Modulation Index and Phase Imbalance of Dual-Sideband Optical Carrier Suppression (DSB-OCS) in Optical Millimeter-Wave System

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Article Modulation Index and Phase Imbalance of Dual-Sideband Optical Carrier Suppression (DSB-OCS) in Optical Millimeter-Wave System

Syamsuri Yaakob 1,* , Rawa Muayad Mahmood 1 , Zuraidah Zan 1, Che Beson Mohd Rashidi 2, Azwan Mahmud 3 and Siti Barirah Ahmad Anas 1

1 Wireless and Photonic Networks Research Centre of Excellence, Department of Computer and Communication Systems Engineering, Faculty of Engineering, Universiti Putra Malaysia (UPM), Serdang 43400, Selangor, Malaysia; [email protected] (R.M.M.); [email protected] (Z.Z.); [email protected] (S.B.A.A.) 2 Advanced Communication Engineering, Centre of Excellence (ACE-CoE), School of Computer and Communication Engineering, Universiti Malaysia Perlis, Kangar 01000, Perlis, Malaysia; [email protected] 3 Faculty of Engineering, Multimedia University, Cyberjaya 63000, Selangor, Malaysia; [email protected] * Correspondence: [email protected]

Abstract: This paper presents a Dual-sideband Optical Carrier Suppression (DSB-OCS) technique which is used to generate an optical millimeter-wave (mm-wave) signal in radio over fiber (RoF) systems. The proposed system employs a Dual-Electrode Mach-Zehnder Modulator (DE-MZM) and a carrier of 40 GHz mm-wave for data transmission through the RoF systems. Characteristics determining the performance of the system, among which are the modulation index, phase imbalance and dispersion parameters, are included. The performance evaluations of the system show that Citation: Yaakob, S.; Mahmood, the mm-wave signal output power follows MZM’s transfer function when the modulation index is R.M.; Zan, Z.; Rashidi, C.B.M.; Mahmud, A.; Anas, S.B.A. raised. Moreover, the generated optical mm-wave signal power is affected by phase imbalance and Modulation Index and Phase optical splitting ratio. It is observed that the optical fiber dispersion influences the DSB-OCS system Imbalance of Dual-Sideband Optical by decreasing the amplitude of the mm-wave and the signal-to-noise ratio (SNR). Carrier Suppression (DSB-OCS) in Optical Millimeter-Wave System. Keywords: mm-wave; RoF; dual sideband optical carrier suppression; optical modulation index; Photonics 2021, 8, 153. https:// optical fiber dispersion; phase imbalance doi.org/10.3390/photonics8050153

Received: 20 March 2021 Accepted: 29 April 2021 1. Introduction Published: 4 May 2021 The millimeter-wave (mm-wave) and radio over fiber (RoF) capabilities have been developed to improve the speed and the ability to meet high traffic requirements in Publisher’s Note: MDPI stays neutral the last few years. The mm-wave signal generation is essential to achieve high data with regard to jurisdictional claims in rates, in addition to other advantages such as low attenuation, large bandwidth, and published maps and institutional affil- low cost [1–3]. The massive increase in bandwidth with the huge number of connected iations. devices causes problems in most communications systems. Therefore, the mm-wave RoF has been promoted as a bandwidth and infrastructure solution that can meet the requirements of high-speed wireless systems such as fifth generation (5G) networks [4,5]. The features of mm-wave RoF systems, such as low latency and high bit rates, are suitable Copyright: © 2021 by the authors. to realize the vision of the centralized-radio access network (C-RAN) [6]. In addition, the Licensee MDPI, Basel, Switzerland. RoF system is also an option for the integrated system to connect with C-RAN fronthaul This article is an open access article networks [7]. Recently, several studies have suggested distributing the wireless signal with distributed under the terms and different techniques over the mm-wave RoF downstream systems. The techniques were conditions of the Creative Commons Attribution (CC BY) license (https:// developed based on the heterodyne optical phase-locked loop [8], Stimulation Brillouin creativecommons.org/licenses/by/ Scattering (SBS) [9,10], the optical single side band (OSSB) [11–13] and Dual-side band 4.0/).

Photonics 2021, 8, 153. https://doi.org/10.3390/photonics8050153 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 153 2 of 12

