On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote:>>>Only a fractionally spaced equalizer will resolve timing phase. If you >use >>>a T spaced equalizer you had better have good symbol timing recovery >prior >>>to the equalizer. Carrier reference can be resolved after equalization >>>unless the frequency offset is large in which case you can split the >loop >>>and recover frequency prior to the equalizer and phase/remaining >frequency >>>after the equalizer. >>>-Doug >> >> >>One of the disadvantages of using the EQ to correct timing is that it >>consumes degrees of freedom usually intended for combating the channel >>distortion. Those degrees of freedom cost complexity, and usually >>the complexity of the EQ is much greater than the complexity of a >>decent timing recovery system. So, in my experience, the systems >>that try to use the EQ for synchronization or to aid in >>synchronization wind up with a heavier complexity load than >>alternative methods. Separating the two tasks can be advantageous for >>the performance of both the EQ and the timing recovery as well as >>complexity. >> >> >> >>Eric Jacobsen >>Anchor Hill Communications >>http://www.anchorhill.com >> > >That's true, but if the channel highly distorted, there is no TED that is >going to give you the optimum timing phase - that can only be gotten from a >fractionally spaced equalizer. Still use timing recovery as normal, but >let the FSE optimize the timing phase - it can synthesize the proper >fractional delay. >-DougDo you know any examples of a channel where that would be necessary? Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Adaptive equalization and synchronization
Started by ●May 30, 2013
Reply by ●June 3, 20132013-06-03
Reply by ●June 3, 20132013-06-03
On 6/3/2013 9:58 AM, Eric Jacobsen wrote:> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote: > >>>> Only a fractionally spaced equalizer will resolve timing phase. If you >> use >>>> a T spaced equalizer you had better have good symbol timing recovery >> prior >>>> to the equalizer. Carrier reference can be resolved after equalization >>>> unless the frequency offset is large in which case you can split the >> loop >>>> and recover frequency prior to the equalizer and phase/remaining >> frequency >>>> after the equalizer. >>>> -Doug >>> >>> >>> One of the disadvantages of using the EQ to correct timing is that it >>> consumes degrees of freedom usually intended for combating the channel >>> distortion. Those degrees of freedom cost complexity, and usually >>> the complexity of the EQ is much greater than the complexity of a >>> decent timing recovery system. So, in my experience, the systems >>> that try to use the EQ for synchronization or to aid in >>> synchronization wind up with a heavier complexity load than >>> alternative methods. Separating the two tasks can be advantageous for >>> the performance of both the EQ and the timing recovery as well as >>> complexity. >>> >> That's true, but if the channel highly distorted, there is no TED that is >> going to give you the optimum timing phase - that can only be gotten from a >> fractionally spaced equalizer. Still use timing recovery as normal, but >> let the FSE optimize the timing phase - it can synthesize the proper >> fractional delay. >> -Doug > > Do you know any examples of a channel where that would be necessary? >That is typical with phone line modems and highly dispersive channels in general. A TED would produce something like centroid of delay spread; which is not neccessarily corresponding to optimal timing. Vladimir Vassilevsky DSP and Mixed Signal Designs www.abvolt.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 06:48:26 -0500, DougB wrote:>>>Only a fractionally spaced equalizer will resolve timing phase. If you > use >>>a T spaced equalizer you had better have good symbol timing recovery > prior >>>to the equalizer. Carrier reference can be resolved after equalization >>>unless the frequency offset is large in which case you can split the > loop >>>and recover frequency prior to the equalizer and phase/remaining > frequency >>>after the equalizer. >>>-Doug >> >> >>One of the disadvantages of using the EQ to correct timing is that it >>consumes degrees of freedom usually intended for combating the channel >>distortion. Those degrees of freedom cost complexity, and usually the >>complexity of the EQ is much greater than the complexity of a decent >>timing recovery system. So, in my experience, the systems that try to >>use the EQ for synchronization