Feature Articles: All-Photonics Network for Enabling Innovative Optical and Wireless Network (IOWN)

Efficiently Accommodating High-frequency-band Wireless Systems by Using Analog Radio-over-fiber

Kota Ito, Mizuki Suga, Yushi Shirato, Naoki Kita, and
Takeshi Onizawa

Abstract

Large-capacity wireless transmission is possible with high-frequency-band wireless systems, but it is necessary to deploy wireless base stations at high density. Under the assumption that the number of wireless systems will increase to meet diversifying needs, the number of wireless base stations to be installed is expected to increase dramatically. At NTT Access Network Service Systems Laboratories, we propose a system configuration that uses analog radio-over-fiber (RoF) technology to enable multiple high-frequency-band wireless systems to share wireless equipment for drastically reducing the number of wireless base stations and their operating loads. This system configuration and its fundamental technology, namely, our remote-beamforming technology, are introduced in this article.

Keywords: analog RoF, beamforming, efficiently accommodate

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1. Introduction

To further expand wireless-transmission capacity, it is effective to use radio waves in the high-frequency band (such as millimeter waves*1), which can secure a wide bandwidth. However, the propagation distance of radio waves becomes shorter as their frequency becomes higher. Therefore, to cover a wide area with a high-frequency-band wireless system, it is necessary to install wireless base stations with high density. In the past, wireless base stations had to be installed for each wireless system. Accordingly, as high-frequency-band wireless systems diversify to meet diversifying needs, an enormous number of wireless base stations will need to be installed. To drastically reduce the number of wireless base stations to be installed and effectively manage their operation, we propose a system configuration that allows multiple wireless systems to share a wireless base station. This system configuration and our remote-beamforming technology—which is indispensable when accommodating high-frequency-band wireless systems with this system configuration—are introduced in this article.

*1 Millimeter wave: A radio (wireless) wave with a very short wavelength of 1 to 10 mm and frequency of 30 to 300 GHz.

2. Separation of functions and simplification of base stations by using analog RoF

Analog radio-over-fiber (RoF)*2 technology modulates the intensity of an optical signal with a wireless signal and transmits the optical signal in the form of a wireless signal via an optical fiber. The transmitted optical signal is then subjected to optical-to-electrical (O/E) conversion*3 to extract the original wireless signal (Fig. 1).


Fig. 1. Analog RoF.

By applying analog RoF, the functions of a conventional wireless base station can be separated into two locations, i.e., a central station (accommodating the signal-processing unit) and the base station (accommodating the antenna unit) (Fig. 2). Conventional wireless base stations have multiple pieces of equipment and functions: antenna, amplifier, electrical-to-optical (E/O) and O/E conversion, and signal processing. By applying analog RoF to consolidate signal processing at the central station, it is possible to simplify the functions of the base station. As a result of such consolidation, installation flexibility and economic efficiency will improve by reducing the size and power consumption of base stations.


Fig. 2. Function consolidation by using analog RoF.

By having only signal processing (which depends on the wireless system) be in the central station, the common functions that do not depend on the wireless system can be left at the base station. Therefore, it is possible for multiple wireless systems to share a base station as long as its frequency range is compatible with the station’s antenna and amplifier. New wireless systems can be installed or renewed, and operations can be carried out on the central-station side only; thus, efficient deployment and operation of wireless systems will become possible. It is also expected that the number of wireless base stations and operational costs can be drastically reduced.

*2 RoF: A technology for transmitting the waveform information of wireless signals via optical fiber. Analog RoF converts the waveform into an analog signal as is, and digital RoF converts the waveform to a digital signal before transmitting via optical fiber. Compared to digital RoF, analog RoF does not require A/D (analog-to-digital) or D/A (digital-to-analog) conversion, and the required optical transmission bandwidth can be narrowed.
*3 O/E conversion: A photodiode is generally used to convert an optical signal into an electrical signal.

