With the right filtering, an FSK signal can be viewed as two complementary OOK (on-off keyed) signals. Is the optimal FSK demodulator more optimal, less optimal, or equivalent to two optimal OOK demodulators with their outputs combined? -- Randy Yates % "Watching all the days go by... Digital Signal Labs % Who are you and who am I?" mailto://yates@ieee.org % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO
FSK Versus OOK Demodulation
Started by ●June 21, 2011
Reply by ●June 21, 20112011-06-21
Randy Yates wrote:> With the right filtering, an FSK signal can be viewed as two > complementary OOK (on-off keyed) signals.There is more than that. The phase is continuous.> Is the optimal FSK > demodulator more optimal, less optimal, or equivalent to two optimal > OOK demodulators with their outputs combined?Optimal FSK demodulator is more optimal then two OOK demodulators as it makes use of the continuity of the phase. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●June 21, 20112011-06-21
On Jun 21, 2:51=A0pm, Randy Yates <ya...@ieee.org> asked:> With the right filtering, an FSK signal can be viewed as two > complementary OOK (on-off keyed) signals. Is the optimal FSK > demodulator more optimal, less optimal, or equivalent to two optimal > OOK demodulators with their outputs combined?An optimal FSK demodulator compares the outputs of two filters F0 and F1 and decides 0 or 1 depending on which filter has the larger output. An optimal OOK demodulator compares the output of a filter to a fixed threshold and decides ON or OFF depending on whether the output exceeds the threshold or not. And while we are at it, the optimum choice of threshold is NOT the half the average output when the ON signal is being transmitted. So now consider two optimal OOK demodulators with filters F0 and F1 respectively (these filters are the same as the ones the optimal FSK demodulator is using). Suppose the (optimal) threshold is 0.4. Suppose also that a 0 is being transmitted. If filter F0 has output 1.3 while filter F1 has output 0.45, then (since 1.3 > 0.45), the optimal FSK demodulator correctly decides that 0 is being transmitted. But the two optimal OOK detectors both think that their respective signals are ON. How do we now combine these ON outputs to determine whether a 0 or a 1 was transmitted? Oh you mean we look at the outputs 1.3 and 0.4 of the two filters and choose the larger? and where are the *optimal OOK detectors* when you do so? What are their thresholds? etc. Making decisions *individually* from the filter outputs (and thereby ignoring the complementary aspect of the signals) as is done by the optimal OOK detector does not work for demodulating FSK. If a joint decision is being made, then the optimal OOK detector has disappeared, and the question asked is moot. Hope this helps. Dilip Sarwate
Reply by ●June 21, 20112011-06-21
On Jun 21, 5:37=A0pm, dvsarwate <dvsarw...@yahoo.com> wrote:> On Jun 21, 2:51=A0pm, Randy Yates <ya...@ieee.org> asked: > > > With the right filtering, an FSK signal can be viewed as two > > complementary OOK (on-off keyed) signals. Is the optimal FSK > > demodulator more optimal, less optimal, or equivalent to two optimal > > OOK demodulators with their outputs combined? > > An optimal FSK demodulator compares the > outputs of two filters F0 and F1 and decides > 0 or 1 depending on which filter has the larger > output.I don't know about that. I have dabbled with an I Q data stream for FSK and I looked at whether the I was leading Q or vice versa to determine the state. =A0An optimal OOK demodulator compares> the output of a filter to a fixed threshold and > decides ON or OFF depending on whether the > output exceeds the threshold or not. =A0And while > we are at it, the optimum choice of threshold is > NOT the half the average output when the ON > signal is being transmitted. =A0So now consider > two optimal OOK demodulators with filters F0 > and F1 respectively =A0(these filters are the same > as the ones the optimal FSK demodulator is > using). =A0Suppose the (optimal) threshold is 0.4. > Suppose also that a 0 is being transmitted. > If filter F0 has output 1.3 while filter F1 has > output 0.45, then (since 1.3 > 0.45), the > optimal FSK demodulator correctly decides > that 0 is being transmitted. =A0But the two > optimal OOK detectors both think that their > respective signals are ON. =A0How do we now > combine these ON outputs to determine whether > a 0 or a 1 was transmitted? > > Oh you mean we look at the outputs 1.3 and 0.4 > of the two filters and choose the larger? =A0and > where are the *optimal OOK detectors* when you > do so? =A0What are their thresholds? etc. > > Making decisions *individually* from the filter > outputs (and thereby ignoring the complementary > aspect of the signals) as is done by the optimal > OOK detector does not work for demodulating > FSK. =A0If a joint decision is being made, then the > optimal OOK detector has disappeared, and the > question asked is moot. > > Hope this helps. > > Dilip Sarwate
