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

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

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

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

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 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)

AC coupling calculations Assumptions: LSR = 20 dB ER = 10 dB R=50 ohm Maximum TC change from loud to soft within guard + preamble (<0.01458 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 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

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

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

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

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

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

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

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

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)

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

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

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

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

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.

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 4857.6 ns Duration (bits) Duration (ns)* CID immunity 72 bits 28.8 ns * Assuming 2.5 GBits/s data rate

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)

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!

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

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

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