Proposal for Distribution of a Low-Phase-Noise Oscillator Signal in Forthcoming Fifth-Generation Mobile Network by Rradio-Over-Fibre Technology Mehmet Alp Ilgaz, Bostjan Batagelj University of Ljubljana, Faculty of Electrical Engineering, ICT Department, Radiation and Optics Laboratory Trzaska cesta 25, 1000 Ljubljana, Slovenia mehmet.ilgaz@fe.uni-lj.si Abstract - This article presents a solution that needs only one low-phase-noise oscillator for many cellular network base-stations. The proposed approach can provide a solution that is especially suitable for the forthcoming fifth-generation (5G) high-capacity radio system based on millimetre-wave (mmW) frequency bands, where the phase noise is one of main limiting parameters. The stable and low-phase-noise signal, which can be generated by an advanced opto-electronic oscillator (OEO), is distributed to remote antenna units over a passive optical network infrastructure by radio-over-fibre (RoF) technology. Besides the cost effectiveness, this solution can decrease the size and complexity of base-stations, the number of which will increase in 5G cellular networks due to a reduction in the cell size. This article also presents the key building block for such a RoF system, which is a high-stability OEO. Keywords - Radio Over Fibre; Opto-Electronic Oscillator; Low Phase Noise; Millimetre-Wave; 5G Mobile Network I. INTRODUCTION The forthcoming fifth-generation (5G) mobile network is expected to handle a very large amount of data traffic with sub-ms latency [1]. For such an efficient broadband mobile network, frequencies in the millimetre-wave (mmW) range (30–300 GHz) need to be used [2]. Since the atmospheric scattering and attenuation of mmWs is high, signal propagation is limited to a small area and the cell size needs to be reduced to a picocell (100 m) or even a femtocell (10 m). In contrast to the upgrade from the 3G to the 4G mobile network, where the number of cells did not increase dramatically [3], it is expected that the upgrade to the 5G mobile network will increase the number of cell base-stations. Smaller base cells can be applied even with today’s technology, but at the same time mobile operators are anticipating that the complexity, energy consumption and cost of the base-stations will decrease. As we can see, one of the possibilities presented in this paper is to lower the system costs while increasing its performance by introducing distributed network elements. Different modulation schemes and multiplexing techniques are proposed for spectra saving and increasing the wireless system’s capacity [4], [5]. In all these cases it is clear that the phase noise of the transmitted signal is playing an important role [6]. Today’s mobile-communications transceivers employ oscillators that provide the periodic signal needed for frequency translation in the transceiver circuits and for the timing of the digital circuits. When radio interfaces move to the mmW region [7] a low phase noise is even more difficult to achieve [8] since the phase-noise floor is increasing when the oscillators are operating at high frequencies. In our opinion a low-phase-noise frequency oscillator in the range of mmWs is critical for 5G and is a key technology enabler. This paper presents a technical solution for the distribution of a low-phase-noise oscillator signal in the forthcoming 5G mobile networks. First, the benefits of radio-over-fibre technology are described. In the third section the opto-electronic oscillator (OEO) is presented as the main device in the proposed system solution because of its low phase noise. Besides a microwave output it has an optical output that can be used to distribute the oscillator signal to the base-stations, as described in the fourth section, which is followed by the conclusion in the fifth section. II. BASICS OF RADIO OVER FIBRE TECHNOLOGY It is well known that a fibre-optic communication link is superior to a wireless radio connection in terms of both the main important parameters that determine the value of telecommunication lines [9]. The first is the range of the telecommunication line measured in units of length, where optical fibre is the absolute winner with an attenuation of 0.2 dB per kilometre, in comparison to the wireless radio link, where the signal attenuation is higher and increases with frequency. The second parameter is the link capacity, which measures the amount of information transmitted in a time frame, where the optical fibre’s broadband properties make it a practically bandwidth-unlimited medium with a slow approach to the “fibre-wall” [10]. The basic idea of radio-over-fibre (RoF) technology, which uses the transmission of radio frequency (RF) signals over fibre- optic links, is to take advantage of fibre’s low attenuation and high bandwidth [11]. Since a fibre-optic line has a large bandwidth, it allows the transmission of RF, microwave or even mmW signals. There are several principles behind the various RF signal transport methods in RoF technology [11]. The simplest method for optically distributing a RF signal is simply to directly modulate the intensity of the laser-diode (LD) source with the RF signal and to use direct detection (DD) at the photodiode (PD) to receive