Reply by ●June 21, 20112011-06-21
On 06/21/2011 02:13 PM, Vladimir Vassilevsky wrote:> > > Randy Yates wrote: > >> With the right filtering, an FSK signal can be viewed as two >> complementary OOK (on-off keyed) signals. > > There is more than that. The phase is continuous.Not necessarily, unless phase continuity is specified (and the specification complied to). Old teletype FSK was sometimes accomplished with a pair of audio-frequency oscillators, their outputs switched, and fed to a SSB transmitter.>> Is the optimal FSK >> demodulator more optimal, less optimal, or equivalent to two optimal >> OOK demodulators with their outputs combined? > > Optimal FSK demodulator is more optimal then two OOK demodulators as it > makes use of the continuity of the phase.Even without phase continuity it's more optimal in that it doesn't make the receiver guess at a zero level -- for the case that I remember, a sufficient statistic is the difference in signal strengths out of a pair of matched filters. In as much as the two signals are nominally of the same amplitude, the sufficient statistic was a signed number with the decision point at 0. I suspect that any FSK you're going to run across today will be of continuous phase, but I'm not going to _trust_ that unless I see it in the specification! -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●June 21, 20112011-06-21
On 06/21/2011 03:41 PM, Tim Wescott wrote:> On 06/21/2011 02:13 PM, Vladimir Vassilevsky wrote: >> >> >> Randy Yates wrote: >> >>> With the right filtering, an FSK signal can be viewed as two >>> complementary OOK (on-off keyed) signals. >> >> There is more than that. The phase is continuous. > > Not necessarily, unless phase continuity is specified (and the > specification complied to). Old teletype FSK was sometimes accomplished > with a pair of audio-frequency oscillators, their outputs switched, and > fed to a SSB transmitter. > >>> Is the optimal FSK >>> demodulator more optimal, less optimal, or equivalent to two optimal >>> OOK demodulators with their outputs combined? >> >> Optimal FSK demodulator is more optimal then two OOK demodulators as it >> makes use of the continuity of the phase. > > Even without phase continuity it's more optimal in that it doesn't make > the receiver guess at a zero level -- for the case that I remember, a > sufficient statistic is the difference in signal strengths out of a pair > of matched filters. In as much as the two signals are nominally of the > same amplitude, the sufficient statistic was a signed number with the > decision point at 0. > > I suspect that any FSK you're going to run across today will be of > continuous phase, but I'm not going to _trust_ that unless I see it in > the specification! >Sorry -- it's 'better' in a BER vs. peak transmitted power sense because you're always transmitting energy. So (if it's good FSK) you have a pair of orthogonal signals that are sqrt(2) * (signal amplitude) apart, instead of a signal that's (signal amplitude) away from zero. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●June 21, 20112011-06-21
Tim Wescott <tim@seemywebsite.com> wrote: (snip on FSK demodulation)> Not necessarily, unless phase continuity is specified (and the > specification complied to). Old teletype FSK was sometimes accomplished > with a pair of audio-frequency oscillators, their outputs switched, and > fed to a SSB transmitter.Reminds me that I used to know that the Bell 103 modem was supposed to work up to 600 baud, though most I knew were only used up to 300. For old teletypes at 110 baud or lower, the demodulator doesn't have to be so good. It gets harder at higher rates, especially with only 200Hz frequency difference. I will guess that phase continuity is more important at higher baud rates. The modem designs that I knew, such as one published in Popular Electronics, used a PLL demodulator. -- glen
