Дисертації з теми "Doppler spreads"

In a WCDMA transmission systems, the properties of the radio transmissionchannels can be strongly aected by the movement of User Equipment (UE) orthe surrounding objects. The estimation of Doppler spread is therefore of greatimportance since it is closely related to the mobile speed, as it can also be usedto characterize the fast fading in the radio channel. Thus the Doppler spread estimation can have wide range of applications and the relative research on thistopic has drawn much attention. Many Doppler spread estimation algorithms has been proposed in the literature. In this report, these algorithms are divided into four categories, and the comparison is performed from both performanceand implementation point of view to compare these four types of estimators. During the investigation, the Rayleigh fading and Rician fading model withdierent mobile speed and SNR are simulated to analyze the performance of estimation algorithms under dierent conditions. The effect of Rician factor and angle of arrival of Line Of Sight (LOS) component is also taken into consideration in the evaluation. Furthermore, the computational complexity of each algorithm is calculated. Based on simulation results, the underlying reasons for their performances are analyzed individually. The Moser's estimator (correlation based estimator) demonstrates the best performance among these four estimators from the perspective of estimation accuracy. And it also shows the great value for the implementation in real systems.
I ett kommunikationssystem baserat p WCDMA kan egenskaperna hos radiotransmissionskanalernapverkas starkt av mobilens eller omgivningens rrelser.Estimering av Doppler spridningen r drfr av stor betydelse eftersom den r nrabeslktad med mobilens hastighet och den kan ocks anvndas fr att karakteriseraden snabba fdning i radiokanalen. Sledes kan estimat av Dopplerspridning ha ettbrett anvndningsomrde och forskningen om detta mne har varit intesiv. Mngaalgoritmer fr estimatering av Doppler spridning har freslagits i litteraturen. Idenna rapport, dessa algoritmer r indelade i fyra kategorier och analysen utfrsbde med avseende p prestanda och implementerbarhet. Simuleringar r utfrdamed Rayleigh fading och Rician modeller med olika hastigheter hos mobilen,och olika signal till brus frhllanden. Simuleringar har ocks gjorts fr olika vrdenav Ricianfaktorn och vinkel fr inkommande signal. Vidare har den berkningsmssigakomplexiteten av varje algoritm analyserats. Utifrn simuleringsresultathar de bakomliggande orsakerna till de olika algoritmernas prestatandadiskuterats. Dopplerspridningsestimat med Mosers metod (korrelation baseradestimator) visar det bsta resultatet bland dessa fyra estimator med avseendep skattningsnoggrannhet. Denna metod har ven frhllandevis lg berkningskomplexitetvilket gr den implementeringsvnlig.

International Telemetering Conference Proceedings / October 25-28, 1999 / Riviera Hotel and Convention Center, Las Vegas, Nevada
Data from ARTM channel sounding test flights is examined to characterize the dynamic channel behavior of aeronautical telemetry channels. The dynamic behavior is characterized using the Doppler power spectrum. The width of the Doppler power spectrum is the Doppler bandwidth of the channel which indicates the required bandwidth of adaptive detection techniques such as adaptive equalization, adaptive modulation, adaptive channel selection and adaptive error control coding. Data collected from ARTM Flight 11 suggest a Doppler bandwidth exceeding 6.7 Hz for the channel, but greater accuracy and resolution will only be possible with more data.

