Optical code-division multiple access system and optical signal processing

This thesis presents our recent researches on the development of coding devices, the investigation of security and the design of systems in the optical cod-division multiple access (OCDMA) systems. Besides, the techniques of nonlinear signal processing used in the OCDMA systems fire our imagination, thus some researches on all-optical signal processing are carried out and also summarized in this thesis. Two fiber Bragg grating (FBG) based coding devices are proposed. The first coding device is a superstructured FBG (SSFBG) using ±π/2-phase shifts instead of conventional 0/π-phase shifts. The ±π/2-phase-shifted SSFBG en/decoders can not only conceal optical codes well in the encoded signals but also realize the reutilization of available codes by hybrid use with conventional 0/π-phase-shifted SSFBG en/decoders. The second FBG based coding device is synthesized by layer-peeling method, which can be used for simultaneous optical code recognition and chromatic dispersion compensation. Then, two eavesdropping schemes, one-bit delay interference detection and differential detection, are demonstrated to reveal the security vulnerability of differential phase-shift keying (DPSK) and code-shift keying (CSK) OCDMA systems. To address the security issue as well as increase the transmission capacity, an orthogonal modulation format based on DPSK and CSK is introduced into the OCDMA systems. A 2 bit/symbol 10 Gsymbol/s transmission system using the orthogonal modulation format is achieved. The security of the system can be partially guaranteed. Furthermore, a fully-asynchronous gigabit-symmetric OCDMA passive optical network (PON) is proposed, in which a self-clocked time gate is employed for signal regeneration. A remodulation scheme is used in the PON, which let downstream and upstream share the same optical carrier, allowing optical network units source-free. An error-free 4-user 10 Gbit/s/user duplex transmission over 50 km distance is reazlied. A versatile waveform generation scheme is then studied. A theoretical model is established and a waveform prediction algorithm is summarized. In the demonstration, various waveforms are generated including short pulse, trapezoidal, triangular and sawtooth waveforms and doublet pulse. ii In addition, an all-optical simultaneous half-addition and half-subtraction scheme is achieved at an operating rate of 10 GHz by using only two semiconductor optical amplifiers (SOA) without any assist light. Lastly, two modulation format conversion schemes are demonstrated. The first conversion is from NRZ-OOK to PSK-Manchester coding format using a SOA based Mach-Zehnder interferometer. The second conversion is from RZ-DQPSK to RZ-OOK by employing a supercontinuum based optical thresholder

DIGITAL ENCODING OF TELEVISION SIGNALS USING THE PULSE WIDTH MODULATOR

An attempt is made to quantify the circuit complexity and mean circuit speed of linearly quantized straight PCM video encoding techniques. Any significant reduction in circuit complexity (i.e. the number of active and passive devices to be integrated) is considered important since this determines: chip area and yield if the encoder is to be fully integrated. Analysis indicates that the complexity of the more highly developed straight PCM video encoders can be reduced by typically a factor 3 using either non-programmed sequential encoding, pulse width modulator encoding or programmed sequential encoding (closed loop successive approximation). The encoder studied in this work is an 8-bit pulse width modulator video encoder using a 2-step production line technique and a detailed design procedure for a prototype encoder is given. This encoder is considered to achieve 7-bit resolution at a sampling rate of 13.3MHZ. A mathematical model of the encoder-decoder system is developed for numerical evaluation of the effect of encoder errors and white Gaussian noise upon a coded and decoded video signal. A triangular wave test is applied to examine the effect of encoder errors upon the statio transfer characteristic of the encoder. Dynamic errors are investigated by simulating colour subcarrier at the model input and observing the phase and gain errors at the filtered codec output. Using differential phase and gain, an attempt is made to determine a circuit design and alignment criterion such that most practical codecs will fall within specific bounds on these parameters (taken as ±6° and ±6% respective1y). In the absense of dither, Monte Carlo analysis indicates that the maximum voltage error incurred by each encoder error source should have a high probability (95%) of being less than a half quantum if 85 - 90% of codecs measured are to fall within the above bounds. If white Gaussian noise is used as a simple dither signal then the probability of a codec falling within the above bounds may increase to about 95%. Improvements to the encoder are discussed, including several automatic error correction techniques which combat instrumental errors and give a more robust PWM encoder. Also, by predetermining the most significant bit for each set of 4 coded bits it is possible to halve the encoder clock frequency (to 133MHZ) without significantly changing the encoder complexity