Reply by ●June 21, 20112011-06-21
dvsarwate wrote:> On Jun 21, 2:51 pm, Randy Yates <ya...@ieee.org> asked: > >>With the right filtering, an FSK signal can be viewed as two >>complementary OOK (on-off keyed) signals. Is the optimal FSK >>demodulator more optimal, less optimal, or equivalent to two optimal >>OOK demodulators with their outputs combined? > > > An optimal FSK demodulator compares the > outputs of two filters F0 and F1 and decides[...] Doctor Dilip Matlabi, Forget about minor technical details of crippled algorithms for a moment. Optimal is about maximum likelihood and conditional probabilities. The only difference between optimal (2 x OOK) and optimal FSK is that the phase relationship between "mark" and "space" is not set with OOK. As such, FSK provides for potentially better BER/Eb/No. And here is a practical realization of the near optimal OOK receiver: http://www.abvolt.com/research/publications1.htm Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●June 21, 20112011-06-21
On 06/21/2011 03:53 PM, Tim Wescott wrote:> On 06/21/2011 03:41 PM, Tim Wescott wrote: >> On 06/21/2011 02:13 PM, Vladimir Vassilevsky wrote: >>> >>> >>> Randy Yates wrote: >>> >>>> With the right filtering, an FSK signal can be viewed as two >>>> complementary OOK (on-off keyed) signals. >>> >>> There is more than that. The phase is continuous. >> >> Not necessarily, unless phase continuity is specified (and the >> specification complied to). Old teletype FSK was sometimes accomplished >> with a pair of audio-frequency oscillators, their outputs switched, and >> fed to a SSB transmitter. >> >>>> Is the optimal FSK >>>> demodulator more optimal, less optimal, or equivalent to two optimal >>>> OOK demodulators with their outputs combined? >>> >>> Optimal FSK demodulator is more optimal then two OOK demodulators as it >>> makes use of the continuity of the phase. >> >> Even without phase continuity it's more optimal in that it doesn't make >> the receiver guess at a zero level -- for the case that I remember, a >> sufficient statistic is the difference in signal strengths out of a pair >> of matched filters. In as much as the two signals are nominally of the >> same amplitude, the sufficient statistic was a signed number with the >> decision point at 0. >> >> I suspect that any FSK you're going to run across today will be of >> continuous phase, but I'm not going to _trust_ that unless I see it in >> the specification! >> > Sorry -- it's 'better' in a BER vs. peak transmitted power sense because > you're always transmitting energy. So (if it's good FSK) you have a pair > of orthogonal signals that are sqrt(2) * (signal amplitude) apart, > instead of a signal that's (signal amplitude) away from zero.And of course, if you _can_ count on phase continuity, then you have even more information to work from and can make an even better detector. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●June 21, 20112011-06-21
On 06/21/2011 07:34 PM, Vladimir Vassilevsky wrote:> > > dvsarwate wrote: > >> On Jun 21, 2:51 pm, Randy Yates <ya...@ieee.org> asked: >> >>> With the right filtering, an FSK signal can be viewed as two >>> complementary OOK (on-off keyed) signals. Is the optimal FSK >>> demodulator more optimal, less optimal, or equivalent to two optimal >>> OOK demodulators with their outputs combined? >> >> >> An optimal FSK demodulator compares the >> outputs of two filters F0 and F1 and decides > > [...] > > Doctor Dilip Matlabi, > > Forget about minor technical details of crippled algorithms for a moment. Optimal is about maximum likelihood and conditional > probabilities. The only difference between optimal (2 x OOK) and optimal FSK is that the phase relationship between "mark" and > "space" is not set with OOK. As such, FSK provides for potentially better BER/Eb/No. > > And here is a practical realization of the near optimal OOK receiver: > > http://www.abvolt.com/research/publications1.htmCool - thanks Vlad. --RY> > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultant > http://www.abvolt.com >-- Randy Yates % "Watching all the days go by... Digital Signal Labs % Who are you and who am I?" mailto://yates@ieee.org % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO
