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Low-cost serial 40-GbE over 40 km SMF


Title: Long-range 40-GbE featuring full-field EDC
 

 
Start date: April 2011
Client: Tyndall National Institute
Investigator Dr David Nugent
david.nugent@elucidare.co.uk

 


Introduction

Researchers at the Photonic Systems Group, Tyndall National Institute and the Department of Physics, University College Cork, have developed technique for transmitting serial 40-Gb/s signals inexpensively over metropolitan distances. Simulations have demonstrated chromatic dispersion (CD) tolerance of ±800 ps/nm at 18 dB OSNR, sufficient for 130 km SMF transmission at 1330 nm, or 45 km at 1550 nm. We expect demand for long-range serial 40-GbE will increase following the recent ratification of the 802.3bg standard, which defines serial 40-Gb/s transmission over a 2 km range.


Technology

Similar to previous studies, the serial 40-Gb/s technique uses a single asymmetric Mach-Zehnder interferometer and two single photodiodes to extract both the optical amplitude and instantaneous frequency information. This information is used to reconstruct the full-field representation of the distorted signal. A novel multi-level phase-modulated format based on full-field reconstruction and multi-chip (MC) differential detection plus 16-state maximum likelihood sequence estimation (MLSE) techniques are used to recover the data bits.

Multi-chip deconvolution leverages the phase coherence between consecutive symbols to improve the performance of subsequent soft decision circuits such as MLSE. Originally applied to DPSK demodulation, multi-chip processes are comparable to the multi-symbol techniques already exploited in the wireless arena.

The marriage of full-field reconstruction with MC-MLSE enables Tyndally to achieve the same information spectral densty (ISD) and improved chromatic dispersion range as conventional DQPSK, but with simpler transmitter pre-coding, a single asymmetric Mach-Zehnder interferometer and two independent photodiodes. Consequently this solution can address the unfulfilled requirements of long-range serial 40-GbE whilst honoring the low-cost demands of campus and metropolitan network operators.

A conference paper is available which describes Offset-DQPSK in more detail.


Addressable market

The 40-GbE market is served by two complementary IEEE standards. Recently ratified 40GBASE-FR specifies a physical medium dependent (PMD) sublayer for serial 40-Gb/s operation over up to 2 km of single-mode fiber. The transmitter wavelength is 1530 - 1565 nm, whilst two wavelength windows are supported at 1290 - 1330 nm and 1530 - 1565 nm. Secondly the 40GBASE-LR4 standard is based on four different CWDM wavelengths in the 1300 nm window, each transmitting at 10.3-Gbps and supporting a maximum distance of 10 km over standard single-mode fiber.

Although well suited to campus and data centre applications, 40GBASE-FR standard satisfies only a small portion of the metropolitan area network market. According to China Telecom, 2 km covers only 7.3 per cent of its inter-site connections; whereas 10 km would cover 69.5 per cent and 20 km would extend to 95.1 per cent.

Figure 1: Cumulative distribution of inter-site span lengths in China Telecom network

Source: China Telecom. 40-Gb/s Ethernet Single-mode Fibre PMD Task Force

 

Whilst 40GBASE-LR4 can operate up to 10 km SMF, it is generally agreed that the price of serial 40-GbE will drop below below CDWM during 2013. Herein lays the opportunity to Tyndall to develop a long-range 40-GbE solution based on low-cost direct detection and established EDC methodologies.

Table 1: Comparison of CDWM versus serial 40-GbE

 

CDWM (4 x 10 GbE)

Serial (1 x 40 GbE)

Cost

Cost is lower initially. Leverages 10G technology

Cost can be lowered by adoption of more advanced technologies. Could ultimately be the lower cost solution.

Size/power Feasible to implement Feasible to implement
Technical risks Very low for CWDM. Serial requires risky technical development to achieve cost projections  
Link budget Technically feasible Technically feasible
Operational issues CDWM complicates operational issues due to monitoring >1 channel in link Consistent with traditional operational procedures and tools
Market requirements Low cost is required quickly to enable market. Higher costs will delay or skip adoption  
Source: Based on http://grouper.ieee.org/groups/802/3/ba/public/sep08/nowell_01_0908.pdf

 


Documents available for download

ECOC 2011 paper "Full-Field Detection Based Multi-Chip MLSE for Offset-DQPSK Modulation Format"

Click here