Embedded coding algorithms applicable to time variable channels

This thesis investigates new design and implementation techniques applicable to modern communication systems operating over time variable channels. Three areas of interest are investigated. These include, source coding in conjunction with real-time channel evaluation, channel coding and modem design. An investigation of source coding methods has led to the development of a new embedded real time channel evaluation, based on statistical techniques. The performance of this technique is examined using simulation techniques for channels with and without memory. Existing channel coding schemes applicable to time variable channels have been examined. This led to the formulation of a new coding technique, termed embedded encoding. Two implementations of such codes, embedded array codes and embedded convolutional codes, were developed. The theoretical and practical performance of these codes has been investigated. The final area of investigation has been the development of a 4-tone multi-frequency shift keying modem. In keeping with the intention of totally digital system design, the demodulator has been im­plemented on a single digital signal processing card. The demodulation method developed employs an embedded synchronisation technique, termed Code-Assisted Bit Synchronisation. The demodulator performs symbol synchronisation by utilising the convolutional code used for the purpose of channel coding. It thus performs the combined functions of the demodulator, decoder and symbol timing recovery, which are normally found as separate sub­systems. In combining these subsystems a more efficient modem has been developed

Evaluating and improving the performance of video content distribution in lossy networks

The contributions in this research are split in to three distinct, but related, areas. The focus of the work is based on improving the efficiency of video content distribution in the networks that are liable to packet loss, such as the Internet. Initially, the benefits and limitations of content distribution using Forward Error Correction (FEC) in conjunction with the Transmission Control Protocol (TCP) is presented. Since added FEC can be used to reduce the number of retransmissions, the requirement for TCP to deal with any losses is greatly reduced. When real-time applications are needed, delay must be kept to a minimum, and retransmissions not desirable. A balance, therefore, between additional bandwidth and delays due to retransmissions must be struck. This is followed by the proposal of a hybrid transport, specifically for H.264 encoded video, as a compromise between the delay-prone TCP and the loss-prone UDP. It is argued that the playback quality at the receiver often need not be 100% perfect, providing a certain level is assured. Reliable TCP is used to transmit and guarantee delivery of the most important packets. The delay associated with the proposal is measured, and the potential for use as an alternative to the conventional methods of transporting video by either TCP or UDP alone is demonstrated. Finally, a new objective measurement is investigated for assessing the playback quality of video transported using TCP. A new metric is defined to characterise the quality of playback in terms of its continuity. Using packet traces generated from real TCP connections in a lossy environment, simulating the playback of a video is possible, whilst monitoring buffer behaviour to calculate pause intensity values. Subjective tests are conducted to verify the effectiveness of the metric introduced and show that the results of objective and subjective scores made are closely correlated

Representing Variable-Length Codes in Fixed-Length T-Depletion Format in Encoders and Decoders

