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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Reception of QPSK Signal Using Digital Coherent Receiver

Chen, Shr-Jie 11 July 2011 (has links)
The coherent system has been extensively studied in recent years. The reasons are that receiver sensitivity is better than Intensity Modulation with Direct Detection (IM/DD) and the spectral efficiency of wavelength division multiplexing (WDM) is increased. The modulation formats of the coherent system are Amplitude shift keying (ASK), Phase shift keying (PSK), and Frequency shift keying (FSK). The detection techniques are Homodyne detection and Heterodyne detection, both of them need a laser light source in the receiver called as the Local oscillator (LO). In the previous study, the Quadrature phase shift keying (QPSK) modulation format with Pseudo Random Binary Sequence (PRBS) 27-1 to transmit 51km was investigated, and the pilot carrier method realized the Homodyne Detection. The merit of the pilot carrier is that the LO is not necessary in the receiver. In this master thesis, the optical signal of the QPSK modulation format with PRBS 215-1 is transmitted over 500km using the pilot carrier method. A WDM system demonstration is also conducted to increase the system capacity. The Bit error rate (BER) is calculated by the MATLAB program. The BER performance of 20G bit/s, 500 km transmission system using the QPSK with single channel and multiplexed channels were measured.
2

Limitations and Improvement of Subcarrier Multiplexed Systems over Optical Fiber

Tebben, Daniel James 24 April 2006 (has links)
Optical coherent techniques are used to eliminate the power fading found in optical subcarrier multiplexed systems. In a double-side band optical subcarrier system the signal experiences a periodic power fading that is dependent on the fiber dispersion and subcarrier frequency. This power fading is manifested during the direct detection of the subcarrier system using a square-law photodetector. Using a modified optical local oscillator to coherently detect the subcarrier channel this power fading can be eliminated. An optical local oscillator is centered at the optical carrier in order to perform homodyne detection. However, the local oscillator is modulated by a term equal the subcarrier frequency of interest. This is then a dual-frequency optical local oscillator. By controlling the phases of the local oscillator and the local subcarrier oscillator independently in the homodyne detection scheme, both the phase error and power fading of the detected subcarrier channel can be eliminated. This technique also allows the subcarrier to be selected optically, before the optical-to-electrical conversion. Analytical and simulation results are given to show the benefits of optical coherent detection in double-sideband subcarrier systems. By eliminating the periodic power loss found in the double-sideband subcarrier system the signal becomes dispersion limited and not power limited. A comparison of double-sideband and single-sideband subcarrier systems is presented. Multiple subcarriers and subcarrier spacing are also investigated for both double sideband and single sideband subcarrier systems. Optical phase and modulator noise are also considered in the analysis and simulation of coherent detection using a dual frequency optical local oscillator. Since the implementation used to eliminate the power fading is a phase correction based process, the phase noise of both the source and local oscillator lasers must be considered and the technique compared to typical direct and coherent detection techniques. Also, the effects of modulator nonlinearity are simulated for multichannel subcarrier multiplexed systems and comparisons made between the performance of using the dual-frequency local oscillator and typical detection techniques. It is shown that the advantages of the dual-frequency LO are retained in the presence of both phase noise and modulator nonlinearity. / Ph. D.
3

