 Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 01 - 03 1 Hybrid OFDM/OOK Modulations in OCDMA Scheme for Free Space Optics Chih-Ta Yen * , Chia-Yu Liu Department of Electrical Engineering, National Formosa University, Yunlin, Taiwan . Received 01 February 2016; received in revised form 15 March 2016; accept ed 02 April 2016 Abstract The study proposes hybrid analog/digital transmission format scheme wh ich integrated optical code-division mult iple -access (OCDMA) and polarization mu lt iple xing technique in free space optics (FSO) trans mission. Orthogonal frequency division mu ltiple xing (OFDM ) transmits as the analog format and (on -off key- ing) OOK transmits as digita l format in the study, respectively. In the proposed hybrid OCDMA system, it has high-speed transmission, signal security and low cost…etc. The multip le access interference (MAI) can be e fficiently e liminated by using the balanced detection scheme at the receive end. Keywor ds : Optical Code-Division Multi- ple-Access (OCDMA), Free Space Optics (FSO), Orthogonal Fre- quency Division Multiplexing (OFDM), Multiple Access Inter- ference (MAI) 1. Introduction In recent years, peoples become closer and closer relationship between the networks. Be - cause of hybrid coaxia l cable transmission net- work change into a fiber optical trans mission network, making people have a better internet e xperience. In the current study, we use the method of different states of polarizations to transmit analog/digital signals and th en into optical code-division mult iple -access (OCDMA) networks. Due to polarization modulator without using electronic circuit or impressed voltage to control, and has the same intensity of light sig- nals in the outputs, which can mit igate fiber nonlinear e ffects and ma king the overa ll struc- ture greatly improve the reliability of the tran s- mission side. At present, wired transmission interface is using fiber or coa xial cable to transmit signal. But in some place because of terrain effect (e.g., cliffs desert, both ends of the bridge). Wireless transmission system is one of the solutions to solve topographical constraints , but it is always disturbed by channel interfe rence and shelter, severely reducing its effectiveness . We take advantage of the free space optics (FSO) ap- proach as the way for last mile trans missions [1]. In this paper, we present optical spectral amp litude coding OCDMA (SAC-OCDMA) combin ing with polarization mu lt iple xing to transmit analog signal of orthogonal frequency division mu ltiple xing (OFDM) and dig ital signal of on-off keying (OOK) signals at the sa me time . In the past year, SAC-OCDMA system is at- tracted more attention because of the multip le access interference (MAI) elimination and pre - serve the orthogonality among the users in the system [2]. OFDM technique has been widely adopted in broadband wired and wireless co m- municat ion systems , because it is robustness to the inter-symbol interfe rence (ISI) and mul- ti-path fading by a dispersive channel [3]. 2. System Structure Description Fig. 1 and 2 illustrate the structure of the proposed base stations (transmitters) and control station (receivers). This OCDMA mult iple xing structure can transmit analog signal and digita l signal at the same time and still preserve the ability of MAI e limination. We transmit OFDM signal as analog signal and OOK signal as digita l signal. In this hybrid system, each base station needs a spectral encoder and a decoder in the receiver. The DFB laser array source is co n- nected a 1 ( 1)N  splitter which p roviding light source to each base station, shown in Fig. 1. The DBF laser source performs as optical carri- * Corresponding aut hor. Email: chihtayen@gmail.com Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 01 - 03 2 Copyright © TAETI ers pass through a polarizer a polarization co n- trolle r (PC), and a polarization bea m splitter (PBS). The PBS allocates the orthogonal hori- zontal and vertical states of polarizat ions (SOPs) upper and lower arms, respectively. Between base station and control station, FSO is used to be this structure channel. The control station received a composite signal comprising the transmitted chips of all the active base station. As shown Fig. 2, the