# A 100 Gbps OFDM-Based 28 GHz Millimeter-Wave Radio over Fiber Fronthaul System for 5G
## 1 Introduction  
Recently, there has been an exponential increase in mobile data traffic and it is expected to increase by a factor of 1000 over the next decade [1, 2]. The proliferation of internet-enabled devices such as smartphones, tablets, etc. consuming a large amount of data and emerging bandwidth \"hungry\" applications and services such as cloud computing, ultra-high definition (UDH) video streaming, augmented and virtual reality (AR and VR) etc. as well as the emergence of the Internet of Things (IoT) are driving this explosion of mobile data traffic, and have put severe pressure on the telecommunication network infrastructure to provide high-capacity (high-speed data rates) links to support emerging cellular systems such as 5G at a low cost [3, 4]. Due to the limited availability of bandwidth in the sub-6 GHz spectrum throughout the world, the current standards do not support higher data rates. However, the increasing demand for high-speed broadband data may be addressed by scaling up to the millimeter wave (mm-wave) radio frequency spectrum where the massive amount of unused bandwidth is available [4, 5]. Although mm-waves are capable of providing huge bandwidth [6], the propagation characteristics limit their application to a short distance, thus requires numerous base stations (BS) to extend coverage thereby increasing capital (capex) and operational (opex) cost. To facilitate the deployment of fifth-generation (5G) systems, cloud radio access networking (C-RAN) which involves a shift in complexity of expensive equipment and centralizing them at a centralized office (CO) has been proposed to deal with this capex and opex concerns. Figure 1a illustrates C-RAN architecture. Also, due to the expensive nature and complexity of generating mm-wave signals, photonic techniques to generate mm-wave signals while facilitating convergence through fiber-wireless integration (Radio over Fiber) have attracted much interest [7]. To harness the huge bandwidth offered by optical fiber and the flexibility features presented via the wireless, RoF (illustrated in Figure 1b) based optical-wireless networks have been considered the most promising solution to increase bandwidth, capacity and coverage [8, 9]. RoF allows the centralization of the baseband processing units (BBU) while simplifying the remote radio heads (RRH) as inexpensive radio access points. The optical transport link between the BBU and the RRH is known as the fronthaul link. As mm-waves suffer various challenges such as high propagation loss, multi-path delay spread effects, scattering losses, etc. multiplexing schemes such as filter bank multi- carrier (FBMC), orthogonal frequency division multiplexing (OFDM), subcarrier multiplexing (SCM), etc. and higher modulation formats such phase shift keying (PSK), quadrature-phase shift keying (Q-PSK) or quadrature amplitude modulation (QAM) have become stringent requirements to achieve high spectral efficiency for 5G systems. In 5G, many aspects of the current generation mobile networks in terms of speed, bandwidth, energy consumption, latency, etc. will be improved [10]. Another key aspect is that, 5G will aid to incorporate different technologies like multi-input and multiple out (MIMO), heterogeneous networks, IoT and cognitive radio (CR) [11]. Currently, various ranges of mm-wave frequencies are being investigated for the physical interface for 5G. Researchers have shown keen interest in, and conducted extensive research on the frequency spectrums of 28 GHz, 38 GHz, 60 GHz, 75 GHz, 82 GHz, 90 GHz and 110 GHz in realizing a high data rate in Gbits/s to support 5G system. For instance, the authors in [12] demonstrated the transmission of carrier aggregated OFDM signal over a 28 GHz RoF fronthaul link for data rates of 1.2 and 4.8 Gbits/s over respective bandwidths of 491.5 MHz and 1.96 GHz.