Η Ορθογώνια Πολυπλεξία Διαίρεσης Συχνότητας (OFDM) έχει προταθεί ως τεχνική διαμόρφωσης σε οπτικά δίκτυα, λόγω της καλής φασματικής απόδοσής της, της ευελιξίας και της ανοχής της σε βλάβες. Η διαμόρφωση OFDM επιτρέπει την ελαστική ανάθεση φάσματος, χρησιμοποιώντας μεταβλητό πλήθος υποφερουσών, καθώς και την επιλογή του κατάλληλου επιπέδου διαμόρφωσης με βάση την απόσταση της μετάδοσης. Το «Πρόβλημα Δρομολόγης και Ανάθεσης Φάσματος» (RSA) έχει αποδειχθεί ότι είναι ένα NP-πλήρες πρόβλημα, γεγονός που υποδηλώνει τη χρήση γραμμικού προγραμματισμού για τη λύση του. Στόχος της διπλωματικής εργασίας είναι η βελτίωση της απόδοσης του υπάρχοντος αλγορίθμου ακέραιου γραμμικού προγραμματισμού, με χρήση μεταευριστικών, έτσι ώστε στο ίδιο χρονικό διάστημα να υπολογίζεται αποδοτικότερη χρησιμοποίηση του συνολικού απαιτούμενου φάσματος, για το σύνολο των μεταδόσεων στο δίκτυο. / Orthogonal Frequency Division Multiplexing (OFDM) has been proposed as a modulation technique for optical networks, because of its good spectral efficiency, flexibility, and tolerance to impairments. OFDM modulation allows elastic spectrum allocation, using a variable number of subcarriers and choosing an appropriate modulation level, taking into account the transmission distance. The “Routing and Spectrum Allocation” (RSA) problem has been proved to be a NP-complete problem, which suggests the usage of linear programming in order to be solved. This diploma thesis aims to improve the efficiency of the existing integer linear programming algorithm, by using metaheuristics, so that at the same time period a more efficient utilization of the required spectrum is computed, for all network transmissions.

Mixed-signal processing systems especially data converters can be reliably tested at high frequencies using on-chip testing schemes based on memory. In this thesis, an on-chip testing strategy based on shift registers/memory (2 k bits) has been proposed for digital-to-analog converters (DACs) operating at 5 GHz. The proposed design uses word length of 8 bits in order to test DAC at high speed of 5 GHz. The proposed testing strategy has been designed in standard 65 nm CMOS technology with additional requirement of 1-V supply. This design has been implemented using Cadence IC design environment. The additional advantage of the proposed testing strategy is that it requires lower number of I/O pins and avoids the large number of high speed I/O pads. It therefore also solves the problem of the bandwidth limitation that is associated with I/O transmission paths. The design of the on-chip tester based on memory contains no analog block and is implemented entirely in digital domain. In the proposed design, low frequency of 1 MHz has been used outside the chip to load the data into the memory during the write mode. During the read mode, the frequency of 625 MHz is used to read the data from the memory. A multiplexing system is used to reuse the stored data during read mode to test the intended functionality and performance. In order to convert the parallel data into serial data at high frequency at the memory output, serializer has been used. By using the frequencies of 1.25 GHz and 2.5 GHz, the serializer speeds up the data from the lower frequency of 625 MHz to the highest frequency of 5 GHz in order to test DAC at 5 GHz.

