Optical CDMA Coder/Decoders with Spectral-Amplitude Coding Schemes over Fiber and Waveguide Gratings

• Main Authors: Chao-Chin Yang, 楊朝欽
• Other Authors: Jen-Fa Huang
• Format: Others
• Language: en_US
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/09155576207183181539

博士 === 國立成功大學 === 電機工程學系碩博士班 === 92 === Optical code division multiple-access (OCDMA) offers high statistical multiplexing gain in a bursty traffic environment and is thought to be a more suitable solution in local-area network. Early incoherent optical CDMA systems used pseudo-orthogonal sequences to encode signals in the time domain, but the codes were long and multiple access interference (MAI) limited the number of simultaneous users. Thus, spectral-amplitude-coding (SAC) optical CDMA systems were proposed to eliminate the influence of MAI. In this dissertation, the evolution of one and two-dimensional OCDMA system from radio CDMA system are reviewed, then encoder/decoder based on fiber Bragg gratings (FBG) is experimentally demonstrated. One compensation scheme is proposed to eliminate the interference arising from the non-ideal filter response of FBG, and one new FBG-based encoder/decoder pair not suffered from non-ideal filter response is presented and analyzed. However, when the number of total users supported by the system becomes large, the lengths of each user’s signature sequences grow longer and the FBG encoders/decoders should have more individual FBGs cascaded. This introduces more insertion losses and the physical sizes of coding devices become impractical. In addition, the finite bandwidths of broadband sources also limit the number of total users supported by the system. By utilizing the cyclic property of arrayed-waveguide grating (AWG) router and the characteristic property of adopted codes, two SAC-OCDMA systems based on AWG routers are proposed. The first system utilizes the cyclic property of M-sequence codes, and the proposed codec pair can encode/decode multiple codewords of M-sequence code while retaining the ability for MAI cancellation. The total system becomes more compact and simple because all users in the network can use a single encoder/decoder pair. By adopting the relation between complementary Walsh-Hadamard (CWH) codes and the associated complement codes, one mirrored AWG router is used to produce either amplitude spectrum of complementary codewords in the second system, and each user requires only one AWG router to implement spectral complementary encoding and decoding, respectively. These two configurations reduce the physical size of the coder, making the implementation of systems more realizable. To increase the number of total users in the system under the limit of finite source bandwidth, the users’ information can be encoded and decoded in both the spectral and spatial domain. We extend one-dimensional (1-D) M-sequence codes to demonstrate that the two-dimensional (2-D) maximal-area matrices (M-matrices) codes can be operated without any multiple-access interference. The associated decoding scheme is developed to make the decoder reject other users’ interferences and extract the information of desired user. This system can be implemented with cheap broadband sources and FBGs, and the physical size of encoder/decoder is smaller as compared to that for 1-D system. The performance degradation due to beat noise in 1-D M-sequences coding can be improved with the 2-D M-matrices decoding architecture However, the power that arrives at the photo-diodes during the decoding process is relatively large, thus the beat noise arising in the photo-diodes is still serious. Two different methods are proposed to suppress the beat noise in the 2-D systems. The first method is to disperse the power of the same wavelength contributed by all interfering codewords to distinct photo-diodes of the decoder. We propose the modified version of the M-matrix code, so called Permuted M-matrix, whose spectral encoder/decoder is derived from the 1-D AWG router-based encoder proposed. The second method is to use sequences with low cross correlation as the spectral or spatial signature sequences in the 2D codeword. After modifying the well-known modified prime codes, one new 2-D Prime/M-matrix code family is proposed by utilizing the resulting 1-D codes as the spectral signature sequences. Both schemes not only retain the advantage of the M-matrix, but also improve the performance degradation due to the beat noise in the 2-D system. Thus OCDMA systems with larger capacity are achieved and a larger number of active users are allowed to access simultaneously in the system.