712 research outputs found
A NOVEL CONSTRUCTION OF VECTOR COMBINATORIAL (VC) CODE FAMILIES AND DETECTION SCHEME FOR SAC OCDMA SYSTEMS
There has been growing interests in using optical code division multiple access
(OCDMA) systems for the next generation high-speed optical fiber networks. The
advantage of spectral amplitude coding (SAC-OCDMA) over conventional OCDMA
systems is that, when using appropriate detection technique, the multiple access
interference (MAI) can totally be canceled. The motivation of this research is to
develop new code families to enhance the overall performance of optical OCDMA
systems. Four aspects are tackled in this research. Firstly, a comprehensive discussion
takes place on all important aspects of existing codes from advantages and
disadvantages point of view. Two algorithms are proposed to construct several code
families namely Vector Combinatorial (VC). Secondly, a new detection technique
based on exclusive-OR (XOR) logic is developed and compared to the reported
detection techniques. Thirdly, a software simulation for SAC OCDMA system with
the VC families using a commercial optical system, Virtual Photonic Instrument,
“VPITM TransmissionMaker 7.1” is conducted. Finally, an extensive investigation to
study and characterize the VC-OCDMA in local area network (LAN) is conducted.
For the performance analysis, the effects of phase-induced intensity noise (PIIN), shot
noise, and thermal noise are considered simultaneously. The performances of the
system compared to reported systems were characterized by referring to the signal to
noise ratio (SNR), the bit error rate (BER) and the effective power (Psr). Numerical
results show that, an acceptable BER of 10−9 was achieved by the VC codes with 120
active users while a much better performance can be achieved when the effective
received power Psr > -26 dBm. In particular, the BER can be significantly improved
when the VC optimal channel spacing width is carefully selected; best performance
occurs at a spacing bandwidth between 0.8 and 1 nm. The simulation results indicate
that VC code has a superior performance compared to other reported codes for the
same transmission quality. It is also found that for a transmitted power at 0 dBm, the
BER specified by eye diagrams patterns are 10-14 and 10-5 for VC and Modified
Quadratic Congruence (MQC) codes respectively
Advanced optical modulation and fast reconfigurable en/decoding techniques for OCDMA application
With the explosive growth of bandwidth requirement in optical fiber communication
networks, optical code division multiple access (OCDMA) has witnessed tremendous
achievements as one of the promising technologies for optical access networks over the
past decades. In an OCDMA system, optical code processing is one of the key
techniques. Rapid optical code reconfiguration can improve flexibility and security of
the OCDMA system. This thesis focuses on advanced optical modulations and
en/decoding techniques for applications in fast reconfigurable OCDMA systems and
secure optical communications.
A novel time domain spectral phase encoding (SPE) scheme which can rapidly
reconfigure the optical code and is compatible with conventional spectral domain phase
en/decoding by using a pair of dispersive devices and a high speed phase modulator is
proposed. Based on this scheme, a novel advanced modulation technique that can
simultaneously generate both the optical code and the differential-phase-shift-keying
(DPSK) data using a single phase modulator is experimentally demonstrated. A
symmetric time domain spectral phase encoding and decoding (SPE/SPD) scheme using
a similar setup for both the transmitter and receiver is further proposed, based on which
a bit-by-bit optical code scrambling and DPSK data modulation technique for secure
optical communications has been successfully demonstrated. By combining optical
encoding and optical steganography, a novel approach for secure transmission of time
domain spectral phase encoded on-off-keying (OOK)/DPSK-OCDMA signal over
public wavelength-division multiplexing (WDM) network has also been proposed and
demonstrated.
To enable high speed operation of the time domain SPE/SPD scheme and enhance the
system security, a rapid programmable, code-length variable bit-by-bit optical code
shifting technique is proposed. Based on this technique, security improvements for
OOK/DPSK OCDMA systems at data rates of 10Gb/s and 40Gb/s using reconfigurable
optical codes of up to 1024-chip have been achieved.