or to aid in synchronization wind up with >>a heavier complexity load than alternative methods. Separating the two >>tasks can be advantageous for the performance of both the EQ and the >>timing recovery as well as complexity. >> >> >> >>Eric Jacobsen >>Anchor Hill Communications >>http://www.anchorhill.com >> >> > That's true, but if the channel highly distorted, there is no TED that > is going to give you the optimum timing phase - that can only be gotten > from a fractionally spaced equalizer. Still use timing recovery as > normal, but let the FSE optimize the timing phase - it can synthesize > the proper fractional delay.I considered just letting the equalizer take care of delay. It would certainly make my life easier in many respects. My problem with this system is that I can't count on a perfect match between transmit and receive clocks, so the change in delay from the start of a run to the end would require an equalizer with an unfeasible amount of delay. Hence, I seek a reliable way to synchronize that does not leave the equalizer holding the bag. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 14:58:47 +0000, Eric Jacobsen wrote:> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote: > >>>>Only a fractionally spaced equalizer will resolve timing phase. If >>>>you >>use >>>>a T spaced equalizer you had better have good symbol timing recovery >>prior >>>>to the equalizer. Carrier reference can be resolved after >>>>equalization unless the frequency offset is large in which case you >>>>can split the >>loop >>>>and recover frequency prior to the equalizer and phase/remaining >>frequency >>>>after the equalizer. >>>>-Doug >>> >>> >>>One of the disadvantages of using the EQ to correct timing is that it >>>consumes degrees of freedom usually intended for combating the channel >>>distortion. Those degrees of freedom cost complexity, and usually the >>>complexity of the EQ is much greater than the complexity of a decent >>>timing recovery system. So, in my experience, the systems that try to >>>use the EQ for synchronization or to aid in synchronization wind up >>>with a heavier complexity load than alternative methods. Separating >>>the two tasks can be advantageous for the performance of both the EQ >>>and the timing recovery as well as complexity. >>> >>> >>> >>>Eric Jacobsen >>>Anchor Hill Communications >>>http://www.anchorhill.com >>> >>> >>That's true, but if the channel highly distorted, there is no TED that >>is going to give you the optimum timing phase - that can only be gotten >>from a fractionally spaced equalizer. Still use timing recovery as >>normal, but let the FSE optimize the timing phase - it can synthesize >>the proper fractional delay. >>-Doug > > Do you know any examples of a channel where that would be necessary?Actually, I'm somewhat concerned that's what I may be dealing with. It appears that a conventional TED would work in my case for the examples that I have -- but I haven't verified this, and I'm concerned that an example with more dispersion or noise might break the TED. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 11:58:34 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Mon, 03 Jun 2013 14:58:47 +0000, Eric Jacobsen wrote: > >> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote: >> >>>>>Only a fractionally spaced equalizer will resolve timing phase. If >>>>>you >>>use >>>>>a T spaced equalizer you had better have good symbol timing recovery >>>prior >>>>>to the equalizer. Carrier reference can be resolved after >>>>>equalization unless the frequency offset is large in which case you >>>>>can split the >>>loop >>>>>and recover frequency prior to the equalizer and phase/remaining >>>frequency >>>>>after the equalizer. >>>>>-Doug >>>> >>>> >>>>One of the disadvantages of using the EQ to correct timing is that it >>>>consumes degrees of freedom usually intended for combating the channel >>>>distortion. Those degrees of freedom cost complexity, and usually the >>>>complexity of the EQ is much greater than the complexity of a decent >>>>timing recovery system. So, in my experience, the systems that try to >>>>use the EQ for synchronization or to aid in synchronization wind up >>>>with a heavier complexity load than alternative methods. Separating >>>>the two tasks can be advantageous for the performance of both the EQ >>>>and the timing recovery as well as complexity. >>>> >>>> >>>> >>>>Eric Jacobsen >>>>Anchor Hill Communications >>>>http://www.anchorhill.com >>>> >>>> >>>That's true, but if the channel