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Article Optical Transmission of an Analog TV-Signal Coded at 2.24 GHz and Its Distribution by Using a Radiating Cable

Ana Gabriela Correa-Mena 1,2 , Jorge Alberto Seseña-Osorio 1, Melissa Eugenia Diago-Mosquera 3, Alejandro Aragón-Zavala 3 and Ignacio Enrique Zaldívar-Huerta 1,* 1 Departamento de Electrónica, Instituto Nacional de Astrofísica, Óptica y Electrónica, Calle Luis Enrique Erro No.1, Tonantzintla, Puebla 72840, Mexico; [email protected] (A.G.C.-M.); [email protected] (J.A.S.-O.) 2 Departamento de Ciencias de la Computación y Electrónica, Universidad Técnica Particular de Loja, San Cayetano Alto, Loja 1101608, Ecuador 3 Escuela de Ingeniería y Ciencias, Tecnológico de Monterrey, Av. Epigmenio González 500, Fracc. San Pablo Querétaro 76130, Mexico; [email protected] (M.E.D.-M.); [email protected] (A.A.-Z.) * Correspondence: [email protected]

 Received: 7 May 2020; Accepted: 25 May 2020; Published: 1 June 2020 

Abstract: In this work, an alternative to extend coverage beyond the conventional methods of providing propagation coverage is presented. The use of a radiating cable is proposed for difficult-to-reach areas. In this regard, an indoor radiating cable is successfully employed for the distribution of an analog electric signal in a fiber-radio scheme using a photonic filter. A filtered microwave band-pass window located at 2.24 GHz is used as an electrical carrier to transmit an analog TV-signal of 67.25 MHz over an of 25.28 km. Measurements are carried out in an indoor environment. Experimental results demonstrate that the recovered signal is of good quality in each measurement location, exhibiting on average a signal-to--ratio (SNR) of around 31.60 dB.

Keywords: indoor propagation; microwave photonic filter; microwave signal; radiating cable

1. Introduction The widespread use of mobile with applications has increased in recent years. This trend has led to an increase in the demand for the delivery of data and video services to a large number of users in optical and wireless access services, and a greater concentration of mobile devices inside buildings, e.g., university campus, shopping centers, airports, and underground environments. Considering the challenges of this explosive growth, wireless optical communications (WOC) are a good option to provide large for users under these scenarios [1]. WOC offers a bundle of advantages, e.g., the is not licensed in the optical band, thus spectrum licensing fees are avoided and system cost can be reduced. Optical in the or visible range is easily contained by opaque boundaries. As a result, interference between adjacent devices can be minimized easily and economically. Although this contributes to the security of wireless optical links and reduces interference, it also impacts rather stringently on the mobility of such devices [2]. Wireless optical links transmit information by using an optoelectronic light modulator and allow its distribution through fiber to users located in indoor environments. In order to provide optimal coverage levels inside buildings, usually, the final distribution in a WOC is implemented through omnidirectional antennas placed at strategic locations. The deployment of these antennas, mainly to address hot-spot areas, may not be possible in outage zones, due to the impossibility of their

Electronics 2020, 9, 917; doi:10.3390/electronics9060917 www.mdpi.com/journal/electronics Electronics 2020, 9, 917 2 of 9