In cellular communication systems, the estimation of Doppler spread has a wide range of applications such as handoff, channel assignment scheme, adaptivetransmission, power control, etc. A great quantity of Doppler spread estimation algorithms have been proposed in the literature. But there has been few investigations which gives a comprehensive comparison of these algorithms. Therefore, it is of great signicance to compare and evaluate the performance of the existing algorithms in the same simulation framework. In this report, the uplink of WCDMA is considered. Four different types of Doppler spread estimation algorithms are evaluated and compared in a link level baseband simulator. The performance and the ability to implement are considered as the metrics for evaluation. Both Rayleigh and Rician fading channel model are applied, and the effect of speed, signal to noise ratio, Rician factor and the angle of arrived line of sight component are also tested. Moreover, the computational complexity is analysed to evaluate the practical value for implementation.
Estimatering av en mobils hastighet i form av Dopplerspridning har ett brett spektrum av tillmpningar i cellulra kommunikationssystem ssom fr yttningen avmobiler mellan celler, kanaltilldelningsschema, adaptiv sndning, eektstyrning,etc. En stor mngd algoritmer fr estimering av Dopplerspriding har frslagitsi litteraturen, men det r ovanligt med heltckande jmfrelser mellan med dessaalgoritmer. Drfr r det av stor betydelse att jmfra och utvrdera resultaten avbentliga algoritmer inom ramen fr samma simuleringsvertyg.I denna rapport anvnds upplnken fr WCDMA fr utvrdering av fyra olikatyper av algoritmer fr estimering av Dopplerspridning. Metriker fr utvrderingenr prestanda och implementeringsvnlighet. Bde Rayleigh och Rician fdningskanalmodeller har utvrderas, samt eekten av mobilens hastighet, signaltill brus frhllande, Rician faktor och infallsvinkel i ppet flt scenario. Dessutomhar den berkningsmssiga komplexiteten analyseras fr att utvrdera den praktiskaanvndbarheten i riktiga system.

International Telemetering Conference Proceedings / October 26-29, 1998 / Town & Country Resort Hotel and Convention Center, San Diego, California
In any satellite communication, the Doppler shift associated with the satellite’s position and velocity must be calculated in order to determine the carrier frequency. If the satellite state vector is unknown then some estimate must be formed of the Doppler-shifted carrier frequency. One elementary technique is to examine the signal spectrum and base the estimate on the dominant spectral component. If, however, the carrier is spread (as in most satellite communications) this technique may fail unless the chip rate-to-data rate ratio (processing gain) associated with the carrier is small. In this case, there may be enough spectral energy to allow peak detection against a noise background. In this paper, we present a method to estimate the frequency (without knowledge of the Doppler shift) of a spread-spectrum carrier assuming a small processing gain and binary-phase shift keying (BPSK) modulation. Our method relies on a simple, averaged discrete Fourier transform along with peak detection. We provide simulation results indicating the accuracy of this method. In addition, we will describe an all-digital hardware design based around a Motorola DSP56303 and high-speed A/D which implements this technique in real-time. The hardware design is to be used in NMSU’s implementation of NASA’s demand assignment, multiple access (DAMA) service.

ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, California
Radio transmission channel influences greatly the quality of transmitted voice and data signal in terms of data rate and robustness. This degradation is as a result of many factors, notable amongst them are having multiple replica of the transmitted signal at the receiver (multipath), changes of frequency as a result of the movement of the aircraft (Doppler shift) and noise. This paper characterizes the scattered components of the aeronautical channel in terms of delay spread. Geometric representation is used to derive expressions for the maximum delay spread using the 2-ray model and the three dimensional model of the scattered path. Furthermore, the delay and Doppler frequencies are described as a function of the horizontal distance to the specular reflection point between a ground station and a test article. The simulated results are compared to measured data of related articles and the value of the maximum delay spread is compared with the proposed intersymbol guard band for Orthogonal Frequency Division Multiplexing (OFDM) in the Integrated Network Enhanced Telemetry (iNET) program to see if this proposition can be adapted to the aeronautical channel.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第17901号
農博第2024号
新制||農||1017(附属図書館)
学位論文||H25||N4797(農学部図書室)
30721
京都大学大学院農学研究科地域環境科学専攻
(主査)教授 近藤 直, 准教授 小川 雄一, 教授 飯田 訓久
学位規則第4条第1項該当

International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California
Parameterized data from ARTM channel sounding test flights are used to derive a parameter-based estimate of the scattering function. The Doppler bandwidth and Doppler spread, important measures of channel dynamics, can be estimated from such a scattering function. Data collected from ARTM Flight 11 suggest that the Doppler bandwidth is larger than 6.7 Hz. Even for very small collections of parameterized data, surprising agreement is shown to non-parametric scattering function estimates. This confirms modeling assumptions and offers a way to achieve significant reductions of storage requirements.

