Ultrahigh-speed Transmission Technology for Future High-capacity Transport Networks

1. High-capacity transport networks and applications of ultrahigh-speed communications

Common broadband network services such as video streaming and electronic commerce (e-commerce) are now available all over the world via personal computers (PCs) and smartphones in daily life. The novel transmission technologies have thus become essential for future network infrastructure in order to achieve further network service evolutions. The fifth-generation (5G) mobile communications service will start in fiscal year 2019, and broadband communications at a line rate up to 20 Gbit/s with low latency will be expected to support emerging new services such as self-driving vehicles and factory automation. Furthermore, it is expected that the application of new technologies such as machine learning and artificial intelligence will facilitate the emergence of new applications such as detailed weather forecasting and preventive medicine, thanks to the Internet of Things technology achieved through recent advances in cost-effective low-power semiconductor integrated circuits used in sensor technology. We believe that future transport networks will have to support the creation of such new application services supporting our daily life.

The application area of ultrahigh-speed transmission technology in today’s high-capacity transport networks is shown in Fig. 1. In radio transmission, significant advances in digital modulation/demodulation techniques have been made in fixed microwave transmission systems, and these had been used to support long-haul core networks until the commercial installation of optical fiber transmission systems began in the 1980s [1]. Further advances have been made in these fixed microwave transmission system technologies to enable economical link systems in areas where it is difficult to install a wired cable transport system.

Fig. 1. Application area of ultrahigh-speed transmission technology.

In addition, dramatic technological advances have been made in mobile communications, achieving high-speed wireless local area networks and mobile phones in the last quarter of a century. Today, wireless access services using PCs and smartphones (4G) with a throughput of at least 1 Gbit/s have spread throughout the world. Furthermore, next-generation 5G systems will offer faster wireless access with the high reliability and low latency required for self-driving vehicles and factory automation.

An ultrahigh-speed optical fiber transmission system was first installed in NTT’s core network in the 1980s, and remarkable progress has been made over the last 40 years. Today, optical fiber is widely used for transoceanic systems over 10,000 km, long-haul national backbone networks, metropolitan networks, and access networks. Optical fiber transmission systems have also become indispensable recently for increasing the capacity of networks such as datacenter (DC) networks and mobile backhauls. Optical fiber transmission systems have so far mostly used single-mode fiber (SMF) as a transmission medium. SMF is designed to have only one optical waveguide (core) per fiber supporting a single waveguide mode. Recent long-haul core networks support a capacity in excess of 10 Tbit/s per fiber by using 100-channel wavelength division multiplexing (WDM) of 100-Gbit/s channels [2]. Furthermore, low-power optical communications with a compact configuration and a channel capacity over 200 Gbit/s have recently been introduced in DC interconnection networks [3].

2. Technical challenges in ultrahigh-speed communications technology

This section describes the common technical challenges in introducing ultrahigh-speed communications technologies in both radio transmission systems and optical fiber transmission systems. The system capacity C of a communications system is generally expressed by the following equation according to Shannon’s well-known theorem:

C = N • B • log (1 + SNR),

where N represents the number of multiplexed channels, B represents the signal bandwidth, and SNR represents the signal-to-noise ratio. There are three major approaches to improving the system capacity C of a communications system as expressed by this equation:

To increase the channel speed in both wireless and optical fiber communications, it is very important to choose the right combination of the feasible abovementioned approaches considering their application area and the maturity of the technologies.

The future technical trends of the abovementioned approaches in line-of-sight millimeter-wave radio transmission and optical fiber transmission are shown in Fig. 2. In the case of using single-channel polarization-division multiplexed (PDM) quadrature phase-shift keying (QPSK) (Approach 1 only) to realize channel capacities over 1 Tbit/s, it is necessary to achieve a symbol rate of at least 300 Gbaud. However, it is very difficult to achieve such a high symbol rate over 300 Gbaud using the latest front-end devices and digital signal processing (DSP) circuits with analog-to-digital converter (ADC) circuits and digital-to-analog converter (DAC) circuits. We therefore need to reduce the required symbol rate by adopting higher-order QAM and/or by using subcarrier multiplexing in the frequency or spatial domain.

Fig. 2. Three approaches in ultrahigh-speed communications.

An example of the latest digital coherent optical transceiver circuit configuration in an optical fiber transmission system is illustrated in Fig. 3 [2, 3]. In this system, each channel is modulated in PDM-QPSK format, and approximately 100 channels of 100-Gbit/s optical signals are wavelength-division-multiplexed to form a 10-Tbit/s aggregate capacity signal and transmitted through an optical inline amplified link over 1000 km.

Fig. 3. Example configuration of digital coherent optical transceiver circuit in an optical fiber transport system.

Here, the optical transmitter consists of a transmitter DSP circuit and an optical transmitter front-end circuit. Each polarization component from a continuous-wave signal laser diode (LD) is independently modulated in QPSK format at a symbol rate of the order of 32 Gbaud, and 100-Gbit/s PDM-QPSK signals (approximately 64 Gbit/s per polarization in gross capacity with error-correcting coding at a coding rate of R = 5/6) can be generated. If the modulation scheme is adaptively changed to 16QAM, it is possible to double the channel capacity to 200 Gbit/s at the same symbol rate.