3. Remote-beamforming technology

Beamforming*4 is an essential technology for high-frequency-band wireless systems—which can only achieve short propagation distances. For conventional wireless base stations, beamforming is executed by their signal-processing units. When functions are separated and base stations are simplified by applying analog RoF, the problem of how to carry out beamforming at base stations that do not have signal-processing units arises. Therefore, we previously proposed and have been investigating a technology called remote beamforming that can remotely control the beam formed at the base station from the central station [1, 2].

Our remote-beamforming technology is explained with the example of using a receiver in Fig. 3. When a wireless signal arrives at a base station with multiple antenna elements, each element receives a wireless signal with a phase difference. While this phase difference is maintained, the wireless signals received by each antenna element are converted into optical signals with different wavelengths and transmitted to the central station via optical fiber by wavelength-division multiplexing (WDM). The central station de-multiplexes the wavelength-multiplexed signals with each wavelength, adjusts the phases of these optical signals, converts the optical signals to electrical ones (O/E conversion), and combines the electrical signals. The original wireless signals are then combined in a state of matching phases and strengthened to form a receiving beam in the direction of the arrival of the wireless signals. Although the optical signals are phase-adjusted in Fig. 3, it is also possible to adjust the phases of the electrical signals after O/E conversion and synthesize the signals. The transmitting beam can be formed on the basis of the same principle. At this time, the base station only performs O/E and E/O conversions of the received signal and does not require any control.


Fig. 3. Remote-beamforming technology (receiving side).

There are two conventional remote-beamforming technologies: one assigns a separate optical fiber (a separate core in the case of multi-core fiber) to each antenna element [3] and the other [4, 5] uses chromatic dispersion*5 to switch the beam direction by changing the wavelength assigned to each antenna element. By overcoming the problems of these conventional technologies (by fixing the wavelength assigned to each antenna element), our remote-beamforming technology has four advantages: (i) only one optical fiber (core) is used; (ii) optical-fiber-distance information is unnecessary; (iii) control of the optical filter at the base station is unnecessary; and (iv) the format of the wireless signal is unrestricted even if a high-frequency band and long-distance optical fiber are used.

Our remote-beamforming technology not only ensures communication quality of high-frequency-band wireless systems but also enables a base station to simultaneously accommodate multiple wireless terminals by executing space-division multiplexing (SDM). Since the beam direction can be controlled remotely, it is not necessary to physically adjust the antenna direction when configuring the base station.

We demonstrated our remote-beamforming technology in a receiving system at the NTT R&D Forum 2019 (Fig. 4).


Fig. 4. Setup of exhibition.

*4 Beamforming: A technology for electrically controlling the directivity of a wireless signal (beam) by using an array antenna with multiple arranged antenna elements. By controlling the phase of the radio waves transmitted and received by each antenna element, it is possible to enhance the transmission of a radio wave in a specific direction (“transmitting beam”) and to receive and enhance the radio wave arriving from a specific direction (“receiving beam”).
*5 Chromatic dispersion: A phenomenon by which propagation time differs because the speed of light propagating in an optical fiber varies according to the wavelength of the light. It occurs because the refractive index of an optical fiber also depends on the wavelength.

4. Future outlook

We will improve wavelength-utilization efficiency by improving our remote-beamforming technology and work with researchers of optical communications to study practical applications of this technology.