received sig- nal in the control station is divided in to (N−1) -th parts by using the 1×(N−1) splitter, and passed into each receiver. The signal was pro- duced vertical and horizontal po larization states by using PBS. Two SOPs of vertica l and hori- zontal output from the PBS which are split into an upper branch and the lower branch. The two diffe rent signals of vertical and horizontal passed balance detector, wh ich co mputed the correlation d iffe rence between the two spectra and eliminate the interfe rence of base station with un-matching codeword ind ividually. The upper branch uses OFDM de modulator to revert analog origina l signal and lo wer branch uses digital OOK de modulator to revert dig ital OOK signal. encoder #j OFDM Transmitter #l PC PBS#1 Vertical SOP Horizontal SOP MZM MZM ( ) l C i Base Station #l 1 x k C o m b in e rDFB Laser Array Source 1xk Splitter PBC Base Station #1 Base Station #(N-1) Digital Signal Transmitter #l FSO Channel Fig. 1 The transmitter of proposing system 1x2 Splitter 1x2 Splitter ( ) l C i ( )lC i ( ) l C i ( )lC i + + - - OFDM Demodulator #l Desired Data #l Receiver #l decoder #j Horizontal SOP 1 x (N -1 ) S p litte r PBS#2 Vertical SOP Receiver #1 Receiver #(N-1) Control Station Digital Signal Demodulator #l Desired Data #l PD #l, v1 PD #l, v2 PD #l, h1 PD #l, h2 Fig. 2 The receiver of proposing system Before analysis of the proposed system [4], the OFDM signal of the k-th base station is )(, tO RFk : )2cos()( ,,, tfAtO RFkRFkRFk  (1) where ,k RFA is the a mplitude of OFDM signal, and ,k RFf is the carrier frequency. And the analog OFDM signal is modulated by a mult i-wavelength DFB laser as optical carriers and then pass through AWG encoder is:     cN i kRFksk irectiCtOPtO 1 , )()()(1)(  (2) where ( )kC i denote the i-th element of the k -th WHC codeword,  is the modulation inde x, sP is the transmitted optical power, and ( )rect i function is given by:     0 0 ( ) 2 2 2 2 2 c c c c rect i u v v N i N u v v N i N                        (3) At the receiver end, the instantaneous power spectral density (PSD) of the vertical and hori- zontal SOP’s received optical signals can be expressed by the following: , 1 1 ( , ) 1 ( ) 2 ( ) ( ) cNK sr v s O RF k i k P r v t P Q t C i rect i             (4)       K k Nc i kk sr h irectiCb P vr 1 1 )()( 2 )(  (5) where Psr is the received effective power at the input port of each receiver,  is the light source bandwidth, K is the number of base sta- tion. The relat ionship between bit-error-rate (BER) can employ as:   1 / 8 2 BER erfc SNR (6) 3. Performance Analysis and Discus- sion FSO performance by several para meters for reference including geo metric loss, link marg in, the received power and bit error rate. In this paper, it considers the received power and bit error rate [5]. Fig. 3(a) and 3(b ) illustrates the bit error rate (BER) re lative to active users in d if- Proceedings of Engineering and Technology Innovation , vol. 3, 2016, pp. 01 - 03 3 Copyright © TAETI ferent bit rate at the analog/digital signals, re - spectively. It had more active user when it had low bit rate. (a) analog signal (b) digital signal Fig. 3 Illustrates the bit error rate (BER) rela - tive to active users in different bit rate 4. Conclusions In this study, we present hybrid an a- log/digital spectral OCDMA system. Th is sys- tem transmits two different kinds of format signal wh ich contain analog and dig ital signals by using polarizat ion multip le xing technique. The proposed hybrid system can transmit analog and digital signals at the same time such as dig- ital T V and mob ile phone system simu ltaneously. FSO is the main t ransmission channel in the study, which has the advantages of low cost, easy construction and terrain without re- strictions … etc. The simu lation results show that both OFDM signal as analog signal and OOK modulation as digital signal in the pro- posed hybrid OCDMA system perfo rm good performance in FSO transmission. Acknowledgement This study was supported in part by the Ministry of Sc ience and Technology MOST 104-2221-E-150-044. References [1] P. T jondronegoro, “Free-space optics for fixed wire less broadband,” pp. 2-20, Oct. 2004. [2] Z. Wei, H. M. H. Shalaby, and H. 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