ElectronicsElectronics 2020 2020, ,9 9, ,x x FOR FOR PEER PEER REVIEW REVIEW 22 of of 9 9 installation in specific regions inside venues, their characteristics, and the maximum transmitradiationradiation power pattern pattern limitations. characteristics, characteristics, Therefore, and and the radiatingthe maximum maximum cables transmit transmit have been power power demonstrated limitations. limitations. toTherefore, Therefore, be a viable radiating radiating option tocablescables overcome havehave suchbeenbeen constraints, demonstrateddemonstrated since toto their bebe aa coverage viableviable optionoption footprint toto overcome isovercome uniform suchsuch and canconstraints,constraints, increase since thesince signal theirtheir strengthcoveragecoverage in footprint footprint indoor environments is is uniform uniform and and [3 –can can6]. increase increase the the signal signal strength strength in in indoor indoor environments environments [3–6]. [3–6]. AAA radiating radiatingradiating cable cablecable is isis a aa slotted slottedslotted coaxial coaxialcoaxial cable cablecable run runrun along alongalong specific specificspecific indoor indoorindoor areas, areas,areas, which whichwhich emits emitsemits and andand receivesreceivesreceives radioradio radio waves,waves, waves, operating operating as asas an anan extended extendedextended . antenna.antenna. The The The cable cable cable is is is leaky, leaky, leaky, i.e., i.e., i.e., it it ithas has has gaps gaps gaps or or or slots slots slots in in initsits its outer outer outer conductor conductor conductor to to to allow allow allow the the the radio radio radio signal signal signal to to to leak leak leak into into or or out outout of ofof the thethe cable cablecable along alongalong its its entire entire length.length. length. BecauseBecauseBecause of ofof this thisthis leakage leakageleakage of ofof signal, signal,signal, line lineline amplifiers amplifiersamplifiers areareare required requiredrequired to toto be bebe inserted insertedinserted at atat regular regularregular intervals, intervals,intervals, typicallytypicallytypically everyevery every 350 350 350 meters meters meters to to to 500 500 500 meters, meters, meters, to to enhanceto enhance enhance the the the signal signal signal back back back up up toup acceptable to to acceptable acceptable levels levels levels [7]. On[7]. [7]. theOn On otherthethe other other hand, hand, hand, an inherentan an inherent inherent feature feature feature of Microwaveof of Microwave Microwave Photonic Ph Photonicotonic Filters Filters Filters (MPFs) (MPFs) (MPFs) is is is that that that microwave microwave microwave signalssignals signals areareare directlydirectly directly processed processed in inin the the optical optical domain, domain, domain, exploiting exploiting exploiting advantages advantages advantages inherent inherent inherent to to tophotonics photonics such such such as as low low as lowloss,loss, loss, high high highbandwidth, bandwidth, bandwidth, immunity immunity immunity to to electromagneti electromagneti to electromagneticcc interference, interference, interference, and and tunability tunability and tunability [8], [8], making making [8], making them them a a themveryvery interesting ainteresting very interesting choice choice compared compared choice compared to to conventional conventional to conventional el electricalectrical electricalfilters. filters. The The filters. factors factors The previously previously factors previously described, described, described,togethertogether with with together the the increasing increasing with the increasing demand demand for demandfor multiple multiple for communications multiple communications communications applications applications applications with with a a great great with amount aamount great amountofof associatedassociated of associated information,information, information, justifyjustify justify thethe introductionintroduction the introduction ofof MPFsMPFs of MPFs intointo into thethe the accessaccess access opticaloptical optical networks networksnetworks [[9].9[9].]. Furthermore,Furthermore,Furthermore, a a comparisona comparisoncomparison between betweenbetween our our proposedour propospropos fiber-radioeded -radiofiber-radio scheme schemescheme with the withwith WOC’s thethe works WOC’sWOC’s reported worksworks inreportedreported the last in in few the the yearslast last few few is presentedyears years is is presented presented in Table1 in .in TheTa Tableble parameters 1. 1. The The parameters parameters considered considered considered are the frequencyare are the the frequency of the electricalofof the the electrical electrical carrier, carrier, carrier, the signal-to-noise-ratio the the signal-to- signal-to-noise-rationoise-ratio of the of recoveredof the the recovered recovered signal signal signal (SNR RX(SNR (SNR), theRXRX),), fiber the the fiber length,fiber length, length, and and theand transmisionthethe transmision transmision and and receptionand reception reception of the of of signalthe the signal signal by using by by using using radiating radiating radiating cable cable cable and antennas,and and antennas, antennas, respectively. respectively. respectively.

TableTableTable 1.1. 1. RelevantRelevant Relevant worksworks works thatthat that considerconsider consider wirelesswireless wireless opticaloptical optical communicationscommunications communications (WOC).(WOC). (WOC).