T-Codes are a class of variable-length codes. Their self-synchronization properties are useful in compression and communication applications where error recovery rather than error correction is at issue, for example, in digital telephony. T-Code s may be used without error correction or additional synchronization mechanisms. Typically, the representation of variable-length codes is a problem in computers based on a fixed-length word architecture. This presents a problem in encoder and decoder applications. The present paper introduces a fixed-length format for storing and handling variable-length T-Code codewords, the T-depletion codewords, which are derived from the recursive construction of the T-Code codewords. The paper further proposes an algorithm for the conversion of T-Code codewords into T-depletion codewords that may be used as a decoder for generalized T-Codes. As well as representing all codewords of a T-Code set (the leaf nodes in the set s decoding tree), the T-depletion code format also permits the representation of "pseudo-T codewords" --- strings that are not in the T-Code set. These strings are shown to correspond uniquely to all proper prefixes of T-Code codewords, thus permitting the representation of both intermediate and final decoder states in a single format. We show that this property may be used to store arbitrary finite and prefix-free variable-length codes in a compact fixed-length format. 1.) The authors' research is supported by the Department of Computer Science, the Division of Science and Technology (Tamaki Campus), and the Graduate Research Fund, all of The University of Auckland, the Centre for Discrete Mathematics and Theoretical Computer Science (CDMTCS) of the University of Auckland and the University of Waikato, and by the Deutsche Forschungsgemeinschaft (DFG). 2.) Proceedings of the First Japan-New Zealand Workshop on Logic in Computer Science, special issue editors D.S. Bridges, C.S. Calude, M.J. Dinneen and B. Khoussainov

Representing Variable-Length Codes in Fixed-Length T-Depletion Format in Encoders and Decoders

T-Codes are a class of variable-length codes. Their self-synchronisation properties are useful in compression and communication applications where error recovery rather than error correction is at issue, for example, in digital telephony. T-Codes may be used without error correction or additional synchronisation mechanisms. Typically, the representation of variable-length codes is a problem in computers based on a fixed-length word architecture. This presents a problem in encoder and decoder applications. The present paper introduces a fixed-length format for storing and handling variable-length T-Code codewords, the T-depletion codewords, which are derived from the recursive construction of the T-Code codewords. The paper further proposes an algorithm for the conversion of T-Code codewords into T-depletion codewords that may be used as a decoder for generalized T-Codes. As well as representing all codewords of a T-Code set (the leaf nodes in the set's decoding tree), the T-depletion co..

Implementation of a VLC HDTV Distribution System for Consumer Premises

A unidirectional, visible light communication (VLC) system intended for the distribution of Digital Video Broadcasting (DVB), high-definition television (HDTV) content to DVB compatible TVs within consumer premises is presented. The system receives off-air HDTV content through a consumer grade DVB-T/T2 terrestrial set-top-box (STB) and re-encodes its Moving Picture Experts Group (MPEG) transport stream (TS) using a pulse position modulation (PPM) scheme called inversion offset PPM (IOPPM). The re-encoded TS is used to intensity modulate (IM) a blue light-emitting diode (LED) operating at a wavelength of 470 nm. Directed line-of-sight (DLOS) transmission is used over a free-space optical (FSO) channel exhibiting a Gaussian impulse response. A direct-detection (DD) receiver is used to detect the transmitted IOPPM stream, which is then decoded to recover the original MPEG TS. A STB supporting a high-definition multimedia interface (HDMI) is used to decode the MPEG TS and enable connectivity to an HD monitor. The system is presented as a complementary or an alternative distribution system to existing Wi-Fi and power-line technologies. VLC connectivity is promoted as a safer, securer, unlicensed and unregulated approach. The system is intended to enable TV manufacturers to reduce costs by, firstly, relocating the TV’s region specific radio frequency (RF) tuner and demodulator blocks to an external STB capable of supporting DVB reception standards, and, secondly, by eliminating all input and output connectors interfaces from the TV. Given the current trend for consumers to wall-mount TVs, the elimination of all connector interfaces, except the power cable, makes mounting simpler and easier. The operation of the final system was verified using real-world, off-air broadcast DVB-T/T2 channels supporting HDTV content. A serial optical transmission at a frequency of 66 MHz was achieved. The system also achieved 60 Mbit/s, error free transmission over a distance of 1.2 m without using error correction techniques. The methodology used to realise the system was a top-down, modular approach. Results were obtained from electrical modelling, simulation and experimental techniques, and using time-domain and FFT based measurements and analysis. The modular approach was adopted to enable design, development and testing of the subsystems independently of the overall system