Hi, I made a complete DSSS demodulator with DBPSK and DQPSK demodulation. In the case of DQPSK demodulation I've got the same PN_code on I and Q branchs. What is the advandatage to put different PN_code on I and Q ? And what is the advantage to make a complex spreading like this : At spreading (modulation) : (I+jQ).(PN_I+jPN_Q) So I signal becomes : I.PN_I-Q.PN_Q and Q signal becomes Q.PN_I+I.PN_Q I'm looking for best performance. I've got these sensitivity on a 2.4 GHz radio with lenght 11 of the PN_code and a 44 MHz radio band : -92 dBm at 2 Mbps (theory says -94 dBm) in DBPSK at 1E-5 BER -88 dBm at 4 Mbps (theory says - 91 dBm) in DQPSK at 1E-5 BER Thanks...
DSSS modulation DQPSK
Started by ●October 26, 2005
Reply by ●October 26, 20052005-10-26
patrick.melet@dmradiocom.fr wrote:> Hi, > > I made a complete DSSS demodulator with DBPSK and DQPSK demodulation. > > In the case of DQPSK demodulation I've got the same PN_code on I and Q > branchs. > > What is the advandatage to put different PN_code on I and Q ? > > And what is the advantage to make a complex spreading like this : > At spreading (modulation) : (I+jQ).(PN_I+jPN_Q) > So I signal becomes : I.PN_I-Q.PN_Q and Q signal becomes Q.PN_I+I.PN_Q > > I'm looking for best performance. > > I've got these sensitivity on a 2.4 GHz radio with lenght 11 of the > PN_code > and a 44 MHz radio band : > -92 dBm at 2 Mbps (theory says -94 dBm) in DBPSK at 1E-5 BER > -88 dBm at 4 Mbps (theory says - 91 dBm) in DQPSK at 1E-5 BER > > Thanks...usually you judge modulation and coding performance against a signal to noise ratio such as C/N or Es/No or Eb/No. The performance measured against input power in dBm will be a function of some of the receiver hardware perfromance such as gain and noise figure, not just the modulation and coding. Mark
Reply by ●October 27, 20052005-10-27
We can calculate the sensitivity if we know the (S/N) at the input of the demodulator and the noise floor and the noise figuire of the receiver like this : Sensitivty=Noisefloor + NoiseFigure-Gp+(S/N)demod Gp : process gain for DSSS and for DBPSK a BER of 1E-5 is achieved with a S/N of 10.5dB and DQPSK with S/N of 13.5 dB
Reply by ●October 27, 20052005-10-27
ok so are you achiving that performance at those S/N at the demod input and does that agree with theory? Mark
Reply by ●October 28, 20052005-10-28
we have 2 dB less performance for sequence of lenght 11 but with long sequence (63) whe have 5dB less than theory we think that our radio is less performant when we approach the noise floor....
Reply by ●October 28, 20052005-10-28
I try to seperate the perfromance of the "radio" from the perfromance of the "demod". The performance of the "radio" is a function of noise figure, phase noise etc. If you measure the S/N at the INPUT TO THE DEMOD (not the radio) and the BER out of the demod and conpare to theory, that will give you the performance of the demod and the coding. If the S/N at the input to the demod is not what it should be, then you need to improve the performance of the "radio" , i.e. improve the noise figure. Mark
Reply by ●October 31, 20052005-10-31
So I repost my question : I made a complete DSSS demodulator with DBPSK and DQPSK demodulation. In the case of DQPSK demodulation I've got the same PN_code on I and Q branchs. What is the advandatage to put different PN_code on I and Q ? And what is the advantage to do a complex spreading like this : At spreading (modulation) : (I+jQ).(PN_I+jPN_Q) So I signal becomes : I.PN_I-Q.PN_Q and Q signal becomes Q.PN_I+I.PN_Q