SNRSNRRXRX FiberFiberFiber Coil CoilCoil Ref.Ref.Ref. CarrierCarrierCarrier (GHz) (GHz) (GHz) SNR (dB) RadiatingRadiatingRadiating Cable Cable Wireless Wireless Wireless Reception Reception (dB)RX(dB) (km)(km) 200720072007 [ 10[10] [10]] 60 60 60 26 26 26 SMF: SMF: 20 2020   2008 [11] 4 - SMF: 4 20082008 [11] [11] 4 4 - - SMF: SMF: 4 4   2015 [12] 2.27; 4.54 38.25; 37.83 SM-SF: 25.24 201620152015 [[12]3 [12]] 2.27;0.9–2.3 2.27; 4.54 4.54 38.25; 38.25;39 37.83 37.83 SM-SF: SM-SF: SM-SF: 25.24 25.2425.24   ≈ 201720162016 [13 [3] [3]] 0.9–2.3 2.40.9–2.3 38.8≈≈ 39 39 SM-SF: SM-SF: SM-SF: 25.24 25.24 30   2018 [14] 60 - SMF: 2.2 201820172017 [ 15[13] [13]] 60 2.4 2.4 38.8 38.8- SM-SF: SM-SF: SMF: 2.2 30 30   This20182018 work [14] [14] 2.24 60 60 36.53 - - SM-SF: SMF: SMF: 2.2 25.282.2   20182018 [15] [15] 60 60 - - SMF: SMF: 2.2 2.2   Considering the main goal of this work is to demonstrate the feasibility of distribution by a ThisThis work work 2.24 2.24 36.53 36.53 SM-SF: SM-SF: 25.28 25.28   radiating cable of an analog TV-signal in a fiber-radio scheme inside a building, a band-pass window at 2.24 GHz generated by a microwave photonic filter is used to code on the analog TV signal of ConsideringConsidering the the mainmain goalgoal ofof this this workwork isis to to demonstratedemonstrate thethe feasibility feasibility ofof distributiondistribution byby aa 67.25 MHz. This TV signal is transmitted over an optical link of 25.28 km for its further distribution by radiatingradiating cable cable of of an an analog analog TV-signal TV-signal in in a a fiber-radio fiber-radio scheme scheme inside inside a a building, building, a a band-pass band-pass window window a radiating cable of 5.50 m length. The novelty of this work resides in the integration of the radiated atat 2.24 2.24 GHz GHz generated generated by by a a microwave microwave photonic photonic filter filter is is used used to to code code on on the the analog analog TV TV signal signal of of cable technique with the fiber scheme to transmit and distribute electrical signals, using optoelectronic 67.2567.25 MHz. MHz. This This TV TV signal signal is is transmitted transmitted over over an an optical optical link link of of 25.28 25.28 km km for for its its further further distribution distribution techniques and its further radio distribution in an indoor environment. Thus, this fiber-radio scheme byby a a radiating radiating cable cable of of 5.50 5.50 m m length. length. The The novelty novelty of of this this work work resides resides in in the the integration integration of of the the radiated radiated could be used as part of a wireless optical system. cablecable techniquetechnique withwith thethe fiberfiber schemescheme toto transmittransmit andand distributedistribute electricalelectrical signals,signals, usingusing The paper is organized as follows: Section2 provides the basic theory of the MPF operation optoelectronicoptoelectronic techniquestechniques andand itsits furtherfurther radioradio distdistributionribution inin anan indoorindoor environment.environment. Thus,Thus, thisthis principle. Moreover, the experimental frequency response of the characterized MPF is presented. fiber-radiofiber-radio scheme scheme could could be be used used as as part part of of a a wireless wireless optical communication system. system. The experimental procedure for the transmission, distribution, and recovery of the analog TV-signal is TheThe paper paper is is organized organized as as follows: follows: Section Section 2 2 pr providesovides the the basic basic theory theory of of the the MPF MPF operation operation described in Section3. Finally, the main conclusions are included in Section4. principle.principle. Moreover, Moreover, the the experimental experimental frequency frequency resp responseonse of of the the characterized characterized MPF MPF is is presented. presented. The The 2.experimentalexperimental Microwave procedure Photonicprocedure Filter for for the the tran transmission,smission, distribution, distribution, and and recove recoveryry of of the the analog analog TV-signal TV-signal is is describeddescribed in in Section Section 3. 3. Finally, Finally, the the main main conclusions conclusions are are included included in in Section Section 4. 4. This section is divided in two subsections. First, the main equations that describe the frequency response2.2. Microwave Microwave of the Photonic Photonic MPF are Filter detailed.Filter In the second subsection, the MPF is characterized to obtain the electrical parameters of the filtered band-pass windows that will be used as electrical carriers. ThisThis section section is is divided divided in in two two subsections. subsections. First, First, the the main main equations equations that that describe describe the the frequency frequency responseresponse of of the the MPF MPF are are detailed. detailed. In In the the second second su subsection,bsection, the the MPF MPF is is characterized characterized to to obtain obtain the the electricalelectrical parameters parameters of of the the filtered filtered band-pass band-pass wi windowsndows that that will will be be used used as as electrical electrical carriers. carriers.