Signature Stability in Laser Doppler Vibrometry

Iverson, Thomas Z. 24 August 2017 (has links)
No description available.
4

Frequency control of terahertz quantum cascade lasers : theory and measurement

Folland, Thomas January 2017 (has links)
Terahertz (THz) technology stands to solve a number of problems in everyday life, from next generation wireless communication to spectroscopic identification and imaging. However it is technically challenging to make a high power, compact source for terahertz radiation. The Quantum Cascade Laser (QCL), which produces gain at THz frequencies by exploiting inter-sub-band transitions in quantum wells, offers one solution to this problem. However controlling and detecting the emission from such sources remains a major challenge. This thesis investigates the theory and measurement of emission frequencies from aperiodic lattice THz QCLs. Crucially, realising both frequency control and detection provides a complete system for coherent THz characterisation of devices at precise, user defined frequencies. The author starts by studying the emission frequencies and threshold of discretely tuned aperiodic lattice lasers. This is achieved using a numerical transfer matrix method (TMM), which allows the calculation of the aperiodic lattice threshold spectrum for the first time. Calculations reveal that the low threshold modes of aperiodic lattice lasers form at peaks in the electromagnetic density of modes. This shows that lasing in aperiodic lattices arises from slow light propagation induced by multiple photonic band gaps, leading to both band edge and defect laser modes. Frequency selective lasing is maintained even under the influence of external facet feedback, albeit at the cost of precise knowledge of the mode frequency. Importantly this framework allows the understanding of essentially any aperiodic lattice laser system. Most significantly, the TMM is exploited in order to understand how graphene can be used to control a THz laser. Graphene interacts strongly with THz waves, and can be easily integrated with semiconductor structures such as lasers and waveguides. Here, numerical calculations reveal that graphene can be introduced into the waveguide of a THz QCL, generating electrically tunable THz surface plasmons. Such surface plasmons couple into an aperiodic lattice to change the scattering strength of each individual grating element. The TMM reveals that this change in scattering strength controls the modal selectivity of an aperiodic lattice THz QCL. This hypothesis successfully explains both earlier experiments and those performed by the author. Crucially, this model was central to a publication in the journal Science. Finally, this thesis demonstrates a novel coherent detection system for the characterisation of THz QCL emission. The technique exploits non-linear up-conversion of THz waves to a telecoms frequency side-band, a process shown to be sensitive to THz waveguide dispersion. By mixing the up-converted THz wave with a near infra-red local oscillator laser, coherent detection of QCL emission using all fibre coupled components is demonstrated for the first time. This measurement allows for the characterisation of laser emission with high frequency and temporal resolution. Specifically sub-microsecond pulses of THz emission and transients can be detected. When taken as a whole, the work of this thesis constitutes a major step towards realising cost effective THz characterisation and spectroscopy using QCLs.
5

Reach Enhancement in both Direct-Detection and Coherent Detection Optical Fiber Communication Systems

Sarkis, Charles 03 1900 (has links)
Early methods of optical fiber communication systems haven't been much promising in terms of efficiency. The presence of various impairments in the fiber channel has forced researchers to uncover solutions in order to minimize those effects. With the advancement of technology, optical solutions were finally easier to implement in the system. To this day, optical compensation methods are still found to be as the best way to minimize fiber impairments. However, such technique does introduce enormous complexity to the system, in addition to a large cost. For that reason, the main focus had to shift to an alternative method. Electrical compensation techniques have provided the factor of simplicity to the optical communication system, not to mention that they are relatively cheaper than optical compensators. Furthermore, electrical schemes were found to handle fiber impairments in a relatively efficient manner. In this thesis, an optical fiber communication scheme using the direct-detection method is simulated. A frequency shifter in the optical domain will be used for the system to have a coherent like detection. At the receiver's side, a linear equalizer is realized to offset the linear effects caused by the fiber. To our knowledge, this will mark the first direct detection transmission system to pass the one thousand kilometre mark in fiber length. Furthermore, we simulate another optical fiber communication design using the coherent detection. A nonlinear compensator adapting the Volterra approach will be used to offset nonlinear impairments. Such performance will be compared to that of a linear compensator. Design trade-offs will be analyzed, and the nonlinear compensator is found to a improve performance when a dispersion compensation fiber (DCF) is introduced in the optical domain. / Thesis / Master of Applied Science (MASc)
6

NON-COHERENTLY DETECTED FQPSK: RAPID SYNCHRONIZATION AND COMPATIBILITY WITH PCM/FM RECEIVERS

Park, Hyung Chul, Lee, Kwyro, Feher, Kamilo 10 1900 (has links)
International Telemetering Conference Proceedings / October 22-25, 2001 / Riviera Hotel and Convention Center, Las Vegas, Nevada / A new class of non-coherent detection techniques for recently standardized Feher patented quadrature phase-shift keying (FQPSK) systems is proposed and studied by computer aided design/simulations and also verified by experimental hardware measurements. The theoretical concepts of the described non-coherent techniques are based on an interpretation of the instantaneous frequency deviation or phase transition characteristics of FQPSK-B modulated signal at the front end of the receiver. These are accomplished either by Limiter-Discriminator (LD) or by Limiter-Discriminator followed by Integrate-and-Dump (LD I&D) methods. It is shown that significant BER performance improvements can be obtained by increasing the received signal’s observation time over multiple symbols as well as by adopting trellis-demodulation. For example, our simulation results show that a BER=10^-4 can be obtained for an E(b)/N(0)=12.7 dB.
7

