Journal of Physics: Conference Series. Related content. To cite this article: A García-Juárez et al 2011 J. Phys.: Conf. Ser.

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1 Journal of Physis: Conferene Series Coherent demodulation of mirowave signals by using optial heterodyne tehnique with appliations to point to point indoor wireless ommuniations systems To ite this artile: A Garía-Juárez et al 0 J. Phys.: Conf. Ser Related ontent - Photoni filtering of mirowave signals in the frequeny range of GHz using a Fabry-Perot filter G Aguayo-Rodríguez, I E Zaldívar-Huerta, A Garía-Juárez et al. - Seure optoeletroni ommuniation using laser diode driving by haoti Rössler osillators R Jaimes-Reátegui, R Sevilla-Esoboza, A N Pisarhik et al. - Color multiplexing using diretional holographi gratings and linear polarization L I Lugo, A Rodríguez, G Ramírez et al. View the artile online for updates and enhanements. This ontent was downloaded from IP address on /04/08 at 06:48

2 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 Coherent demodulation of mirowave signals by using optial heterodyne tehnique with appliations to point to point indoor wireless ommuniations systems. A Garía-Juárez, I E Zaldívar-Huerta, G Aguayo-Rodríguez, J Rodríguez- Asomoza 3, M R Gómez-Colín, A G Rojas-Hernández Universidad de Sonora (Méxio). Instituto Naional de Astrofísia, Óptia y Eletrónia (Méxio). 3 Universidad de las Amérias-Puebla (Méxio). E mail: agaria@ifus.uson.mx Abstrat. An optial ommuniations system using a ouple mirostrip antennas for distributing point to point analog TV with oherent demodulation based on optial heterodyne in lose viinity is reported in this paper. In the proposed experimental setup, two optial waves at different wavelengths are mixed and applied to a photodetetor. Then a beat signal with a frequeny equivalent to the spaing of the two wavelengths is obtained at the output of the photodetetor. This signal orresponds to a mirowave signal loated at.5 GHz, whih it is used as a mirowave arrier in the transmitter and as a loal osillator in the reeiver of our optial ommuniation system. The feasibility of this tehnique is demonstrated transmitting a TV signal of 66-7MHz. Keywords: Mirowave photonis, optial heterodyne, mirowave generation, mirowave arriers, wireless ommuniations, proessing of mirowave signals, oherent demodulation..-introdution Over the past few years, there has been an inreasing effort in researhing new design of indoor wireless ommuniations systems, due to onnetivity that they show in a room or in a building. Currently, several ompanies of teleommuniations use purely omnidiretional antennas in their wireless routers to transmit data to laptops in lose viinity []. The properties of mirostrip path antennas and arrays with their planar onfiguration exhibit an attrative option for indoor ommuniations where the gain is onsiderably enhaned. On the other hand, the generation of mirowave and millimetre-wave (mm-wave) signals by using photoni tehnique are being used in radio-over-fibre (RoF) systems, distribution antenna systems, broadband wireless aess networks, and radar systems et. In all these appliations the mirowave signals are generated at a remote entral station and distributed transparently to several simplified antenna stations via optial fiber []. Further author information: (orresponding author Alejandro Garía Juárez) agaria@ifus.uson.mx Published under liene by IOP Publishing Ltd