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2.1. Principle of Operation 2.1. Principle of Operation Previously, the authors of [16] reported the use of a basic architecture of MPFs that consist of Previously, the authors of [16] reported the use of a basic architecture of MPFs that consist of four four basic components: an optical source, a modulator, a fiber coil, and a photodetector. The basic components: an optical source, a modulator, a fiber coil, and a photodetector. The frequency frequency response of this MPF is constituted of a series of microwave band-pass windows that response of this MPF is constituted of a series of microwave band-pass windows that depend on the depend on the fiber chromatic , fiber length, and Fourier transform of the spectral density fiber chromatic dispersion, fiber length, and Fourier transform of the spectral density of the optical of the optical source. In particular, when a Multimode Diode (MLD) is used as an optical source. In particular, when a Multimode (MLD) is used as an optical source, the theoretical source, the theoretical center frequency of the nth filtered band-pass window is computed as [16] center frequency of the nth filtered band-pass window is computed as [16] 1 =   1 (1) fn = n (1) where n is a positive integer (n = 1, 2, …), δλ (nm) is DLtheδλ Free Spectral Range (FSR) between the modes ofwhere the MLD,n is a positiveand L (km) integer and (Dn =(ps/nm·km)1, 2, . . . ), δλ are(nm) the islength the Free and Spectral the chromatic Range (FSR)dispersion between of the the Single modes Mode-Standardof the MLD, and FibreL (km) (SM-SF), and D (psrespectively./nm km) are The the bandwidth length and at the −3dB chromatic of the dispersionnth filtered of band-pass the Single · windowMode-Standard is obtained Fibre as (SM-SF),[16] respectively. The bandwidth at 3dB of the nth filtered band-pass − window is obtained as [16] 4ln(2) Δ = p (2) Δ4 ln(2) ∆ fbp = (2) where Δλ (nm) is the spectral width of the optical source.πDL∆λ where ∆λ (nm) is the spectral width of the optical source. 2.2. Experimental MPF Characterization 2.2. Experimental MPF Characterization Figure 1 shows the microwave photonic filter scheme that is characterized. The MPF is composedFigure of1 showsan optical the microwave source MLD, photonic a Polariza filter schemetion Controller that is characterized. (PC), a Mach–Zehnder The MPF is composed Modulatorof an optical (MZ-IM), source a MLD, Microwave a Signal Generator Controller (MSG), (PC), aan Mach–Zehnder SM-SF, and a Photo Intensity Detector Modulator (PD). The(MZ-IM), Electrical a Microwave Spectrum Signal Analyzer Generator (ESA) (MSG),is used anto SM-SF,measure and the a frequency Photo Detector response (PD). of The the Electrical system. TheSpectrum MLD is Analyzer driven at (ESA) 25 °C is by used a to measure thecontroller frequency and response operated of with the system.a well-stabilized The MLD injection is driven currentat 25 ◦C of by 20 amA. temperature Under this controller condition, and its op operatedtical characteristics with a well-stabilized are central injection current λ0 = 1547.2 of 20 nm, mA. ΔλUnder = 7.31 this nm, condition, and δλ = its 1.1 optical nm. characteristics are central wavelength λ0 = 1547.2 nm, ∆λ = 7.31 nm, and δλ = 1.1 nm.

Figure 1. Scheme of the microwave photonic filter.

