Deutsche Telekom Partners with ZTE for Ultra 100G Trial

Background

Deutsche Telekom AG is the largest telecommunications carrier in Europe and the fourth largest telecommunications carrier in the world. In June 2010, DT established the world’s first commercial 100 Gbit/s optical network connecting a high-performance data center at Technische Universität Dresden to another at Technische Universität Bergakademie, Freiberg. As the internet and terminal data services evolve, DT expects that increased backbone network bandwidth will create strong demand for ultra 100G technologies after 2014. 

DT is eager to obtain research and experimental data about ultra 100G technologies. This will allow them to determine the development direction of transmission technologies and to deploy ultra 100G systems by using appropriate technologies. The company will be able to cement its leadership of the industry.

For years, ZTE has been committed to researching and developing 100 Gbit/s, 400 Gbit/s, and 1 Tbit/s technologies and has made important breakthroughs in these technologies. ZTE has achieved a single-channel transmission rate of 11.2 Tbit/s, which eclipsed the previous world record of 1 Tbit/s for single-channel transmission. The optical signals were transmitted over 640 km standard single-mode fiber (SSMF). ZTE also achieved a signal transmission rate of 24 Tbit/s (24 × 1.3 Tbit/s) using wavelength-division multiplexing. 

In February 2012, a team of experts from ZTE and DT conducted an ultra 100G trial.

Process and Results

The trial was based on standard single-mode fibers (G.652 SSMF) that already connected eight densely populated cities—including Darmstadt, Stuttgart, and Nuremberg—in the south of Germany. To test the long-haul transmission capabilities of different technologies, laboratory SSMF was also used to extend the optical transmission distance.

The lowest fiber span loss was 20 dB, and the highest fiber span loss was 24.1 dB (Fig. 1). Eight 100 km G.652 fiber spans were added in Stuttgart, extending the total transmission distance to 1750 km. A commercial flat-gain in-line erbium-doped fiber amplifier (EDFA) was used to compensate for the fiber span loss, and no other gain equalizers were used. The average fiber loss for all 22 spans was 21.6 dB. Transmitters and receivers were installed at the test center, which was located in DT’s Darmstadt R&D center.

ZTE conducted two experiments on the trial network.

The first experiment was designed to test long-haul transmission of eight 216.8 Gbit/s PDM-CSRZ-QPSK signals with 50 GHz spacing (Fig. 2).

In the signal system, eight tunable lasers were used as light sources. The eight lasers were divided into two groups of four: odd and even. The lasers in each group were spaced at 50 GHz, and the spacing between the groups was 100 GHz. Optical signals generated by the lasers in each group were coupled by an optical coupler before being modulated by an I/Q polarization-multiplexed modulator. The modulator was driven by two 54.2 Gbit/s binary signals. Then, the optical signals underwent carrier-suppressed return-to-zero (CSRZ) modulation before being sent to the polarization multiplexer and EDFA. Finally, the optical signals of the two groups met in the WSS for coupling and filtering before being transmitted to the line.

A digital post-filter was introduced into digital signal processing (DSP) at the receiver in order to convert binary signals into duobinary signals. This was necessary to reduce noise and crosstalk between channels and to enhance the performance of 1 bit maximum likelihood sequence estimation (MLSE) DSP.

The signals had a record spectrum efficiency of 4 bit/s/Hz. After they were transmitted across more than 22 SSMF spans over 1750 km, the bit error rates ( BERs) of all signals was less than 3.8 × 10^-3, which is lower than the forward error correction (FEC) threshold. This experiment proved that baud rate and channel capacity can be doubled and ultralong-haul transmission can be achieved by using polarization-multiplexed quadrature phase-shift keying (QPSK) with 50 GHz spacing.

The second experiment was designed to test hybrid transmission at 100 Gbit/s, 400 Gbit/s, and 1 Tbit/s and to demonstrate ZTE’s 400G and 1T transmission capability in the DT laboratory. 

In this experiment, Nyquist-WDM was used to generate a 400 Gbit/s super channel using four 112 Gbit/s PDM-QPSK signals that were multiplexed after filtering. A 1 Tbit/s channel with more than 13 subchannels was created using optical orthogonal frequency division multiplexing (OFDM). Each subchannel occupied 25 GHz, and the total signal bandwidth was 325 GHz.

Using these two superchannels and ZTE’s two commercial 100G line cards, hybrid transmission was possible. After the signals were transmitted over 1750 km, the BERs of all signals was less than 2 × 10^-3. When the length of an optical fiber was extended to 2450 km using a commercial 100G line card, the BER was 1.1 × 10^-3. This indicated that there was still considerable room for improvement.

This experiment proved that SD-FEC 100G technology can be used in ultralong-haul transmission, and Nyquist-WDM is feasible for strengthening ultra 100G channel capacity in long-haul transmission. In addition, ZTE’s 400G and 1T technologies are compatible with the original commercial 100G technologies.

Significance of the Field Trial

This trial was a significant milestone in the development of 400G and 1T transmission technologies. It demonstrated ZTE’s ability to design 100G, 400G, and 1T transmission systems. Because these technologies are compatible and scalable, they can help expand network capacity, reduce the cost per bit, and reduce difficulties in implementation. The field trial also showed ZTE’s strength in helping network carriers address exponential increase in backbone network traffic that has arisen as a result of a boom in data services.

ZTE and DT have cooperated closely on the implementation and development of ultra 100G technologies and have laid a solid foundation for future exchanges.