Transcript Slide 1

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
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Settle in less time than preamble with acceptable BER
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Hold signal for longer than CID with acceptable BER
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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
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AC coupling
Data lost
Data received within BER limits

AC threshold set at the midpoint assuming even mark-space
ratio
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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
Guard + Preamble
(68.8 ns at 2.5Gbps)
Settle to within the
upper threshold level
Loudest
Softest
Minimum time constant determined by maximum CID period
CID (ones) (72 bits or 28.8ns at 2.5Gbps)
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June 2006
Remain within the
upper threshold level
AC coupling calculations
Assumptions:
LSR = 20 dB
ER = 10 dB
R=50 ohm
Maximum TC
Minimum TC*
change from loud to soft within guard +
preamble (<0.01458 V after 68.8 ns)
requirement to remain within limits during a
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
=> R=50 ohms, C<120 pF
28.8 ns
=> 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 front end designs
Feedback control
Pre-amp
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Amplitude recovery done in preamp
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Requires differential preamp –
making the design more
complicated and expensive
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Feedback inherently more stable
than feedforward therefore more
reliable
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Slower settling time between
bursts than FFW – important for
reset circuitry
+
Peak
detector
Feedforward control
Feedforward front end designs
Pre-amp
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Can use a conventional DC
coupled preamp - amplitude
recovery in post-amp
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Less stable - more prone to
oscillation
+
Peak
detector
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
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Edge response vs AC coupling
V
Tbit
0
t
V
TC>>Tbit
Conventional
AC coupling
0
t
V
TC<<Tbit
0
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t
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
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t
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)
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Noise/sensitivity analysis
V
tpeak
Vpeak
f(V)
Vc(tdec)
Vth+
t
Vthtdec
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Pe
Bit error probability calculations
Tdec increasing
Plot of bit error probability against input SNR (Pin/N0) for increasing decision time
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CID treatment
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A full BER analysis requires special treatment of multiple bits
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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)
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Can be reduced to a ER enhancement factor taking the first
bit as a special case
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This work should conclude by the end of June ready for
publication
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Conclusions
 AC
with long time constants not compatible with
GPON due to the close pre-amble and CID durations
 DC
coupled 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
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June 2006
END
NOC conference, Berlin
June 2006