15 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,
âVPITM TransmissionMaker 7.1â 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 10â9 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
Fiber-optic code division multiple access : multi-class optical orthogonal codes, optical power control, and polarization encoding
Ever since the mid- 1980s when the single-mode fiber-optic media were believed to become the main highways of future telecommunications networks for transporting high-volume high-quality multipurpose information, the need for all-optical multi-access networking became important. An all-optical multi-access network is a collection of multiple nodes where the interconnection among various nodes is via single- or multi-mode fiber optics and for which they perform all their essential signal processing requirements such as switching, add-drop, multiplexing/demultiplexing and amplification in the optical domain. Optical CDMA networking is one possible technique that allows multiple users in local area networks to access the same fiber channel asynchronously with no delay or scheduling.
Optical CDMA networks are not without their own problems. Search for codes suitable to the optical domain is one of the important topics addressed in the literature on optical CDMA. Existing codes developed in the late 80's are limited to single class traffic or can support multiclass traffic but with restrictions on code lengths and weights. Also the number of generated codes is severely limited due to orthogonality issues. In this thesis, we pay particular attention to propose new codes that can support multiclass traffic with arbitrary code weights and lengths. Therefore, data sources with varying traffic demands can be accommodated by optical CDMA networks using the proposed codes. We also present a simple generation technique for the proposed multiclass codes and analyze their performance. The number of users supported by the proposed multiclass codes will be limited since it is an extension of existing code designs with such limitation. We then propose the use of polarization dimension in order to double the number of supported users.
On the other hand, incoherent optical CDMA systems are considered as positive systems meaning that only unipolar codes can be considered for such systems. Therefore, multiple access interference will be quite high in optical CDMA due to the nature of incoherent power detection. Reducing the effect of the interference on the performance of optical CDMA is an important topic. We propose the use of power control to decrease the effects of interference in optical star networks in which users' fiber lengths and data rates are not equal. We consider the case of optically amplified network with amplifier noise as the main source. We then elaborate by considering the nonlinearity in the photodetection process and propose the use of an iterative algorithm to find the solution of the non-linear optical power control problem.
Finally, we propose an optical CDMA system based on polarization encoding. Since the encoding is performed in the spatial domain, therefore, positive and negative levels can be realized. This approach leads to increasing the number of supported users of optical CDMA by the use of known codes, such as Gold and Hadamard codes, with enhanced performance.reviewe
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,
âVPITM TransmissionMaker 7.1â 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 10â9 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
Applications of perfect difference codes in fiber-optics and wireless optical code-division multiplexing/multiple-access systems
After establishing itself in the radio domain, Spread spectrum code-division
multiplexing/multiple-access (CDMA) has seen a recent upsurge in optical
domain as well. Due to its fairness, flexibility, service differentiation and
increased inherent security, CDMA is proved to be more suitable for the bursty
nature of local area networks than synchronous multiplexing techniques like
Frequency/Wavelength Division Multiplexing (F/WDM) and Time Division
Multiplexing (TDM). In optical domain, CDMA techniques are commonly known
as Optical-CDMA (O-CDMA). All optical CDMA systems are plagued with the
problem of multiple-access interference (MAI). Spectral amplitude coding (SAC)
is one of the techniques used in the literature to deal with the problem of MAI.
The choice of spreading code in any CDMA system is another way to ensure the
successful recovery of data at the receiving end by minimizing the effect of MAI
and it also dictates the hardware design of the encoder and decoder.
This thesis focuses on the efficient design of encoding and decoding hardware.
Perfect difference codes (PDC) are chosen as spreading sequences due to their
good correlation properties. In most of the literature, evaluation of error
probability is based on the assumptions of ideal conditions. Such assumptions
ignore major physical impairments such as power splitting losses at the
multiplexers of transmitters and receivers, and gain losses at the receivers, which
may in practice be an overestimate or underestimate of the actual probability of
error.