ITC/USA 2011 Conference Proceedings / The Forty-Seventh Annual International Telemetering Conference and Technical Exhibition / October 24-27, 2011 / Bally's Las Vegas, Las Vegas, Nevada
Acquisition based on Fast Fourier Transform (FFT) can acquire Pseudo-random code phase quickly and improve the performance of the satellite navigation receivers. In the paper Real-time receiver adopts the FPGA to realize the function of FFT and uses DSP processor to control the implementation process of Acquisition. For increasing the sensitivity of Acquisition incoherent accumulation were used in the process. Also, in the paper we have discussed the process method for decreasing the negative influence of signal power changes and carrier's Doppler frequency.

International Telemetering Conference Proceedings / October 28-31, 1996 / Town and Country Hotel and Convention Center, San Diego, California
NASA's Space Network (SN) provides both single access (SA) and multiple access (MA) services through a pre-scheduling system. Currently, a user's spacecraft is incapable of receiving service unless prior scheduling occurred with the control center. NASA is interested in efficiently utilizing the time between scheduled services. Thus, a demand assignment multiple access (DAMA) service study was conducted to provide a solution. The DAMA service would allow the user's spacecraft to initiate a service request. The control center could then schedule the next available time slot upon owner approval. In this paper, the basic DAMA service request design and integration is presented.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第22783号
農博第2426号
新制||農||1081(附属図書館)
学位論文||R2||N5303(農学部図書室)
京都大学大学院農学研究科地域環境科学専攻
(主査)教授 近藤 直, 教授 飯田 訓久, 准教授 小川 雄一
学位規則第4条第1項該当

"Many current and future medical devices are wearable, using the human body as a conduit for wireless communication, which implies that human body serves as a crucial part of the transmission medium in body area networks (BANs). Implantable medical devices such as Pacemaker and Cardiac Defibrillators are designed to provide patients with timely monitoring and treatment. Endoscopy capsules, pH Monitors and blood pressure sensors are used as clinical diagnostic tools to detect physiological abnormalities and replace traditional wired medical devices. Body-mounted sensors need to be investigated for use in providing a ubiquitous monitoring environment. In order to better design these medical devices, it is important to understand the propagation characteristics of channels for in-body and on- body wireless communication in BANs. The IEEE 802.15.6 Task Group 6 is officially working on the standardization of Body Area Network, including the channel modeling and communication protocol design. This thesis is focused on the propagation characteristics of human body movements. Specifically, standing, walking and jogging motions are measured, evaluated and analyzed using an empirical approach. Using a network analyzer, probabilistic models are derived for the communication links in the medical implant communication service band (MICS), the industrial scientific medical band (ISM) and the ultra- wideband (UWB) band. Statistical distributions of the received signal strength and second order statistics are presented to evaluate the link quality and outage performance for on-body to on- body communications at different antenna separations. The Normal distribution, Gamma distribution, Rayleigh distribution, Weibull distribution, Nakagami-m distribution, and Lognormal distribution are considered as potential models to describe the observed variation of received signal strength. Doppler spread in the frequency domain and coherence time in the time domain from temporal variations is analyzed to characterize the stability of the channels induced by human body movements. The shape of the Doppler spread spectrum is also investigated to describe the relationship of the power and frequency in the frequency domain. All these channel characteristics could be used in the design of communication protocols in BANs, as well as providing features to classify different human body activities. Realistic data extracted from built-in sensors in smart devices were used to assist in modeling and classification of human body movements along with the RF sensors. Variance, energy and frequency domain entropy of the data collected from accelerometer and orientation sensors are pre- processed as features to be used in machine learning algorithms. Activity classifiers with Backpropagation Network, Probabilistic Neural Network, k-Nearest Neighbor algorithm and Support Vector Machine are discussed and evaluated as means to discriminate human body motions. The detection accuracy can be improved with both RF and inertial sensors."