References

[1] K. Ito, M. Suga, Y. Shirato, N. Kita, and T. Onizawa, “A Novel Centralized Beamforming Scheme for Radio-over-fiber Systems with Fixed Wavelength Allocation,” IEICE Communications Express, Vol. 8, No. 12. pp. 584–589, 2019.
[2] M. Suga, K. Ito, Y. Shirato, N. Kita, and T. Onizawa, “Fiber Length Estimation Method for Beamforming at Millimeter Wave Band RoF-FWA System,” IEICE Communications Express, Vol. 8, No. 11, pp. 428–433, 2019.
[3] T. Nagayama, K. Furuya, S. Akiba, J. Hirokawa, and M. Ando, “Millimeter-wave Antenna Beam Forming by Radio-over-fiber with 1.3 µm Light Source and Variable Delay Line,” The 22nd Opto-Electronics and Communications Conference (OECC 2017) and the 5th Photonics Global Conference (PGC 2017), pp. 1–2, Singapore, July/Aug. 2017.
[4] M. Tadokoro, T. Taniguchi, and N. Sakurai, “Optically-controlled Beam Forming Technique for 60 GHz-ROF System Using Dispersion of Optical Fiber and DFWM,” Proc. of the 30th Optical Fiber Communication Conference and Exposition and National Fiber Optic Engineers Conference (OFC/NFOEC 2007), pp. 1–3, Anaheim, USA, Mar. 2007.
[5] S. Akiba, M. Oishi, Y. Nishikawa, K. Minoguchi, J. Hirokawa, and M. Ando, “Photonic Architecture for Beam Forming of RF Phased Array Antenna,” The 37th Opto-Electronics and Communications Conference (OFC 2014), pp. 1–3, San Francisco, USA, Mar. 2014.
Kota Ito
Research Engineer, NTT Access Network Service Systems Laboratories.
He received a B.S. and M.S. from Tokyo Institute of Technology in 2013 and 2015. He joined NTT EAST in 2015 and has been with NTT Access Network Service Systems Laboratories since 2017. He is currently engaged in researching RoF systems. He received the Young Researcher’s Award in 2019 from the Institute of Electronics, Information and Communication Engineers (IEICE). He is a member of IEICE.
Mizuki Suga
Research Engineer, NTT Access Network Service Systems Laboratories.
She received a B.E. and M.E. from Chiba University in 2012 and 2014. In 2014, she joined NTT Access Network Service Systems Laboratories. She is currently researching high-frequency-band systems for wireless communications. She received the Young Researcher’s Award in 2017 from IEICE. She is a member of IEICE.
Yushi Shirato
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received a B.E., M.E., and D.E. in electrical engineering from Tokyo University of Science in 1990, 1992, and 2018. Since joining NTT Wireless Systems Laboratories in 1992, he has been engaged in research and development (R&D) of adaptive equalizers, modems for fixed wireless access systems, and software-defined radio systems. He is currently engaged in R&D of millimeter-wave-band very high throughput fixed wireless backhaul systems. He received the Best Paper Award in 2018 and the Young Engineer’s Award in 2000 from IEICE and the 18th Telecom System Technology Award from the Telecommunications Advancement Foundation in 2003. He is a senior member of IEICE.
Naoki Kita
Senior Research Engineer, Supervisor, Group Leader, NTT Access Network Service Systems Laboratories.
He received a B.E. from Tokyo Metropolitan Institute of Technology in 1994 and received an M.E. and Ph.D. from Tokyo Institute of Technology in 1996 and 2007. Since joining NTT in 1996, he has been researching the propagation characteristics of wireless access systems and developing future satellite communication systems. From 2009 to 2010, he was a visiting scholar at Stanford University, CA, USA. From 2013 to 2014, he was a visiting research scholar at Waseda University, Tokyo. He is currently researching and developing future wireless relay systems for 5G mobile communications systems. He received the IEICE Young Researcher’s Award, the IEICE Communications Society Best Paper Award, and IEICE Best Paper Award in 2002, 2010, and 2014, respectively. He is a senior member of IEICE and a member of IEEE.
Takeshi Onizawa
Executive Research Engineer, Supervisor, Executive Manager - Wireless Entrance Systems Project, NTT Access Network Service Systems Laboratories.
He received a B.E., M.E., and Ph.D. from Saitama University in 1993, 1995, and 2003. Since joining NTT in 1995, he has been engaged in the R&D of personal communication systems, high data rate wireless LAN systems, future wireless access systems, and radio propagation. He is currently an executive manager of Wireless Entrance Systems Project, NTT Access Network Service Systems Laboratories and in charge of management strategies of wireless communication systems. He received the Best Paper Award, Young Investigators Award, and Achievement Award from IEICE in 2000, 2002, and 2006, respectively. He received the Maejima Hisoka Award in 2008 and the Best Paper Award at the International Symposium on Antennas and Propagation (ISAP) in 2016. He is a senior member of IEICE and a member of IEEE.

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