Optical Carrier Suppression (DSB-OCS) [14,15]. All these studies generated the mm- wave optically and utilized it in signal transmission. It was, however, noted that none of them examined the effect of modulation index on the system or the effects of optical fiber dispersion on optical mm-wave networks as well as the phase imbalance, which is crucial. Therefore, there are more parameters and more efficient techniques to be investigated to ensure an excellent system for the future. On the other hand, the DSB-OCS technique is worth exploring for this reason because it generates the mm-wave signal using just a fraction of the local oscillator (LO) at the base station (BS) [16]. Additional advantages of the DSB-OCS system are an ability to generate mm-wave signals with tunable frequencies [17], greater spectral efficiency [18], and lower mm-wave component requirements to generate the mm-wave carriers [19]. Overall, the choice of using the mm- wave method is to reduce the central station (CS) model complexity and reduce the remote antenna unit (RAU) size. Several studies have suggested that both Analog-RoF (A-RoF) and Digital-RoF (D-RoF) technologies have their advantages and disadvantages [6,20,21]. However, this paper presents the DSB-OCS technique in A-RoF with the intention to improve its performance, reduce the RAU complexity and enhancing system flexibility [22]. Moreover, it is still possible to convert A-RoF to D-RoF [23], or combine both A-RoF and D-RoF in one system [24]. This way, the benefits for both the analog and digital can be shared together with vast bandwidth in the optical domain to be offered to the wireless counterpart. Prior research on DSB-OCS has pointed towards the need to enhance the current techniques to reach higher mm-wave frequency with higher bandwidth and capability through RoF systems. A high-order RF multiplier was used to generate the mm-wave in the DSB modulation for the Maximum Transmission Bias Point (MATB) in [25], which focused on the odd harmonics. In that study, a dual-parallel Mach–Zehnder modulator was used which led to some insertion losses and managed to generate 30 GHz and 60 GHz signals. On the other hand, this study focuses on determining the right modulation index to produce the maximum mm-wave output with the help of Bessel function in the DSB- OCS system, and studying the effect of dispersion and phase imbalance in such a system. The modulation index and the dispersion of the DSB-OCS parameters are considered as essential parameters to evaluate the hybrid network performance [26]. Firstly, when the power of the broadband signal injected into the system is increased, more intermodulation products are produced, which results in interference. Secondly, the dispersion may induce power loss and signal broadening, which results in poor signal detection to recover the original transmitted data. Thirdly, high power injected into the system may also cause different phase imbalances, which have a great influence on the performance of the mm- wave signal and directly affect the system performance. Therefore, it is imperative to maximize the mm-wave output and study the modulation index and dispersion as well as the phase imbalance in this type of system. This study is arranged as follows. The following section describes the system design of the DSB-OCS technique. Section3 models the system at various modulation indexes. Section4 presents the effects of phase imbalance on the generated optical mm-wave in RoF systems. Finally, the effect of dispersion on different fiber lengths is explained in Section5.

2. System Design An MZM schematic diagram of the DSB-OCS system is illustrated in Figure1. The DSB-OCS technique is established using a Dual-Electrode Mach-Zehnder Modulator (DE- MZM) by receiving the optical signal with a wavelength of Ω from the continuous wave (CW) laser diode (LD). Then, the optical signal is divided into two arms, the upper and lower DE-MZM electrodes. The arms contain lithium niobate (Nio3) materials, which cause disturbance to the optical signal to perform the optical modulation. Next, both arms are driven by the RF signal from the local oscillator (LO) with a frequency of ωrf, except that only one of the arms is driven with a 180◦ phase-shifter. As a result, the system achieves a Dual-Sideband Optical Carrier Suppression as the output, where Ω + ωrf and Ω PhotonicsPhotonics2021 2021, 8,, 8 153, 153 33 of of 12 12 Photonics 2021, 8, 153 3 of 12

