# Effect of Phase Noise on the Optical Millimeter-Wave Signal in the DWDM-RoF System
## 1 Introduction  
During the past few years, the communication industry has experienced rapid expansion in the number of worldwide users and an increase in the speed of transmission data [1]. Therefore, it will be necessary for future access networks to merge both photonic technology and radio-over-fiber (RoF) technology to provide high-flexibility and sufficient systems to the end-user. Dense wavelength division multiplexing (DWDM) and RoF technologies have been proven to support cost-effective and high-capacity access for the transmission data networks. The technology of DWDM is based on RoF, combining the benefits of optical fiber and millimeter-wave wireless systems. The millimeter-wave (mm-wave) band has great potential in RoF technology due to its wide transmission bandwidth and unlicensed frequency band. In addition, the 5G traffic requirements will necessitate improvements to the design of radio access networks (RANs), which depend on unused ranges of frequencies such as the mm-wave range. Different applications, such as complementing terrestrial RANs with unmanned aerial vehicles (UAVs), can be cost-effective and flexible strategies with the 5G radio access networks, as reported in [2]. Furthermore, the mm-wave technology is capable of meetingthe increasing demands of wireless applications [3; 4; 5; 6]. However, in the electrical domain, the generation of mm waves with high frequency remains a challenge due to the current restrictions on frequency responses of electronic devices and equipment. Therefore, several researchers are focusing on mm-wave generation using photonic techniques [7; 8]. Several techniques have been reported to improve the capacity of RoF communication systems [9; 10; 11]. The performance of multiple modulation formats, such as dual-polarization quadrature phase-shift keying (DP-QPSK), dual-polarization 16-ary quadrature amplitude modulation (DP-16 QAM), DP-64 QAM, and DP-256 QAM, was studied in C-band systems with different transmission distances. It was found that the capacity for enhancing achievable data rates strongly depends on signal modulation formats as well as the transmission distances [11]. Additionally, the DWDM technique enhanced the capability of traffic data by using multiple transmission channels in the photonic communication networks. The DWDM is a cost-effective technique that avoids the need for additional fiber for transmission. Furthermore, the advantages of DWDM networks make them suitable for long-haul transmission data [12; 13; 14; 15]. However, some equipment hinders the implementation of cost-effective DWDM networks rather than the RoF systems. Some studies utilized a large number of modulators in the system, such as [16; 17], in which many Mach Zehnder Modulators (MZMs) are used to generate the optical mm-wave. Using many modulators in the system increases the cost and complexity of the system. Furthermore, the MZM performance is limited by the uncontrolled phase, which creates a phase imbalance in the mm-wave signals [18]. Therefore, using a large number of MZMs increases the phase imbalance and creates a significant insertion loss in the system [18; 19; 20]. In addition, the phase imbalance causes phase noise in the mm-wave signal [20]. Therefore, the use of a large number of MZMs and uncontrolled phases cause the mm-wave signal to face significant phase noise. Alternatively, the use of a single modulator in the transmission data systems reduces the phase imbalance in the system and insertion losses as well as the cost and complexity at the remote antenna unit (RAU), especially in cases where a large number of RAUs is deployed. The optical generation of mm waves using a single modulator results in instability and low phase noise in the system [21]. Methods have been presented for the photonic generation of the mm-wave signal [22; 23; 24; 25; 26; 27]. These studies optically generated the mm wave. However, they did not examine the effects of phase imbalance on the system or the effects of the resulting phase noise.