Similarly, the optical receiver consists of a receiver DSP circuit and an optical receiver front-end circuit. An intradyne reception of PDM-QPSK signals is performed using a wavelength-tunable local oscillating LD that has a frequency offset within a few gigahertz. Four lanes of received electrical tributary signals are fed to ADCs, and linear waveform distortions of transmitted signals after the optical fiber transmission are digitally equalized to demodulate the QPSK (or 16QAM) signals. Finally, the error correcting code recovers the original data bit stream. In this case, the throughput in the DSP circuit exceeds 2 Tbit/s (≈8×2×(6/5)×100 Gbit/s) between the DSP circuit part and the DAC/ADC circuit part, when the channel capacity is 100 Gbit/s assuming 8-bit quantization, a sampling rate of two samples per symbol, and an error correction coding rate of R = 5/6.

In order to implement a compact and cost-effective DSP circuit, it is preferable to integrate the DAC/ADC circuit part and the DSP circuit part on a single-chip large-scale integrated (LSI) circuit. Furthermore, it is essential to suppress the skew of the four parallel broadband analog electrical signals (in-phase and quadrature signal components for each of the X- and Y-polarization planes) between the DSP-ASIC (application-specific integrated circuit) and optical front-end circuits for high-quality digital modulation and demodulation.

3. Fundamental technologies for ultrahigh-speed communications infrastructure

The Feature Articles in this issue focus on the four fundamental technologies of ultrahigh-speed transmission to achieve a capacity of 1 Tbit/s (Fig. 4), and they introduce the current status and future prospects of these technologies.

Fig. 4. Key technologies to achieve future ultrahigh speeds for millimeter-wave radio transmission and optical fiber transmission.

4. Future prospects

This article introduced the latest ultrahigh-speed communications technologies and future trends to realize a channel capacity over 1 Tbit/s in future transport networks. Further advances are expected in highly efficient DSP technologies for ultrahigh-speed channel transmission with careful consideration of practical DSP complexity economically implemented in single-chip LSI circuits. In millimeter-wave communications, a new degree of freedom offered by the OAM of electromagnetic waves in free space is expected to increase the channel speeds to over 1 Tbit/s. In optical fiber communications, long-haul transmission of high-speed channels beyond 1 Tbit/s will be achieved by using a suitable combination of advanced DSP functions and integrated optical front-end technology. Optical parametric amplifier technology and spatial multiplexing technology are promising for achieving further increases in the channel speed while maintaining the current DSP complexity and power consumption in optical fiber communications. Through continuous efforts in research and development and timely commercialization of these new technologies, terabit-per-second-class signals will be able to be easily handled in future transport networks.

References

[1]	K. Kohiyama and S. Komaki, “A 64/256QAM Digital Microwave Transmission System,” J. IEICE, Vol. 68, No. 8, pp. 889–895, 1985 (in Japanese).
[2]	S. Suzuki, Y. Miyamoto, T. Tomizawa, T. Sakano, K. Murata, S. Mino, A. Shibayama, M. Shibutani, K. Fukuchi, H. Onaka, T. Hoshida, K. Komaki, T. Mizuochi, K. Kubo, Y. Miyata, and Y. Kamio, “R&D on the Digital Coherent Signal Processing Technology for Large-capacity Optical Communication Networks,” J. IEICE, Vol. 95, No. 12, pp. 1100–1116, 2012 (in Japanese).
[3]	Y. Kisaka, M. Tomizawa, and Y. Miyamoto, “Digital Signal Processing (DSP) Circuits for Optical Transport Beyond 100G,” NTT Technical Review, Vol. 14, No. 9, 2016. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201609fa2.html
[4]	T. Kobayashi, F. Hamaoka, M. Nakamura, H. Yamazaki, M. Nagatani, and Y. Miyamoto, “Ultrahigh-speed Optical Communications Technology Combining Digital Signal Processing and Circuit Technology,” NTT Technical Review, Vol. 17, No. 5, pp. 12–19, 2019. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201905fa2.html
[5]	T. Umeki, T. Kazama, T. Kobayashi, K. Enbutsu, R. Kasahara, and Y. Miyamoto, “Low-noise Amplification and Nonlinearity Mitigation Based on Parametric Repeater Technology,” NTT Technical Review, Vol. 17, No. 5, pp. 20–26, 2019. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201905fa3.html
[6]	M. Nagatani, H. Wakita, Y. Ogiso, H. Yamazaki, M. Ida, and H. Nosaka, “Ultrahigh-speed Optical Front-end Device Technology for Beyond-100-GBaud Optical Transmission Systems,” NTT Technical Review, Vol. 17, No. 5, pp. 27–33, 2019. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201905fa4.html
[7]	D. Lee, H. Sasaki, Y. Yagi, T. Yamada, T. Kaho, and H. Hamada, “Toward Terabit-class Wireless Transmission: OAM Multiplexing Technology,” NTT Technical Review, Vol. 17, No. 5, pp. 34–39, 2019. https://www.ntt-review.jp/archive/ntttechnical.php?contents=ntr201905fa5.html
[8]	K. Shibahara, T. Mizuno, D. Lee, Y. Miyamoto, H. Ono, K. Nakajima, S. Saitoh, K. Takenaga, and K. Satoh, “DMD-unmanaged Long-haul SDM Transmission over 2500-km 12-core × 3-mode MC-FMF and 6300-km 3-mode FMF Employing Intermodal Interference Cancelling Technique,” Proc. of the 41st Optical Fiber Communication Conference and Exhibition (OFC 2018), Th4C.6, San Diego, CA, USA, Mar. 2018.
[9]	D. Lee, H. Sasaki, H, Fukumoto, Y. Yagi, T. Kaho, H. Shiba, and T. Shimizu, “An Experimental Demonstration of 28 GHz Band Wireless OAM-MIMO (Orbital Angular Momentum Multi-input and Multi-output) Multiplexing,” Proc. of the 2018 IEEE 87th Vehicular Technology Conference (VTC 2018-Spring), Porto, Portugal, June 2018.