Contents I. Introduction
As the fifth generation (5G) mobile technology standards approach the final phases in 2019, the fiber-wireless integrated technology is promising for resilient networking and last-mile access networks [1]. Radio-over-fiber (RoF) technology based on millimeter-wave (MMW) transmission has been proposed to provide flexible wireless delivery when the fiber is unavailable in the two-level, Next Generation Fronthaul Interface (NGFI) architecture [2], [3]. In some situations, when the fiber is accidentally severed due to natural disasters or road construction, 5G-MMW link at V-band carrier frequency is one of the promising candidates to restore the network connection because it provides prompt and opportune delivery with high data bandwidth and rapid deployment [2]–[4]. Also, V-band is unlicensed in most places and available for the mobile network operator. However, in a conventional RoF scheme, RF signals are sensitive to the frequency-selective fading (FSF) due to fiber dispersion. Thus, intermediate frequency (IF) signal over fiber with a relatively higher tolerance of FSF becomes an alternative approach [5]. Meanwhile, coherent detection can be utilized to enhance the receiver power sensitivity and to extend the MMW reach when the received signal experiences electrical signal-to-noise ratio (ESNR) reduction or received optical power (ROP) attenuation [6]. In a combination of the strength of IF over fiber and coherent detection, we proposed a converged fiber-MMW- fiber link with heterodyne detection for resilient Tier-I mobile fronthaul as shown in Fig. 1. Optical fiber covers most of the distance from the distributed unit (DU) to remote radio units (RRU) and delivers the IF signal to avoid FSF. For the locations in the absence of the optical fiber, MMW is utilized. However, The MMW at V-band frequency suffers from more power attenuation than fiber link and results in ESNR reduction. Moreover, the worst case is that there are multiple locations where MMW is needed, which severely degrades the quality of the received signal. Therefore, the cost-efficient heterodyne coherent detection is utilized to enhance the tolerance to ESNR and ROP degradation. In cases of fiber delivery, conventional Intensity modulation and direct detection (IM-DD) scheme is employed.
Conceptual schematic of agile deployment of the converged fiber-wireless-fiber link with coherent detection in tier-i mobile fronthaul.