The light issued of theFigure MLD 1. (Thorlabs, Scheme of LPS-1550-FC) the microwave passesphotonic through filter. the PC. Subsequently, it is injected to the MZ-IM (MXAN-LN-20, insertion loss of 2.7 dB, operating wavelength of 1530 nm to 1580The nm) light where issued it is intensity-modulated of the MLD (Thorlabs, by an LPS-1550- RF signalFC) supplied passes bythrough the MSG the in PC. the Subsequently, frequency range it is of injected0.01 GHz to to the 10 MZ-IM GHz at an(MXAN-LN-20, electrical power insertion of 5 dBm. loss The of 2.7 modulated dB, operating light travels wavelength along 25.28of 1530 km nm of theto 1580SM-SF nm) (α where= 0.2 dB it is/km, intensity-modulated D = 15.81 ps/nm km by @ an 1500 RF signal nm). At supplied the output by the of theMSG fiber, in the the frequency PD (DR-125G-A, range · of30 0.01 kHz GHz to 12.5 to GHz)10 GHz converts at an electrical the light power to its corresponding of 5 dBm. The photo-current. modulated light This travels experiment along 25.28 is carried km ofout the at SM-SF the maximum (α = 0.2 dB/km, PD frequency D = 15.81 of 10ps/nm·km GHz. Finally, @ 1500 the nm). electrical At the signaloutput at of the the PD’s fiber, output the PD is (DR-125G-A,amplified and 30 connected kHz to 12.5 to the GHz) ESA converts (Anritsu, the MG3692, light to frequency its corresponding range 0.01 photo-current. GHz to 20 GHz) This to experiment is carried out at the maximum PD frequency of 10 GHz. Finally, the electrical signal at measure the MPF’s frequency response. This experimental frequency response is formed by four the PD’s output is amplified and connected to the ESA (Anritsu, MG3692, frequency range 0.01 GHz microwave band-pass windows, as shown in the curve of Figure2. The center frequency of to 20 GHz) to measure the MPF’s frequency response. This experimental frequency response is each filtered band-pass window is f 1 = 2.24 GHz, f 2 = 4.40 GHz, f 3 = 6.62 GHz, and f 4 = 8.86 GHz. formed by four microwave band-pass windows, as shown in the blue curve of Figure 2. The center On the same graph, the black curve corresponds to the theoretical frequency response obtained by frequency of each filtered band-pass window is f1 = 2.24 GHz, f2 = 4.40 GHz, f3 = 6.62 GHz, and using the VPIphotonics software [17], and it is in good concordance with the experimental response. f4 = 8.86 GHz. On the same graph, the black curve corresponds to the theoretical frequency response obtained by using the VPIphotonics software [17], and it is in good concordance with the

ElectronicsElectronics 2020,2020 9, x, FOR9, 917 PEER REVIEW 4 of 94 of 9 experimental response. Additionally, the increase in observed in the frequency response plot asAdditionally, frequency the increases increase is in justified attenuation by observedthe influence in the of frequency the envelope response of plotthe assource frequency power increases spectrum, is justified by the influence of the envelope of the source power spectrum, as was demonstrated by the as was demonstrated by the authors of [13]. The interested reader can see this reference for a detailed authors of [13]. The interested reader can see this reference for a detailed explanation. explanation.

FigureFigure 2. Frequency 2. Frequency response response of of the microwave photonic photonic filter. filter.

Table2 summarizes the electrical characteristics of the filtered band-pass windows. Relevant Table 2 summarizes the electrical characteristics of the filtered band-pass windows. Relevant parameters shown are: the center frequency and bandwidth of each band-pass window computed parameters shown are: the center frequency and bandwidth of each band-pass window computed by by using Equations (1) and (2), respectively; the Signal-to-Noise-Ratio (SNR) and the related error using Equations (1) and (2), respectively; the Signal-to-Noise-Ratio (SNR) and the related error percentage between the theoretical and experimental center frequency fn are calculated as percentage between the theoretical and experimental center frequency fn are calculated as

fn,theoretical −fn, experimental %error, f = ,theoretical− ,experimental100% (3) % error, n = × 100% fn theoretical × (3) ,,theoretical

TableTable 2. Electrical 2. Electrical characteristics characteristics of thethe filteredfiltered band-pass band-pass windows. windows. Theoretical Experimental Theoretical Experimental Experimental FrequencyFrequency Theoretical Experimental Theoretical Experimental % % Experimental fn fnf n (GHz) ffnn (GHz) ∆f bpΔf(MHz)bp ∆f bpΔ(MHz)fbp Error SNR (dB)SNR fn Error f 1 (GHz)2.27 (GHz) 2.24 (MHz) (MHz) 1.32 13.27(dB) f 2 4.55 4.40 3.29 12.79 f1 2.27 2.24 362.82 350 1.32 13.27 f 3 6.82 6.62 2.93 12.11 f 4 9.10 8.86 2.63 11.15 f2 4.55 4.40 3.29 12.79 362.82 350 f3 6.82 6.62 12.11 Additionally, from Table2 we notice that the average of the electrical bandwidth2.93 exhibited by the

