1. Burst-mode receivers for GPON and LRPON applications
J.J. Lepley and S.D. Walker
University of Essex

2. 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

3. BMR – Front End Design requirements
Large dynamic range (20 dB) sensitive (-28 dBm) front end receiver with rapid (tens of ns) decision threshold setting
Maximum Loud-soft ratio must be defined – suggest this be maximum of 20 dB
Front end is the primary focus of attention as this is potentially the most difficult design problem
2 main design techniques based on AC and DC coupling
Amplification + Threshold Detection
Timing Recovery
APD/PIN
TIA

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

5. 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

6. 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

7. AC coupling – upper/lower limits
Maximum time constant determined by settling time between loudest and softest bursts
Settle to within the upper threshold level
Loudest
Guard + Preamble (68.8 ns at 2.5Gbps)
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)

8. AC coupling calculations
Assumptions: LSR = 20 dB ER = 10 dB R=50 ohm
Maximum TC change from loud to soft within guard + preamble (< V after 68.8 ns)
Minimum TC* 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
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

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

10. DC coupling – basic designs
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
Feedback control
Pre-amp + -
Peak detector
Feedforward control
Pre-amp + -
Peak detector

11. 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

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

13. Trans- impedance amplifier
Edge detector design
Photodiode
Trans- impedance amplifier
Transient detector
Comparator
+Vcc
Pin
ER D(t) R C
V(t)
Vc(t)
isig + A - +
Vout(t) R
±Vth +
inri
f=RC

14. Edge response vs AC coupling
Tbit t V t
TC>>Tbit
Conventional AC coupling t V
TC<<Tbit

15. Edge detector – impulse characteristics
Use a very short time constant (less than duration of 1 bit) V
Tr(t)
Tf(t)
Vc(t)
Tbit t

16. Requirements
The pulse must exceed the comparator threshold voltage prior to the decision time
The noise present on the signal must not trigger a false reading following the first trigger and prior to the decision time
The statistical noise present on the comparator must be considered for a full analysis – although this is relatively insignificant c.f. amplified RX noise
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)
PE necessarily improves for larger signals – maximum input power limited by TIA overload (typ. Few mA)

17. Noise/sensitivity analysis
tpeak
Vpeak
Vc(tdec)
f(V)
Vth+
Vth- t
tdec Pe

18. Bit error probability calculations
Tdec increasing
Plot of bit error probability against input SNR (Pin/N0) for increasing decision time

19. SNR sensitivity of receiver
Plot of minimum SNR required to attain a bit error probability of 10-4 as Tdec increases

20. 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

21. 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

22. END

23. AC coupling AC coupled front end – summary of the main issues
Advantageous as well established in market
Easiest and cheapest to implement – requires no feedback and decision level setting
Long time constants (in the order of ms) are used in conventional ac coupled receivers (such as SDH) – would therefore exceed the burst duration!
Smaller capacitors on front end will lower the time constant but low freq components of signal will be filtered out – resulting in ISI
Assumes line coding such as 8B/10B to reduce long strings of consecutive identical digits (CIDs) suitable for EPON
Scrambling an alternative to line coding with redundancy but requires longer time constant (and preamble) as protection against long CIDs is only statistical
Long CIDs cause level drifting and CDR malfunction so TC has to be longer than the longest CID acceptable to the CDR
Gig ethernet employs 8B/10B, Ethernet uses Manchester (1B/2B). With 8B10B each ten bits contains either 5 ones and 5 zeroes or six of one and 4 of the other.

24. Design criteria – Timing
Name
Guard time
64 bits
25.6 ns
Preamble
108 bits
43.2 ns
Burst duration (Ethernet type II)
64 to 1518 bytes
204.8 to ns
Duration (bits)
Duration (ns)*
CID immunity
72 bits
28.8 ns
* Assuming 2.5 GBits/s data rate

25. AC coupling – Eye settling
Decision threshold settling
Vhigh
THhigh
THopt
THlow
Vlow
Assuming AWGN limited only then:
BER=0.5*erfc(Q/sqrt(2))
For a minimum BER=1e-4 (Q=3.719)
Therefore THhigh=0.78 (Vhigh-Vlow)

26. AC coupling calculations…conclusions
AC coupling can work under same conditions if for example the CID requirement is reduced from a maximum of 72 bits to a maximum of 12 bits
AC may be useable if CID reduction is insisted – is built into most PHY layer formats, for example GigE employs 8B/10B encoding which ensures no more than max of 6 CIDs
Even well chosen TC will result in some ISI due to the relative filtering effect at lower frequencies
Situation worse for soft to loud case
The calculations are also under ideal conditions, with eye distortion this will be reduced further
Therefore AC coupling problems are intractable without changes to standards!

27. DC coupled front-ends (amplitude recovery)
Advantages:
Overcomes the TC problem of AC coupled RXs (which as seen in above is intractable without changes to standards), employs detection of signal amplitude to determine decision level
Difficulties
DC coupled RXs more complicated than AC coupled receivers - research effort is having difficulty in increasing the speed beyond top EPON rates of 1.25 Gbps (which has >400 ns settling time – not the 68.8 ns of GPON)
Need to reset the threshold level with each packet and reset in less than the guard time period
Decision settling is not assisted by guard duration – must complete within preamble time (43.2 ns at GPON 2.5 Gbps)
Feedback control also necessary to reduce pulse width distortion (PWD) which increases BER due to reduced sampling duration (time setting) and may also lead to CDR malfunction since CDR assumes equal ones/zeroes pulse widths

28. BMR research status
EPON BMRs of up to 1.25 Gbps have been demonstrated using both DC and AC coupled front ends (permissable in EPON due to relaxed CID requirements)
To my knowledge, nobody has yet implemented a burst-mode receiver capable of more than 1.25 Gbps (for PON standards) and I can see no way forward based on conventional AC or DC coupled front ends – although a highly integrated GaAs/SiGe/InP solution may push the boundaries to 2.5 Gbps

29. Signal shape pk-pk pulse amplitude Fall time response of edge detector
Rise time associated with RX bandwidth
Residue term from preceding pulses