− ωrf emerge as the sideband from the original signal Ω, as shown in the block diagram in − ωrf emerge as the sideband from the original signal Ω, as shown in the block diagram − ωrf emerge as the sideband from the original signal Ω, as shown in the block diagram in inFigureFigure 11 byby manipulating manipulating the modulator’s DCDC biasbias voltage.voltage. TheThe DSB-OCS DSB-OCS system system was was Figure 1 by manipulating the modulator’s DC bias voltage. The DSB-OCS system was theoreticallytheoretically clarified clarified inin [[27].27]. A A baseband baseband signal signal of of1.25 1.25 Gbps Gbps was was injected injected or directly or directly mod- theoretically clarified in [27]. A baseband signal of 1.25 Gbps was injected or directly mod- modulatedulated via via the the CW CW LD LD and and simulated simulated the the syst systemem performance viavia OptisystemOptisystem software software ulated via the CW LD and simulated the system performance via Optisystem software versionversion 17. 17. The The simulation simulation was was performed performed using using the the setup setup shown shown in in Figure Figure2 to2 to generate generate version 17. The simulation was performed using the setup shown in Figure 2 to generate thethe mm-wave mm-wave withwith DSB-OCSDSB-OCS technique. technique. A A dire directct modulation modulation laser laser diode diode (DMLD) (DMLD) of 1550 of the mm-wave with DSB-OCS technique. A direct modulation laser diode (DMLD) of 1550 1550nm was nm wasused used to produce to produce the CW the optical CW optical signal signalwith an with amplitude an amplitude signal around signal − around10 dBm. nm was used to produce the CW optical signal with an amplitude signal around −10 dBm. −The10 dBm. relative The intensity relative noise intensity (RIN) noise as set (RIN) at −140 as setdB/Hz. at − 140In order dB/Hz. to ensure In order a full to ensureDE-MZM a The relative intensity noise (RIN) as set at −140 dB/Hz. In order to ensure a full DE-MZM fullcoupling DE-MZM and couplingto minimize and the to minimizepolarization-dependent the polarization-dependent loss, a polarization loss, a controller polarization (PC) coupling and to minimize the polarization-dependent loss, a polarization controller (PC) controllerwas used (PC) in the was circuit. used Then, in the an circuit. erbium-dop Then, aned erbium-doped fiber amplifier fiber 1 (EDFA1) amplifier was 1 utilized (EDFA1) to was used in the circuit. Then, an erbium-doped fiber amplifier 1 (EDFA1) was utilized to wasamplify utilized the to signal amplify and the to overdrive signal and it to before overdrive sending it before to a 20 sending GHz RF to signal. a 20 GHz A 3 RFdB signal.splitter Aamplifydivided 3 dB splitter thethe signal RF divided signal and thetointo overdrive RF two signal equal it into before parts two sending equalwith a parts 180° to a with20phase-shifter GHz a 180 RF◦ signal.phase-shifter in one A 3 of dB the insplitter oneelec- ofdividedtrodes. the electrodes. Then,the RF both signal Then, parts into both drove two parts theequal drove upper parts the and upperwith lower a and180° DE-MZM lower phase-shifter DE-MZM electrodes. in electrodes.one The of powerthe Theelec- of trodes. Then, both parts drove the upper and lower DE-MZM electrodes. The power of powerRF signal of RF was signal 20 dBm was 20at dBmfrequency at frequency of 20 GHz. of 20 The GHz. half-wave The half-wave voltage voltage of the ofmain the V mainπ was RF signal was 20 dBm at frequency of 20 GHz. The half-wave voltage of the main Vπ was Vdeemedπ was deemed to be 5 toV, be VDC 51 V, wasVDC biased1 was at biased 5 V, while at 5 V, V whileDC2 is VbiasedDC2 is at biased 0 V. at 0 V. deemed to be 5 V, VDC1 was biased at 5 V, while VDC2 is biased at 0 V.

FigureFigure 1. 1. DSB-OCS schematic diagram. Figure 1.DSB-OCS DSB-OCS schematic schematic diagram. diagram.

Figure 2. DSB-OCS simulation setup. Figure 2. DSB-OCS simulation setup. Figure 2. DSB-OCS simulation setup. The DE-MZM output was amplified by EDFA2. After that, the signal was linked to an opticalTheThe DE-MZM DE-MZM fiber of output0.21 output dB/km was was amplifiedloss, amplified and 16 by byps/(nm.km) EDFA2. EDFA After2. After chromatic that, that, the the signaldispersion. signal was was linked At linkedRAU, to an the to opticalanphotodetector optical fiber fiber of 0.21(PD) of 0.21 dB/km with dB/km 0.6 loss, A/W loss, and was and 16 used16 ps/(nm.km) ps/(nm.km) to detect chromatic the chromatic signals dispersion. whichdispersion. were At Atthen RAU, RAU, filtered the the photodetectorphotodetectorby a lowpass (PD)filter (PD) with(LPF) with 0.6 0.6with A/W A/W cutoff waswas frequency usedused to to detect detect of 0.75x the the BitRate. signals signals which Finally,which were were the thenoutput then filtered filtered signal bybywas a a lowpass lowpassconnected filter filter to (LPF)an (LPF) analyzer with with cutoff cutoffto collect frequency frequency the data of of and 0.75x 0.75x to BitRate. performBitRate. Finally, Finally,analysis the the of output theoutput signal. signal signal waswas connected connected to to an an analyzer analyzer to to collect collect the the data data and and to to perform perform analysis analysis of of the the signal. signal. 3. Modulation Index 3.3. Modulation Modulation Index Index The DSB-OCS system has been modelled to investigate the RoF system performance The DSB-OCS system has been modelled to investigate the RoF system performance and Thethe signalDSB-OCS power system of the has mm-wave been modelled at various to investigate modulation the indexesRoF system (m). performance At the DSB- andand the the signal signal power power of of the the mm-wave mm-wave atvarious at various modulation modulation indexes indexes (m). At(m). the At DSB-OCS the DSB-

