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Re: [8023-10GEPON] Optical Overload Ad-Hoc announcement



Dear Dr. effenberger, Hamano-san, Nagahori-san, All,

Please see attached simulation result.

I beleive that the burst overhead time of the feedback based AGC needs about
5x or more times longer than that of the predicted value estimated by the 
feedforward equivalent
AC-coupling.

So, I agree with Hamano-san's idea of  Treceiver_setting = 800-ns(max) as
follows,

----
> I thought that the total timing assignment of 'Treceiver_settling + Tcdr'
> up to
> 800ns was a good idea.  But if the separation of the two parameters is the
> major
> opinion of the task force members, then I suggest to have the
> Treceiver_settling
> (max) alone of 800ns.
> When a short-burst-timing receiver can be achieved, sync_time parameter
> exchange will adjust the proper timing between OLT and ONU, and nothing
> will
> bother the system function and the signal efficiency.


Regards,

Naoki Suzuki
Mitsubishi Electric


----- Original Message ----- 
From: "EffenbergerFrank 73695" <feffenberger@HUAWEI.COM>
To: <STDS-802-3-10GEPON@LISTSERV.IEEE.ORG>
Sent: Tuesday, April 08, 2008 3:03 PM
Subject: Re: [8023-10GEPON] Optical Overload Ad-Hoc announcement