Finally, a novel tunable two-dimensional coherent optical en/decoder which can
simultaneously perform wavelength hopping and spectral phase encoding based on
coupled micro-ring resonator is proposed and theoretically investigated. The techniques
included in this thesis could be potentially used for future fast reconfigurable and secure
optical code based communication systems
IR-UWB for multiple-access with differential-detection receiver
Impulse-Radio Ultra-Wideband (IR-UWB) emerged as a new wireless technology because of its unique characteristics. Such characteristics are the ability to support rich-multimedia applications over short-ranges, the ability to share the available spectrum among multi-users, and the ability to design less complex transceivers for wireless communication systems functioning based on this technology. In this thesis a novel noncoherent IR-UWB receiver designed to support multiple-access is proposed. The transmitter of the proposed system employs the noncoherent bit-level differential phase-shift keying modulation combined with direct-sequence code division multiple-access. The system is investigated under the effect of the additive white Gaussian noise with multiple-access channel. The receiver implements bit-level differential-detection to recover information bits. Closed-form expression for the average probability of error in the proposed receiver while considering the channel effects is analytically derived. This receiver is compared against another existing coherent receiver in terms of bit error rate performance to confirm its practicality. The proposed receiver is characterized by its simple design requirements and its multiple-access efficiency
Simulated Assessment of Interference Effects in Direct Sequence SpreadSpectrum (DSSS) QPSK Receiver
This research developed and validated a generic simulation for a direct sequence spread spectrum (DSSS), using differential phase shift keying (DPSK) and phase shift keying (PSK) modulations, providing the flexibility for assessing intentional interference effect using DSSS quadrature phase shift keying receiver (QPSK) with matched filtering as a reference. The evaluation compares a comprehensive pool of jamming waveforms at pass-band that include continuous wave (CW) interference, broad-band jamming, partial-band interference and pulsed interference. The methodology for jamming assessment included comparing the bit error rate (BER) versus required jamming to signal ratio (JSR) for different interferers using the Monte Carlo approach. This thesis also analyzes the effect of varying the jammer bandwidth for broad-band jammers including broad-band noise (BBN), frequency hopping interference (FHI), comb- spectrum interference (CSI), multi-tone jamming (MTJ), random frequency modulated interference (RFMI) and linear frequency modulated interference (LFMI). Also, the effect of changing the duty cycle for pulsed CW waveforms is compared with the worst case pulsed jamming equation. After the evaluation of different interferers, the research concludes that pulsed binary phase shift keying (BPSK) jamming is the most effective technique, whereas the CW tone jamming and CW BPSK interference result are least effective. It is also concluded that by finding an optimum bandwidth, FHI and BBN improves the required JSR by approximately 2.1 dB, RFMI and LFMI interference by 0.9 and 1.5 dB respectively. Alternately, MTJ and CSI improves their effectiveness in 4.1 dB and 3.6 dB respectively, matching the performance of the pulsed BPSK jammer
Chirp-rate quasi-orthogonality based DSSS-CDMA system for underwater acoustic channel
Abstract(#br)Underwater network node positioning is a key supporting technology for underwater networks. Generally, nodes at known locations (anchor nodes) are used to transmit location information to the node to be located, and the node to be located performs location calculation according to the arrival time of the received information, which requires multiple access communication between the anchor nodes and the node to be located. In order to reduce the multiple access interference (MAI) and distinguish the information of different anchor nodes, it is necessary to study the underwater multiple access method. The GPS positioning system uses direct sequence spread spectrum code division multiple access(DSSS-CDMA) technology, and the positioning satellite transmits a signal formed by the BPSK modulation by transmitting a pseudo-random sequence (PN sequence) to the binary code sequence corresponding to the navigation message to the user equipment to be located. The underwater acoustic (UWA) channel has a complex multipath structure and Doppler effect, which causes a large interference to the communication system. Compared with narrowband communication, spread spectrum communication has a strong anti-interference ability, and can maintain the reliability of the communication system in the UWA channel. Based on this, this paper proposes a quasi-orthogonal Chirp-rate based DSSS-CDMA method under UWA channel, and carries out simulation analysis and experimental verification
COSSAP simulation model of DS-CDMA indoor microwave ATM LAN
This thesis presents an original work in the area of designing and implementing a simulation testbed for modelling a high speed spread spectrum Asynchronous Transfer Mode (ATM) Local Area Network (LAN). The spread spectrum technique used in this LAN model is Direct Sequence Code Division Multiple Access (DS-CDMA). The simulation model includes at least a physical layer of such a LAN, embedded into the COSSAP1 simulation environment, and has been fully tested. All the newly developed building blocks are comprised of standard blocks from the COSSAP libraries or compatible user-built primitive blocks (only where it is absolutely necessary), and are flexible enough to allow the modification of simulation or model parameters; such as the number of signal channels, modulation method used, different spreading code sequences and so on. All these changes can be made with minimal effort. Another significant contribution made in this thesis is the extended research into evaluating the Bit Error Rate (BER) performance of different spread spectrum COMA coding schemes for an indoor microwave A1M LAN [8]. Different spread spectrum CDMA coding schemes are compared for their transmission error rate in Additive White Gaussian Noise (AWGN) channel with varying transmitted signal power and at different channel Signal to Noise Ratio (SNR) levels. Since a wireless microwave channel is very prone to transmission errors, a major contribution of the simulation testbed developed in this thesis is its use in the finding of an optimal physical layer transmission scheme with the best Bit Error Rate (BER) performance in an indoor environment
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.
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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
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