El sistema de comunicació ràdio d’alta freqüència (HF, en anglès) és usat arreu del món per agències governamentals i no governamentals sempre que calgui una alternativa a les comunicacions via satèl•lit: vaixells a alta mar, avions fora de cobertura de xarxes ràdio amb visió directa, operacions militars, zones on la infraestructura ha estat destruïda per algun tipus de desastre o bé zones llunyanes sense cap altre tipus de comunicació. La ràdio HF representa una alternativa, o un sistema de backup al satèl•lit per a comunicacions de llarg abast i en redueix els costos, evita la vulnerabilitat i els problemes de sobirania. En aquesta tesi s’ha estudiat l’enllaç HF entre la base antàrtica espanyola Juan Carlos I, situada a l’illa Livingston a l’arxipèlag de les Shetland del Sud, i Espanya. L’objectiu d’aquest treball és estudiar els problemes que afecten la propagació; és a dir, la relació senyal a soroll i interferència, la dispersió multicamí i la dispersió per efecte Doppler, i dissenyar la capa física d’un enllaç HF de baixa velocitat, poca potència i llarg abast. Pel que fa aquest últim punt es fan un parell de propostes: espectre eixamplat per seqüència directa (DSSS, en anglès) i multiplexació per divisió en freqüència ortogonal (OFDM, en anglès). El repte que es planteja és el de la definició de les característiques dels símbols que millor encaixen en aquest canal per tal d’obtenir un benefici de la diversitat temporal i freqüencial que ofereix el canal. Des de l’any 2003 diverses campanyes han permès estudiar aquest canal HF, però no va ser fins la campanya 2009/2010 que s’obtingué un foto de les característiques, diürnes i nocturnes, de la ionosfera. En els articles que es presenten en aquesta tesi hem estès el rang freqüencial d’estudi respecte a investigacions prèvies i hem mostrat diferències de comportament entre el dia i la nit. Hem usat els resultats de la caracterització del canal per a dissenyar i comparar la bondat dels símbols DSSS i OFDM. Ambdues possibilitats han resultat ser candidates a implementar l’enllaç HF entre l’Antàrtida i Espanya. Tot i així, ambdues tècniques representen visions diferents de la implementació del mòdem: mentre que DSSS obté bons resultats a baixa velocitat en entorns amb baixa relació senyal a soroll, OFDM aconsegueix tasses de velocitat més elevades en canals més benignes.
Los sistemas de radio de alta frecuencia (HF, en inglés) son usados por agencias gubernamentales y no gubernamentales en todo el mundo siempre que se necesite una alternativa a las comunicaciones por satélite: barcos en alta mar, aviones fuera del rango de cobertura de las redes radio de visión directa, operaciones militares, zonas donde la infraestructura ha sido destruida por algún desastre. Ésta ofrece una alternativa, o representa un sistema de backup, a las comunicaciones vía satélite, evitando los costes, la vulnerabilidad y los problemas de soberanía de las comunicaciones por satélite. En esta tesis se ha estudiado el enlace HF entre la base antártica española Juan Carlos I en la isla Livingston, en las Shetland del sur y España. El objetivo de este trabajo es el estudio de las limitaciones de la propagación ionosférica (como la relación señal a ruido e interferencia, la dispersión multicamino y la dispersión por efecto Doppler) y el diseño de la capa física de un enlace HF de baja velocidad, baja potencia y largo alcance. Se han estudiado un par de propuestas para este enlace, como son el espectro ensanchado por secuencia directa (DSSS, en inglés) y la multiplexación por división en frecuencia ortogonal (OFDM, en inglés). El reto ha sido definir las características que mejor se adecuan a este enlace para poder aprovechar la diversidad temporal y frecuencial que ofrece el canal HF. Desde el año 2003 diversas campañas de sondeo han permitido estudiar el canal HF pero no es hasta la campaña 2009/2010 que se consigue una fotografía de la actividad ionosférica tanto nocturna como diurna. En los artículos que se presentan en esta tesis hemos extendido los estudios previos a todo el rango de frecuencias HF y hemos mostrado las diferencias entre el día y la noche. Hemos usado estos resultados de caracterización del canal para diseñar y comparar símbolos DSSS y símbolos OFDM. Ambas posibilidades han resultado ser posibles candidatas para implementar un enlace HF de baja velocidad entre la Antártida y España. Sin embargo ambas técnicas representan dos aproximaciones distintas a la implementación del módem. Mientras que DSSS consigue un buen funcionamiento a baja velocidad en escenarios con baja relación señal a ruido, OFDM consigue tasas de transmisión más altas en escenarios más benignos.
High Frequency (HF) radio is used by governmental and non nongovernmental agencies worldwide whenever an alternative to satellites for sky wave communication is needed: ships at sea, aircraft out of range of line-of-sight radio networks, military operations, disaster areas with communication infrastructure destroyed or distant regions lacking other communications. It offers an alternative to satellites, or a backup, for long-haul communications, thus avoiding the costs, vulnerabilities and sovereignty concerns of satellite communications. In this thesis the HF link between the Antarctic Spanish Station Juan Carlos I in Livingston Island, South Shetlands and Spain is studied. The aim of this study is to address the impairments that affect HF propagation (i.e., signal-to-noise plus interference ratio, multipath and Doppler shift and spread) and to design the physical layer of a low rate, low power and long-haul HF link. Some proposals regarding this last issue are addressed, i.e., direct sequence spread spectrum (DSSS) and orthogonal frequency division multiplexing (OFDM). The challenge is to define the symbol characteristics that best fit the link to benefit from time and frequency diversity that offers the HF channel. Since 2003 several transmission campaigns have allowed to study the HF channel but it is not until the 2009/2010 campaign that we have achieved a whole picture of both diurnal and nocturnal ionospheric activity. In the papers presented in this thesis we have extended the previous research to the whole range of HF frequencies and we have shown the differences on performance between day and night. We have used the results from channel characterization to design and compare the performance of DSSS and OFDM symbols. Both techniques have turned out to be possible candidates to implement a low rate HF link between Antarctica and Spain. However, both techniques stand for different approaches of the modem: DSSS achieves good performance at low data rate in low SNR scenarios, whereas OFDM achieves higher data rate in benign channels

