OFDM for Next Generation Optical

Access Networks

Neda Cvijetic

NEC Laboratories America

ECOC 2010 Access Workshop

September 19, 2010

Orthogonal Frequency Division Multiplexing (OFDM)

▐ Key idea: use many narrow-band orthogonal subcarriers to transmit

symbols in parallel

▐ By orthogonality spectra may partially overlap for high spectral

efficiency

High resistance to linear dispersion; efficient DSP implementation

Advance modulation can be exploited in high-speed fiber transmission

2

OFDM Sub-carriers

in frequency domainOFDM Sub-carriers

in time domain

……

.

The Rise of Optical OFDM

▐ 2006— present: exponential growth of optical OFDM

publications

▐ OFDM shown superior physical layer performance for next-

generation fiber systems

Linear dispersion tolerance, spectral efficiency, efficient channel

estimation/equalization

▐ However, for 100 Gb/s long-haul transmission

OFDM requires digital transmitter, QPSK does not

Blind equalization simple + sufficiently good (for now…)

Single-carrier QPSK leading candidate for 100 Gb/s long-haul

Why Optical OFDM in Access?

▐ Fiber-to-the-home emerging as future-proof access solution

▐ Bandwidth driver: digital video

▐ Point-to-multipoint passive optical network (PON) expected to play

leading role in next-generation access

▐ Global deployment of Gigabit/Gigabit Ethernet PON (G/GE PON)

▐ Ratification of 10G/GE PON standards

▐ Bandwidth flexibility between users/applications of premium value in

future access

▐ Services include mix of digital, analog, circuit and packet-switched,

legacy and emerging applications

▐ Orthogonal Frequency Division Multiple Access (OFDMA)

OFDMA-PON Technology

▐ Several candidates for future passive optical network (PON)

OFDMA, TDM, WDM, hybrid TDM/WDM…

▐ OFDMA-PON differentiator: tackle key challenges in electronic domain

through digital signal processing (DSP)

▐ Leverage advanced DSP to achieve superior performance, rapid and

robust network re-configurability, cost reduction

▐ OFDMA-PON: novel DSP-based platform for speed, flexibility and cost-

efficiency in future high-speed PON access systems

▐ Critical that future PON technologies be highly cost-efficient to remain

attractive and practical

Re-use existing optical distribution network (ODN)

Upgrade with advanced modulation and digital signal processing (DSP)

OFDM-Based PON for Next Generation Optical Access

6

System is transparent, flexible and extensible to any emerging applications

OFDM sub-carriers become transparent pipes for delivery of arbitrary signals

(e.g. T1/E1, Ethernet, RF mobile backhaul, IPTV, VPN, etc..)

OLT

Business area

Mobile station

Residential area

Splitter ONU 1

ONU 2

ONU 3

OFDMA frame

Fre

qu

en

cy

Time

ONU-3

ONU-2

ONU-1

1

2

3 f

f

11 1

Analog baseband T1/E1 signal

Ethernet packets

Analog wireless RF signal

Ethernet packets3 3 3

11

ONU-1

f

3 3 3

11 1 1 1 1

11 1

t

f

f

Integrated DSP-based transmission and control planes

Bandwidth dynamically assigned to different services in different time slots

OFDM-based PON Flavors for Multi-User Access

▐ OFDMA-PON: Different users assigned different OFDM

subcarriers within one OFDM band of total N subcarriers

▐ OFDMA + TDMA PON: Different users assigned

different OFDM subcarriers and TDM slots within one OFDM

band; 2-dimensional dynamic bandwidth allocation

▐ OFDMA + TDMA + WDMA PON: Different users assigned

different OFDM subcarriers, and TDM slots and WDM

7

Frequency (sub-carriers)

Packet service TDM service

Frequency (sub-carriers)

Packet service TDM service

Frequency (sub-carriers)

Packet service TDM service

Frequency (sub-carriers)

Packet service TDM service

1 2 M…

…

8

Practical OFDM-PON Implementation

▐ Mathematically, each data carrier can be

described as a complex wave

▐ However, through advanced digital

signal processing (DSP), practical

implementation is simple and efficient

through Fast Fourier Transform (FFT)knNj

N

k

pp ekXN

nx )/2(1

0

][1

][

Constellation

mapperIFFT

P/S

& D/A

DC

offsetMZM… .