DSP based Chromatic Dispersion Equalization and Carrier Phase Estimation in High Speed Coherent Optical Transmission Systems

Xu, Tianhua January 2012 (has links)
Coherent detection employing multilevel modulation formats has become one of the most promising technologies for next generation high speed transmission systems due to the high power and spectral efficiencies. Using the powerful digital signal processing (DSP), coherent optical receivers allow the significant equalization of chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise (PN) and nonlinear effects in the electrical domain. Recently, the realizations of these DSP algorithms for mitigating the channel distortions in the coherent transmission systems are the most attractive investigations. The CD equalization can be performed by the digital filters developed in the time and the frequency domain, which can suppress the fiber dispersion effectively. The PMD compensation is usually performed in the time domain with the adaptive least mean square (LMS) and constant modulus algorithms (CMA) equalization. Feed-forward and feed-back carrier phase estimation (CPE) algorithms are employed to mitigate the phase noise (PN) from the transmitter (TX) and the local oscillator (LO) lasers. The fiber nonlinearities are compensated by using the digital backward propagation methods based on solving the nonlinear Schrödinger (NLS) equation and the Manakov equation. In this dissertation, we present a comparative analysis of three digital filters for chromatic dispersion compensation, a comparative evaluation of different carrier phase estimation methods considering digital equalization enhanced phase noise (EEPN) and a brief discussion for PMD adaptive equalization. To implement these investigations, a 112-Gbit/s non-return-to-zero polarization division multiplexed quadrature phase shift keying (NRZ-PDM-QPSK) coherent transmission system with post-compensation of dispersion is realized in the VPI simulation platform. In the coherent transmission system, these CD equalizers have been compared by evaluating their applicability for different fiber lengths, their usability for dispersion perturbations and their computational complexity. The carrier phase estimation using the one-tap normalized LMS (NLMS) filter, the differential detection, the block-average (BA) algorithm and the Viterbi-Viterbi (VV) algorithm is evaluated, and the analytical predictions are compared to the numerical simulations. Meanwhile, the phase noise mitigation using the radio frequency (RF) pilot tone is also investigated in a 56-Gbit/s NRZ single polarization QPSK (NRZ-SP-QPSK) coherent transmission system with post-compensation of chromatic dispersion. Besides, a 56-Gbit/s NRZ-SP-QPSK coherent transmission system with CD pre-distortion is also implemented to analyze the influence of equalization enhanced phase noise in more detail. / QC 20120528
8

Data Detection and Channel Estimation of OFDM Systems Using Differential Modulation

Khizir, Zobayer Abdullah 13 August 2009
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique which is robust against multipath fading and very easy to implement in transmitters and receivers using the inverse fast Fourier transform and the fast Fourier transform. A guard interval using cyclic prefix is inserted in each OFDM symbol to avoid the inter-symbol interference. This guard interval should be at least equal to, or longer than the maximum delay spread of the channel to combat against inter-symbol interference properly.<p> In coherent detection, channel estimation is required for the data detection of OFDM systems to equalize the channel effects. One of the popular techniques is to insert pilot tones (reference signals) in OFDM symbols. In conventional method, pilot tones are inserted into every OFDM symbols. Channel capacity is wasted due to the transmission of a large number of pilot tones. To overcome this transmission loss, incoherent data detection is introduced in OFDM systems, where it is not needed to estimate the channel at first. We use differential modulation based incoherent detection in this thesis for the data detection of OFDM systems. Data can be encoded in the relative phase of consecutive OFDM symbols (inter-frame modulation) or in the relative phase of an OFDM symbol in adjacent subcarriers (in-frame modulation). We use higher order differential modulation for in-frame modulation to compare the improvement of bit error rate. It should be noted that the single differential modulation scheme uses only one pilot tone, whereas the double differential uses two pilot tones and so on. Thus overhead due to the extra pilot tones in conventional methods are minimized and the detection delay is reduced. It has been observed that the single differential scheme works better in low SNRs (Signal to Noise Ratios) with low channel taps and the double differential works better at higher SNRs. Simulation results show that higher order differential modulation schemes don¡¯t have any further advantages. For inter-frame modulation, we use single differential modulation where only one OFDM symbol is used as a reference symbol. Except the reference symbol, no other overhead is required. We also perform channel estimation using differential modulation. Channel estimation using differential modulation is very easy and channel coefficients can be estimated very accurately without increasing any computational complexity. Our simulation results show that the mean square channel estimation error is about ¡¼10¡½^(-2) at an SNR of 30 dB for double differential in-frame modulation scheme, whereas channel estimation error is about ¡¼10¡½^(-4) for single differential inter-frame modulation. Incoherent data detection using classical DPSK (Differential Phase Shift Keying) causes an SNR loss of approximately 3 dB compared to coherent detection. But in our method, differential detection can estimate the channel coefficients very accurately and our estimated channel can be used in simple coherent detection to improve the system performance and minimize the SNR loss that happens in conventional method.
9