A software GPS receiver is a functional GPS receiver in software. It has several advantages compared to its hardware counterparts. For instance, improvements in receiver architecture as well as GPS system structure can be easily adapted to it. Furthermore, interaction between nearby sensors can be coordinated easily. In this thesis, a SGR (software GPS receiver) is presented from a practical point of view. Major components of the SGR are implemented in Matlab environment. Furthermore, some alternative algorithms are implemented. SGR implementation is considered in two main sections namely a signal processing section and a navigation section. Signal processing section is driven by the raw GPS signal samples obtained from a GPS front-end of NordNavTM R-25 instrument. The conventional and the block adjustment of synchronizing signal (BAAS) processing methods are implemented and their performances are compared in terms of their speed and outputs. Signal processing section outputs raw GPS measurements and navigation data bits. Since the output data length is insufficient in our case, navigation section input is fed from AshtechTM GPS receiver for a moving platform and TrimbleTM GPS Receiver for a stationary platform. Satellite position computation, pseudorange corrections, Kalman filter and LSE (least squares estimation) are implemented in the navigation section. Kalman filter and LSE methods are compared in terms of positioning accuracy for a moving as well as a stationary platform. Results are compared with the commercial GPS outputs. This comparison shows that the software navigation section is equivalent to the commercial GPS in terms of positioning accuracy.