highly distorted, there is no TED that >>>is going to give you the optimum timing phase - that can only be gotten >>>from a fractionally spaced equalizer. Still use timing recovery as >>>normal, but let the FSE optimize the timing phase - it can synthesize >>>the proper fractional delay. >>>-Doug >> >> Do you know any examples of a channel where that would be necessary? > >Actually, I'm somewhat concerned that's what I may be dealing with. It >appears that a conventional TED would work in my case for the examples >that I have -- but I haven't verified this, and I'm concerned that an >example with more dispersion or noise might break the TED.Can you say much about your channel characteristics? Is it even well characterized (i.e., is there a good description or model of the channel)? I've yet to encounter a channel suitable for single-carrier communication that couldn't be handled with a good timing recovery system prior to the EQ (and independent of it), so I'm curious as to the cases where people claim it can't or shouldn't be done. Wireline and cable channels tend to be tamer than wireless because they're less dynamic, and my experience is mostly wireless. There are certainly conditions where aggressive channels will make a single-carrier system impractical, but in those conditions one typically just uses something other than a single-carrier modulation. IMHO, anyway.>-- >My liberal friends think I'm a conservative kook. >My conservative friends think I'm a liberal kook. >Why am I not happy that they have found common ground? > >Tim Wescott, Communications, Control, Circuits & Software >http://www.wescottdesign.comEric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 17:59:26 +0000, Eric Jacobsen wrote:> On Mon, 03 Jun 2013 11:58:34 -0500, Tim Wescott <tim@seemywebsite.com> > wrote: > >>On Mon, 03 Jun 2013 14:58:47 +0000, Eric Jacobsen wrote: >> >>> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote: >>> >>>>>>Only a fractionally spaced equalizer will resolve timing phase. If >>>>>>you >>>>use >>>>>>a T spaced equalizer you had better have good symbol timing recovery >>>>prior >>>>>>to the equalizer. Carrier reference can be resolved after >>>>>>equalization unless the frequency offset is large in which case you >>>>>>can split the >>>>loop >>>>>>and recover frequency prior to the equalizer and phase/remaining >>>>frequency >>>>>>after the equalizer. >>>>>>-Doug >>>>> >>>>> >>>>>One of the disadvantages of using the EQ to correct timing is that it >>>>>consumes degrees of freedom usually intended for combating the >>>>>channel distortion. Those degrees of freedom cost complexity, and >>>>>usually the complexity of the EQ is much greater than the complexity >>>>>of a decent timing recovery system. So, in my experience, the >>>>>systems that try to use the EQ for synchronization or to aid in >>>>>synchronization wind up with a heavier complexity load than >>>>>alternative methods. Separating the two tasks can be advantageous >>>>>for the performance of both the EQ and the timing recovery as well as >>>>>complexity. >>>>> >>>>> >>>>> >>>>>Eric Jacobsen >>>>>Anchor Hill Communications >>>>>http://www.anchorhill.com >>>>> >>>>> >>>>That's true, but if the channel highly distorted, there is no TED that >>>>is going to give you the optimum timing phase - that can only be >>>>gotten from a fractionally spaced equalizer. Still use timing >>>>recovery as normal, but let the FSE optimize the timing phase - it can >>>>synthesize the proper fractional delay. >>>>-Doug >>> >>> Do you know any examples of a channel where that would be necessary? >> >>Actually, I'm somewhat concerned that's what I may be dealing with. It >>appears that a conventional TED would work in my case for the examples >>that I have -- but I haven't verified this, and I'm concerned that an >>example with more dispersion or noise might break the TED. > > Can you say much about your channel characteristics? Is it even well > characterized (i.e., is there a good description or model of the > channel)?Part of my job is to develop that model, so no, I don't feel that it is really well characterized at the moment. I'm pretty sure that the dominant features are a low-pass filter with at least one and possibly more poles with time constants that are significantly longer than a bit time, plus tons of noise that, fortunately, doesn't have a lot of power at DC.