Data Detection and Channel Estimation of OFDM Systems Using Differential Modulation

Khizir, Zobayer Abdullah 13 August 2009 (has links)
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique which is robust against multipath fading and very easy to implement in transmitters and receivers using the inverse fast Fourier transform and the fast Fourier transform. A guard interval using cyclic prefix is inserted in each OFDM symbol to avoid the inter-symbol interference. This guard interval should be at least equal to, or longer than the maximum delay spread of the channel to combat against inter-symbol interference properly.<p> In coherent detection, channel estimation is required for the data detection of OFDM systems to equalize the channel effects. One of the popular techniques is to insert pilot tones (reference signals) in OFDM symbols. In conventional method, pilot tones are inserted into every OFDM symbols. Channel capacity is wasted due to the transmission of a large number of pilot tones. To overcome this transmission loss, incoherent data detection is introduced in OFDM systems, where it is not needed to estimate the channel at first. We use differential modulation based incoherent detection in this thesis for the data detection of OFDM systems. Data can be encoded in the relative phase of consecutive OFDM symbols (inter-frame modulation) or in the relative phase of an OFDM symbol in adjacent subcarriers (in-frame modulation). We use higher order differential modulation for in-frame modulation to compare the improvement of bit error rate. It should be noted that the single differential modulation scheme uses only one pilot tone, whereas the double differential uses two pilot tones and so on. Thus overhead due to the extra pilot tones in conventional methods are minimized and the detection delay is reduced. It has been observed that the single differential scheme works better in low SNRs (Signal to Noise Ratios) with low channel taps and the double differential works better at higher SNRs. Simulation results show that higher order differential modulation schemes don¡¯t have any further advantages. For inter-frame modulation, we use single differential modulation where only one OFDM symbol is used as a reference symbol. Except the reference symbol, no other overhead is required. We also perform channel estimation using differential modulation. Channel estimation using differential modulation is very easy and channel coefficients can be estimated very accurately without increasing any computational complexity. Our simulation results show that the mean square channel estimation error is about ¡¼10¡½^(-2) at an SNR of 30 dB for double differential in-frame modulation scheme, whereas channel estimation error is about ¡¼10¡½^(-4) for single differential inter-frame modulation. Incoherent data detection using classical DPSK (Differential Phase Shift Keying) causes an SNR loss of approximately 3 dB compared to coherent detection. But in our method, differential detection can estimate the channel coefficients very accurately and our estimated channel can be used in simple coherent detection to improve the system performance and minimize the SNR loss that happens in conventional method.
10

APSK Transmission Experiment with Homodyne Receiver Using Carrier Phase Recovery

Kung, Hui-Hsuan 28 June 2011 (has links)
In the current transmission systems, the transmission capacity is still not enough. The information bandwidth of the optical fiber communication system is limited by the optical amplifier bandwidth, and more efficient use of the bandwidth is a very important issue. Therefore, the amplitude and phase shift keying (APSK) is one attractive method of multi-bit per symbol modulation scheme to improve the spectral efficiency, and it can effectively increase the transmission capacity. To improve the capacity and the spectral efficiency, the advanced modulation format is effective, and the coherent detection scheme is also effective. However, an optical phase-locked loop (PLL) to lock the local oscillator (LO) phase and the signal phase required for the homodyne detection is still difficult to realize and it makes the receiver circuit complicated. Using the digital coherent receiver, the optical carrier phase information can be recovered by means of the digital signal processing (DSP), and this scheme enables to eliminate the optical PLL circuit by the phase estimation algorithm through the DSP. The stored data can be offline processed by using the MATLAB program. This master thesis is focusing on studying the transmission performance of the APSK format using the DSP in the digital coherent receiver. 497km transmission experiment has been conducted. Subsequently, the stored data are offline processed by the algorithms of the DSP. Then, the APSK performances between back-to-back and 497km transmission are compared.

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