In order to meet the ever-growing demand for mobile data, a number of different technologies have been adopted by the fourth generation standardization bodies. These include multiple access schemes such as spatial division multiple access (SDMA), and efficient modulation techniques such as orthogonal frequency division multiplexing (OFDM)-based modulation. The specific objectives of this theses are to develop an effective feedback method for interference management in smart antenna SDMA systems and to design an efficient OFDM-based modulation technique, where an additional dimension is added to the conventional two-dimensional modulation techniques such as quadrature amplitude modulation (QAM). In SDMA time division duplex (TDD) systems, where channel reciprocity is maintained, uplink (UL) channel sounding method is considered as one of the most promising feedback methods due to its bandwidth and delay efficiency. Conventional channel sounding (CCS) only conveys the channel state information (CSI) of each active user to the base station (BS). Due to the limitation in system performance because of co-channel interference (CCI) from adjacent cells in interference-limited scenarios, CSI is only a suboptimal metric for multiuser spatial multiplexing optimization. The first major contribution of this theses is a novel interference feedback method proposed to provide the BS with implicit knowledge about the interference level received by each mobile station (MS). More specifically, it is proposed to weight the conventional channel sounding pilots by the level of the experienced interference at the user’s side. Interference-weighted channel sounding (IWCS) acts as a spectrally efficient feedback technique that provides the BS with implicit knowledge about CCI experienced by each MS, and significantly improves the downlink (DL) sum capacity for both greedy and fair scheduling policies. For the sake of completeness, a novel procedure is developed to make the IWCS pilots usable for UL optimization. It is proposed to divide the optimization metric obtained from the IWCS pilots by the interference experienced at the BS’s antennas. The resultant new metric, the channel gain divided by the multiplication of DL and UL interference, provides link-protection awareness and is used to optimize both UL and DL. Using maximum capacity scheduling criterion, the link-protection aware metric results in a gain in the median system sum capacity of 26.7% and 12.5% in DL and UL respectively compared to the case when conventional channel sounding techniques are used. Moreover, heuristic algorithm has been proposed in order to facilitate a practical optimization and to reduce the computational complexity. The second major contribution of this theses is an innovative transmission approach, referred to as subcarrier-index modulation (SIM), which is proposed to be integrated with OFDM. The key idea of SIM is to employ the subcarrier-index to convey information to the receiver. Furthermore, a closed-form analytical bit error ratio (BER) of SIM OFDM in Rayleigh channel is derived. Simulation results show BER performance gain of 4 dB over 4-QAM OFDM for both coded and uncoded data without power saving policy. Alternatively, power saving policy maintains an average gain of 1 dB while only using half OFDM symbol transmit power.

Structured light illumination (SLI) is the process of projecting a series of light striped patterns such that, when viewed at an angle, a digital camera can reconstruct a 3-D model of a target object's surface. But by relying on a series of time multiplexed patterns, SLI is not typically associated with video applications. For this purpose of acquiring 3-D video, a common SLI technique is to drive the projector/camera pair at very high frame rates such that any object's motion is small over the pattern set. But at these high frame rates, the speed at which the incoming video can be processed becomes an issue. So much so that many video-based SLI systems record camera frames to memory and then apply off-line processing. In order to overcome this processing bottleneck and produce 3-D point clouds in real-time, we present a lookup-table (LUT) based solution that in our experiments, using a 640 by 480 video stream, can generate intermediate phase data at 1063.8 frames per second and full 3-D coordinate point clouds at 228.3 frames per second. These achievements are 25 and 10 times faster than previously reported studies. At the same time, a novel dual-frequency pattern is developed which combines a high-frequency sinusoid component with a unit-frequency sinusoid component, where the high-frequency component is used to generate robust phase information and the unit-frequency component is used to reduce phase unwrapping ambiguities. Finally, we developed a gamma model for SLI, which can correct the non-linear distortion caused by the optical devices. For three-step phase measuring profilometry (PMP), analysis of the root mean squared error of the corrected phase showed a 60х reduction in phase error when the gamma calibration is performed versus 33х reduction without calibration.

The increasing demand for high data rate transmission over broadband radio channels has imposed significant challenges in wireless communications. Accurate channel estimation has a major impact on the whole system performance. Specifically, reliable estimate of the channel state information (CSI) is more challenging for orthogonal frequency division multiplexing (OFDM) systems in doubly selective fading channels than for the slower fading channels over which OFDM has been deployed traditionally. With the help of a basis expansion model (BEM), a novel multivariate autoregressive (AR) process is developed to model the time evolution of the fast fading channel. Relying on pilot symbol aided modulation (PSAM), a novel Kalman smoothing algorithm based on a second-order dynamic model is exploited, where the mean square error (MSE) of the channel estimator is near to that of the optimal Wiener filter. To further improve the performance of channel estimation, a novel low-complexity iterative joint channel estimation and symbol detection procedure is developed for fast fading channels with a small number of pilots and low pilot power to achieve the bit error rate (BER) performance close to when the CSI is known perfectly. The new channel estimation symbol detection technique is robust to variations of the radio channel from the design values and applicable to multiple modulation and coding types. By use of the extrinsic information transfer (EXIT) chart, we investigate the convergence behavior of the new algorithm and analyze the modulation, pilot density, and error correction code selection for good system performance for a given power level. The algorithms developed in this thesis improve the performance of the whole system requiring only low ratios of pilot to data for excellent performance in fast fading channels. / Graduate

In this thesis, various pilot symbol aided channel estimation and tracking methods are investigated and their performances are compared for an OFDM system with packet based communication on HF channel. For the HF channel, Watterson HF channel model is used. The compared methods are least squares (LS) channel estimation, linear minimum mean square error (LMMSE) channel estimation, least mean squares (LMS) channel tracking, recursive least squares (RLS) channel tracking, constant position model based Kalman filter channel tracking, and constant velocity model based Kalman filter channel tracking. For LMS and RLS methods some adaptive approaches are also investigated.

