Burst-mode receivers for GPON and LRPON applications J.J. Lepley and S.D. Walker University of...
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Transcript of Burst-mode receivers for GPON and LRPON applications J.J. Lepley and S.D. Walker University of...
Burst-mode receivers for GPON and LRPON applications
J.J. Lepley and S.D. WalkerUniversity of Essex
NOC conference, Berlin June 2006
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?
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
AC coupled front end
C
R
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
Softest
Loudest
Guard + Preamble(68.8 ns at 2.5Gbps)
Maximum time constant determined by settling time between loudest and softest bursts
CID (ones) (72 bits or 28.8ns at 2.5Gbps)
Minimum time constant determined by maximum CID period
Settle to within the upper threshold level
Remain within the upper threshold level
NOC conference, Berlin June 2006
AC coupling calculations
Assumptions: LSR = 20 dB ER = 10 dB R=50 ohm
V
Maximum TCchange from loud to soft within guard +
preamble (<0.01458 V after 68.8 ns)
68.8 ns
0.01458
V
Minimum TC*
requirement to remain within limits during a max CID period (>0.44 V after 28.8 ns)
28.8 ns
0.44
=> 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!
AC coupling will not meet the GPON/LRPON specifications…this conclusion will scale to any data rate!
Calculation complicated by changing target levels with onset of burst after 25.6 ns
* 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
+
-
Peak detector
Peak detector
+
-
Feedforward control
Feedback front end designs Amplitude recovery done in pre-
amp Requires differential preamp –
making the design more complicated and expensive
Feedback inherently more stable than feedforward therefore more reliable
Slower settling time between bursts than FFW – important for reset circuitry
Feedforward front end designs Can use a conventional DC
coupled preamp - amplitude recovery in post-amp
Less stable - more prone to oscillation
Pre-amp
Pre-amp
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
t
V Tbit
0
Edge response vs AC coupling
V
t0
TC>>Tbit
t
V
0TC<<Tbit
Conventional AC coupling
NOC conference, Berlin June 2006
V
0tTbit
Vc(t)
Tr(t)Tf(t)
Edge detector – impulse characteristics
Use a very short time constant (less than duration of 1 bit)
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
t
tpeak
Vth+
Vth-
V
Vpeak
Vc(tdec)
tdec
f(V)
Pe
Noise/sensitivity analysis
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
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
NOC conference, Berlin June 2006
END