PAM8 & FEC Options
Vipul Bhatt – Cisco
Sudeep Bhoja – Inphi
Arash Farhood – Cortina
Gary Nicholl – Cisco
IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force, Nov 2012
2
Supporters
Brain Welch, Luxtera
Mark Nowell, Cisco
Beck Mason, JDSU
3
PAM8 Introduction
http://www.ieee802.org/3/100GNGOPTX/public/jul12/schell_01_0712_optx.pdf
10 GBaud
28 GBaud 32 Gbaud, 8 PAM electrical eye, using DAC
http://www.ieee802.org/3/bm/public/sep12/lewis_01_0912_optx.pdf
• PAM-8 measurements results have been presented at .bm
• EML is used as light source and external modulator
• In this presentation, we propose a mapping and FEC options for PAM8
Follow on to presentation from bhatt_01a_0912
4
Option 1: PAM8 Coded Modulation
000 001 010 011 100 101 110 111
• PAM8 is partitioned into 2 subsets, Blue & Green
• High rate code is applied on the 2 MSB’s.
.bj RS FEC for 2 out of 3 bits. Throughput 76Gb/s
Lower power than .bj FEC running at 100Gb/s
2 MSB’s gain 6dB in distance after set partitioning.
.bj FEC provides additional 5.7dB in addition to 6dB set
partitioning gain
• LSB is coded with a low rate strong hard decision FEC
11.2dB FEC code is applied at 38Gb/s
5
PAM8 Coded Modulation Example
PAM8
Mapping
Strong FEC
R = 0.68
38.16GBaud
• Combined rate: (2 * 514/528 + 0.6836) / 3 = 0.8769
• Ethernet Rate = 100/3 * 257/256 * 1/0.8769 = 38.16 GBaud
LSB
RS(528, 514)
802.3bj 2 MSBs
6
Option 1: Coded Modulation Sim Results
16 18 20 22 24 26 28 30 3210
-18
10-16
10-14
10-12
10-10
10-8
10-6
10-4
10-2
SNR
BE
R
PAM8 Coded Modulation
PAM8 Uncoded
PAM4 Uncoded
PAM4 with .bj RS FEC
• PAM8 coded modulation FEC delivers 11.2dB coding gain
11.2dB
7
Option 2: PAM8 with low latency
PAM8
Gray
Mapping
BCH(3018, 2646, t=31)
3 38.17GBaud
• Gray Mapping
• Total FEC latency is 115ns. BCH targets 2E-3 input BER
• Ethernet Rate = 3018 / 2646 * 100 * 257/256 / 3 = 38.17G
8
Option 2: PAM8 with BCH
20 22 24 26 28 30 3210
-18
10-16
10-14
10-12
10-10
10-8
10-6
10-4
10-2
Slicer SNR (dB)
BE
R
PAM8 Uncoded
PAM8 with t = 31 BCH
1E-15
• PAM8 with BCH FEC delivers 9.2dB coding gain
9
PAM8 Slicer SNR Summary
PAM8 Option1
(Coded Mod)
PAM8 Option 2
with BCH
PAM4 with .bj
RS
Baud rate 38.16G 38.17G 51.56G
FEC Target BERi
for 1e-15 BERo 9E-3 2E-3 2.2E-5
FEC Latency 600ns 115ns 100ns
Ideal Required
Slicer SNR 19.9dB 21.9dB 19.1dB
Relative SNR
penalty -0.8dB -2.8dB Ref
Relative Noise
BW penalty Ref Ref -1.27dB
Total Penalty -0.8dB -2.8dB -1.27dB
Relative Margin Ref Ref-2dB Ref-0.47dB
10
PAM Options Comparison
PAM8 Coded
Modulation
PAM8 BCH PAM4 .bj RS
Baud rate 38.16G 38.17G 51.56G
FEC Target BERi for
1e-15 BERo 9E-3 2E-3 2.2E-5
Latency Target <600ns <115ns <100ns
Coding Overhead 14.04% 14.04%
2.72%
Mapping / Coding gain 11.2dB 9.2dB 5.8dB
11
Aug, 2012
12
Simulation Parameters
Tx_RJ = 150fS
Tx_BW = 22GHz
ER = 6dB
Tx_SNR = 30dB
RIN = -144 dB/Hz
Resp = 0.85 A/W
NEP = 15 pA/sqrt(Hz)
Rx_BW = 18GHz
Rx_RJ = 150fS
OMA = 0dBm
PJ = 2ps @ 100MHz
13
PAM8 Simulation Results
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 16
7
8
9
10
11
12
13
14
OMA [dBm]
Segm
ent
SN
R (
dB
)
________________________________________________________________________________________________________________________________________
Segment SNR is defined in Backup
14
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1
10-4
10-3
10-2
10-1
TP3 OMA [dBm]
BE
R
PAM8 Simulation Results
Allocate 1dB MPI penalty allocation for -35dB RL (See farhood_01_1112_optx)
3dB Optical margin for Option 1 Coded Modulation FEC
1.5dB Optical margin for Option 2 BCH FEC
Option 1: Coded Modulation
Option 2: BCH
15
Summary
We have proposed two options for coding and mapping of 38G PAM8
PAM8 coded modulation with 600ns latency solution provides a robust optical link with 3dB margin
PAM8 and BCH with 115ns latency offers a lower latency solution with 1.5dB margin
We have a high degree of confidence in technical feasibility for -35dB Return Loss cable plant
16
Backup
17 17
SNR Channel Model
FEC
Encoder
Laser
Optical
Modulator Driver
PIN
TIA CDR FEC
Decoder
Data Encoded
Data Fiber
FEC
Encoder
Driver &
Optical
Modulator Encoded
Data
h(t) R |E-field |2 + +
h(-t) FEC
Decoder
PIN
N(t)
y(t)
y(t) _
E field,
Optical power = |E-field|2
= {0, 1, …, M-1}D
TIA output levels
= {-(M-1)/2, -(M-3)/2, …, -1/2,
1/2,…(M-1)/2} RD
TIA
CDR
Current levels
= {0, 1, …, M-1}RD
Fiber Data
Physical Channel
Equivalent AWGN Channel Model
- +
18 18
General Case SNR
N(t) is a white Gaussian random process which has both signal independent thermal noise
and signal dependent components due to laser RIN and shot noise. For optimum receiver
thresholds, the symbol error probability is:
DD
D
D
2
0
2102
2
@,
@,
,
1
)(1
1
2
)()(
bandwidth)Rx W ()102(
,
)1(
12
)12)(1(
6
1
2
2-M0,...,k),(/ R Q(k)
M
k
s
RIN
Thk
TIAoutputelectricalavg
TIAinputelectricalavg
opticalavg
kk
kPM
P
kQerfckP
WRkqRkN
M
P
MM
P
M
RP
R
A single sigma is not valid
Each individual level has its own noise variance
Define Segment SNR for each eye, Q(k)
-0.5 0 0.5-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Time (UI)
Am
plit
ude
PAM4 @50G with BT4 25G BW
0
1
2
3
DR
DR
DR