NOC BURST by CpA177

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									Burst-mode receivers for GPON and LRPON applications




                                J.J. Lepley and S.D. Walker
                                University of Essex
Overview


   Burst-mode receivers are a key component in passive optical
    networks
   Solutions now off-the-shelf for Ethernet based PONs (such as
    EPON and GEPON), but GPON and the future LRPON
    standards are proving difficult
   This paper discusses some of the issues involved and presents
    a possible solution based on edge-triggered receivers




NOC conference, Berlin   June 2006
Overview




Burst Mode Receiver for LR-PON
          AC coupling
          DC coupling
          Edge detection




NOC conference, Berlin   June 2006
AC coupling – what are the requirements?
                                     AC coupled front end


                                       C
                                               R



   Settle in less time than preamble with acceptable BER
   Hold signal for longer than CID with acceptable BER
   At 2.5 Gbps
    •   Guard time (64 bits) = 25.6 ns
    •   Preamble (108 bits) = 43.2 ns
    •   CID (72 bits) = 28.8 ns



NOC conference, Berlin   June 2006
AC coupling

                                     Data lost   Data received within BER limits




   AC threshold set at the midpoint assuming even mark-space
    ratio
   Large change in burst amplitude requires finite settling time
    during which data will not be received


NOC conference, Berlin   June 2006
   AC coupling – upper/lower limits

Maximum time constant determined by settling time between loudest and softest bursts


                                                                                  Settle to within the
                                                    Guard + Preamble
                                                   (68.8 ns at 2.5Gbps)
                                                                                 upper threshold level
                               Loudest


                                                                             Softest



 Minimum time constant determined by maximum CID period                           Remain within the
                                                                                 upper threshold level
                                 CID (ones) (72 bits or 28.8ns at 2.5Gbps)




   NOC conference, Berlin   June 2006
AC coupling calculations

Assumptions:              LSR = 20 dB               ER = 10 dB           R=50 ohm

                     Maximum TC                                                      Minimum TC*
          change from loud to soft within guard +                      requirement to remain within limits during a
           preamble (<0.01458 V after 68.8 ns)                           max CID period (>0.44 V after 28.8 ns)
     V                                                               V
                    Calculation complicated by
                    changing target levels with
                    onset of burst after 25.6 ns
                                                                     0.44
0.01458

                         68.8 ns                                                28.8 ns
              => R=50 ohms, C<120 pF                                             => R=50 ohms, C>701 pF


         AC coupling will not meet the GPON/LRPON specifications…this
                      conclusion will scale to any data rate!
 *An additional factor is that shorter TCs will result in some level drifting which may impair the performance of the CDR,
 therefore the CDR may impose some limitation on min TC


NOC conference, Berlin     June 2006
Overview




Burst Mode Receiver for LR-PON
          AC coupling
          DC coupling
          Edge detection




NOC conference, Berlin   June 2006
DC coupling – basic designs

 Feedback control                                  Feedback front end designs
                                                      Amplitude recovery done in pre-
                                                       amp
               Pre-amp
                                                      Requires differential preamp –
               +                                       making the design more
               -                                       complicated and expensive
                                                      Feedback inherently more stable
                    Peak                               than feedforward therefore more
                   detector                            reliable
                                                      Slower settling time between
                                                       bursts than FFW – important for
 Feedforward control                                   reset circuitry
                                                   Feedforward front end designs
                Pre-amp
                                                      Can use a conventional DC
                                               +       coupled preamp - amplitude
                                               -       recovery in post-amp
                                                      Less stable - more prone to
                                     Peak
                                    detector
                                                       oscillation




NOC conference, Berlin        June 2006
DC coupled - Main design considerations

   Peak detector may need to detect both high and low levels to
    prevent mark-space distortion, especially when the extinction
    ratio is poor
   Fast resets needed for recovery from bursts within guard
    period
     •   Feedforward type favoured here as feedback settling seen as too slow
     •   less of a problem with fixed packet length formats (ATM) but particularly
         important with variable burst length standards such as Ethernet.
   Nobody has yet implemented a PON compatible DC coupled
    BMR capable of operating at 2.5 or 10 Gbps




NOC conference, Berlin   June 2006
Overview




Burst Mode Receiver for LR-PON
          AC coupling
          DC coupling
          Edge/Impulse detection




NOC conference, Berlin   June 2006
Edge response vs AC coupling

                    V                Tbit




                    0                       t
                    V

  TC>>Tbit                                      Conventional
                                                AC coupling

                    0                       t
                    V

  TC<<Tbit          0                       t




NOC conference, Berlin   June 2006
Edge detector – impulse characteristics

          Use a very short time constant (less than duration of 1 bit)
  V


                                     Tr(t)
                          Tf(t)




                Vc(t)




  0
                                             Tbit                        t

NOC conference, Berlin   June 2006
Requirements


1.    The pulse must exceed the comparator threshold voltage prior to the
      decision time
2.    The noise present on the signal must not trigger a false reading
      following the first trigger and prior to the decision time
3.    The statistical noise present on the comparator must be considered
      for a full analysis – although this is relatively insignificant c.f.
      amplified RX noise
4.    Sensitivity determined when Vpeak = Vth for a noise contribution
      that results in an error probability of 10-4 (abs min values of ~-30dBm
      using typical data and assuming thermal noise limited)
5.    PE necessarily improves for larger signals – maximum input power
      limited by TIA overload (typ. Few mA)



NOC conference, Berlin   June 2006
   Noise/sensitivity analysis

   V

            tpeak
  Vpeak




Vc(tdec)                                          f(V)

  Vth+

  Vth-                                   t
                                  tdec
                                             Pe




   NOC conference, Berlin   June 2006
 Bit error probability calculations



                                             Tdec increasing




Plot of bit error probability against input SNR (Pin/N0) for increasing decision time

 NOC conference, Berlin   June 2006
CID treatment


     A full BER analysis requires special treatment of multiple bits
     Multiple bits provide a special problem for this receiver design
      because if bit m from a sequence of n bits triggers a false
      level on the comparator then the subsequent n-m bits will be
      in error as well (unless another positive error is triggered)
     Can be reduced to a ER enhancement factor taking the first
      bit as a special case
     This work should conclude by the end of June ready for
      publication




NOC conference, Berlin   June 2006
Conclusions



 ACwith long time constants not compatible with
  GPON due to the close pre-amble and CID durations
 DCcoupled receivers still striving to exceed the
  GEPON 1.25 Gbps level
 Edge  detection looking very promising and
  preliminary models are indicating it is capable of
  several Gbps operation




NOC conference, Berlin   June 2006
                                     END




NOC conference, Berlin   June 2006

								
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