QPSK Communication
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From: Avier on 22 Jan 2010 02:47 >>>On Jan 21, 5:04=A0am, "steveu" <ste...(a)coppice.org> wrote: >>>> > i am working on a software defined radio kit. building a frequency >>>> hopping >>>> >system using QPSK (quadrature phase shift keying) modulation >>>> >>>> >i have got a problem on which i am stuck for weeks now and cant get >>help >>>> >anywhere >>>> >>>> >presently i have made system for SINGLE FREQUENCY , i have got two >>kits >>>> >(transmitter and receiver) i can transmit and receive simultaneously >>on >>>> a >>>> >single kit so i first tested system on single kit and was successful >>in >>>> >hearing voice (DSP ----DAC ----RF(transmit)----RF(receive)---ADC >>>> ----DSP) >>>> >>>> >the problem comes that when i try to hear voice on second kit i get >>>> >nothing >>>> >>>> >i have checked the delays,, which for my case are controllable >(both, >>>> >which locks phase of RF signal with IF down conversion signal =A0 >>=A0and >>>> >getting the right bits to form one word(framing) ) >>>> >>>> >can you help me in this matter >>>> >>>> There could be many possible reasons, but my guess would be..... >>>> >>>> With one system everything runs from the same clock, and some >>>> synchronisation issues just don't arise. With 2 systems you may need >>to >>>> achieve proper carrier recovery (depends if you do things >>differentially = >>>or >>>> not), and you will need to achieve proper symbol synchronisation. >Maybe >>y= >>>ou >>>> don't. >>>> >>>> Steve >>>> >>>> Steve- Hide quoted text - >>>> >>>> - Show quoted text - >>> >>>I second Steve's concerns. A receiver must allow for frequency offsets >>>between the Tx and the Rx. Usually this is done via some type of >>>frequency and/or phase locking. >>> >> >> >>the kits are identical so i have checked the frequency responses and >also >>in RF module PLLs are present which locks the frequency ,,,so this is >also >>catered ,,,, > >Maybe. Without knowing more about your system's RF design I can't say. > >>why do i need symbol sync. > >That's pretty fundamental. If you don't know the answer to that, find >yourself a digital comms book. > >On one kit even with single clock when signal >>goes in air and is received back the point of sampling may not be same >this >>shows that even if iam receiving on one kit then why it was not needed >on >>single kit > >You may not be sampling at the ideal point within the symbol, but you are >probably sampling at a consistent point within each symbol if everything is >running from the same clock. With a really simple modulation like QPSK, and >a strong signal, you can sample way off symbol centre, and still demodulate >OK - provided you sample each symbol at around the same point. > >Steve > by saying that "why i need symbol sync" i dont meant that i dont know it i mean to say i am catering the two factors (discussed in first message)
From: steveu on 22 Jan 2010 04:56
>>>>On Jan 21, 5:04=A0am, "steveu" <ste...(a)coppice.org> wrote: >>>>> > i am working on a software defined radio kit. building a frequency >>>>> hopping >>>>> >system using QPSK (quadrature phase shift keying) modulation >>>>> >>>>> >i have got a problem on which i am stuck for weeks now and cant get >>>help >>>>> >anywhere >>>>> >>>>> >presently i have made system for SINGLE FREQUENCY , i have got two >>>kits >>>>> >(transmitter and receiver) i can transmit and receive >simultaneously >>>on >>>>> a >>>>> >single kit so i first tested system on single kit and was >successful >>>in >>>>> >hearing voice (DSP ----DAC ----RF(transmit)----RF(receive)---ADC >>>>> ----DSP) >>>>> >>>>> >the problem comes that when i try to hear voice on second kit i get >>>>> >nothing >>>>> >>>>> >i have checked the delays,, which for my case are controllable >>(both, >>>>> >which locks phase of RF signal with IF down conversion signal =A0 >>>=A0and >>>>> >getting the right bits to form one word(framing) ) >>>>> >>>>> >can you help me in this matter >>>>> >>>>> There could be many possible reasons, but my guess would be..... >>>>> >>>>> With one system everything runs from the same clock, and some >>>>> synchronisation issues just don't arise. With 2 systems you may need >>>to >>>>> achieve proper carrier recovery (depends if you do things >>>differentially = >>>>or >>>>> not), and you will need to achieve proper symbol synchronisation. >>Maybe >>>y= >>>>ou >>>>> don't. >>>>> >>>>> Steve >>>>> >>>>> Steve- Hide quoted text - >>>>> >>>>> - Show quoted text - >>>> >>>>I second Steve's concerns. A receiver must allow for frequency offsets >>>>between the Tx and the Rx. Usually this is done via some type of >>>>frequency and/or phase locking. >>>> >>> >>> >>>the kits are identical so i have checked the frequency responses and >>also >>>in RF module PLLs are present which locks the frequency ,,,so this is >>also >>>catered ,,,, >> >>Maybe. Without knowing more about your system's RF design I can't say. >> >>>why do i need symbol sync. >> >>That's pretty fundamental. If you don't know the answer to that, find >>yourself a digital comms book. >> >>On one kit even with single clock when signal >>>goes in air and is received back the point of sampling may not be same >>this >>>shows that even if iam receiving on one kit then why it was not needed >>on >>>single kit >> >>You may not be sampling at the ideal point within the symbol, but you >are >>probably sampling at a consistent point within each symbol if everything >is >>running from the same clock. With a really simple modulation like QPSK, >and >>a strong signal, you can sample way off symbol centre, and still >demodulate >>OK - provided you sample each symbol at around the same point. >> >>Steve >> > >by saying that "why i need symbol sync" i dont meant that i dont know it > >i mean to say i am catering the two factors (discussed in first message) I don't seen anything in your original message that says you have handled these synchronisation issues properly. You talk about delay, and I'm not clear what you mean by that. Delay doesn't mean much in an affine space (i.e. between two independent boards). If you really have handled carrier and symbol synchronisation properly, and the basic sanity of your receiver has been verified by running transmit and receive on the same board, you really ought to be seeing a sane bit stream coming out of your receiver. Regards, Steve |