This thesis aims to investigate thoroughly with the consideration of practical
impairments the applications of PDCs and other spreading sequences in optical
communications systems based on spectral-amplitude coding and utilizing codedivision
as multiplexing/multiple-access technique. This work begins with a
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general review of optical CDMA systems. An open-ended practical approach has
been used to evaluate the actual error probabilities of OCDM/A systems under
study. It has been concluded from results that mismatches in the gains of
photodetectors, namely avalanche photodiode (APDs), used at the receiver side
and uniformity loss in the optical splitters results in the inaccurate calculation of
threshold level used to detect the data and can seriously degrade the system bit
error rate (BER) performance. This variation in the threshold level can be
compensated by employing techniques which maintain a constant interference
level so that the decoding architecture does not have to estimate MAI every time
to make a data bit decision or by the use of balanced sequences.
In this thesis, as a solution to the above problem, a novel encoding and decoding
architecture is presented for perfect difference codes based on common zero code
technique which maintains a constant interference level at all instants in CDM
system and thus relieves the need of estimating interference. The proposed
architecture only uses single multiplexer at the transmitters for all users in the
system and a simple correlation based receiver for each user. The proposed
configuration not only preserves the ability of MAI in Spectral-Amplitude Coding
SAC-OCDM system, but also results in a low cost system with reduced
complexity. The results show that by using PDCs in such system, the influence of
MAI caused by other users can be reduced, and the number of active users can be
increased significantly.
Also a family of novel spreading sequences are constructed called Manchestercoded
Modified Legendre codes (MCMLCs) suitable for SAC based OCDM
systems. MCMLCs are designed to be used for both single-rate and Multirate
systems. First the construction of MCMLCs is presented and then the bit error rate
performance is analyzed.
Finally the proposed encoding/decoding architecture utilizing perfect difference
codes is applied in wireless infrared environment and the performance is found to
be superior to other codes
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
Hybrid fibre and free-space optical solutions in optical access networks
This thesis evaluates the potentials of hybrid fibre and free space optical (FSO) communications access networks in providing a possible solution to an all optical access network. In such network architectures, the FSO link can extend the system to areas where an optical fibre link is not feasible, and/or provide limited mobility for indoor coverage. The performance of hybrid fibre and FSO (HFFSO) networks based on digital pulse position modulation (DPPM), for both the indoor and outdoor environments of the optical access network, are compared with the performance of such a network that is based on conventional on-off keying non-return-to-zero (OOK NRZ) modulation using results obtained through computational and analytical modelling. Wavelength division multiplexing (WDM) and/or code division multiple access (CDMA) are incorporated into the network for high speed transmission and/or network scalability.
The impacts of optical scintillation, beam spreading and coupling losses, multiple access interference (MAI), linear optical crosstalk and amplified spontaneous emission noise (ASE) on the performance of hybrid fibre and FSO (HFFSO) access networks are analysed, using performance evaluation methods based on simple Gaussian approximation (GA) and more complex techniques based on moment generating function (MGF), including the Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA). Results in the form of bit error rate (BER), power penalty, required optical power and outage probability are presented, and both the CB and MCB, which are upper bounds, are suggested as safer methods of assessing the performance of practical systems.
The possibility of using a CDMA-based HFFSO network to provide high speed optical transmission coverage in an indoor environment is investigated. The results show a reduction in transmit power of mobile devices of about 9 â 20 dB (depending on number of active users) when an optical amplifier is used in the system compared to a non-amplified system, and up to 2.8 dB improvement over OOK NRZ receiver sensitivity is provided by a DPPM system using integrate and compare circuitry for maximum likelihood detection, and at coding level of two, for minimum bandwidth utilization.
