A NOVEL APPROACH FOR GENERATION OF ALL-OPTICAL OFDM USING DISCRETE COSINE TRANSFORM BASED ON OPTICAL COUPLERS IN A RADIO-OVER-FIBER LINK

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1 International Journal of Advanced Research in Engineering and Technology (IJARET) Volume 8, Issue 3, May - June 2017, pp , Article ID: IJARET_08_03_006 Available online at ISSN Print: and ISSN Online: IAEME Publication A NOVEL APPROACH FOR GENERATION OF ALL-OPTICAL OFDM USING DISCRETE COSINE TRANSFORM BASED ON OPTICAL COUPLERS IN A RADIO-OVER-FIBER LINK Sreedevi Prasanna Department of Electronics and Communication Mar Baselios College of Engineering and Technology, Trivandrum, India Vijayakumar Narayanan Department of Electronics and Communication, Govt. Engineering College, Barton Hill, India ABSTRACT A novel method for100gbpsall-optical OFDM using Discrete Cosine Transform in a Radio-Over-Fiber link is proposed. The system is designed simply using both symmetric and asymmetric passive optical couplers. DCT is achieved all-optically by adjusting the length and splitting ratio of the couplers. The performance of the system is compared with all-optical OFDM based on Discrete Fourier, Discrete Cosine and Discrete Wavelet Transform. A BER of is achieved using all-optical cosine transform. The system is designed and studied using Optisystem and Matlab. Key words: All-optical OFDM, Discrete Cosine Transform (DCT), Discrete Fourier Transform (DFT), Discrete Hartley Transform (DHT), Discrete Wavelet Transform (DWT), Fast Fourier Transform (FFT).. Cite this Article: Sreedevi Prasanna and Vijayakumar Narayanan. A Novel Approach for Generation of All-Optical OFDM Using Discrete Cosine Transform Based on Optical Couplers in a Radio-Over-Fiber Link. International Journal of Advanced Research in Engineering and Technology, 8(3), 2017, pp INTRODUCTION The wide spread acceptability of Orthogonal Frequency Division Multiplexing(OFDM) have paved way for the boom of orthogonal transforms in the field of optical communication [1].Most of the works in this area were concentrated on generating OFDM using Discrete Fourier Transforms and a few in DHT [2]. The possibility of employing high performance DCT has been less exploited in the area of OFDM. The simplicity in the structure and high spectral efficiency enables DCT to emerge as an alternative for DFT [3]. OFDM incorporated on to a RoFsystem is considered to be the future of wired/wireless communication [4].High speed, long distance transmission is possible through a RoF-OFDM 43 editor@iaeme.com

2 A Novel Approach for Generation of All-Optical OFDM Using Discrete Cosine Transform Based on Optical Couplers in a Radio-Over-Fiber Link link. Replacing OFDM with all-optical OFDM in the RoF network contributes to the enhancement of higher multigigabyte transmission over long distance. There has been literatures in the area of all-optical transforms.realization of DFT and DHT using planar Lightwave circuits (PLC) has been proposed by researchers[5][6][7]. To reduce the complexity in fabricating DFT using PLC s, Fast Fourier Transform (FFT) has been implemented [8]. Research work has been undergoing in the area of all-optical OFDM due to the speed limitation imposed on electronic OFDM systems. The OFDM data rate limited by the throughput of electronics boosted the research in generating all-optical OFDM.OFDM both in the electronic and all-optical domain is most commonly generated using Fourier transform [9] [10]. The complexity of all-optical Fourier transform led to the study of generating orthogonal signals using Discrete Hartley Transform (DHT) and Discrete Cosine Transform (DCT) in the optical domain [11]. The power spectra of the transmitted data are computed faster by Fast Hartley Transform (FHT) than FFT. FHT, the real valued transform requires less computation compared to the complex FFT transform [12]. While DCT is derived effectively from FHT by simply reordering the elements and adding few stages butterfly stages, the chances are not much exploited[13].similarly Discrete Wavelet Transform(DWT) is also emerging as a competitor for generating OFDM in future high speed systems[14]. The work in this paper focuses on generating orthogonal signals using DCT. A 100Gbps All-optical OFDM is generated using passive optical couplers both symmetric and asymmetric couplers. The splitting ratios of the optical couplers are chosen such that the average peak power of the transmitting signal has been step down. Finally the performance of the system is compared with Fourier, Hartley and Wavelet based all-optical OFDM. The system is designed using Optisystem 13.0 and Matlab. 2. DISCRETE COSINE BASED OFDM In general DCT of a sequence()is defined as [15] y(k) = 2 () () (2 + 1) 2 ; = 0,1,,,, ( 1) () = $ Similarly DHT is defined as 1 2 = 0 ( 1 0 < ( 1) h(k) = * 1 + (), ; = 0,1,,,, ( 1) DCT can be related to DHT using equ.3 /() / 0 () = 1 2 cos 4 0 sin sin 4 0 cos h 0() h 0 () ; (1) (2) = 0,1,,,, * 2 1+ (3) 44 editor@iaeme.com

