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