> I've yet to encounter a channel suitable for single-carrier > communication that couldn't be handled with a good timing recovery > system prior to the EQ (and independent of it), so I'm curious as to the > cases where people claim it can't or shouldn't be done. Wireline and > cable channels tend to be tamer than wireless because they're less > dynamic, and my experience is mostly wireless.The communication is riding on a power supply. So I don't have a lot of freedom to change it -- it's very much a "make the best of what you have" sort of channel. All in all, I think my best "carrier" is baseband, for a variety of reasons.> There are certainly conditions where aggressive channels will make a > single-carrier system impractical, but in those conditions one typically > just uses something other than a single-carrier modulation. IMHO, > anyway.Like I said, my carrier is almost certainly going to be cos(0). -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 13:11:15 -0500, Tim Wescott <tim@seemywebsite.com> wrote:>On Mon, 03 Jun 2013 17:59:26 +0000, Eric Jacobsen wrote: > >> On Mon, 03 Jun 2013 11:58:34 -0500, Tim Wescott <tim@seemywebsite.com> >> wrote: >> >>>On Mon, 03 Jun 2013 14:58:47 +0000, Eric Jacobsen wrote: >>> >>>> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> wrote: >>>> >>>>>>>Only a fractionally spaced equalizer will resolve timing phase. If >>>>>>>you >>>>>use >>>>>>>a T spaced equalizer you had better have good symbol timing recovery >>>>>prior >>>>>>>to the equalizer. Carrier reference can be resolved after >>>>>>>equalization unless the frequency offset is large in which case you >>>>>>>can split the >>>>>loop >>>>>>>and recover frequency prior to the equalizer and phase/remaining >>>>>frequency >>>>>>>after the equalizer. >>>>>>>-Doug >>>>>> >>>>>> >>>>>>One of the disadvantages of using the EQ to correct timing is that it >>>>>>consumes degrees of freedom usually intended for combating the >>>>>>channel distortion. Those degrees of freedom cost complexity, and >>>>>>usually the complexity of the EQ is much greater than the complexity >>>>>>of a decent timing recovery system. So, in my experience, the >>>>>>systems that try to use the EQ for synchronization or to aid in >>>>>>synchronization wind up with a heavier complexity load than >>>>>>alternative methods. Separating the two tasks can be advantageous >>>>>>for the performance of both the EQ and the timing recovery as well as >>>>>>complexity. >>>>>> >>>>>> >>>>>> >>>>>>Eric Jacobsen >>>>>>Anchor Hill Communications >>>>>>http://www.anchorhill.com >>>>>> >>>>>> >>>>>That's true, but if the channel highly distorted, there is no TED that >>>>>is going to give you the optimum timing phase - that can only be >>>>>gotten from a fractionally spaced equalizer. Still use timing >>>>>recovery as normal, but let the FSE optimize the timing phase - it can >>>>>synthesize the proper fractional delay. >>>>>-Doug >>>> >>>> Do you know any examples of a channel where that would be necessary? >>> >>>Actually, I'm somewhat concerned that's what I may be dealing with. It >>>appears that a conventional TED would work in my case for the examples >>>that I have -- but I haven't verified this, and I'm concerned that an >>>example with more dispersion or noise might break the TED. >> >> Can you say much about your channel characteristics? Is it even well >> characterized (i.e., is there a good description or model of the >> channel)? > >Part of my job is to develop that model, so no, I don't feel that it is >really well characterized at the moment. > >I'm pretty sure that the dominant features are a low-pass filter with at >least one and possibly more poles with time constants that are >significantly longer than a bit time, plus tons of noise that, >fortunately, doesn't have a lot of power at DC.So you're equalizing distortion rather than signal reflections? Unless it's really horrible distortion, usually that lends itself well to having a timing recovery loop in front of the EQ. In a bursty sytem it can be turned on after initial timing is estimated. Of course I say that without any knowledge of the channel other than what you just briefly described, but usually timing synchronization trouble comes from reflections coming and going dynamically, and it sounds like you don't have that. If there's any reasonably recognizable eye pattern in the received signal at all, a TED should be able to lock to it. It can be very rattty, since the loop BW can be very narrow, there just needs to be a roughly recognizable eye pattern that can be locked to. It doesn't even need to be the optimal sampling point, just something there to steer the loop. I hope that makes sense.