problem about transmission two BPSK in one frequency

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ddt694

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hello,

we know QPSK can be seen as two BPSK, the carriers are cos(wt) and sin(wt),
say, Icos(wt) + Qsin(wt). The I and the Q has the same amplitude and data rate.

My question is can i transmit two different kinds of signal, their data rate and amplitude are all different. The signal is A(t)cos(wt) + B(t)sin(wt).

thanks
 

Hi,
If you are transmitting the digital signa, there is no problem with the amplitude and if the rates are multiples then also you donot have a problem. However if you are transmitting signal with varying amplitude levels than the interference among the quadrature signal increase and you have higher error in case of signal with lower amplitude.
brmadhukar
 

Hi, brmadhukar
I do want to transmit two channel digital signal, the data rates are not multiples and each channel data should be spread spectrum signal, the chip rate is the same that is, the spread spctrum rate is different between the two channel. So at the receiver, after despreaded, the soft signal amplitude is not equal.

For example, a channel with data rate is 10kbps, the other channel is 1.6kbps, after spread spectrum, the two channel both have the same chip rate 2Mchips/s. If the two signals are transmited by QPSK, can they be despreaded or demodulated ?

thanks
 

Yes, it could be demodulated.
Such modulation is called unbalanced QPSK and some papers about the question
you could find by the Google's search.
When signals has different spreading rates quadrature summation is realized with unequal power.
One of such signals is GPS L1 signal.
Best regards
cat_basilio
 


hi, cat_basilio
Thanks. Because the high spread spectrum rate of the GPS (43dB for C-code), we can directly despread/demodulate one channel GPS signal by using BPSK method, the quadrature channel seen as noise. But if the spread spectrum is not so high, the quadrature channel is seen as interfere and the despread/demod will be failed.

The key problem may be if the two channel signal has different data rate and power, say, the two channel signals are complete different. Can we transmit them by using two quadrature carrier?

Best regards
ddt694
 

Hi,
You can use them as at the point of modulation there is no difference inpowers of I and Q arms. (If there were detection would be difficult as laready mentioned). Do not worry about the softsymbol amplitude. On air the I and Q amplitudes must be nearly same.

B R
Madhukar
 

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