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simulationOCS simulation setup, setup, the parameters the parameters were were kept atkept constant at constant values values and theand fiber the fiber length length was setwas to set 20 to km. 20 km. The The important important findings findings were were collected collected and and are are plotted plotted in Figuresin Figures3 and 3 and4. The4. The result result in Figurein Figure3 shows 3 shows the relationshipthe relationship between between the mm-wavethe mm-wave signal signal power power and theand modulationthe modulation index. index. In this In this figure, figure, the amplitudethe amplitude of the of LO the signals LO signals ranges ranges from from 0 V to 0 20V V,to and the modulation index ranges from 0 to 4. It can be seen that all curves perform the 20 V, and the modulation index ranges from 0 to 4. It can be seen that all curves perform same behavior in the graph with small differences of around 0.6%, starting at a modulation the same behavior in the graph with small differences of around 0.6%, starting at a mod- index of 2.6. However, the 10 GHz and 20 GHz curves have the same output for the ulation index of 2.6. However, the 10 GHz and 20 GHz curves have the same output for whole iteration. the whole iteration. Initially, 10.7 dBm signal power of the mm-wave was generated at a modulation index Initially, 10.7 dBm signal power of the mm-wave was generated at a modulation in- of 0. When the modulation index as raised, the curves increased gradually to reach the dex of 0. When the modulation index as raised, the curves increased gradually to reach maximum value at 11.8 dBm for modulation indexes from 0.8 to 1. This case can be clarified the maximum value at 11.8 dBm for modulation indexes from 0.8 to 1. This case can be by Equation (1): clarified by Equation (1): ∞ n πm Eout = −Eo cos(Ωt) ∑ (−1) J2n+1 · cos[(2n + 1)ωRFt] (1) =− cos(Ωt) (−1) 2 . cos[(2 + 1) ] (1) n=0 2 where J2π+1 (πm/2) refers to Bessel functions of the first kind of n order where n = 0, 1, 2, 3 where (πm/2) refers to Bessel functions of the first kind of n order where = and the related functions J1 , J3 and J5 can be represented by Equation (2): 0, 1, 2, 3 and the related functions , and can be represented by Equation (2): n+2k ∞ (−(−1)1)k x ( ) J (x) = 2 2 (2) n () =∑ ( + + ) (2) k=0 k!!Γ Γn ( +k +1 1)

wherewhere xx= = ππmm/2,/2, m isis thethe modulationmodulation index, index, or orV VDCDC/V/Vππ of the DSB-OCS,DSB-OCS, VVDCDC is the DC biasbias voltage,voltage, n and k are integers,integers, and ωRF is is thethe opticaloptical carriercarrier frequency.frequency. FigureFigure4 4 displays displays thethe theoretical Bessel function with J1 ,,J3 and J5 for for comparison.comparison. DueDue toto lowlow bias applied, only oddodd BesselBessel componentscomponents areare considered.considered. Theoretically, the J1 portionportion isis dominantdominant compared toto J3and andJ5 at modulationat modulation indexes indexes of 0.6of 0.6 and and 1.1, 1.1, while whileJ0 and other and other components compo- arenents suppressed are suppressed in the DSB-OCSin the DSB-OCS system. system.J1 is considered J1 is considered as the dual as the sideband dual sideband components, com- whileponents, the while dual sidebandthe dual sideband components components for J3 and forJ5 canJ3 and be J regarded5 can be regarded as noise. as Theoretically, noise. The- theoretically, term J1 the− J 3term− J5 Jis1 − the J3 − output J5 is the result output of the result system. of the It cansystem. be seen It can that be the seen theoretical that the Jtheoretical1 − J3 − J5 J1term, − J3 − orJ5 term, the output or the resultoutput of result the system, of the system, shows maximumshows maximum mm-wave mm-wave signal powersignal power at a modulation at a modulation index index of 0.6 of to 0.6 1.1 to in 1.1 Figure in Figure4, and 4, this and result this result coincides coincides with with the simulationthe simulation result result in Figure in Figure3. 3.

Figure 3. SignalSignal power power of of optical optical mm-wave mm-wave as asa function a function of modulation of modulation index index (m) (form) different for different LO LOfrequencies. frequencies.

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FigureFigure 4. 4. BesselBessel functions functions of of the the first ﬁrst kind. kind.