This thesis aims to contribute to the developments of wireless communication systems. The work generally consists of three parts: the first part is a discussion on general digital communication systems, the second part focuses on wireless channel modelling and fading mitigation techniques, and in the third part we discuss the possible application of advanced digital signal processing, especially time-frequency representation and blind source separation, to wireless communication systems. The first part considers general digital communication systems which will be incorporated in later parts. Today's wireless communication system is a subbranch of a general digital communication system that employs various techniques of A/D (Analog to Digital) conversion, source coding, error correction, coding, modulation, and synchronization, signal detection in noise, channel estimation, and equalization. We study and develop the digital communication algorithms to enhance the performance of wireless communication systems. In the Second Part we focus on wireless channel modelling and fading mitigation techniques. A modified Jakes' method is developed for Rayleigh fading channels. We investigate the level-crossing rate (LCR), the average duration of fades (ADF), the probability density function (PDF), the cumulative distribution function (CDF) and the autocorrelation functions (ACF) of this model. The simulated results are verified against the analytical Clarke's channel model. We also construct frequency-selective geometrical-based hyperbolically distributed scatterers (GBHDS) for a macro-cell mobile environment with the proper statistical characteristics. The modified Clarke's model and the GBHDS model may be readily expanded to a MIMO channel model thus we study the MIMO fading channel, specifically we model the MIMO channel in the angular domain. A detailed analysis of Gauss-Markov approximation of the fading channel is also given. Two fading mitigation techniques are investigated: Orthogonal Frequency Division Multiplexing (OFDM) and spatial diversity. In the Third Part, we devote ourselves to the exciting fields of Time-Frequency Analysis and Blind Source Separation and investigate the application of these powerful Digital Signal Processing (DSP) tools to improve the performance of wireless communication systems.

Pulsed radars have an ambiguous relationship between range and velocity which is proportional to the pulse repetition frequency (PRF), leading to potential tradeoffs. High PRFs are necessary to avoid velocity aliasing but suffer at the expense of unambiguous range. Obscuration due to ambiguous range foldover from distant clutter echos seriously degrades target detectability. For the case of skywave HF over-the-horizon (OTH) radar, ionospheric motion causes spreading of surface clutter in Doppler space and coupled with range folded clutter, introduces the effect of so-called 'separated' spread Doppler clutter (SDC). Selection of a nonrecurrent waveform (NRWF) with a quadratic phase interpulse code has been shown to mitigate long-range SDC by folding the multi-hop returns into known disassociated Doppler regions.

Utilizing multiple receive elements, spatial processing can be preformed to exploit the correlation between spatial frequency and Doppler shift produced by ionospheric winds. Adaptive beamforming is known to provide asymptotically optimal array gain if sufficient training data is available. In highly nonstationary environments however, obtaining this asymptotic performance is difficult as neither knowledge of the target wavefront nor signal-free training data is easily obtainable for training. A blind adaptive spatial processing (BASP) technique has been proposed, combining minimum variance (MV) adaptive beamforming and blind source separation (BSS). The unique idea of BASP is the formulation of a signal-free covariance matrix from BSS clutter and noise components at a single range bin, and utilizing it in adaptive beamforming to suppress clutter.

This research explores a clutter mitigation method that will combine NRWF and BASP in order to recover targets masked by Doppler-spread surface backscatter from points beyond the radar's maximum unambiguous range while maintaining target detectability elsewhere in Doppler. Current methods for mitigating range-folded clutter, such as reducing the pulse-repetition frequency or the use of non-recurrent waveforms, suffer loss in the usable Doppler space available for target detection or a reduction in target revisit rate. The proposed research uses BSS methods to exploit the known Doppler separation afforded by NRWF in order to estimate the spatial wavefront of the clutter across a linear receive array. Spatial adaptive processing using this estimated wavefront is then used to suppress range-folded clutter at each range bin without sacrificing the radar timeline or usable Doppler space.

Simulation is conducted to understand the NRWF and its ability to separate range-folded clutter in Doppler. The BASP method is applied to the NRWF and its results demonstrate performance improvement in terms of achievable signal-to-clutter and noise ratio gain. Laboratory experimental results show the NRWF's ability to separate range-folded clutter into designed Doppler regions. BASP is then applied and demonstrated to mitigate the separated range folded clutter and recover usable Doppler space.

