NEC Breaks Spectral Efficiency Record of Submarine Optical Transmission

The Japan-based NEC Corporation has demonstrated a groundbreaking transmission capacity of optical fiber. The team was able to transmit 34.9 terabits per second (Tbps) on a single optical fiber over a distance greater than 6,300 kilometers. This innovation will have tremendous applications, especially in the future cross-continent communication using submarine fiber optic cables. NEC’s achievement breaks the spectral efficiency record for transoceanic transmission, achieving 8.3 bit/s/Hz using the C-band spectrum. This is a 16.9% improvement on the previous record. According to NEC, the previous world record was 7.1 bit/s/Hz.

A press release from NEC says the current results come very close to the Shannon limit, the fundamental spectral efficiency limit of optical communications. Maximizing spectral efficiency is one of the primary goals in the design of submarine fiber optic cable networks. Highest possible capacity per fiber and reduction in terminal equipment cost, space, and energy consumption are ensured by maximizing the spectral efficiency. This demonstration of NEC’s technologies comes within 0.5 decibels (dB) of the theoretical maximum value.

Shannon was a researcher in Bell labs who developed a mathematical theory for communication at a time when engineers were attempting to design telephone systems to carry a maximum amount of information and correct the distortions in communication lines. Shannon introduced a measure of the amount of information in a message in order to analyze the transmission quantitatively. Shannon showed that the amount of information is closely related to the chance of one of several messages being transmitted. Shannon introduced the entropy rate to measure the information production rate of the source. Entropy rate is a measure of the information carrying capacity or channel capacity.

Shannon could show that if the entropy rate exceeds the channel capacity there were unavoidable and uncorrectable errors in the transmission. He also showed that if the sender’s entropy rate is below the channel capacity, then there is a way to encode the information so that it can be received without errors. This is true even if the channel distorts the message during transmission.

The General Manager of the Submarine Network Division at NEC Corporation told that the company is proud to have come so close to Shannon’s cornerstone of communication theory and added that NEC’s research and development teams will continue to explore the limits of even greater subsea capacity, flexibility, and cost-effectiveness.

Back in 1975, the first optical fiber communication system operated at a bit rate of 45 Mbps. The current transmission capacity achieved NEC is more than 755,555 times higher than the first optical system. We have traveled a lot from our initial system. That’s quite interesting! Scientists have reported achievement of higher transmission speeds using different fiber types such as multi-core fibers. For example, the National Institute of Information and Communication in Tokyo reported 109 Terabits per second. The team developed a special fiber having seven cores inside a single cladding. Each core of the fiber was able to carry 15.6 terabits per second.

Another record is set by NEC’s Dayou Qian, when he used 370 separate lasers to transmit light pulses at 101.7 terabits per second instead of using multicore fibers. Each laser contained a small packet of information but combined to form a massive amount of data transfer. Dayou used 165 kilometers of optical fiber to demonstrate optical transfer rate.

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