A novel bidirectional half-duplex fronthaul system using MMW/RoF and analog network coding

Abstract

A novel bidirectional half-duplex fiber-wireless fronthaul system is proposed in this paper. Bidirectional relaying transmission is deployed in both optical fiber and wireless link thanks to millimeter-wave radio over fiber (MMW/RoF) and analog network coding (ANC) techniques. The architecture of the proposed fronthaul system is simplified significantly since it requires only an optical fiber with a single wavelength and a wireless link with a single MMW frequency. The detailed architectures of the subsystems such as the central station, the remote antenna unit, and the remote radio head are also designed. Mathematical expressions for the performance analysis of the proposed fronthaul system are derived considering the effects of various physical layer impairments. The numerical results demonstrate not only the feasibility of our proposed system but also the gain in terms of throughput of ANC-based relaying compared to conventional and digital network coding (DNC)-based relaying.

Introduction

Radio access network (RAN) is a term usually used to describe connectivity between the core network and base stations (BSs). The capacity required for next-generation (the fifth generation or 5G) RANs will be drastically increased due to the rising number of mobile subscribers and the availability of mobile high-speed data services. As a consequence, an important requirement in 5G RANs is to be able to forward massive traffic to/from a high-density and large number of BSs into the core network [1]. Recently, cloud-based RANs (C-RANs) have been receiving significant interest to provide a cost-effective and energy-efficient solution for the 5G cellular network [2]. Generally, a C-RAN consists of two sections including backhaul and fronthaul as shown in Fig. 1 [3]. Fronthaul refers to the connection between the pool of baseband units (BBUs) located at the central station CS (or central office—CO) to and remote radio heads (RRHs). To meet the requirement of high-capacity fronthaul networks, the use of using an optical fiber with radio-over-fiber (RoF) technique and millimeter-wave (MMW) wireless connection is considered as key technologies [4]. Recently, the hybrid backhaul system using both optical fiber and the wireless connection was also proposed [5]. In this proposal, MMW radio signals are optically transmitted from CS to remote antenna unit (RAU) and then wirelessly forwarded to remote radio heads (RRHs) [5]. Accordingly, RAU plays a role as a relay node while BBU pool and RRH are the source node and the destination node for the downlink and vice versa for the uplink, respectively.

In spite of the fact that there is a lot of attention paid to RoF downlink and uplink architectures separately, the bidirectional architectures for RoF systems are still dominant research issues. Using two optical fibers or one optical fiber with two/three different wavelengths would be a simple solution to this issue [6]. This solution, however, requires the large physical resources (i.e., the number of optical fibers or wavelengths) to support a large number of BSs. Alternatively, bidirectional RoF systems with one optical fiber can be achieved by using optical frequency conversion techniques, where intermediate frequency (IF) signals are transmitted over the fiber and frequency up- and down-converted at the RAUs [[7], [8], [9], [10]]. In [7] and [8], the authors proposed two-way cost-effective 60 GHz RoF system using cascaded semiconductor optical amplifier (SOA)-electroabsorption modulator (EAM) as frequency up- and down-converter. This solution, however, requires optical local oscillator (OLO) at RAUs. Another up- and down-conversion approach is performed by using InP/InGaAs heterojunction phototransistors (HPT) optoelectronic or SOA without any OLO at RAUs [[9], [10]].

Nevertheless, above-mentioned studies demonstrated bi-directional transmission only for fiber-based backhaul or fronthaul [[6], [7], [8], [9], [10]]. Practically, the cost and challenge of fiber installation may be prohibitive, especially in urban areas. The use of fiber cable is also lack of flexibility and unsuitable in many applications and situations such as after large disasters. In this paper, we, therefore, propose to apply hybrid fiber-wireless system using MMW/RoF to a fronthaul network that is more flexible and scalable. In the proposed fronthaul architecture, CS is connected to RAU via a MMW/RoF link while the connection between RAU and RRH is based on MMW wireless link. Although our proposal is similar to the idea presented in [5], it has a novel contribution in terms of the architecture improvement. More specifically, it is able to provide bi-directional transmission for the fronthaul link in both optical fiber and MMW wireless link.

Generally, the bidirectional full-duplex transmission provides larger throughput compared to bidirectional half-duplex transmission. It, however, requires more physical resources such as more number of optical fibers and/or wavelengths at the optical link as well as RF frequencies at the wireless link. In order to reduce the required physical resources, in this paper, we, therefore, propose to use apply bidirectional half-duplex relaying to fiber-wireless systems using MMW/RoF, where RAU plays a role as a relay node between CS and RRH. As a result, our proposed architecture is cost-effective as it requires only one optical fiber with a single wavelength and one MMW frequency for bidirectional transmission in both downlink and uplink.

In addition, conventional half-duplex bidirectional relaying required four time slots to complete sending the data (i.e., two packets) between two terminal nodes, A and C, via a relay node (R) as shown in Fig. 2. The number of time slots can be reduced to three with digital network coding (DNC), which is implemented in accordance with decode-and-forward (DF) relaying scheme [[11], [12], [13], [14], [15]]. By using amplify-and-forward (AF) based on analog network coding (ANC) at the RAU, the number of required time slots could be reduced to two time slots. As a consequence, the throughput is twice as large as that of the conventional half-duplex system. As shown in Fig. 3, CS and RRH could transmit their data to RAU at the same time, which consumes one time slot. Consequently, RAU receives the interfered signals which are the sum of transmitting signals from CS and RAU, i.e., $s_{RAU}\left(t\right)=s_{CS}\left(t\right)+s_{RRH}\left(t\right)$. Then, RAU simply amplifies and broadcasts the combined signal to CS and RRH in the second time slot. Since CS knows the data it transmitted, i.e., $s_{CS}\left(t\right)$, it can, therefore, subtract $s_{CS}\left(t\right)$ from the received interfered signal $s_{RAU}\left(t\right)$ to get $s_{RRH}\left(t\right)$. Similarly, RRH can recover the signal $s_{CS}\left(t\right)$ from CS.

The performance of the proposed bidirectional half-duplex fronthaul system using MMW/RoF and ANC is analyzed in terms of throughput considering the effects of various physical layer impairments originated from the optical and RF receivers, optical fiber, and wireless channel. These impairments include noise, fiber dispersion, and fading. Two network coding (NC)-based relaying techniques including DNC and ANC are considered and compared to conventional relaying one without network coding. The numerical results demonstrate that fronthaul system using ANC is able to provide higher throughput than the one using conventional or DNC-based relaying.

The rest of this paper is organized as follows. Section 2 describes the architecture and principle of the bidirectional half-duplex fronthaul system using MMW/RoF and ANC. Performance analysis and numerical results will be presented in Sections 3 Performance analysis, 4 Numerical results, respectively. Finally, the last section concludes the paper.