filteredf4 bandpass9.10 windows is around8.86 350 MHz. This bandwidth is enough to code2.63 an analog TV-signal 11.15 of 6 MHz on any filtered band-pass [18]. In this work, the first microwave band-pass window of 2.24 GHz is selected as an electrical carrier. However, potentially the other band-pass windows could Additionally, from Table 2 we notice that the average of the electrical bandwidth exhibited by be used as electrical carriers. the filtered bandpass windows is around 350 MHz. This bandwidth is enough to code an analog TV-signal3. Experimental of 6 MHz Results on any filtered band-pass [18]. In this work, the first microwave band-pass window Forof 2.24 a better GHz understandingis selected as an of electrical the results, carrier. this sectionHowever, is dividedpotentially in threethe other subsections. band-pass windowsFirst, could the experimental be used as setupelectrical for thecarriers. transmission of the analog TV-signal by radiating cable is presented. The characteristics of the indoor environment as well as the position of the omnidirectional 3. Experimental Results For a better understanding of the results, this section is divided in three subsections. First, the experimental setup for the transmission of the analog TV-signal by radiating cable is presented. The characteristics of the indoor environment as well as the position of the omnidirectional antenna that will receive the signal transmitted by the radiating cable are described next. Finally, the measured electrical spectrum for the transmitted and recovered analog TV-signal are shown.

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3.1. Experimentalantenna that willSetup receive the signal transmitted by the radiating cable are described next. Finally, the measured electrical spectrum for the transmitted and recovered analog TV-signal are shown. Figure 3 depicts the proposed experimental setup for the transmission of the analog TV-signal using3.1. radiating Experimental cable. Setup This setup is based on the scheme of Figure 2; in order to have the complete fiber-radioFigure scheme,3 depicts some the devices proposed are experimental added. Hence, setup the for light the transmissionissued by the of MLD the analog passes TV-signal through the PC andusing is radiatinginjected cable.to the ThisMZ-IM. setup The is based MSG on provides the scheme the of RF Figure signal2; in of order 2.24 to GHz have at the 15 complete dBm that is mixedfiber-radio (Mixer scheme,1) with somethe amplified devices are TV-signal added. Hence, of 67.25 the light MHz issued (Channel by the MLD4) delivered passes through by the the NTSC (BK PCPrecision, and is injected Model to 1249B) the MZ-IM. generator The MSG (National provides the RF signal System of 2.24 Committee) GHz at 15 dBm [18]. that The is mixed resulting electrical(Mixer signal 1) with used the amplifiedto modulate TV-signal the light of 67.25 given MHz by (Channel the MLD 4) is delivered connected by the to NTSCthe RF (BK port Precision, of the MZ- IM. TheModel modulated 1249B) generator optical (Nationalsignal is Televisionlaunched Systeminto the Committee) coil SM-SF [18 of]. 25.28 The resulting km. Afterward, electrical the signal optical signalused travels to modulate 25.28 km, thelight and givenit is injected by the MLD to the is connected PD that toconverts the RF port the of optical the MZ-IM. to the The electrical modulated signal. This opticalfinal signal signal is is amplified launched into and the matched coil SM-SF to of5.50 25.28 m km.of the Afterward, radiating the cable optical (RADIAFLEX® signal travels 25.28 model km, RCF 12–50and J, itmanufactured is injected to the by PD Radio that converts Frequency the optical Systems, to the maximum electrical signal. operating This final frequency signal is amplified 6 GHz) [19]. and matched to 5.50 m of the radiating cable (RADIAFLEX®model RCF 12–50 J, manufactured by The Leaky (LCX) is terminated with a matched 50 Ω load to avoid any unwanted Systems, maximum operating frequency 6 GHz) [19]. The Leaky Coaxial Cable (LCX) reflections. On the other hand, the receiving stage is composed of an omnidirectional antenna is terminated with a matched 50 Ω load to avoid any unwanted reflections. On the other hand, the (Kathrein,receiving gain stage G = is 2 composed dBi). The ofelectrical an omnidirectional signal captured antenna by (Kathrein, the isG amplified= 2 dBi). The and electrical plugged to Mixersignal 2 in capturedorder to bysuppress the antenna the carrier is amplified signal and of plugged2.24 GHz to and Mixer recover 2 in order the toTV-signal. suppress theThe carrier quality of the recoveredsignal of 2.24 TV-signal GHz and is recover measured the TV-signal. by the ESA. The quality of the recovered TV-signal is measured by the ESA.