3 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 The main goal of these systems is to redue infrastruture ost and to overome the apaity bottlenek in wireless aess networks, allowing, at the same time, flexible merging with onventional optial aess networks. Thus, in order to design a reliable RoF-based aess network infrastruture, RoF tehniques must be apable of generating the mirowave signals and allow a reliable mirowave signals transmission over the optial link. For this purpose, several RoF tehniques have been proposed in the past few years. For broadband wireless systems and distribution antenna systems operating at mirowave and millimeter-wave arriers, several photoni tehniques for generating mirowave signals have been proposed. Among the most ommon used tehniques are: optial heterodyning [3], optial injetion loking [4], optial frequeny/phase loked loops (OFLL/OPLL) [5], mirowave generation using external modulation [6]. Optial injetion loking [7] and optial phase-loked loops (OPLL) [8] are expensive in pratie. The use of external intensity modulation generates frequeny doubling or quadrupling of the driven RF sinusoid signal [9]. This method requires an external modulator whih inreases both loss and ost, and is more suseptible to bias drifting of the modulators, whih an affet the output spetrum. The key advantage for generating mirowave or millimeter-wave signals by optial means is that very high-frequeny signals with very low phase noise and high purity an be generated. By using optial heterodyne tehnique it is very easy to tune frequenies with a spetral linewidth of a few ten MHz and over a wide range by simply tuning the wavelength of the two optial input signals; the obtained frequenies are limited only by the photodetetor bandwidth [0]. Besides, the generated signals by using this tehnique an be generally used as both information arriers, and as a loal osillator for transmitting and reeiving both analog and digital information signals by using not only RF shemes but also through an optial fiber. In this sense, an optial ommuniations system using a ouple mirostrip antennas for transmitting and reeiving information with oherent demodulation based on optial heterodyne sheme is proposed in this paper, where our main goal is to show potential appliations of the photoni generation of mirowave signals for distributing point to point analog TV signals by using mirostrip antennas. The remainder of this paper is strutured as follows. A brief theoretial desription of the optial heterodyne tehnique and the experimental sheme for generating mirowave signals is presented in setion. A theoretial desription of the modulation and demodulation will be presented in setion 3. The proposed transmission system is desribed in setion 4. Finally, we show our onlusion to this work in setion 5..- Optial heterodyne tehnique The basi priniple for generating mirowave arriers is based on optial heterodyne tehnique, it represents a physial proess alled optial beating or frequeny beating, where two phase-loked optial soures with angular frequenies and are superimposed and injeted into a high frequeny photodetetor that permits to obtain a photourrent at a frequeny. To explain this in more detail, let us onsider the relation between the generated eletrial output signal and the two superimposed optial input waves from a more physial point of view. For simpliity, we assume that the two optial input waves are linearly polarized monohromati plane waves in the infrared whih propagate in the +z diretion. Let and E E ˆ E exp ˆ E exp t k z e i, () t k z e i, () be the omplex eletrial field vetors of the two optial waves, with field amplitudes Ê and Ê, angular frequenies and and wave numbers k and k. The phase of eah optial input wave is onsidered by and and e and e are the unit vetors determining the orientation of the

4 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 eletrial field vetor of the linearly polarized optial input waves. The intensities of the onstituent waves are given by the magnitude of their Poynting vetors and are therefore given by [] I o E r. (3) o I o E r. (4) o If the two inident optial waves are perfet plane waves and have preisely the same polarization ( e = e ), the resulting eletrial field Eo of the optial interferene signal is the sum of the two onstituent input fields and hene we an write Eo E E. Taking the squared absolute value of the optial interferene signal we obtain E o E E E E EE E E. (5) E E E E os ( ) t ( ) * * From equation (5) and by using equations (3) and (4), it follows that the intensity of the interferene signal I o is given by [] I I os( ) t ( ) I o I. (6) I By launhing this optial interferene signal into a photodetetor, a photourrent i is generated whih an be expressed as [] oq oq i P P hf hf f q P P h f f os ( ) t ( ), (7) where q is the eletron harge and P and P denote the optial power levels of the two onstituent optial input waves. The photodetetor s DC and high-frequeny quantum effiienies are represented by and. It is of ourse important to onsider that the detetor s quantum effiieny is not o f independent of the frequeny. Several intrinsi and extrinsi effets suh as transit time limitations or mirowave losses will eventually limit the high-frequeny performane of the detetor and thus the detetor s DC responsivity o is typially muh larger than its high-frequeny responsivity f. In our ase, we an further simplify the photourrent equation (equation (7)) by onsidering the fat that the two optial input waves are lose in frequeny ( f f ) whereas the differene frequeny f is by far smaller ( f f f f, f ). If we further assume for simpliity that the power levels of the two optial input waves are equal ( P opt P P ), equation (7) beomes [] i s P s P os f t. (8) o opt f opt Where. Here s oq f q o and s f are the photodetetor s DC and high hf hf frequeny responsivities given in A/W. Equation (8) is the fundamental equation desribing optial heterodyning in a photodetetor. The first term is the DC photourrent generated by the onstituent 3

