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



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

Frank,

I dont want to enter the debate on what the time constant of the PON should be, but your e-mail (below) on what penalty you get for AC coupling caught my eye.

I agree with you that you need to take the probability of occurrence in to account for this, but I dont think that looking at CID is the right way to approach the calculation.

When you AC couple a binary signal you are filtering out some of the signal energy.  This is equivalent to adding this noise-like signal (with the opposite polarity) to a perfect transmitted eye.  To quantify the magnitude of this effect, I simulated a large amount of 64B/66B coded random data with an AC coupling of bit rate / 200 (20 ns).  The signal was represented as +1 for a one and -1 for a zero and the value of the mid point between the ones and zeros for the AC coupled signal captured for each successive bit.  Then for a range of baseline wander values I calculated the probability of any bit having a baseline wander higher than this.  This plot is attached.

<<baseline wander.pdf>>

By fitting to the shape of this curve we get an estimate of the magnitude of this noise-like signal.  In this case it is equivalent to a linear Q value of 20.5

A BER of 1E-3 is equivalent to a Q value of 3.29.  I believe that if we have a source with an effective Q of 20.5, then the Q we would have had if the baseline wander had not been present is 1/(1/3.29 1/20.5) = 3.92  (i.e. a receiver with a Q of 3.92 using a perfect signal is degraded to a Q of 3.29 by AC coupling at bit rate / 200).  If I have done my sums right, this is equivalent to a power penalty of 0.7 dB

Regards,

Pete Anslow

Nortel Networks UK Limited, London Rd, Harlow, Essex CM17 9NA, UK

External +44 1279 402540 ESN 742 2540

Fax +44 1279 402543

_____________________________________________
From: EffenbergerFrank 73695 [mailto:feffenberger@HUAWEI.COM]
Sent: 08 April 2008 07:04
To: STDS-802-3-10GEPON@LISTSERV.IEEE.ORG
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.

> Nagahoris

> >> 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

> >> -----------------------------------------

> >>

> >>

>

baseline wander.pdf