Figure 3. Experimental setup of the proposed fiber-radio scheme. Figure 3. Experimental setup of the proposed fiber-radio scheme. 3.2. Measurement Locations Map 3.2. MeasurementFigure4 corresponds Locations Map to the architectonic layout of the laboratory where the electrical measurements were carried out. According to the bibliography [20], it is very important to indicate the materials usedFigure in the4 constructioncorresponds of to the the laboratory, architectonic objects, andlayo distributionut of the of laboratory furniture placed where at thethe interior electrical measurementsto observe thewere effects carried of absorption, out. According dispersion, to the and bibliography of the [20], signal. it is The very measurements important to of indicate the the materialsanalog TV-signal used in transmissionthe construction by using of the the labora radiatingtory, cable objects, are madeand distribution inside the INAOE’s of furniture optical placed at thecommunications interior to observe lab. The wallsthe andeffects floor of are absorp made withtion, drywall dispersion, and vinyl and tile, fading respectively. of the The signal. ceiling The measurementsis constructed of the of steel analog decks TV-signal and metallic transmission beams. It by is 4using m high the and radiating has a suspended cable are made false ceiling inside the INAOE’sat 2.60 optical m. The communications radiating cable is lab. installed The walls over theand false floor ceiling, are made and it with is laid drywall along one and path vinyl of tile, respectively.5.50 m. Inside The theceiling laboratory, is constructed there are opticalof steel and decks electrical and equipment,metallic beams. computers, It is desks,4 m high and chairs.and has a suspendedMoreover, false the ceiling points inat 2.60 show m. theThe position radiating where cable the is omnidirectional installed over antenna the false is installed ceiling, toand receive it is laid along one path of 5.50 m. Inside the laboratory, there are optical and electrical equipment, computers, desks, and chairs. Moreover, the points in red show the position where the omnidirectional antenna is installed to receive the TV signal delivered by the radiating cable. These measuring locations (ml) are labeled from 1 to 6. Between each, there is 1 m of distance to cover all the available lab’s area.

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Electronicsthe TV 2020 signal, 9, x FOR delivered PEER REVIEW by the radiating cable. These measuring locations (ml) are labeled from 1 to 6.6 of 9 Between each, there is 1 m of distance to cover all the available lab’s area. Electronics 2020, 9, x FOR PEER REVIEW 6 of 9

FigureFigure 4.4. Measurement locations map.map. Figure 4. Measurement locations map. 3.3. Results 3.3. Results 3.3. ResultsThe technical criteria to validate the feasibility of the distribution of an analog TV-signal by using The technical criteria to validate the feasibility of the distribution of an analog TV-signal by using a radiating cable are focused on evaluating the SNRRX of the recovered signal for each ml. Figure5a,b aThe radiating technical cable criteria are focused to validate on evaluating the feasibility the SNR ofRX theof the distribution recovered signalof an analogfor each TV-signal ml. Figure by5a,b using shows the measured electrical spectrum for the transmitted as well as the recovered analog TV-signal a radiatingshows thecable measured are focused electrical on evaluatingspectrum for the the SNR transmRX ofitted the as recovered well as the signal recovered for eachanalog ml. TV-signal Figure 5a,b of 67.25 MHz. The measured average of the SNR value for the transmitted and recovered analog showsof the67.25 measured MHz. The electrical measured spectrum average forof the the SNR transm valueitted for as the well transmitted as the recovered and recovered analog analog TV-signal TV-signal was 61.95 dB and 31.60 dB, respectively. The obtained SNRRX values are in good agreement of 67.25TV-signal MHz. was The 61.95 measured dB and 31.60average dB, respectively.of the SNR Thevalue obtained for the SNR transmittedRX values are and in good recovered agreement analog with the works reported in references [3,16]. The results demonstrate that the recovered signal presents TV-signalwith the was works 61.95 reporteddB and 31.60 in references dB, respectively. [3,16]. Th Thee results obtained demonstrate SNRRX values that the are recovered in good agreementsignal a good quality in each ml, highlighting that the best location is in ml4 with a SNRRX of 36.53 dB and a presents a good quality in each ml, highlighting that the best location is in ml4 with a SNRRX of 36.53 dB withreceived the works electrical reported power in of references22 dBm, approximately. [3,16]. The results demonstrate that the recovered signal and a received electrical power− of −22 dBm, approximately. presents a good quality in each ml, highlighting that the best location is in ml4 with a SNRRX of 36.53 dB and a received electrical power of −22 dBm, approximately.

(a) (b) Figure 5. Cont. (a) (b)

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(c) (d)

(e) (f)

Figure 5. Electrical spectrum for the transmitted and recovered analoganalog TV-signal forfor eacheach ml.ml. (a) ml1:: SNRRX == 30.9930.99 dB; dB; ( (bb)) ml ml22: :SNR SNRRXRX = =26.0426.04 dB; dB; (c ()c ml) ml3: 3SNR: SNRRXRX = 33.67= 33.67 dB; dB; (d) ( dml) ml4: SNR4: SNRRX =RX 36.53= 36.53 dB; dB;(e) (mle)5 ml: SNR5: SNRRX = RX32.59= 32.59 dB; and dB; ( andf) ml (f6:) SNR ml6:RX SNR = 29.82RX = dB.29.82 dB.