Dissertation

With the increasing demand for more wireless multimedia applications, it is desired to design a wireless system with higher data rate. Furthermore, the frequency spectrum has become a limited and valuable resource, making it necessary to utilize the available spectrum efficiently and coexist with other wireless systems. Orthogonal frequency division multiplexing (OFDM) modulation is widely used in communication systems to meet the demand for ever increasing data rates. The major advantage of OFDM over single-carrier transmission is its ability to deal with severe channel conditions without complex equalization. However, OFDM systems suffer from a high peak to average power ratio, and they are sensitive to carrier frequency offset and Doppler spread. This dissertation first focuses on the development of intercarrier interference (ICI) reduction and signal detection algorithms for OFDM systems over time-varying channels. Several ICI reduction algorithms are proposed for OFDM systems over doubly-selective channels. The OFDM ICI reduction problem over time-varying channels is formulated as a combinatorial optimization problem based on the maximum likelihood (ML) criterion. First, two relaxation methods are utilized to convert the ICI reduction problem into convex quadratic programming (QP) problems. Next, a low complexity ICI reduction algorithm applicable to $M$-QAM signal constellations for OFDM systems is proposed. This formulates the ICI reduction problem as a QP problem with non-convex constraints. A successive method is then utilized to deduce a sequence of reduced-size QP problems. For the proposed algorithms, the QP problems are solved by limiting the search in the 2-dimensional subspace spanned by its steepest-descent and Newton directions to reduce the computational complexity. Furthermore, a low-bit descent search (LBDS) is employed to improve the system performance. Performance results are given to demonstrate that the proposed ICI reduction algorithms provide excellent performance with reasonable computational complexity. A low complexity joint semiblind detection algorithm based on the channel correlation and noise variance is proposed which does not require channel state information. The detection problem is relaxed to a continuous non-convex quadratic programming problem. Then an iterative method is utilized to deduce a sequence of reduced-size quadratic programming problems. A LBDS method is also employed to improve the solution of the derived QP problems. Results are given which demonstrate that the proposed algorithm provides similar performance with lower computational complexity compared to that of a sphere decoder. A major challenge to OFDM systems is how to obtain accurate channel state information for coherent detection of the transmitted signals. Thus several channel estimation algorithms are proposed for OFDM systems over time-invariant channels. A channel estimation method is developed to utilize the noncircularity of the input signals to obtain an estimate of the channel coefficients. It takes advantage of the nonzero cyclostationary statistics of the transmitted signals, which in turn allows blind polynomial channel estimation using second-order statistics of the OFDM symbol. A set of polynomial equations are formulated based on the correlation of the received signal which can be used to obtain an estimate of the time domain channel coefficients. Performance results are presented which show that the proposed algorithm provides better performance than the least minimum mean-square error (LMMSE) algorithm at high signal to noise ratios (SNRs), with low computational complexity. Near-optimal performance can be achieved with large OFDM systems. Finally, a CS-based time-domain channel estimation method is presented for OFDM systems over sparse channels. The channel estimation problem under consideration is formulated as a small-scale $l_1$-minimization problem which is convex and admits fast and reliable solvers for the globally optimal solution. It is demonstrated that the magnitudes as well as delays of the significant taps of a sparse channel model can be estimated with satisfactory accuracy by using fewer pilot tones than the channel length. Moreover, it is shown that a fast Fourier transform (FFT) matrix of extended size can be used as a set of appropriate basis vectors to enhance the channel sparsity. This technique allows the proposed method to be applicable to less-sparse OFDM channels. In addition, a total-variation (TV) minimization based method is introduced to provide an alternative way to solve the original sparse channel estimation problem. The performance of the proposed method is compared to several established channel estimation algorithms.
Graduate