… .….

DSP OFDM MODULATOR

Laser

OPTICAL

MOD.

data inTo fiber & ODN

Transmitter

Architecture

PD A/Dfrom fiber

DSP OFDM DEMODULATOR data out

Receiver

Architecture

Advanced DSP enables practical, cost-efficient, multi-user implementation

)]([)()(

ttwj

ccccetAts

amplitude

phase

Key Benefits of OFDM-PON Transmission

▐ SPEED & DISTANCE

Up to 100 Gb/s/ downstream transmission

Up to 100 Gb/s/ upstream transmission

Up to 100km reach for PON, 1000km for metro

▐ FLEXIBILITY

Adaptive modulation and FEC on subcarrier basis

Dynamic bandwidth allocation in time and frequency

Transparency to arbitrary services

Optically-transparent ONUs

▐ COST-EFFICIENCY

Colorless architecture

Stable, accurate DSP-based operation

Non-disruptive to legacy ODN

9

Summary of Experimental Demonstrations

▐ (NEC Labs, ECOC 2007): 10 Gb/s Bi-Directional OFDMA-PON

▐ (NEC Labs, OFC 2008): Heterogeneous OFDMA-PON

▐ (NEC Labs, Paper OMV3, OFC 2009): 40 Gb/s OFDMA-PON

▐ (NEC Labs, Paper OTuO7, OFC 2009): 1000km OFDM-DD

Transmission

▐ (NEC Labs, Paper PDPD5, OFC 2009):108 Gb/s OFDMA-PON

▐ (NEC Labs, ECOC 2009): 36 Gb/s/ OFDMA-PON over 100km

▐ (NEC Labs, ECOC 2009 PD 3.3): 108 Gb/s/ Upstream OFDMA-

PON

▐ (NEC Labs, OFC 2010): 44 Gb/s/ Polarization Insensitive

Upstream OFDMA-PON

▐ (NEC Labs, OFC 2010 PDPD9): 41.25 Gb/s Real-Time, Variable-

Rate WDM-OFDMA-PON

“OFDM is a great technology…”

Orthogonal Frequency Division Multiplexing (OFDM) PON

very well-suited for future PON systems

Transparent to emerging heterogeneous applications

Highly-flexible, dynamic bandwidth allocation

Non-disruptive to legacy ODN

Recent demonstrations of ultra high-speed OFDMA PON

Speed: 108 Gb/s/ downstream and upstream

Feasible on class C+ ODN (30+ dB power budget)

Highly dispersion tolerant (60km-100km transmission)

“But it requires advanced Digital

Signal Processing (DSP)”

DSP Component Characteristics

▐ Silicon platform

ADC/DAC + DSP integration, mass production, cost-

efficiency

▐ Low power consumption

65nm, 40nm, 28nm CMOS processes

▐ Cost-efficient packaging options

Component cost profile driven by volume

▐ DSP complexity: IFFT/FFT dominates, ~log(N) scaling

Optimized, readily available algorithms

Technology Trends and Next Steps

▐ DSP-based system cost can be significantly and rapidly reduced

by component integration and mass production

▐ Optimized OFDM algorithms re-used from wireless and

wireline building blocks

▐ Next generation 100 Gb/s long-haul fiber transmission will be

heavily DSP-based

50+ GS/s, 2 channel ADC chips commercially available

10-30GS/s, 2 channel DAC chips commercially available

Intensive on-going effort in parallelized, real-time DSP

architectures

▐ Aggressive 100 Gb/s DSP development for fiber transmission

expected to have favorable effect on ultra high-speed OFDM-

PON

Summary and Conclusions

OFDMA PON very well-suited for high-speed future PON

Transparent to emerging heterogeneous applications

Highly-flexible, dynamic bandwidth allocation

DSP-based for stable, cost-efficient implementation

Key challenges tackled in DSP

2 channel ADC/DAC, DSP processor key components

Favorable technology trends in terms of cost profile

41.25 Gb/s real-time, variable-rate receiver for WDM-OFDMA-PON

demonstrated

Key OFDMA-PON advantages : high flexibility and cost-efficiency

with record transmission rates/distances