3 Sreedevi Prasanna and Vijayakumar Narayanan DCT can be preferred over DFT in all-optical transmission; as less computations are required for real transforms with less complexity [16]. 3. MATHEMATICAL MODELLING OF DCT BASED ALL-OPTICAL OFDM The electric field of light at the output of optical modulator is [17] : ; (<) = =>? A B (C) + >? A D (C) E>?F G (C) (4) For the proposed system : I; (<) = =>? A JB (C) + >? A JD (C) E>?F G (C) : - for 1,2,,,, N (5) 4 I (<) and 4 IR (t)denotes the phase change at the input and output port of the i th optical modulator.dct is implemented using optical couplers as shown in Figure 1. The signals orthogonalised at the end of the couplers, by means of inverse DCT is given by V : () [ : ; U : R () Z = \ : ] ^ R; _ (6) U: W () Z : W; T: X ()Y : X; V : ; () (: [ ; + : R; + : W; + : X; ) U : R; () Z (: = ^ ; + : R; : W; : X; ) _ ^ U: W; () Z (: ; : R; + : W; : X; ) 0 0 1/6 1/6 _ (7) T: X; ()Y (: ; : R; : W; + : X; ) 0 0 5/6 5/6 : ; (), : R; (), : W; (), : X; () denotes the orthogonalised signals across the coupler. While equ.(7) describes the inverse DCT to be performed at the transmitting end of the proposed system, at the receiving end forward DCT is performed to retrieve the frequency domain transmitted signal : ; V : ; () [ : ^ R; 1 _ = : W; \ ] U : R; () Z (8) U: W; () Z : X; T: X; ()Y Equ.(7) and equ.(8) clearly show just a difference in the constant value for performing forward and inverse cosine transforms, which reduces the hardware complexity of the structure. Figure 1 Hardware implementation of all-optical model of DCT using optical couplers

4 A Novel Approach for Generation of All-Optical OFDM Using Discrete Cosine Transform Based on Optical Couplers in a Radio-Over-Fiber Link 4. MODELLING OF DCT BASED ALL-OPTICAL OFDM IN RADIO OVER FIBER A short optical Gaussian pulse centralised at frequency of 193.1THz is generated to modulate the incoming four channel QAM modulated data which in turn is orthogonalised optically by means of Inverse Discrete Cosine.At the receiving end all-optical OFDM symbols are sampled for each time interval and transmitted on to individual paths. In order to incorporate the entire all-optical OFDM spectrum the sampled symbols are kept broadened to strip the frequency information of the received spectrum. Thereafter the channels are demultiplexed by performing forward DCT on these sampled signals. Model of transmitter and receiver is shown in Figure 2 and Figure 3, respectively along with the eye pattern at the bottom most part. Figure 2 All-optical transmitter Figure 3 All-optical Receiver

5 Sreedevi Prasanna and Vijayakumar Narayanan 5. RESULT AND DISCUSSION The transmitted 4X25Gbps all-optical OFDM is shown in Figure 4and the demultiplexed channels at the receiving end are shown in Figure 5. The demultiplexer circuit of DCT is almost same as that of the inverse DCT as it is derived from the real Hartley transform.wide openings seen at the center of the eye, clearly points to the fact that the demultiplexed channels are retrieved at the detecting end within 100km. In Fig.4, the transmitted channels and the corresponding demultiplexed channels at the receiver is shown in Fig.5. Figure 4 Transmitted channels It is seen in Figure 6, while varying OSNR between 15 and 45dB, the BER of cosine transmitter is varying form 10-4 to Also, it is seen Fourier, Hartley and Wavelet based transmitters modeled for all-optical OFDM transmission achieved the same BER with less OSNR, compared to DCT transmitter. A power penalty of nearly 4dB is possessed by Cosine all-optical transmitter with respect to the Wavelet. The BER performance is highlighted in its Q-factor variation. Between 15dB and 45dB OSNR, the Q-factor is seen increasing from 3 to 18dB, as in Figure 7. Even though for lower values of OSNR, DCT is prone towards dispersion, it gives comparatively closer performance with respect to other transforms. At higher OSNR, the signal is high enough to tolerate dispersion. But above 50dB, the input power level of the optical source introduces clipping in all-optical OFDM signal, causing the OSNR to fall which in turn will reduce the BER performance. Hence a tradeoff is to be maintained between input power level and BER. Figure 5 De-multiplexed channels

6 A Novel Approach for Generation of All-Optical OFDM Using Discrete Cosine Transform Based on Optical Couplers in a Radio-Over-Fiber Link Figure 6 OSNR versus BER Figure 7 OSNR versus Q-factor 6. CONCLUSIONS This paper provides a novel and simple approach to design an all-optical OFDM based on DCT transform. DCT transform is designed simply using, both symmetric and asymmetric optical couplers. The BER and Q-factor performance of DCT finds to be competitive with alloptical DFT/DHT/DWT transform, which in turn can simplify the architecture of the alloptical OFDM system. The tradeoff between input power and BER is suggested as future work. In general, all-optical systems improve energy efficiency and reduce the emission of greenhouse gases. So from the results obtained, the system is not only on a par with the existing systems, but also improves the energy efficiency. REFERENCES [1] W Shieh, H. Bao, and Y. Tang, Coherent optical OFDM: Theory and design, Opt. Express, vol. 16, pp , Jan [2] G. D. Mandyam, Sinusoidal transforms in OFDM systems, IEEE Trans. Broadcasting, vol. 50, no. 2, pp , Jun

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