Increasing the capacity of optical transmission

A multilevel modulation format developed by University of Nottingham Malaysia Campus researchers shows clear advantage in achieving longer fibre-transmission distances.

One particular issue in optical fibre networks is that of chromatic dispersion, where the optical pulse lengthens as it travels down the fibre. Pulses launched too close together can then interfere with each other, with a resulting loss in information.

High-speed communications are the backbone of the modern technological age, with the ability to transfer data using optical fibres often forming the skeleton upon which these systems are built. Increasingly, metropolitan or ‘campus’ networks carry more traffic, with a CISCO report estimating that these networks will account for 62% of all IP traffic by 2018.

Often, the enterprise, university or government operating these networks almost entirely own their infrastructure. Given the constraints on the cost of operating and replacing this equipment, something of an ‘arms race’ is developing in the optical communications world to provide the fastest, cheapest data transmission.

The very simplest form of fibre-optic communication, ‘on-off keying’, allows the transmission of data over long distances via a series of binary ‘1’s and ‘0’s created by the presence or absence of a high-frequency electromagnetic wave called a ‘carrier wave’. The process is analogous to the use of Morse code over radio
frequencies.

The rate at which this carrier wave is changed is known as its modulation. When viewed visually, the example above would look like a square wave as the signal changes between on and off. This form of modulation only allows one data stream to be carried at a time.

Research has focused on systems that use pulse-amplitude modulation (PAM) to encode information by placing the information along the rising and descending crest, or amplitude, of regularly timed electromagnetic pulses. PAM allows different data streams to be encoded in a single data stream, similar to how the bass and lead guitars combine different signals at the same time in a song. This system does present some difficulties, with signal noise (erroneous signals picked up) requiring sensitive receivers to regenerate the data without error at the end of the optical fibre.

Recently, a modulation known as eight-level PAM (8-PAM) attracted considerable interest, as it was hoped it would triple data rates. Now, researchers from the University of Nottingham Malaysia Campus (UNMC), led by Professor Amin Malek Mohammadi, have proposed a modulation format known as absolute added correlative coding (AACC) that is proving to be more robust against long-distance fibre transmission than the leading PAM methods.

One particular issue in optical fibre networks is that of chromatic dispersion, where the optical pulse lengthens as it travels down the fibre. Pulses launched too close together can then interfere with each other, with a resulting loss in information.

Professor Mohammadi’s AACC method was less prone to chromatic dispersion when compared with 8-PAM. In comparison to 4-PAM, AACC demonstrated around 7.8 decibels improvement in terms of the minimum power the receiver needs to regenerate the data without error, called receiver sensitivity.

Such an improvement in the receiver sensitivity in 100 gigabyte/second optical systems would lead to cheaper transmission components and reducing costs.

The UNMC team continues to develop the AACC modulation in the hope of improving and further combining it into other existing technologies. The team’s ultimate goal is to increase the capacity of existing optical transmission systems by at least a factor of three.

------------------------------------
Did you know?
Global Internet traffic in 2018 will be 64 times the volume of the entire global Internet in 2005.
------------------------------------

Further information
Professor Amin Malek Mohammadi | E-mail: [email protected]
Faculty of Engineering
The University of Nottingham Malaysia Campus

Published: 03 May 2017

Contact details:

The University of Nottingham Malaysia Campus Jalan Broga, 43500 Semenyih Selangor Darul Ehsan Malaysia

+6(03) 8924 8778 (International) or +6(03) 8924 8000 (Malaysia)
Country: 
News topics: 
Academic discipline: 
Websites: