QPSK Communication
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From: Avier on 21 Jan 2010 04:40 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 and getting the right bits to form one word(framing) ) can you help me in this matter
From: steveu on 21 Jan 2010 05:04 > 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 and >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 you don't. Steve Steve
From: Clay on 21 Jan 2010 16:04 On Jan 21, 5:04 am, "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 and > >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 you > 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. Clay
From: Avier on 21 Jan 2010 22:43 >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 ,,,, why do i need symbol sync. 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
From: steveu on 21 Jan 2010 23:20
>>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 |