> Dear Dr. Nagahori,
>
> Thank you for your document.  I can now see how you have figured your
> numbers.  However, I wonder about your calculation of penalty.  We must
> keep in mind that the penalty is a function of the BER at which you are
> measuring the penalty.
>
> Your calculation basically says that the penalty is equal to the amount of
> eye closure that happens during a CID event.  And that is true *during
> that event*.  But, we need to consider the likelihood of such events,
> because Bit Error Rate is a probability game.  A CID of 60 bits happens
> with a frequency of 10^-18.  That is very infrequently.  When it does
> happen, you will likely see a short run of errors near the tail end of the
> CID event.  (So much the better for us, since we are using a code that is
> good at correcting burst errors.)  We can also see the direct evidence of
> this in your BER curves, because the CID events are the source of the
> error floors.
>
> So, I would claim that you could build a system that has a tau of 20ns
> (the first row on your chart).  But, the optical penalty you would see is
> not 1.5 dB, as you have there.  The penalty is zero.  The reason is that
> we are operating at a BER way above the error floor that the CID events
> cause.
>
> Don't you think?
>
> Sincerely,
> Frank E
>
>
> ----- Original Message -----
> From: Takeshi Nagahori <t-nagahori@AH.JP.NEC.COM>
> Date: Tuesday, April 8, 2008 1:04 pm
> Subject: Re: [8023-10GEPON] Optical  Overload Ad-Hoc announcement
>
>> Dear Dr. effenberger, Hamano-san, All,
>>
>> On the US 10G burst mode timing discussion at January and March
>> meeiting,there were not enough discussions about trade off between
>> aquisition time
>> and power penalty as a function of lower cut off in AC coupling.
>>
>> On DC balance limit with 64B/66B coding, I presentated that there
>> are few inpact on sensitivity
>> at 1E-3 BER on  March meeting.
>>
>> Attached is a calculated result on CID limit. The calculation is
>> based on simplified AC coupling
>> (1 pole high pass filter)  model, where droop in CID is modeled
>> as 1-exp(-t/tau). The result shows
>> that if we obtain 100ns acquitin time per 1pole with 15dB dynamic
>> range, we have to allow 1.5 dB
>> power penaly per 1 pole. If we allow only 0.5dB power penalty, the
>> acquitin time becomes 350 ns
>> per 1pole.
>>
>> I agree with Dr. Effenbergers comments qualitatively that 350ns+
>> 350ns<<700ns at 2 pole system.
>>
>> Best Regards,
>> Takeshi Nagahori
>>
>> ------------ Original Message ------------
>>
>> >Dear Hiroshi,
>> >
>> >Maybe I am being dumb, but I did not see any evidence that 500 ns
>> is needed.
>> >What I saw was some good data that suggested that short times are
>> possible, followed by some hand waving that suggested the
>> opposite.
>> >
>> >The only way that I can arrive at such a number is to take the
>> theoretically understandable value of 100 ns, and then assume a
>> limited implementation results in a 5x multiplication.  But I have
>> a hard time accepting such a poor implementation for our standard.
>>
>> >
>> >But let's judge this debate on the same standard as that used for
>> the slow start concept.  Various folks have objected to that based
>> on the fact that their implementation doesn't have that problem.
>> Well, I'm saying that my implementation doesn't have this 500ns
>> response time problem.  I'm saying that, even though my
>> implementation uses something essentially similar to the average
>> power AGC.
>> >
>> >I would like to see a detailed explanation of why 500 ns is so
>> necessary.  Show me the delay budget or similar equations that
>> comes out to the result.  And, if it can be shown that the worst-
>> case design results in 500ns time, well, then we should set the
>> value at 500, and not 800!  The number 800 was based on a faulty
>> premise, and should not be used.
>> >
>> >Sincerely,
>> >Frank
>> >
>> >
>> >
>> >
>> >----- Original Message -----
>> >From: Hiroshi Hamano <hamano.hiroshi@jp.fujitsu.com>
>> >Date: Tuesday, April 8, 2008 11:37 am
>> >Subject: Re: [8023-10GEPON] Optical Overload Ad-Hoc announcement
>> >
>> >> Dear Dr. Effenberger,
>> >>
>> >> On issue 2, Treceiver_settling, I support the last Dr.
>> Nagahori’s
>> >> presentation
>> >> targeting a practical 500ns assignment, and his E-mail
>> suggestion
>> >> to specify
>> >> 800ns(max).
>> >>
>> >> As we discussed in the last meeting, I think most of us agree