>> I've yet to encounter a channel suitable for single-carrier >> communication that couldn't be handled with a good timing recovery >> system prior to the EQ (and independent of it), so I'm curious as to the >> cases where people claim it can't or shouldn't be done. Wireline and >> cable channels tend to be tamer than wireless because they're less >> dynamic, and my experience is mostly wireless. > >The communication is riding on a power supply. So I don't have a lot of >freedom to change it -- it's very much a "make the best of what you have" >sort of channel. > >All in all, I think my best "carrier" is baseband, for a variety of >reasons. > >> There are certainly conditions where aggressive channels will make a >> single-carrier system impractical, but in those conditions one typically >> just uses something other than a single-carrier modulation. IMHO, >> anyway. > >Like I said, my carrier is almost certainly going to be cos(0). > >-- >My liberal friends think I'm a conservative kook. >My conservative friends think I'm a liberal kook. >Why am I not happy that they have found common ground? > >Tim Wescott, Communications, Control, Circuits & Software >http://www.wescottdesign.comEric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 3, 20132013-06-03
On Mon, 03 Jun 2013 21:29:48 +0000, Eric Jacobsen wrote:> On Mon, 03 Jun 2013 13:11:15 -0500, Tim Wescott <tim@seemywebsite.com> > wrote: > >>On Mon, 03 Jun 2013 17:59:26 +0000, Eric Jacobsen wrote: >> >>> On Mon, 03 Jun 2013 11:58:34 -0500, Tim Wescott <tim@seemywebsite.com> >>> wrote: >>> >>>>On Mon, 03 Jun 2013 14:58:47 +0000, Eric Jacobsen wrote: >>>> >>>>> On Mon, 03 Jun 2013 06:48:26 -0500, "DougB" <60916@dsprelated> >>>>> wrote: >>>>> >>>>>>>>Only a fractionally spaced equalizer will resolve timing phase. >>>>>>>>If you >>>>>>use >>>>>>>>a T spaced equalizer you had better have good symbol timing >>>>>>>>recovery >>>>>>prior >>>>>>>>to the equalizer. Carrier reference can be resolved after >>>>>>>>equalization unless the frequency offset is large in which case >>>>>>>>you can split the >>>>>>loop >>>>>>>>and recover frequency prior to the equalizer and phase/remaining >>>>>>frequency >>>>>>>>after the equalizer. >>>>>>>>-Doug >>>>>>> >>>>>>> >>>>>>>One of the disadvantages of using the EQ to correct timing is that >>>>>>>it consumes degrees of freedom usually intended for combating the >>>>>>>channel distortion. Those degrees of freedom cost complexity, and >>>>>>>usually the complexity of the EQ is much greater than the >>>>>>>complexity of a decent timing recovery system. So, in my >>>>>>>experience, the systems that try to use the EQ for synchronization >>>>>>>or to aid in synchronization wind up with a heavier complexity load >>>>>>>than alternative methods. Separating the two tasks can be >>>>>>>advantageous for the performance of both the EQ and the timing >>>>>>>recovery as well as complexity. >>>>>>> >>>>>>> >>>>>>> >>>>>>>Eric Jacobsen >>>>>>>Anchor Hill Communications >>>>>>>http://www.anchorhill.com >>>>>>> >>>>>>> >>>>>>That's true, but if the channel highly distorted, there is no TED >>>>>>that is going to give you the optimum timing phase - that can only >>>>>>be gotten from a fractionally spaced equalizer. Still use timing >>>>>>recovery as normal, but let the FSE optimize the timing phase - it >>>>>>can synthesize the proper fractional delay. -Doug >>>>> >>>>> Do you know any examples of a channel where that would be necessary? >>>> >>>>Actually, I'm somewhat concerned that's what I may be dealing with. >>>>It appears that a conventional TED would work in my case for the >>>>examples that I have -- but I haven't verified this, and I'm concerned >>>>that an example with more dispersion or noise might break the TED. >>> >>> Can you say much about your channel characteristics? Is it even well >>> characterized (i.e., is there a good description or model of the >>> channel)? >> >>Part of my job is to develop that model, so no, I don't feel that it is >>really well characterized at the moment. >> >>I'm pretty sure that the dominant features are a low-pass filter with at >>least one and possibly more poles with time constants that are >>significantly longer than a bit time, plus tons of noise that, >>fortunately, doesn't have a lot of power at DC. > > So you're equalizing distortion rather than signal reflections? Unless > it's really horrible distortion, usually that lends itself well to > having a timing recovery loop in front of the EQ. In a bursty sytem it > can be