Outdoor HFFSO networks using only WDM, and incorporating CDMA with WDM, are also investigated. In the presence of atmospheric scintillations, an OOK system is required (for optimum performance) to continuously adapt its decision threshold to the fluctuating instantaneous irradiance. This challenge is overcome by using the maximum likelihood detection DPPM system, and necessitated the derivation of an interchannel crosstalk model for WDM DPPM systems. It is found that optical scintillation worsens the effect of interchannel crosstalk in outdoor HFFSO WDM systems, and results in error floors particularly in the upstream transmission, which are raised when CDMA is incorporated into the system, because of MAI. In both outdoor HFFSO networks (with WDM only and with WDM incorporating CDMA), the optical amplifier is found necessary in achieving acceptable BER, and with a feeder fibre of 20 km and distributive FSO link length of 1500 m, high speed broadband services can be provided to users at safe transmit power at all turbulence levels in clear air atmosphere
Hybrid fibre and free-space optical solutions in optical access networks
This thesis evaluates the potentials of hybrid fibre and free space optical (FSO) communications access networks in providing a possible solution to an all optical access network. In such network architectures, the FSO link can extend the system to areas where an optical fibre link is not feasible, and/or provide limited mobility for indoor coverage. The performance of hybrid fibre and FSO (HFFSO) networks based on digital pulse position modulation (DPPM), for both the indoor and outdoor environments of the optical access network, are compared with the performance of such a network that is based on conventional on-off keying non-return-to-zero (OOK NRZ) modulation using results obtained through computational and analytical modelling. Wavelength division multiplexing (WDM) and/or code division multiple access (CDMA) are incorporated into the network for high speed transmission and/or network scalability.
The impacts of optical scintillation, beam spreading and coupling losses, multiple access interference (MAI), linear optical crosstalk and amplified spontaneous emission noise (ASE) on the performance of hybrid fibre and FSO (HFFSO) access networks are analysed, using performance evaluation methods based on simple Gaussian approximation (GA) and more complex techniques based on moment generating function (MGF), including the Chernoff bound (CB), modified Chernoff bound (MCB) and saddlepoint approximation (SPA). Results in the form of bit error rate (BER), power penalty, required optical power and outage probability are presented, and both the CB and MCB, which are upper bounds, are suggested as safer methods of assessing the performance of practical systems.
The possibility of using a CDMA-based HFFSO network to provide high speed optical transmission coverage in an indoor environment is investigated. The results show a reduction in transmit power of mobile devices of about 9 â 20 dB (depending on number of active users) when an optical amplifier is used in the system compared to a non-amplified system, and up to 2.8 dB improvement over OOK NRZ receiver sensitivity is provided by a DPPM system using integrate and compare circuitry for maximum likelihood detection, and at coding level of two, for minimum bandwidth utilization.
Outdoor HFFSO networks using only WDM, and incorporating CDMA with WDM, are also investigated. In the presence of atmospheric scintillations, an OOK system is required (for optimum performance) to continuously adapt its decision threshold to the fluctuating instantaneous irradiance. This challenge is overcome by using the maximum likelihood detection DPPM system, and necessitated the derivation of an interchannel crosstalk model for WDM DPPM systems. It is found that optical scintillation worsens the effect of interchannel crosstalk in outdoor HFFSO WDM systems, and results in error floors particularly in the upstream transmission, which are raised when CDMA is incorporated into the system, because of MAI. In both outdoor HFFSO networks (with WDM only and with WDM incorporating CDMA), the optical amplifier is found necessary in achieving acceptable BER, and with a feeder fibre of 20 km and distributive FSO link length of 1500 m, high speed broadband services can be provided to users at safe transmit power at all turbulence levels in clear air atmosphere
Application of Asynchronous Transfer Mode (Atm) technology to Picture Archiving and Communication Systems (Pacs): A survey
Broadband Integrated Services Digital Network (R-ISDN) provides a range of narrowband and broad-band services for voice, video, and multimedia. Asynchronous Transfer Mode (ATM) has been selected by the standards bodies as the transfer mode for implementing B-ISDN; The ability to digitize images has lead to the prospect of reducing the physical space requirements, material costs, and manual labor of traditional film handling tasks in hospitals. The system which handles the acquisition, storage, and transmission of medical images is called a Picture Archiving and Communication System (PACS). The transmission system will directly impact the speed of image transfer. Today the most common transmission means used by acquisition and display station products is Ethernet. However, when considering network media, it is important to consider what the long term needs will be. Although ATM is a new standard, it is showing signs of becoming the next logical step to meet the needs of high speed networks; This thesis is a survey on ATM, and PACS. All the concepts involved in developing a PACS are presented in an orderly manner. It presents the recent developments in ATM, its applicability to PACS and the issues to be resolved for realising an ATM-based complete PACS. This work will be useful in providing the latest information, for any future research on ATM-based networks, and PACS