>> that
>> >> simple
>> >> TIA option with average signal detection AGC should not be
>> >> rejected for
>> >> burst-mode receiver implementation.  Shorter-burst-timing
>> receiver
>> >> with peak detection AGC may be preferable, but circuit design
>> >> difficulty
>> >> may result in its high cost and installation delay.
>> >>
>> >> AC-coupling experimental result on P.7 of
>> 3av_0803_nagahori_1.pdf
>> >> shows
>> >> a good example of the relationship between time constant and
>> >> penalty, but
>> >> for average power detection AGC feedback, the penalty may
>> appear
>> >> differently,
>> >> and E-FEC compensation cannot be equally applied.
>> >>
>> >> Besides, 10GE-PON upstream power budget is extremely tight,
>> >> especially
>> >> with the PR30 channel insertion loss, and the penalty by the
>> AGC
>> >> circuitry
>> >> should be negligibly small.
>> >>
>> >> I am not so sure how much penalty your 100ns time constant may
>> >> result in,
>> >> and right now I do not have enough experimental results either
>> to
>> >> confirm
>> >> the preferable timing and the penalty.  I wonder if somebody so
>> >> far has
>> >> the results to confirm them, and then, Treceiver_settling
>> should
>> >> be relaxed
>> >> not to reject the possible options.
>> >>
>> >> I thought that the total timing assignment of
>> 'Treceiver_settling
>> >> + Tcdr' up to
>> >> 800ns was a good idea.  But if the separation of the two
>> >> parameters is the major
>> >> opinion of the task force members, then I suggest to have the
>> >> Treceiver_settling
>> >> (max) alone of 800ns.
>> >> When a short-burst-timing receiver can be achieved, sync_time
>> >> parameter
>> >> exchange will adjust the proper timing between OLT and ONU, and
>> >> nothing will
>> >> bother the system function and the signal efficiency.
>> >>
>> >> Best regards,
>> >> Hiroshi Hamano
>> >>
>> >> %% Frank Effenberger <feffenberger@HUAWEI.COM>
>> >> %% Re: [8023-10GEPON] Optical Overload Ad-Hoc announcement
>> >> %% Wed, 2 Apr 2008 15:45:08 -0400
>> >>
>> >> > Dear All,
>> >> >
>> >> > On issue 2: I think that we agree that a (single pole)
>> settling
>> >> time of
>> >> > 100ns is sufficient for the 20dB dynamic range that we are
>> >> interested in.
>> >> > (That's a time constant of 20ns, and considering 5 time
>> >> constants to be
>> >> > 'settled'.)
>> >> >
>> >> > I agree with Mr. Nagahori that in a practical receiver today,
>> we
>> >> will> actually need to have a "2 pole" filter: One is the AC
>> >> coupling, and the
>> >> > other is some form of AGC feedback.  But, I would suggest
>> that
>> >> the AGC
>> >> > feedback based on average power will have the same time
>> constant
>> >> as the
>> >> > AC-coupling.  That is because it faces the same dilemma of
>> being
>> >> fast enough
>> >> > to see the bursts but slow enough not to see data patterns.
>> >> >
>> >> > The simple math would suggest 100+100=200.  But we all know
>> that
>> >> time> constants don't add that way, it's actually RMS.  So, if
>> >> things are working
>> >> > linearly, then we need 141ns.  We see to have lots of margin.
>>
>> >> We can
>> >> > tolerate even doubling both of the responses of each circuit,
>> >> and it still
>> >> > works.  Just as long as things remain linear and don't go
>> into
>> >> pathological> modes.
>> >> >
>> >> > So, I don't think we need to relax this any further than the
>> >> established> 400ns value.
>> >> >
>> >> > On issue 3: The problem with overloading the circuit is not
>> >> necessarily only
>> >> > one for the LA, but also for the output stage of the TIA, and
>> >> the AGC
>> >> > control loop.  Control loops work best when the signals that
>> >> they are acting
>> >> > on are in their linear range.  If the strong burst suddenly
>> >> comes in and the
>> >> > TIA saturates, then the AGC loop will not behave optimally.
>> Of
>> >> course, this
>> >> > can be allowed for by waiting longer, but isn't that the very
>> >> complaint in
>> >> > issue 2?
>> >> >
>> >> > The whole point of controlling the transmitter rate-of-attack
>> is
>> >> that it
>> >> > helps the receiver settle faster. Given that people are
>> >> concerned with a
>> >> > technology gap for the 10G burst Rx, it seems an obvious
>> cross
>> >> optimization> to make.
>> >> >
>> >> > Now, as to the cost of such a rise-time control - I think it
>> is
>> >> a pretty
>> >> > simple circuit to control the modulation current supply on a