InIn Figure Figure 33,, thethe curvecurve ofof 2020 GHzGHz LOLO recordsrecords thethe minimumminimum amplitudeamplitude valuesvalues whenwhen modulationmodulation indexes indexes range range from from 2.4 2.4 to to 2.8, 2.8, while while the the curves curves for for 10 10 GHz GHz and and 30 30 GHz GHz record record thethe minimum minimum amplitude amplitude values values at at a a modulation modulation index index ranging ranging from from 2.6 2.6 to to 3. 3. The The 20 20 GHz GHz curvecurve had had higher higher amplitude amplitude after after a a modulation modulation index index of of 3 3 than than the the curves curves of of 10 10 GHz GHz and and 3030 GHz. GHz. All All curves curves increased increased gradually gradually after after a a modulation modulation index index of of 3, 3, with with a a difference difference of of 0.650.65 dB dB in in the the amplitude amplitude between between the the 20 20 GHz GHz curve curve and and 10 10 GHz GHz curve. curve. This This can can also also be be explained by referring to the J − J − J term plot in Figure4, where the results coincide explained by referring to the J11 – J3 3– J5 term5 plot in Figure 4, where the results coincide withwith results results in in Figure Figure 3,3 ,and and it ithas has been been shown shown that that in order in order to obtain to obtain the maximum the maximum mm- mm-wave signal power amplitude, the modulation index should be between 0.6 and 1.1. wave signal power amplitude, the modulation index should be between 0.6 and 1.1. 4. Phase Imbalance 4. Phase Imbalance In this section, we discuss the phase imbalance parameter that may affect the per- formanceIn this of section, optical we mm-wave. discuss the This phase comes imba fromlance the parameter inﬂuence that of phase may imbalanceaffect the perfor- on the manceinsertion of optical loss as mm-wave. well as the This DE-MZM comes modulationfrom the influence indexes. of Tophase prove imbalance that, we on assumed the in- sertionthat the loss inputs as well of upper as the and DE-MZM lower DE-MZMmodulation electrodes indexes. were To prove imbalanced. that, we This assumed imbalance that thecomes inputs from of the upper difference and lower in length DE-MZM∆L between electrodes the two were branches. imbalanced. This will This cause imbalance a phase comesimbalance from( φthe) at difference the MZM in scheme length because∆L between of different the two speedsbranches. of beamsThis will inside cause the a phase MZM, imbalancethereforedifferent (ϕ) at the outputs MZM scheme at the MZM. because The of phase different imbalance speeds canof beams be represented inside the by MZM, [28]: therefore different outputs at the MZM. The phase imbalance can be represented by [28]: φ = 2 π n ∆L/λ (3) ϕ =2 π n ∆L/λ (3) wherewhere nn isis the the refractive refractive index index and and λλ isis the the light light wave wave wavelength. wavelength. TheThe change change in in ϕφ andand splitting splitting ratio ratio ( (γγ)) of of the the two two electrodes electrodes (upper (upper and and lower lower DE- DE- MZMMZM electrodes) electrodes) may may cause cause declines declines in in the the generated generated optical optical mm-wave. mm-wave. Considering Considering the the limitationlimitation or or changing changing of of the the γγ andand ϕφ inin both both electrodes, electrodes, the the mm-wave mm-wave generated generated can can be be representedrepresented by: by:

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n h V cos ω t V cos ω t i E(t) = αE eiωct γ γ exp jπ r f m + (1 − γ ) exp −jπ r f m ejϕ + (1 − γ) 1 Vπa 1 Vπa h V cos(ω t+ π ) × γ exp jπ r f m 2 ejϕ 2 Vπb V cos(ω t+ π ) io +(1 − γ ) exp −jπ r f m 2 2 Vπb jωct jm cos ωmt −jm cos ωmt jϕ = α E e γ γ1 e + (1 − γ1)e e −jm sin ωmt jm sin ωmt +(1 − γ) γ2 e + (1 − γ2) e ∞ jωct n jnωmt = α E e γ γ1 ∑ j Jn (m)e + (1 n=−∞ (4) ∞ n jnωmt jϕ −γ1) ∑ (−j ) Jn (m)e e + (1 − γ) n=−∞ ∞ ∞ n jnωmt jnωmt × γ2 ∑ (−1) Jn (m)e + (1 − γ2) ∑ Jn (m)e n=−∞ n=−∞ ∞ jϕ n jϕ n n = α E ∑ γγ1e j + γ (1 − γ1)e (−j) + (1 − γ)γ2 (−1) + (1 − γ)(1 n=−∞ j(ωc+nωm)t −γ2] Jn(m)e ∞ j(ωc+nωm)t = α E ∑n=−∞ An Jn(m)e

where E is the light wave amplitude and α is the modulator insertion loss, ωc is an angular frequency, Vrf is the voltage of local oscillator, m is the modulation index, Vπa and Vπa are the switching voltages of the two upper and lower DE-MZM electrodes, and Va and Vb are the two bias voltages where Va = Vb. The An can be expressed as:

 γ ejϕ + (1 − γ), n = 4m  jϕ  −jγ(1 − 2γ1)e + (1 − γ)(1 − 2γ2), n = 4m + 1 An = jϕ (5)  −jγe + (1 − γ), n = 4m + 2  jϕ jγ(1 − 2γ1)e + (1 − γ)(1 − 2γ2), n = 4m + 3

Assuming that the two electrodes of DE-MZM have a γ of γ1 and γ2, in the case of γ1 = γ2 = 0.5 and φ = 0, Equation (4) becomes Equation (6) [28]. The orders of n = 4m + 1, n = 4m + 2 and n = 4m + 3 can be ignored.

∞ jωct j(ωc+4nωm)t E(t) = α E e ∑ J4n(m)e =− n ∞ n h i jωct j(ωc+4ωm)t j(ωc−4ωm)t (6) ≈ α E Jo(m)e + J4(m) e + e h i o j(ωc+8ωm)t j(ωc−8ωm)t + J8(m) e + e + ··· .

