Transparent Waveform Transfer for Resilient and Low-latency Links

Introduction The Great East Japan Earthquake on 11th March 2011 seriously damaged telecommunication networks, which were indispensable to share and collect information for disaster recovery activities. Telecom operators in Japan used various types of transmission media, including communication satellites, to resume services quickly [1]. Radio-wave communications can have agile deployment capability. However, it is rather difficult to obtain long-haul large capacity wireless links by using radio-waves, because of limitation of radio-wave frequency resources especially in microwave bands. Millimeterwave links can provide high-speed transmission up to a few Gb/s, while the transmission distance is limited by attenuation in the air. Thus, we should rely on optical fiber communication (OFC) links to provide over 100 Gb/s high-speed transmission. To use radio-wave links as backups for optical fiber links, transmission capacity difference between radio-wave and optical fiber links should be reduced to mitigate possible surge of traffic demands. In this article, we review wired and wireless seamless network technology which provides both highspeed transmission and agile deployment capabilities. Digital coherent technology developed for high-speed can mitigate signal deformation in radio-wave links in the air as well as in optical fibers [2]. Radio-over-fiber (RoF) technique, which transmits waveforms for radio-wave links on intensity profiles of lightwaves over fibers, can provide direct waveform transfer between lighwaves and millimeter-waves by using optical-toelectric or electric-to-optical conversion devices. Combination of RoF in millimeter-wave bands and digital coherent with high-performance digital signal processing (DSP) can provide wired and wireless seamless links where bit rate of wireless links would be close to 100 Gb/s [3]. Millimeter-wave transmission distance would be shorter than a few kilometers due to atmospheric attenuation, so that many moderate distance wireless links which are seamlessly connected to optical fiber networks should be required to provide resilient and high-capacity data transmission. Reduction of cost and power consumption at media converters connecting wired and wireless links would be very important to enhance total performance of the seamless networks. Latency reduction in transmission is also one of important issues for particular applications such as data transfer for high-frequency trading, online gaming [4]. Recently, wireless links has been paid attention to achieve low-latency transmission, where data rate would be much smaller than in OFC, but propagation speed is much higher in the air. Thus, combination of wireless and wired links would be useful to mitigate such demands both for high capacity and low latency. RoF-based transparent waveform transfer would provide low latency media conversion between optical and radio-wave signals.