博士
國立中正大學
電機工程研究所
88
Four issues and solutions are investigated in the thesis with regard to the pseudo-noise (PN) code synchronization for single- and multi-carrier direct sequence spread spectrum (DS-SS) systems operating on additive white Gaussian noise (AWGN) and/or multipath fading channels. In the first part, code tracking with code-Doppler compensation for single carrier DS-SS systems on AWGN channels is proposed. Code Doppler compensation is introduced to the traditional noncoherent digital delay lock loop (DLL) in addition to carrier Doppler compensation. Thanks to the code Doppler compensation, the simple-to-implement active correlation with a large correlation length can be employed to improve the loop performance. The code Doppler compensation significantly improve the loop performance with the presence of a large code Doppler. In the second part, new tracking loop analysis with A/D quantization for single carrier DS-SS systems on AWGN channels is considered. In particular, the performance of non-coherent second-order all digital delay lock loops with the presence of Doppler shift is analyzed. A new method based on a regenerative Markov chain modeling of the tracking process is proposed with the loop A/D quantization being taken into account that has been neglected in previous analyses. Numerical results show that A/D quantization has a significant impact on the loop performance, including loop transient responses, lock-in range, mean time to lose lock (MTLL) and mean square error (MSE). In addition, by using saddle point integration for the evaluation of the transition probabilities of the Markov chain, MTLL and MSE is evaluated more accurately than previous methods. In the third part, the code acquisition for multicarrier direct sequence code division multiple access (DS-CDMA) systems on multipath fading channels is investigated. For multipath fading channels, the exact analysis of code acquisition becomes difficult because of the channel memory incurred by multipath fading. In this part, a new analysis is given to accurately evaluate the mean acquisition time performance of the serial search multiple dwell code acquisition for the multicarrier DS-CDMA systems. Our analysis can take the fading correlations into account, the effects of system parameters on the acquisition performance such as carrier and user numbers, fading correlations, dwell''s number, times and thresholds can be easily evaluated, and, therefore, facilitating the design of the acquisition subsystem. Finally, code tacking for multicarrier DS-CDMA systems on multipath fading channels is investigated. In this part, the tracking error performance of the traditional noncoherent DDLL with diversity reception is analyzed for the multicarrier DS-CDMA systems, under the effects of automatic gain control (AGC), multipath fading, and multiple access interference. AGC is used to eliminate the inherent stability problem of a DDLL. And, a new decision-directed noncoherent tracking loop based on a differentially-detected phase detector is proposed and analyzed. The new tracking loop outperforms the traditional DDLL.

博士
國立成功大學
電機工程學系研究所
85
The use of direct-sequence spread-spectrum (DS-SS) in communication systems is considered a promising technique to obtain higher bandwidth efficiency and power efficiency, in addition to its well-knownmerits in the field of secure communications. However, the efficiency of a spread-spectrum system is highly dependent on the capability of the receiver to obtain accurate and fast synchronization between the received and the locally generated PN codes. In severe Doppler environm ent, such asthe low-earth-orbit (LEO) satellite communication, the effectsof D oppler include not only the carrier frequency shift, but alsothe PN code chip rate offset, which may result in the PN code synchronization to be hard to accomplish. Thus, we shall investi gate the effects of Doppler shift on the PNcode synchronization, including the combined acquisition-trackingsynchronization loops and the digital delay lock loops (DDLL). Moreover, we intend to findsome simple and feasible designs to diminish the influencesof Doppler effect. In this thesis, we first analyze the overall acquisition performanceof the combined acqu isition-tracking synchronization loopsin the presence of Doppler shift. Both t he change of effectivesearch rate and the impact onthe detection probability d ue to Doppler are taken into account. Furthermore, the influence of the DDLL acquiring capability on the complete acquisition process is investi gated and some numerical results are presented to demonstratethe acquisition performance of this combined loop whichare quite di fferent from the previous reports. The delay lock loop is a we ll-known technique to track the PN codes for DS-SS system. This thesis considers discrete time analys es of thefirst- and second-order digital delay lock loops (DDLL), and pre sents the results of an investigation concerning the performance degradation d ue to Doppler. The performance measures eva luated include the pull-in capability, the steady-state timing error probability density function (pdf),and the mean time to lose lock (MTLL). The linear approximations are also providedand confirmed by computer simulations.