However, the declination of the original values causes some limitations in the 4m order sidebands and further (4m + 1st), (4m + 2nd), and (4m + 3rd) order sidebands. We can see from these equations that the amplitude mostly depends on the γ and the φ. For instance, the amplitude of the 4m can be expressed by:

2 |A4m| = 1 + 2 γ − 2γ + 2γ(1 − γ) cos ϕ (7)

Figure5 indicates the relationship between the φ and the amplitude when optical γ is varied from 0 to 0.5. The increase in the φ from 0 to 90◦ causes decrease in the 4m order sideband. It can be seen that when γ is equal to zero, the greatest value of amplitude will be formed. This suggests that the influence of φ is greater than γ. According to the theoretical analysis on both γ and φ, we can confirm that both of them affect the amplitude of the generated optical mm-wave. It is worth noting that the influence of phase imbalance is greater than the splitting ratio. It is important to take this point into consideration when generating an optical mm-wave in addition to adjusting the DC bias voltage and the LO voltages. It is obvious that the mm-wave signal power and the phase imbalance are dependent on the insertion losses and the modulation indexes of the DE-MZM. Photonics 2021, 8, 153 7 of 12 Photonics 2021, 8, 153 7 of 12

Figure 5. Effect ofof bothboth opticaloptical splittingsplitting ratioratioγ γand and the the phase phase imbalance imbalanceφ ϕon on the the 4 m4morder order sideband. sideband. 5. Effect of Optical Fiber Dispersion InAccording this section, to the the theoretical effect of dispersion analysis ison studied both γ toand investigate ϕ, we can the confirm performance that both of the of RoFthem system affect the and amplitude the signal of power the generated of the mm-wave optical mm-wave. at different It optical is worth ﬁber noting lengths. that The the simulationinfluence of results phase of imbalance the proposed is greater architecture than the showed splitting acceptable ratio. It is performance important to of take the RoFthis − system:point into the consideration system obtained when a BERgenerating of 2.6 ×an10 optical10 and mm-wave Q factor in of 6.2addition for the to 1.25 adjusting Gbps signal.the DC Figure bias voltage6 shows and the relationshipthe LO voltages. between It is BER obvious and thethat received the mm-wave optical powersignal (ROP)power atand 0 kmthe andphase 20 imbalance km ﬁber length. are dependent From the on graph, the insertion it can be seenlosses that and the the power modulation penalty in- is −9 1.5dexes dBm of betweenthe DE-MZM. the 0 km and 20 km lines at a BER of 10 , where the system achieves error-free transmission. 5. Effect of Optical Fiber Dispersion In this section, the effect of dispersion is studied to investigate the performance of the RoF system and the signal power of the mm-wave at different optical fiber lengths. The simulation results of the proposed architecture showed acceptable performance of the RoF system: the system obtained a BER of 2.6 × 10−10 and Q factor of 6.2 for the 1.25 Gbps signal. Figure 6 shows the relationship between BER and the received optical power (ROP) at 0 km and 20 km fiber length. From the graph, it can be seen that the power penalty is 1.5 dBm between the 0 km and 20 km lines at a BER of 10−9, where the system achieves error-free transmission.

Figure 6. BER vs. the received optical power (ROP) at0 km and 20 km ﬁber length. Figure 6. BER vs. the received optical power (ROP) at 0 km and 20 km fiber length.

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To show the dispersion effects, the optical ﬁber length was varied from 0 km to 40 km and dispersionTo show the wasdispersion considered effects, at the 16 optical ps/(nm/km). fiber length The was parameters varied from for0 km CW to 40 laser, PD, EDFA1km and anddispersion EDFA2 was were considered kept at at the 16 sameps/(nm/km). settings. The The parameters results infor Figure CW laser,7a–e PD, show the effectsEDFA1 of and dispersion EDFA2 were on the kept DSB-OCS at the same system settings. and The how results it contributes in Figure to 7a–e changing show the the output spectrumeffects of dispersion of 40 GHz. on the DSB-OCS system and how it contributes to changing the output spectrum of 40 GHz.

Figure 7. Cont.

