Asymmetric Carrier Divider with an Irregular RF Phase on DD-MZ Modulator for Eliminating Dispersion Power Fading in RoF Communication  
[PERSON]  
1 Department of Electrical Engineering, Universitas Indonesia, Jawa Barat 16424, Indonesia; [EMAIL_ADDRESS] (F.U. ); [EMAIL_ADDRESS] (P.S.P)  
1 Department of Electrical Engineering, Universitas Indonesia, Jawa Barat 16424, Indonesia; febrizal@ui. intensity modulation (IM) is recognized as direct detection (DD), whereas in phase modulation, it is known as coherent detection [1]. The IM on the RoF link produces an optical signal with a double sideband spectrum. The optical double sideband (ODSB) is an optical signal with a spectrum consisting of an optical carrier and upper and lower sidebands located around the optical carrier. When the ODSB signal is transmitted through a fiber link, the chromatic dispersion of the fiber causes the sideband and optical carrier to propagate at different speeds. This leads to the modulated signal at the receiver (\(E_{RX}(t)\)) experiencing a different phase between the sideband component and optical carrier by \(\phi\). The proportion of \(\phi\) follows the length of the fiber (\(L\)), the frequency of the RF signal (\(f_{m}\)) and the wavelength (\(\lambda_{c}\)) used. The phase difference between the sideband and optical carrier causes the O/E process to generate two identical RF signals but with a different phase of \(2\phi\), resulting in constructive and deconstructive interference on the recovered RF signal. Destructive interference causes a reduction in recovered RF signal power, which is known as dispersion power fading (DPF). If \(\phi=\pi/2\), the sideband cancelation effect occurs, which causes a massive loss of power or deep fade [2]. There are several methods for overcoming the DPF, such as carrier phase shift (CPS) [3; 4; 5; 6; 7; 8]. The CPS method uses a carrier phase shift of the \(E_{TX}(t)\) signal at the transmitter. The phase of the carrier (\(\phi\)) is adjusted such that \(\phi\) at the receiver is zero. However, this method has disadvantages because \(\phi\) is different due to the change in \(L,f_{m}\), and \(\lambda_{c}\). Thus, it is always necessary to reset \(\phi\) every system change, and it is not a convenient or robust solution. Furthermore, optical carrier suppression (OCS) was proposed by [9; 10; 11; 12; 13; 14; 15; 16]. The OCS modulation system is modulated with upper and lower sidebands but without carrier components. At the OCS, \(X_{rec}(t)\) is generated from the beating upper sideband and lower sideband with a distance of \(2f_{m}\), so the frequency of \(X_{rec}(t)\) is \(2f_{m}\). However, this method has drawbacks, such as requiring additional devices, such as downconverters, to convert \(X_{rec}(t)\) frequency to its initial value. Machine learning can also be used to overcome DPF. This method has been successfully implemented for short-range transmission [17; 18]. However, it also has some disadvantages, one of which is a complicated configuration on the receiver side. The reduction in DPF can be overcome using optical single-sideband (OSSB) modulation. OSSB modulation has advantages such as not requiring frequency translation [19] or inconvenient phase receiver adjustment. OSSB modulation scheme can be generated by filtering one of the sideband modulated optical signals [20]. This method, however, highly dependents on the optical wavelength. The OSSB modulation scheme can be produced with DD-MZM by driving one of its arms using the RF signal that is given a bias voltage \(\
icefrac{{1}}{{2}}\) of the switching voltage and driving another arm using a signal with a regular phase (\(\theta\)) of \(90^{\circ}\)[15; 21; 22; 23; 24]. DD-MZM is an E/O converter frequently used in RoF links due to its ability to generate various optical schemes with just a simple configuration. The mathematical model of OSSB modulation is usually developed based on the Bessel function [15].