5 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 optial input waves and the seond term is the desired high-frequeny signal osillating at the differene frequeny f (down-onverter) or intermediate frequeny (IF) []. In our ase it represents the mirowave signal that we will use as both information arriers, and as a loal osillator for transmitting and reeiving TV signals in a wireless ommuniation system.. Experimental sheme for generating mirowave signals The heterodyne tehnique for generating mirowave signals has been done using the experimental setup shown in figure. In this experiment we have used two laser diodes emitting at different wavelengths, one of them was a tunable laser (New Fous, model TLB-390) whih an be tuned over the C band with a hannel spaing of 5 GHz, and the other one is a fiber oupled DFB laser soure (Thorlabs, model S3FC550) with a entral wavelength at 550 nm. For the generation of the mirowave signals, the outputs of both lasers are oupled to optial isolators to avoid a feedbak into the lasers and onsequently instabilities to the system. A pair of polarization ontrollers was used to minimize the angle between the polarization diretions of both optial soures. Thus, the polarization of the light issued from eah optial soure is mathed and therefore, there was no degradation of the power levels in the mirowave signals generated from the photodetetor. The output of eah ontroller is launhed to a 3dB oupler to ombine both optial spetrums. After that, an optial output signal is reeived by a fast photodetetor (MITEQ model SCMR-50K6G-0-0-0) with a typial gain of 5 db, and 3 db bandwidth of 6 GHz, The resulting photourrent from the photodetetor orresponds to the mirowave beat signal whih is analyzed with an Eletrial Spetrum Analyzer (ESA), (Agilent model E4407B). The other optial output resulting from optial oupler was applied to an Optial Spetrum Analyzer (OSA) (Anritsu model MS970C), for monitoring the wavelength of the two beams. Figure. Experimental setup for generating mirowave signals by using optial heterodyne tehnique DFB laser an be used to ontrol not only the output power of the fiber oupled laser diode, but also the preise ontrol of the temperature at whih the laser is operating. Both ontrols an be used to tune the fiber oupled laser diode to an optimum operating point, providing a stable output. In this way, we observed that the wavelength of the DFB laser was shifting, by varying its temperature with a sale of ºC. Consequently, the beat signal frequeny was ontinuously tuned from 0 to 0 GHz. Figure (a) illustrates the spetrums of three mirowave signals generated with optial heterodyne tehnique. The generated signals are loated at.4, 4.9 and 9 GHz when the temperature of the DFB laser, with an optial fixed power of.4 mw, was tuned at.8ºc, 3.ºC and 3.7ºC respetively. Figure (b) shows the optial spetrums of the optial signal from both lasers. From this figure, we an see that the wavelength differene between both lasers is 0.07nm and it orresponds to the beat signal frequeny of 9 GHz as shown in figure (a). The frequeny differene from both lasers an be expressed by [3] f, (9) 4

6 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 where and are the wavelengths of the two beams, respetively, and is the differene between the two wavelengths. From the result shown in figure (b), we an observe that the generated mirowave signal is in good agreement with theoretial expression of equation (9). Therefore, when one laser soure is operating at a fixed wavelength and the other is being ontinuously tuned, the beat frequeny will shift orrespondingly. (a) Figure. Mirowave signals and optial mixing. (a) Mirowave spetrums ontinuously tuned from 0 to 0 GHz, (b) optial spetrum orresponding to the two ombined optial beams with spetral separation of 9GHz. 3.-Modulation and demodulation Some form of modulation is always needed in an RF system to translate a baseband signal (e.g., audio, video, data) from its original frequeny bandwidth to a speified RF frequeny spetrum. There are many modulation tehniques, for example, AM, FM, amplitude shift keying (ASK), frequeny shift keying (FSK), phase shift keying (PSK), biphase shift keying (BPSK), quadriphase shift keying (QPSK), 8-phase shift keying (8-PSK), 6- phase shift keying (6-PSK), minimum shift keying (MSK), and quadrature amplitude modulation (QAM). AM and FM are lassified as analog modulation tehniques, and the others are digital modulation tehniques [4]. In this setion we desribe the AM modulation and demodulation due to it was used in our proposed wireless ommuniation system. 3. Amplitude modulation Analog modulation uses the baseband signal (modulating signal) to vary one of three variables: amplitude A, eletrial frequeny ( ) f ; or phase. Aording to equation (8), the obtained arrier signal by using optial heterodyne tehnique an be written by Where f opt p t) A ( ) t A os f t ( os. (0) A s P. In amplitude modulation, if we assume that s(t) is the information signal, and onsidering A, 0, then a modulated signal an be written by (b) g( t) s( t)osf t. () Applying the modulation property of the Fourier transform to equation (), we an find the density spetral of g(t) is 5