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FigureFigure 7. 7. DSB-OCSDSB-OCS electrical electrical output output spectra spectra at different at different optical opticalfiber lengths. fiber ( lengths.a) 0 km; (b (a) )10 0 km; km; (b) 10 km; (c) 20 km; (d) 30 km; (e) 40 km. (c) 20 km; (d) 30 km; (e) 40 km. Figure 7a shows the 40 GHz output spectrum with a 0 dBm peak and 80 dB SNR at 0 Figure7a shows the 40 GHz output spectrum with a 0 dBm peak and 80 dB SNR at km fiber length. Figure 7b shows that the SNR decreases to 54 dB, while the peak of 40 0GHz km remains fiber length. at 0 dBm Figure at 107 kmb shows fiber length. that the At SNRthis length, decreases the components to 54 dB, whileof 20 GHz the peak of 40and GHz 60 GHz remains appear at 0with dBm amplitudes at 10 km fiberof −27 length. dBm and At − this38 dBm, length, respectively. the components Figure 7c of 20 GHz andshows 60 that GHz the appear peak of with40 GHz amplitudes output spectrum of −27 is dBm−5 dBm and and− the38 dBm,SNR value respectively. drops to 44Figure 7c showsdB at 20 that km fiber the peaklength. of The 40 component GHz output amplitudes spectrum of 20 is GHz−5 dBm and 60 and GHz the are SNR −45 dBm value drops toand 44 −36 dB dBm, at 20 respectively. km fiber length. Figure 7d The shows component that the peak amplitudes of the 40 GHz of 20 output GHz spectrum and 60 GHz are −is 45−10 dBm dBm and and −the36 value dBm, of respectively. the SNR is 42 Figure dB at 307d km shows fiber thatlength, the while peak the of thecomponent 40 GHz output spectrumamplitudes is of− 2010 GHz dBm and and 60 GHz the valueare −43 of dBm the and SNR −38 is dBm, 42 dB respectively. at 30 km fiber Finally, length, Figure while the component7e shows that amplitudes the peak of the of 2040 GHzGHz output and 60 spectrum GHz are is− −4314 dBm and and the−38 SNR dBm, is 42 respectively. dB Finally,at 40 kmFigure fiber length,7e shows whereas that the the component peak of the amplitudes 40 GHz output of 20 GHz spectrum and 60 isGHz−14 are dBm −41 and the SNRdBm and is 42 −45 dB dBm, at 40 respectively. km fiber length, It is obvious whereas that thethe 40 component GHz amplitude amplitudes and the ofSNR 20 is GHz and decreasing with the increase in optical fiber length. The amplitude of 40 GHz is reduced 60 GHz are −41 dBm and −45 dBm, respectively. It is obvious that the 40 GHz amplitude to −14 dB after increasing the fiber length to 40 km. Additionally, the amplitude of SNR anddrops the to SNR44 dB isafter decreasing 40 km fiber with length. the increaseTherefore, in the optical effect of fiber optical length. fiber Thedispersion amplitude of 40occurs GHz with is reduced the increase to − in14 optical dB after fiber increasing length. To the investigate fiber length more to into 40 km.dispersion Additionally, pa- the amplituderameter, we ofcan SNR refer drops to the toformula 44 dB [29]: after 40 km fiber length. Therefore, the effect of optical fiber dispersion occurs with the increase in2 optical fiber length. To investigate more into dispersion parameter, we can refer to= the − formula [29]: (8) where D is the dispersion [ps/(nm/km)], C represents2πC the light speed in (ps), λ represents D = − β (8) the wavelength in (nm), and β2 represents the second-orderλ2 2 propagation constant. The D value degrades the RoF system due to the high utilization of optical bandwidth, as dis- whereplayed Din isFigure the dispersion 7. The degradation [ps/(nm/km)], of systemC influencesrepresents more the lighton the speed DSB insystem (ps), thanλ represents thethe OCS. wavelength This because in (nm), the speed and βof2 therepresents sideband the is different second-order from the propagation speed at which constant. the The Dcarriervalue propagates degrades inthe the RoFfiber. systemLater, both due arrive to the with high an introduced utilization delay of optical and inter-mix bandwidth, as displayedwith the optical in Figure signal7 .at The the degradationdetector. Theref ofore, system the system influences would more experience on the power DSB systemloss than theand OCS.signal This broadening, because which the speed makes of it the difficult sideband to decode is different the received from the data. speed Another at which the carrierstudy provides propagates a comprehensive in the fiber. description Later, both of arrive dispersion with and an introducedits influence delayon theand DSB- inter-mix OCS output [30]. with the optical signal at the detector. Therefore, the system would experience power loss Figure 8 illustrates the relationship between the mm-wave signal amplitude in dBm and signal broadening, which makes it difficult to decode the received data. Another study and different optical fiber distance. In this graph, the optical fiber length varies from 0 to provides40 km distance, a comprehensive while the amplitudes description for the of mm-wave dispersion signals and range its influence from 0 dBm on to the −80 DSB-OCS outputdBm. It [can30]. be observed that the graph of 40 GHz is different from 20 GHz and 60 GHz; the 40Figure GHz graph8 illustrates is more the linear relationship than the others between with big the difference mm-wave in signal amplitude amplitude power. in dBm andIn the different graph, the optical 40 GHz fiber level distance. is aboveIn the this other graph, signals, the 20 optical GHz and fiber 60 lengthGHz, with varies a from 0 topower 40 km difference distance, of 28 while dBm theat 10 amplitudes km distance. for Based the on mm-wave this graph, signals we can rangeconclude from that 0 dBm to −the80 DSB-OCS dBm. It system can be is observedcompatible thatmore the with graph a 40 GHz of 40 mm-wave GHz is signal different than fromthe others. 20 GHz and 60The GHz; linearity the 40for GHzsuch grapha system is morecan be linear enhanced than using the others various with techniques, big difference for instance, in amplitude power. In the graph, the 40 GHz level is above the other signals, 20 GHz and 60 GHz, with a power difference of 28 dBm at 10 km distance. Based on this graph, we can conclude that the DSB-OCS system is compatible more with a 40 GHz mm-wave signal than the others. The linearity for such a system can be enhanced using various techniques, for instance, ref. [31] used the direct digital predistortion technique to improve the laser Photonics 2021,, 8,, 153153 1010 of of 12