4. Conclusions A fiber-radiofiber-radio scheme using a microwave photonic filter filter and a radiating cable to distribute an analog TV-signal was successfully demonstrated. The novelty of this work resides in the integration of the radiated cable technique with the fiberfiber schemescheme toto transmittransmit informationinformation using optoelectronic techniques and its further radioradio distributiondistribution inin anan indoorindoor environment.environment. In In particular, particular, the analog TV-signal isis codedcoded onon anan electricalelectrical carriercarrier at 2.242.24 GHz, and it is transmitted through an optical link of 25.28 km for itsits furtherfurther distributiondistribution throughthrough 5.505.50 mm ofof radiatingradiating cable.cable. In this experiment, the location ofof thethe bandpassbandpass windowwindow atat 2.24 2.24 GHz GHz present present in in the the frequency frequency response response of of the the MPF MPF is fixedis fixed at aat particular a particular optical optical fiber fiber length length of 25.28 of km.25.28 However, km. However, the versatility the versatility of the proposed of the proposed scheme resides scheme in theresides fact in that the the fact center that frequencythe center offrequency the nth filtered of the n band-passth filtered windowsband-pass could windows be tuned could to be a particular tuned to value,a particular as is indicated value, as inis Equationindicated (1).in Equation On the other (1). On hand, the in other order hand, to emulate in order the to indoor emulate environment, the indoor theenvironment, distribution the of thedistribution TV-signal of was the carried TV-signa outl insidewas carried the INAOE’s out inside optical the communications INAOE’s optical lab. Experimentalcommunications measurements lab. Experimental have shownmeasurements that the recoveredhave shown TV-signal that the inrecovered all the measuring TV-signal locationsin all the havemeasuring acceptable locations SNR have values acceptable (31.60 dB onSNR average). values (31.60 The transmit dB on poweraverage). level The within transmit the experimentalpower level frequencywithin the rangeexperimental satisfies thefrequency IEEE C95.1-1999 range satisfie Standards the SafetyIEEE LevelsC95.1-1999 for preventing Standard harmfulSafety Levels effects for in humanspreventing exposed harmful to electromagnetic effects in humans fields exposed in the 3to kHz electromagnetic to 300 GHz frequency fields in range the 3 [21kHz]. Furthermore,to 300 GHz thisfrequency fiber-radio range system [21]. Furthermore, operating at this the fiber-radio ISM frequency system band operating (2.4 GHz) at the is characterized ISM frequency by band its short (2.4 coverageGHz) is characterized range and low by power, its short guarantying coverage range noninterference and low power, with other guarantying equipment, noninterference especially, medical with other equipment, especially, medical equipment [22]. Finally, it is important to remark that we

Electronics 2020, 9, 917 8 of 9 equipment [22]. Finally, it is important to remark that we decided to transmit and distribute an analog TV-signal because in Mexico the analog shutdown occurred some years ago. However, this fiber-radio scheme could be used to transmit different IP services, such as high definition television (HDTV), video gaming, and voice.

Author Contributions: Conceptualization and methodology, J.A.S.-O., A.G.C.-M., and I.E.Z.-H.; formal analysis, J.A.S.-O., A.G.C.-M., M.E.D.-M., A.A.-Z., and I.E.Z.-H.; investigation, J.A.S.-O. and A.G.C.-M.; writing—original draft preparation, A.G.C.-M., A.A.-Z., and I.E.Z.-H.; writing—review and editing, M.E.D.-M., A.A.-Z., and I.E.Z.-H.; visualization, M.E.D.-M.; supervision, A.A.-Z. and I.E.Z.-H.; project administration, A.A.-Z and I.E.Z.-H.; funding acquisition, I.E.Z.-H. All authors have read and agreed to the published version of the manuscript. Funding: This research did not receive external funding. Acknowledgments: A. G. Correa-Mena, J. A. Seseña-Osorio, and M. E. Diago-Mosquera wish to thank CONACyT for the student scholarships number 335148, 204357, and 746015, respectively. Conflicts of Interest: The authors declare no conflict of interest.

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