7 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 G( f ) S( f f ) S( f f ). () Amplitude modulation therefore translates the frequeny spetrum of a signal by f hertz, but leaves the spetral shape unaltered. This type of amplitude modulation is alled suppressed-arrier beause the spetral density of g(t) has no identifiable arrier in it, although the spetrum is entered at the frequeny f. 3. Amplitude demodulation Reovery the signal information s (t) from the signal p(t) requires another translation in frequeny to shift the spetrum to its original position. This proess is alled demodulation or detetion. Beause the modulation property of the Fourier transform proved useful in translating spetra for modulation, we try it again for demodulation. Assuming that g( t) s( t)osf t is the transmitted signal, we have g( t)osf t s( t)os f t s( t) os4f t. (3) Taking the Fourier transform of both sides of equation (3) and using the modulation property, we get g ( t)os ft S( f ) S( f f ) S( f f ). (4) 4 4 The mathematial proess desribed in this setion an be obtained by onvolving the spetrum of the reeived signal g (t) with that of os f t (i.e., with impulses at f ). A low-pass filter is required to separate out the double frequeny terms from the original spetral omponents. Obviously we need a filter with a ut frequeny fut fm for proper signal reovery. In this ase f m represents the information frequeny. 3.3 Effets in frequeny and phase variations When the loal osillator at the reeiver, has a small frequeny error f and a phase error, then this signal an be written as pl ( t) os f f t. (5) Assuming again that g( t) s( t)osf t is the transmitted signal; then we have that at the reeiver, the reovered signal an be written by g( t)os ( f f ) t s( t)os(f t)os ( f f ) t os(ft ) os ( f s( t) f ) t. (6) The seond term on the right hand side of equation (6) is entered at f f and an be filtered out by using a low pass filter. The output of this filter s F (t) will then be given by the remaining term in equation (6). 6

8 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 s( t) s F ( t) os ( f ) t os( ) sen ( f ) tsen ( ). (7) As an been from equation (7), the output signal is not s(t), unless both f and are zero. The effets of both frequeny errors and random phase errors render this demodulation of the signal unsatisfatory. It is neessary, therefore, to have synhronization in both frequeny and phase between the transmitter and the reeiver when amplitude modulation is used. The synhronization of the arrier signals presents no major problem when the transmitter and the reeiver are in lose proximity. Reovering the original signal s (t) from the modulated signal g (t) using a synhronized osillator is alled oherent demodulation. In our ase we take advantage of proposed optial heterodyne tehnique permits to obtain mirowave arrier and loal osillator simultaneously in the transmitter and reeiver respetively. 4. Transmission of TV signals by using heterodyne tehnique In order to show a potential appliation of optial heterodyne tehnique in the field of the wireless ommuniations, we have proposed a oherent system that although is not a truly wireless ommuniation system, sine an optial fiber is required to deliver both mirowave arrier and loal osillator for transmitting and reeiving information of TV signals by using a ouple of mirostrip antennas, we have used it as an approximation to point to point indoor wireless ommuniations systems as shown in figure 3. From the photodetetor in the transmitter, a mirowave signal loated at.5ghz was obtained and mixed with an analog TV signal loated at 6.5MHz. Then the resulting signal was amplified before being applied to a mirostrip yagi antenna. After that, the obtained modulated signal was transmitted through a point to point wireless link by using a mirostrip yagi antenna. Finally in the reeiver, another mirostrip yagi antenna reeived the transmitted information, whih it was proessed using optial heterodyne tehnique again to reover in this ase the TV signal (66-7MHz). From the photodetetor in the reeiver, a loal osillator that is synhronized, in frequeny as well as in phase with to that obtained from the photodetetor, was mixed with the reeived signal. Then the resulting signal was filtered and the power spetral density obtained was displayed in an eletrial spetrum analyzer, where it is analyzed to measure the power level of reovered information. Figure 3. Optial point to point wireless link for transmitting and reeiving TV signals 7