[31]chirp used and the ﬁber direct dispersion digital inpredistortion long-haul RoFtechnique links. to However, improve the the process laser chirp of converting and fiber dispersionthe signal fromin long-haul the digital RoF formlinks. toHowever, analog formthe process was found of converting to induce the distortion signal from in the digitalsignal [form32]. Additionally, to analog form it iswas possible found toto avoidinduce a distortion dispersion in penalty the signal using [32]. single-sideband Additionally, it(SSB) is possible modulation to avoid and DSBa dispersion in the system penalt [33y ],using or using single-sideband a tunable dispersion-compensation (SSB) modulation and DSBmodule in the [34 system]. [33], or using a tunable dispersion-compensation module [34].

Figure 8. mm-wavemm-wave amplitudes vs. fiber ﬁber lengths.

6. Conclusions Conclusions We studied the modulation index, phase imbalance,imbalance, and dispersion to observe the effects of these parameters on the performanceperformance of an RoF system through the DSB-OCS technique. The The system system successfully successfully generated generated an an optical optical mm-wave mm-wave by by using using DE-MZM. DE-MZM. Several LO LO frequencies frequencies have have been been applied applied to generate to generate a wireless a wireless carrier carrier for the for data the trans- data missiontransmission through through the RoF the systems RoF systems at different at different fiber lengths. fiber lengths. The performance The performance of the system of the hassystem been has tested been with tested various with various modulation modulation indexes indexes where the where maximum the maximum mm-wave mm-wave signal powersignal poweramplitude amplitude can be canachieved be achieved with a withmodu alation modulation index between index between 0.6 and 0.6 1.1. and The 1.1. phase The imbalancephase imbalance and insertion and insertion loss have loss been have theo beenretically theoretically analyzed. analyzed. The performance The performance evalua- evaluations of the system show that the output mm-wave signal power increased when the tions of the system show that the output mm-wave signal power increased when the mod- modulation index of the MZM was increased. The study also shows that phase imbalance ulation index of the MZM was increased. The study also shows that phase imbalance and and optical splitting ratio affect the power of the generated optical mm-wave. Thus, the optical splitting ratio affect the power of the generated optical mm-wave. Thus, the gen- generated mm-wave signal power and the phase imbalance are dependent on the insertion erated mm-wave signal power and the phase imbalance are dependent on the insertion losses and the modulation indexes of the DE-MZM. It was also found that dispersion of losses and the modulation indexes of the DE-MZM. It was also found that dispersion of the optical fiber influences the performance of the system through reducing the mm-wave the optical fiber influences the performance of the system through reducing the mm-wave amplitude and SNR with the increase in fiber length. The system presented has great amplitude and SNR with the increase in fiber length. The system presented has great po- potential in future optical and wireless access networks. tential in future optical and wireless access networks. Author Contributions: S.Y. and R.M.M. studied and simulated all the data; Z.Z. consulted on the Author Contributions: S.Y. and R.M.M. studied and simulated all the data; Z.Z. consulted on the modulation index section, and review and editing; A.M., C.B.M.R. and S.B.A.A. consulted on the modulation index section, and review and editing; A.M., C.B.M.R. and S.B.A.A. consulted on the dispersion section and reviewed the paper. All authors have read and agreed to the published version dispersion section and reviewed the paper. All authors have read and agreed to the published ver- of the manuscript. sion of the manuscript. Funding: This work supported by the Fundamental Research Grant (FRGS), Ministry of Higher Funding: This work supported by the Fundamental Research Grant (FRGS), Ministry of Higher Ed- Education, Malaysia, No. FRGS/1/2019/TK04/UPM/02/07 and Inisiatif Putra Muda Grant by ucation, Malaysia, No. FRGS/1/2019/TK04/UPM/02/07 and Inisiatif Putra Muda Grant by Universiti Universiti Putra Malaysia, No. GP-IPM/2017/9524100. Putra Malaysia, No. GP-IPM/2017/9524100. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Sanchez, M.G. Millimeter-Wave (mmWave) Communications; MDPI: Basel, Switzerland, 2020. 2.1. Sanchez,Chauhan, M.G. S.; Miglani, Millimeter-Wave R.; Kansal, (mmWave) L.; Gaba, Communications G.S.; Masud, M.; PerformanceMDPI: Basel, analysisSwitzerland, and enhancement 2020. of free space optical links for 2. Chauhan,developing S.; state-of-the-art Miglani, R.; Kansal, smart cityL.; Gaba, framework, G.S.; Masud, photonics. M. PePhotonicsrformance2020 analysis, 7, 132. and [CrossRef enhancem] ent of free space optical links for developing state-of-the-art smart city framework, photonics. Photonics 2020, 7, 132.

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