1. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Chapter 12 Optical CDMA Network 12.1 The Main Concerns 12.2 Fiber-Optic Code-Division Multiple-Access 12.3 Code Families for Optical CDMA 12.3.1 Walsh-Hadamard Codes 12.3.2 Binary M-sequence Codes 12.4 FBG-Based Optical CDMA Codecs 12.5 AWG-Based Optical CDMA Codecs

2. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The main concern for users of the new DWDM system is its reliability and stability over time. We can examine the critical factors limiting the performance of a time-domain multiplexing (TDM) system by placing them on a two-dimensional power-versus-time representation. Figure 12.01 Critical factor limiting TDM performance. 12.1 The Main Concerns

3. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  On the power axis, tile critical issues are laser power, fiber attenuation, and component loss.  On the time axis, the critical factors are fiber PMD, chromatic and (for multimode fibers) modal dispersion, as well as signal jitter and transmission rate.  At the junction of the power and time axes, new factors come into play: laser modulation depth, fiber non- linearity, relative intensity noise (RIN), and bit error rate (BER). 12.1 The Main Concerns

4. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  WDM adds wavelength as a new dimension and complicates considerably the representation of the critical factors (Figure 12.02). Figure 12.02 Addition of the wavelength dimension in WDM system. 12.1 The Main Concerns

5. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  On the wavelength axis, critical elements as spectral stability, EDFA spectral range, central wavelength, and bandwidth.  At the corner of wavelength and time, we encounter laser chirp, chromatic dispersion, stability of the optical frequency, and phase noise (self-phase modulation and cross-phase modulation).  At the corner of wavelength and power, we find EDFA amplified spontaneous emission (ASE), EDFA gain, crosstalk, four-wave mixing, and stimulated Raman forward scattering. Where all three axes meet, we encounter stimulated Brillouin backscattering. 12.1 The Main Concerns

6. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.2 Fiber-Optic Code-Division Multiple-Access  With rapid increases of utility rate in optical communications nowadays, the capacity and the security of one optical system have become a critical issue.  The development of optical CDMA (OCDMA) technique is such attractive since it is possible that allows multiple users in a local area network (LAN) environment to access the same fiber channel asynchronously at all times.  Besides, CDMA systems also offer a security advantage over other multiple access systems.

7. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  In FO-CDMA, data of the users is spread by a signature sequence, which uses to distinguish the desired user from other users.  Major CDMA techniques are direct-sequence (DS), time-hopping (TH), frequency-hopping (FH) and frequency-encoded (FE).  Most of OCDMA systems previously proposed are DS systems, in which short optical pulses are needed. In the case, the dispersion should be taken into consideration and the sequences used can’t be changed dynamically. 12.2 Fiber-Optic Code-Division Multiple-Access

8. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  In order to avoid the dispersion, TH or FH patterns are on the basis of different construction of codes that represent signature sequences.  As shown in Figure 12.1, time-hopping technique divides a data bit slot to several chip slots and represents the code by the slot position of pulse.  On the other hand, frequency-hopping technique represents the code by the wavelength it uses and the similar example is shown in Figure 12.2. 12.2 Fiber-Optic Code-Division Multiple-Access

9. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.1 Graphical representation of an appropriate optical code (time hopping pattern). 12.2 Fiber-Optic Code-Division Multiple-Access

10. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Figure 12.2 Graphical representation of frequency hopping code pattern. 12.2 Fiber-Optic Code-Division Multiple-Access

11. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  Besides, signature sequences used simultaneously in a communication system should be mutually orthogonal or pseudo-orthogonal or have good autocorrelation and cross-correlation properties.  Furthermore, in order to utilize the large bandwidth offered by optical fiber channels, the encoding and detection operations must be performed optically to circumvent the speed limitations of electronic systems. 12.2 Fiber-Optic Code-Division Multiple-Access

12. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The FE-CDMA is similar to the DS-CDMA, but the coding is done in the frequency domain while in the DS-CDMA, the code multiplies the modulation signal in the time domain.  Different wavelength represents different element in the codeword, and the presence of wavelength means “1” while the absence of wavelength means “0”.  A new class of signature sequences known as optical orthogonal codes (OOCs), which has low off-peak autocorrelation and cross-correlation has been proposed for such systems. 12.2 Fiber-Optic Code-Division Multiple-Access

13. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  Asynchronous multiplexing schemes are more efficient than synchronous ones.  Figure 12.3, a typical FO-CDMA communication system is described each receiver correlates its own address code with the received sequence. Fig. 12.3 Schematic diagram of FO-CDMA system with all-optical encoder and decoder connected in a star configuration. 12.2 Fiber-Optic Code-Division Multiple-Access

14. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  Selecting the appropriate code sequence will maximize the autocorrelation and minimize the cross-correlation function and then the receiver would be able to distinguish the correct address.  In order to extract data with the desired optical pulse sequence, we therefore have to design sequences that satisfy some conditions ： Each sequence can be distinguished from a shifted version of itself easily. Each sequence can be distinguished from (a possibly shifted version of) every other sequence in the same set. 12.2 Fiber-Optic Code-Division Multiple-Access

15. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.3 Code Families for Optical CDMA  In most of the earliest proposed OCDMA systems, OOCs were popular with good autocorrelation and cross-correlation properties.  The use of OOCs makes a large number of asynchronous users to transmit information efficiently.  OOCs also have applications in mobile radio, frequency-hopping spread-spectrum communications, and radar-sonar signal design.

16. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.3.1 Walsh-Hadamard Codes  Walsh-Hadamard code is obtained by selecting as codes the row of a Hadamard matrix.  All rows except one contains N/2 zeros and N/2 ones. This code can be constructed using the iterative procedure: where is derived from H n by replacing all entries with their complements. For example ：

17. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  Taking X as being row i and Y as being row j with, it is easy to verify that  We can apply this result to design our proposed receiving decoder for balanced detection for an N x N Hadamard matrix, N - 1 subscriber can be accommodated since the codeword containing all 1’s has to be rejected. 12.3.1 Walsh-Hadamard Codes

18. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.3.2 Binary M-sequence Codes  Maximal-length sequences (M-sequences) were used mainly because of their excellent periodic autocorrelation properties.  The cross-correlation properties of such sequences are as important as autocorrelation properties.  Consider the sequence Y as being Y = T k X, where T k is the operator that cyclically shifts the sequence X = (x 0, x 1, …, x N-1 ). For example, TX = (x 1, x 2, …, x N-1, x 0 ), T 2 X = (x 2, x 3, …, x N-1, x 0, x 1 ), and so on.

19. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The periodic cross-correlation of sequences X and Y is  In other words, half the 1’s in TkX coincide with the 1’s of X while the other half coincides with the 0’s. Note that the sum i + k is taken modulo N. 12.3.2 Binary M-sequence Codes

20. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  We can compute correlation difference 12.3.2 Binary M-sequence Codes

21. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  We can apply this result to design our proposed receiving decoder for balanced detection in the OCDMA network. will be used in our CDMA systems.  By assigning the N cyclic shifts of a single M-sequence to N subscribers and we have a network that can support N simultaneous users without any interference.  We have therefore obtained complete orthogonality between each user in the OCDMA network. 12.3.2 Binary M-sequence Codes

22. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.4 FBG-Based Optical CDMA Codecs  The fiber-optic CDMA encoder/decoder is structured with the spectral coding of incoherent light sources by fiber gratings.  The signature address code imposed on the data encoder in the transmitter end should be matched to the signature code of the decoder in the receiver end.  The coded information sequences are then linked to a common star coupler, and are shared with the same channel bandwidth.  The whole FO-CDMA system is as shown in Fig. 12.4.

23. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.4 Fiber-optic CDMA network. 12.4 FBG-Based Optical CDMA Codecs

24. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The encoders (Fig. 12.5) consist of a series of fiber Bragg gratings.  The broadband optical wave is directed to a fiber Bragg grating. If the central wavelength of the narrowband pulse of the incoming optical wave is equal to the Bragg wavelength, it will be reflected by the FBG, or it will be transmitted.  With a proper written CDMA coding pattern, the reflected light field from FBG will be spectrally encoded onto an M-sequence address code. 12.4 FBG-Based Optical CDMA Codecs

25. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.5 FBG-based optical CDMA encoder. 12.4 FBG-Based Optical CDMA Codecs

26. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The incoherent light sources used in encoder module include Edge-emitting LED, Superluminescent Diodes (SLD), and Erbium-Amplified Spontaneous Emissions (Er-ASE).  These light sources provide broad spectrum, high transmitted power, less driving circuits, low temperature sensitivity, and low cost due to high yields. 12.4 FBG-Based Optical CDMA Codecs

27. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The decoder architecture in the receiver is as shown in Fig. 12.6, which consists of a star coupler, a matched series of FBGs, a balanced photo-detector, and a decision circuitry unit.  Every decoder needs to extract the desired sequences from the interference of other users. The decoder can reject the multiple-access interference (MAI) coming from other users. It decreases the data error rate, and thus increases the system performance of the optical CDMA. 12.4 FBG-Based Optical CDMA Codecs

28. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.6 FBG-based optical CDMA decoder. 12.4 FBG-Based Optical CDMA Codecs

29. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 12.5 AWG-Based Optical CDMA Codecs  An optical CDMA encoder/decoder over arrayed- waveguide grating (AWG) routers is configured with m-sequence coded spectral- amplitude of broadband light sources. In such OCDMA system, each user shares the same AWG routers in the transmitters and receivers.  The signature address imposed on the transmit- encoder should be matched to the signature of the receiver-decoder.  The coded information sequences are linked to a common star coupler to share the same channel bandwidth.

30. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  In Fig. 12.7, the broadband optical signals of each user are directed to the corresponding input port of the grating router.  M-sequence signature will determine the links between AWG router and the star coupler. Fig. 12.7 AWG-based optical CDMA encoder. 12.5 AWG-Based Optical CDMA Codecs

31. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The receiver-decoder architecture is as shown in Fig. 12.8.  Each user has a balanced photo-detector and a decision circuitry unit connected to a pair of common grating routers.  User’s M-sequence signature and complement codes are adopted to determine the links between the star coupler and the upper/lower AWG routers. 12.5 AWG-Based Optical CDMA Codecs

32. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.8 AWG-based optical CDMA decoder. 12.5 AWG-Based Optical CDMA Codecs

33. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  The operating principle of the AWG- based OCDMA coder/decoders is as illustrated in Fig. 12.9.  The grating router demultiplexes the incident wavelength into all the output ports, and the same wavelength signals that are incident from different input ports will go to different output ports in a cyclic manner.  By utilizing the cyclic property of m-sequence code, we can obtain all users’ spectral chips in the selected output ports, which are designed to be of different central wavelength. 12.5 AWG-Based Optical CDMA Codecs

34. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰 Fig. 12.9 Operation principle of waveguide grating routers. 12.5 AWG-Based Optical CDMA Codecs

35. 教育部顧問室光通訊系統教育改進計畫 成功大學 黃振發 編撰  In the receiver end, the decoder can reject the multiple-access interference (MAI) coming from other users and decreases the data error rate, thus increases the system performance of the optical CDMA.  In addition, the total system is compact and low cost, thus can be upgraded to larger capacity easily. 12.5 AWG-Based Optical CDMA Codecs