turned on after initial timing is estimated. > > Of course I say that without any knowledge of the channel other than > what you just briefly described, but usually timing synchronization > trouble comes from reflections coming and going dynamically, and it > sounds like you don't have that. If there's any reasonably > recognizable eye pattern in the received signal at all, a TED should be > able to lock to it. It can be very rattty, since the loop BW can be > very narrow, there just needs to be a roughly recognizable eye pattern > that can be locked to. It doesn't even need to be the optimal sampling > point, just something there to steer the loop. > > I hope that makes sense. >It makes sense, and I'm pretty sure that the low-pass is going to dominate the reflection. So it sounds like I'm safe. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●June 4, 20132013-06-04
>If there's any reasonably >recognizable eye pattern in the received signal at all, a TED should >be able to lock to it. It can be very rattty, since the loop BW can >be very narrow, there just needs to be a roughly recognizable eye >pattern that can be locked to. It doesn't even need to be the optimal >sampling point, just something there to steer the loop. > >Eric Jacobsen >Anchor Hill Communications >http://www.anchorhill.com >Agreed - a proper [NDA] TED will lock onto just about any garbage that is switching at the symbol rate. However, the sampling point or timing phase will not be optimal. If the convolution of the transmit pulse shape and channel is not symmetrical, then the peak moves around depending on the particular string of symbols. Recover timing independent and prior to fractionally spaced equalization. -Doug
Reply by ●June 7, 20132013-06-07
>On Mon, 03 Jun 2013 06:48:26 -0500, DougB wrote: > >>>>Only a fractionally spaced equalizer will resolve timing phase. Ifyou>> use >>>>a T spaced equalizer you had better have good symbol timing recovery >> prior >>>>to the equalizer. Carrier reference can be resolved afterequalization>>>>unless the frequency offset is large in which case you can split the >> loop >>>>and recover frequency prior to the equalizer and phase/remaining >> frequency >>>>after the equalizer. >>>>-Doug >>> >>> >>>One of the disadvantages of using the EQ to correct timing is that it >>>consumes degrees of freedom usually intended for combating the channel >>>distortion. Those degrees of freedom cost complexity, and usually the >>>complexity of the EQ is much greater than the complexity of a decent >>>timing recovery system. So, in my experience, the systems that try to >>>use the EQ for synchronization or to aid in synchronization wind upwith>>>a heavier complexity load than alternative methods. Separating the two >>>tasks can be advantageous for the performance of both the EQ and the >>>timing recovery as well as complexity. >>> >>> >>> >>>Eric Jacobsen >>>Anchor Hill Communications >>>http://www.anchorhill.com >>> >>> >> That's true, but if the channel highly distorted, there is no TED that >> is going to give you the optimum timing phase - that can only be gotten >> from a fractionally spaced equalizer. Still use timing recovery as >> normal, but let the FSE optimize the timing phase - it can synthesize >> the proper fractional delay. > >I considered just letting the equalizer take care of delay. It would >certainly make my life easier in many respects. > >My problem with this system is that I can't count on a perfect match >between transmit and receive clocks, so the change in delay from the >start of a run to the end would require an equalizer with an unfeasible >amount of delay. Hence, I seek a reliable way to synchronize that does >not leave the equalizer holding the bag. > >-- >My liberal friends think I'm a conservative kook. >My conservative friends think I'm a liberal kook. >Why am I not happy that they have found common ground? > >Tim Wescott, Communications, Control, Circuits & Software >http://www.wescottdesign.com >In the given scenario, in my opinion, the TED is not even supposed to lock onto the timing phase; it will lock to the symbol rate (or transmit clock). So any NDA TED like Gardner followed by a fractionally spaced equalizer should do the job for you. A very interesting and practical reference paper is the one by John Treichler, "Practical blind demodulators for high-order QAM signals" published in 1998. The most important part I think is the fractional tap-spacing because a symbol rate equalizer can easily lose the battle due to aliasing.