>> >> 10ns time
>> >> > scale.  In fact, existing circuits could likely be adapted
>> >> simply by the
>> >> > addition of a single capacitor.  Is it really much harder
>> than
>> >> that?  We
>> >> > don't need precision, keep in mind.
>> >> >
>> >> > Sincerely,
>> >> > Frank E.
>> >> >
>> >> >
>> >> > -----Original Message-----
>> >> > From: Takeshi Nagahori [mailto:t-nagahori@AH.JP.NEC.COM]
>> >> > Sent: Wednesday, April 02, 2008 10:40 AM
>> >> > To: STDS-802-3-10GEPON@LISTSERV.IEEE.ORG
>> >> > Subject: Re: [8023-10GEPON] Optical Overload Ad-Hoc announcement
>> >> >
>> >> > Dear Dr. Effenberger,
>> >> >
>> >> > I greatly appreciate your effort taking both damege theshold
>> /
>> >> burst mode
>> >> > timing ad hoc leadership.
>> >> >
>> >> > I would like to comment on toipic 2 and 3 in-line.
>> >> >
>> >> >
>> >> >
>> >> > >2. What dynamic performance can be expected from strong-to-
>> weak
>> >> burst> >reception (the Treceiver_settling question)?
>> >> > >
>> >> > >The Nagahori presentation gives us very useful data.  Let me
>> >> illustrate it
>> >> > >in the following way:  From Nagahori page 7, we can see that
>> a
>> >> tau/T of 210
>> >> > >results in an error curve that has zero penalty at the
>> higher
>> >> bit error
>> >> > >rates that we are working at. (There are signs of an error
>> >> floor, but it
>> >> > >happens at 1E-10, so we don't care).  T, in out case, is 97
>> ps.
>> >> So, the
>> >> > >data says that setting tau to be 20ns is OK.
>> >> > >
>> >> > >Suppose we want to tolerate 20 dB of dynamic range burst to
>> >> burst.  This
>> >> > >means that we need to set the time constant of the AC-
>> coupling
>> >> to be at
>> >> > >least 5 times shorter than the burst-to-burst time.
>> (e^5=148 >
>> >> 20dB).> That
>> >> > >means that the burst to burst time needs to be 100ns.  So
>> far,
>> >> we are not
>> >> > >seeing any problems.  (By the way, the value of 100ns is
>> what I
>> >> put forward
>> >> > >in 3av_0801_effenberger_3-page4.)
>> >> > >
>> >> > >I also think that real circuits will need to allocate time
>> for
>> >> control of
>> >> > >the pre-amplifier stage (setting of the APD bias and/or the
>> TIA
>> >> impedance).> >This should take no longer than an additional
>> 100ns
>> >> of time.
>> >> > >
>> >> > >So, this leaves us with a requirement of 200ns, which has a
>> >> safety margin
>> >> > of
>> >> > >2x below the 400ns that is the proposed value for
>> >> Treceiver_settling.
>> >> > >
>> >> > >Thus, I don't see any reason why we should change the value
>> >> from 400ns,
>> >> > just
>> >> > >like in 1G EPON.  While it is true that Treceiver_settling
>> will
>> >> likely need
>> >> > >to be longer than T_cdr, setting the maximum values of both
>> at
>> >> 400ns will
>> >> > >not preclude any implementations.  I fully expect that real
>> >> systems will
>> >> > >actually do much better than both of these limits.
>> >> >
>> >> >
>> >> >     At first, I would like to enphasize that the limiting
>> factor
>> >> is not
>> >> > in AC coupling between TIA-LIM, but in burst mode AGC in TIA
>> to
>> >> control> transimpedance gain.
>> >> >    The required TIA input dynamic range is estimated to be
>> 23dB for
>> >> > PR-30/PRX-30
>> >> > dual rate. But state-of-the-art data of 10G burst mode TIA
>> >> dynamic is only
>> >> > 15dB with AGC in TIA from published paper in ECOC2007 and
>> >> ISSCC2008.
>> >> > We have to recognize this technology gap at this moment.
>> >> >    In this situation, it is preferred to allow the use of
>> simple
>> >> average
>> >> > detection type TIA AGC, instead of peak detection type AGC
>> that
>> >> was appeared
>> >> >
>> >> > in your and Dr. Ben-Amram's presentation at January meeting,
>> in
>> >> order to
>> >> > reduce
>> >> > the technology gap. Peak detection type AGC is superior to
>> >> avarage detection
>> >> > type
>> >> > AGC in response speed, but it has challenging issues in
>> response
>> >> in
>> >> > peak-detector's response at >1Gbps (not 10Gbps only), in
>> >> addition to
>> >> > dynamic range issue.
>> >> >    Considering the large enough margins for averaging
>> detection
>> >> type TIA AGC
>> >> > and some margin to 200ns for TIA-LIM AC coupling, 400ns is
>> not
>> >> large enough
>> >> > for treceiver_settling. The appropriate value would be less
>> than
>> >> 800ns,> even if we consider the technical gap between required
>> >> spec and
>> >> > ECOC2007/ISSCC2008 state-of-the-art data.
