown in the following image:

   

   The frame is composed of two parts. The first part uses the 2GFSK modulation, and the second part uses 2GFSK or 4GFSK according to the PSF field in the first part: when PSF=0, PSDU is modulated by 2GFSK, and when PSF=2, PSDU is modulated by 4GFSK, and in PSDU, there are no guard bits or sync bits any more.

    When PSF=0, I can receive and decode the frame correctly because I can demodulate and decode all the data in the frame with one demodulation scheme--2GFSK.

   The question is when PSF=2, the PSDU part is modulated with 4GFSK, and how can I retrieve the bitstream of PSDU correctly ?

   Firstly, I thought that maybe I can get the precise offset of PSDU in the sample sequence, and begin to do 4GFSK demodulation from the offset. I tried to  calculate the offset of PSDU by calculating samples/symbol * 128(here samples/symbol is 4), and demodulate the following samples according to value of the PLW field( when demodulating the PSDU part, the reset parameter of the FSK demodulator is set to false because I think it may need some phase information obtained in demodulating PLCP header). However, I found that  even when PSF=0, the demodulated PSDU bitstream is not correct.

   Then I doubt that the sample length of PLCP may be not 128*samples/symbol(here i.e. 512), and I do some experiments to see how many samples do I need to demodulate a correct PLCL header. I find that I need at least 574 samples. Here another question occurs: the bit length of PLCP header is 128, why do I need 574 samples to demodulate PLCP header?

   Can any one give me some good advices? Thanks in advanced!