Orthogonal frequency division multiplexing for optical access networks
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme with numerous advantages that has for years been employed as the leading physical interface in many wired and wireless communication systems. Recently, with advancements made in digital signal processing, there has been a surge of interest in applying OFDM techniques for optical communications. This thesis presents extensive research on optical OFDM and how it is being applied in access networks. With the aid of theoretical analysis, simulations and experiments, it is shown that the system performance of direct-detection optical OFDM (DD-OOFDM) in the presence of MZM non-linear distortion can be improved by proper biasing and selection of appropriate drive to the MZM. Investigations are conducted to illustrate how a variation in the number of subcarriers and the modulation format influences the sensitivity of the DD-OOFDM system to the MZM non-linear distortion. The possibility of improving the spectral efficiency by reduction of the width of the guard band is also investigated. This thesis also looks into the radio-over-fibre (RoF) transmission of Multiband OFDM UWB as a transparent and low-cost solution for distributing multi-Gbit/s data to end-users in FTTH networks. Due to relaxed regulatory requirements and the wide bandwidth available, UWB operation in the 60-GHz band is also considered for this FTTH application scenario. Four techniques for enabling MB-OFDM UWB RoF operation in the 60-GHz band are experimentally demonstrated. The impacts of various parameters on the performance of the techniques as well as the limitations imposed by fibre distribution are illustrated. Finally, a digital pre-distorter is proposed for compensating for the MZM non-linearity. Experimental demonstration of this digital pre-distortion in an UWB over fibre transmission system shows an increased tolerance to the amplitude of the driving OFDM signal as well as an increase in the optimum modulation index of the OFDM signal
Optical code division multiple access systems in AlGaInAs/InP
The rise of photonic integration makes optical code division multiple access (OCDMA) worth revisiting due to its promising role in future all-optical networks. OCDMA has the potential to exploit the surplus bandwidth of optical fibres and to carry over to the optical domain the benefits seen CDMA radio communication systems, such as the effective sharing of the spectrum for multiple network subscribers, and resistance to jamming and eavesdropping. One of the major requirements for the deployment of OCDMA in networks is integration.
This thesis presents a research study of integrated OCDMA systems using the AlGaInAs/InP semiconductor material system. This material is considered due to its useful intrinsic properties such as thermal stability, strong electron confinement, and low threshold, making it suitable for fabricating optoelectronic devices. Two bespoke OCDMA systems are considered for integration: coherent temporal phase coding (TPC), and incoherent wavelength-hopping time-spreading (WHTS) OCDMA systems. TPC systems are excellent for high speed communications due to their static en/decoding enabling features. In this research, a 2Ă2 asymmetric Mach Zehnder interferometer (AMZI) is used to generate a 2-bit phase code, allowing multiplexing for up to four users. A semiconductor mode-locked ring laser is also embedded in the circuit, and using a synchronous mode-locking method, adequate signal en/decoding is achieved. WHTS systems on the other hand fully exploit the spectral and temporal space available in networks by assigning each user with a unique wavelength-time hop sequence for en/decoding data signals. Here, a mode-locked laser array is used with intracavity distributed Bragg reflectors (DBRs) for spectrally tuning each laser, and a 4:1 multimode interference coupler is used to combine the laser signals into a single channel for amplification, modulation and transmission. The integrated system is fully characterised and synchronisation experiments are performed to show the potential for its use in high speed multi-user networks.
Mode-locked lasers play an important role in many OCDMA implementations due to their wide spectrum and discrete temporal properties, which can be easily exploited during data en/decoding. Various mode-locked laser devices have been studied during this research with additional embedded components such as intracavity DBRs and phase controllers for precise tuning of the wavelength and pulse repetition frequency. However, the noisy nature of passively operating mode-locked lasers make them prone to high jitter, which can result in high bit error rates. Synchronisation schemes are thereby explored in order to temporally stabilise the pulse oscillations to make them suitable for use in long haul transmission systems. This includes synchronous and hybrid mode-locking, as well as a passive technique using an optical fibre loop to provide phase feedback, which is shown to promote ultralow RF linewidths in mode-locked lasers