Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

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
-----------------------------------------