burst-mode receivers for gpon and lrpon applications j.j. lepley and s.d. walker university of essex

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


Overview

Burst Mode Receiver for LR-PON

AC coupling

DC coupling

Edge detection


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


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


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


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


Overview


AC coupling

DC coupling

Edge detection


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


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


Overview


AC coupling

DC coupling

Edge/Impulse detection


t

V Tbit

0

Edge response vs AC coupling

V

t0

TC>>Tbit

t

V

0TC<<Tbit

Conventional AC coupling


V

0tTbit

Vc(t)

Tr(t)Tf(t)

Edge detector – impulse characteristics

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


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)


t

tpeak

Vth+

Vth-

V

Vpeak

Vc(tdec)

tdec

f(V)

Pe

Noise/sensitivity analysis


Bit error probability calculations

Tdec increasing

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


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


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


END

burst-mode receivers for gpon and lrpon applications j.j. lepley and s.d. walker university of essex

Documents

noc conference

gbps slide

ns cid

ns preamble

tc slide

threshold detection

ns of gpon

preamble time