9 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 Figure 4, learly shows the spetrums of an analog TV hannel around.5 GHz. We an observe in figure 4(a) that the power level transmitted was approximately of -36 dbm, while the frequeny spetrum of the reeived signal as shown in figure 4(b) was attenuated db. However analog TV hannel transmitted was satisfatorily reovered at the reeiver. (a) Figure 4. Spetrums of TV signal around.5ghz. (a) transmitted, (b) reeived. Figure 5(a) shows the frequeny spetrum of an analog NTSC TV signal around 67.5MHz before being applied to frequeny mixer, while the spetrum in figure 5(b) shows the analog TV hannel 4 at the output of the reeiver. In order to measure the quality of the reeived signal, is neessary to quantify the parameters of signal-to-noise ratio (SNR), differential gain and differential phase. Nevertheless it is not the aim of this paper. Here only we demonstrate that mirowave signals generated by optial heterodyne an be used as arrier information in a wireless ommuniation system and we have used a TV signal of test to verify it. (b) (a) Figure 5. Spetrums of TV signal around 67.5MHz. (a) Transmitted, (b) reeived. (b) 5.- Conlusion Wireless ommuniation systems require ompat soures for the generation of mm-wave signals, that must have high spetral purity (linewidth < 00 khz, phase noise < 00 khz offset), tuneability, low power onsumption and low ost, and although optial heterodyne of two DFB lasers has phase noise of 75 db/hz even at an offset frequeny of 00 MHz and it does not very ompat, we have demonstrated in this work that by using optial heterodyne tehnique, a TV signal was 8

10 XVII Reunión Iberoameriana de Óptia & X Enuentro de Óptia, Láseres y Apliaiones IOP Publishing Journal of Physis: Conferene Series 74 (0) 0008 doi:0.088/ /74//0008 transmitted and reeived satisfatory as a result of our proposed ommuniation system generates a mirowave arrier and a loal osillator simultaneously ensuring synhronization in frequeny as well as in phase between mirowave arrier and a loal osillator and avoiding in this ase the use of an analog phase loked loop in the reeiver to reover the TV information. The authors onsider that the proposed sheme in this paper is not a truly wireless ommuniation system, sine an optial fiber is required to deliver the loal osillator in the reeiver, however in order to obtain a wireless ommuniation systems by using optial heterodyne tehnique, it is neessary to have ollimated beams from optial fiber to photodetetors. On the other hand, due to the fat that the distribution of TV over mirowave signals in the eletrial domain presents loss assoiated with eletrial distribution lines, the authors onsider that the optial fiber is an ideal solution to fulfill this task beause of its extremely broad bandwidth and low loss. In that ase the distribution of TV over mirowave an be diretly by using optial fiber. The results obtained in this work ensure that as an interesting alternative, several modulation shemes an be used for transmitting not only analog information but also digital information. Besides as optial heterodyne tehnique desribed here an generate mirowaves ontinually tuned, we an use this feature to transmit several TV signals using frequeny division multiplexing shemes FDM and wavelength division multiplexing WDM tehniques, not only point to point but also with bidiretional shemes by using simultaneous wired and wireless systems. Aknowledgment This work was supported by CONACyT ( grant No 0046). Referenes [] Gerald R DeJean 008 Mirowave Conferene APMC (Asia-Paifi) p [] D. Wake 00 in Pro. Int. Top. Meeting MWP p. [3] U Gliese, T N Nielsen, S Nørskov, and K E Stubkjaer 998 IEEE Transations on Mirowave Theory and Tehniques 46 p 458 [4] Wen Y J Liu H F Novak D and Ogawa Y 000 IEEE Photonis Tehnology Letters p 058 [5] Langley L N, Elkin M D, Edge C, Wale M J, Gliese X, Huang X, and Seeds A J 999 IEEE Transations On Mirowave Theory and Tehniques 47 p 57 [6] J J O Reilly, P M Lane, R Heidemann, and R Hofstetter 99 Eletron Letters 8 p 309 [7] Goldberg L, Taylor H F, Weller J F, and Bloom D M 983 Eletron Letters 9 p 49 [8] Ramos R T and Seeds A J 99 Eletron Letters 8 p 8 [9] H Shams, P M Anandarajah, and P Perry, L P Barry 009 Optis Communiations 8 p 4789 [0] M Garía Larrodé, A M J Koonen, J J Vegas Olmos and E J M Verdurmen 007 Journal of Lightwave Tehnology 5 p. 37 [] Stavros Iezekiel 009 Mirowave Photonis Devies and Appliations (John Wiley) [] C Gutiérrez Martínez, A Garía Juárez, J A Torres Fórtiz and J Meza Pérez 003 International Mirowave and Optoeletronis Conferene (IMOC) p 83 [3] Ang Miao, Yongqing Huang, Hui Huang, Rui Wang, Song Wang, and Xiaomin Ren 009 Mirowave and optial tehnology letters 5 p 44 [4] Kai Chang 000 RF and Mirowave Wireless Systems (John Wiley) 9