>> >> >
>> >> >
>> >> > >3. What about limiting the rate-of-attack of the burst Tx
>> >> (Ton/Toff)?> >I went to talk with my optical front-end expert,
>> and
>> >> he explained the
>> >> > latest
>> >> > >results that we've been seeing.  The whole motivation of our
>> >> concern is the
>> >> > >large 20dB dynamic range that we are targeting in PON
>> systems.
>> >> The problem
>> >> > >is that the receiver is normally in the maximum gain
>> condition,
>> >> and then a
>> >> > >strong burst comes in that threatens to overload the
>> circuit.
>> >> > >
>> >> > >Initially, we were concerned that the APD and the TIA would
>> be most
>> >> > >sensitive to high burst transients.  However, this seems to
>> be
>> >> not the
>> >> > case.
>> >> > >The APD gain may be self-limiting (saturating), and this
>> helps
>> >> to limit the
>> >> > >signal to some extent.  So, damage to that part of the
>> circuit
>> >> seems> >unlikely.
>> >> > >
>> >> > >However, there still is a problem, and that is that the
>> second
>> >> stage> >amplifier (the one that is driven by the TIA) tends to
>> get
>> >> overloaded by
>> >> > the
>> >> > >strong bursts. (This is understandable, since the signal has
>> >> received more
>> >> > >gain by this point.)  This prevents the output signal from
>> >> being useful
>> >> > (for
>> >> > >control as well as for the actual signal), and the recovery
>> >> from overload
>> >> > is
>> >> > >not well behaved.  So, we'd like to avoid that.
>> >> > >
>> >> > >The simplest way to prevent transient overload is to reduce
>> >> either the APD
>> >> > >gain (by reducing its bias), or reducing the TIA impedance.
>> >> Either of
>> >> > these
>> >> > >methods is essentially a control loop, and it will have a
>> >> characteristic> >speed.  The setting of the speed is bounded on
>> >> both directions just like
>> >> > the
>> >> > >AC coupling speed, and a value of 20ns is good.  Given that
>> we
>> >> have a
>> >> > >control speed of 20ns, the loop will respond only that fast
>> to
>> >> input> >transients.  We can thereby reduce the excursion of the
>> >> control system
>> >> > >output by limiting the "time constant" of the input signal
>> to
>> >> be similar to
>> >> > >that of the control loop.  This is why we suggest a 'rise
>> time'
>> >> on the
>> >> > order
>> >> > >of 20ns.
>> >> > >
>> >> > >I was wrong in extending this to also specifying a 'fall
>> time' -
>> >> there is
>> >> > no
>> >> > >need for controlling the trailing edge, at least, not
>> strictly.
>> >> The reason
>> >> > >is that the receiver will 'know' when the burst is over, so
>> it
>> >> should be
>> >> > >able to manage its withdrawal symptoms.  (Note that this
>> >> implies that the
>> >> > Rx
>> >> > >has certain feedback paths, such as when the CDR declares
>> loss
>> >> of lock.)
>> >> > >
>> >> > >So, that's the reason why we should consider having a
>> >> controlled turn-on
>> >> > for
>> >> > >the transmitter.
>> >> >
>> >> >     At March meeting, impacts of rise time control on
>> transimission>> > efficiency
>> >> > and complexity PON chip were discussed and were concluded
>> that
>> >> there were
>> >> > very
>> >> > few impact on those. But precise rise time control makes
>> >> implementation of
>> >> > Laser
>> >> > driver circuitry in ONU complicated to affects the ONU's cost.
>> >> >
>> >> >     I understood from your explanation that the reason why
>> rise
>> >> time control
>> >> > is needed is only to prevent saturation in LIM.  But if we
>> >> consider actual
>> >> > receiver circuit implementation, TIA does not generate signal
>> >> exceeding> power supply voltage, typically 3.3V, even if AGC in
>> >> TIA is not finished
>> >> > to reduce the transimpedance gain. This means that a large
>> >> signal to
>> >> > saturate LIM would not generated from TIA, so we need not
>> have
>> >> attention> to saturation in LIM. Considering above, I cannot
>> see
>> >> any reason for need
>> >> > for rise time control.
>> >> >
>> >> >
>> >> >
>> >> > Best Regards,
>> >> > Takeshi Nagahori
>> >> > NEC
>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >>
>> >> ---
>> >> -----------------------------------------
>> >> Hiroshi Hamano
>> >> Network Systems Labs., Fujitsu Labs. Ltd.
>> >> Phone:+81-44-754-2641 Fax.+81-44-754-2640
>> >> E-mail:hamano.hiroshi@jp.fujitsu.com
>> >> -----------------------------------------
>> >>
>> >>
>>

Tagc_sim_suzuki.pdf