Interference and jamming severely disrupt our ability to communicate by decreasing the effective signal-to-noise ratio and by making parameter estimation difficult at the receiver. The objective of this research work is to design robust wireless systems and algorithms to suppress the adverse effects of non-intentional co-channel interference (CCI) or intentional jamming. In particular, we develop chip-combining schemes with timing, channel, and noise-power estimation techniques, all of which mitigate CCI or jamming. We also exploit the spatial diversity and iterative receiver techniques for this purpose. Most of the existing timing estimation algorithms are robust against either large frequency offsets or CCI, but not against both at the same time. Hence, we develop a new frame boundary estimation method that is robust in the presence of severe co-channel interference and large carrier-frequency offsets. To solve the high peak-to-average-power ratio problem of a multicarrier code division multiple access (MC-CDMA) system and enhance its robustness against fading and jamming, we propose a constant-envelope MC-CDMA system employing cyclic delay diversity (CDD) as transmit diversity. We analyze the diversity order, coding gain, and bit-error rate upper bound. We also propose a blind, accurate, and computationally efficient signal-to-noise ratio estimator for the proposed system. We propose a configurable robust anti-jam receiver that estimates the frequency- or time-domain jammer state information (JSI) and uses it for chip combining in the corresponding domain. A soft-JSI-based chip-combining technique is proposed that outperforms conventional hard-JSI-based chip combining. We also derive a chip combiner that provides sufficient statistics to the decoder. Channel estimation is necessary for coherent signal detection and JSI estimation. Conversely, knowledge of the jamming signal power and JSI of different subcarriers can improve the accuracy of the channel estimates. Hence, we propose joint iterative estimation of the multiple-input multiple-output (MIMO) channel coefficients, jamming power, and JSI for a coded MC-CDMA MIMO system operating under jamming and a time-varying frequency-selective fading channel. Finally, we reduce the computational complexity of the JSI-based anti-jam receivers by introducing an expectation-maximization-based joint channel and noise-covariance estimator that does not need either the subcarrier JSI or the individual powers of the AWGN and jamming signal.

Distortion-based crest factor reduction (CFR) algorithms were studied in orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) OFDM systems to reduce the nonlinear distortion and improve the power efficiency of the transmitter front-end. First, definitions of peak-to-average-power ratio (PAR) were clarified based on the power efficiency improvement consideration in the MIMO-OFDM systems. Next, error vector magnitude (EVM) was used as the in-band performance-evaluating metric. Statistical analysis of EVM was performed to provide concrete thresholds for the amount of allowable distortions from each source to meet EVM requirements in the standard. Furthermore, an effective CFR technique, constrained clipping, was proposed to drastically reduce the PAR while satisfying any given in-band EVM and out-of-band spectral mask constraints. Constrained clipping has low computational complexity and can be easily extended to the multiple-user OFDM environment. Finally, signal-to-noise-and-distortion ratio (SNDR) analysis for transceiver nonlinearities in the additive white Gaussian noise channel was investigated. An analytical solution was presented for maximizing the transceiver SNDR for any given set of nonlinear transmitter polynomial coefficients. Additionally, mutually inverse pair of transceiver nonlinearities was shown to be SNDR-optimal only in the noise-free case.

Fibre-optic sensors have been the focus of a lot of research, but their associated high cost has stifled their transferral from the laboratory to real world applications. This thesis addresses the issue of multiplexing, a technology that would lower the cost per unit sensor of a sensor system dramatically. An overview of the current state of research of, and the principles behind, multiplexed sensor networks is given. A new scheme of multiplexing, designated W*DM, is developed and implemented for a fibre Bragg grating (FBG) optical fibre sensor network. Using harmonic analysis, multiplexing is performed in the domain dual to that of the wavelength domain of a sensor. This scheme for multiplexing is compatible with the most commonly used existing schemes of WDM and TDM and thus offers an expansion over, and a resultant cost decrease from, the sensor systems currently in use. This research covered a theoretical development of the scheme, a proof of principle, simulated and experimental analysis of the performance of the multiplexed system, investigation into sensor design requirements and related issues, fabrication of the sensors according to the requirements of the scheme and the successful multiplexing of eight devices (thus offering an eightfold increase over current network capacities) using this scheme. Extensions of this scheme to other fibre sensors such as Long Period Gratings (LPGs) and blazed gratings were also investigated. Two LPGs having a moir?? structure were successfully multiplexed and it was shown that a blazed Fabry Perot grating could be used as a tuneable dual strain/refractive index sensor. In performing these tests, it was discovered that moir?? LPGs exhibited a unique thermal switching behaviour, hereto unseen. Finally the application of fibre sensors to the civil engineering field was investigated. The skill of embedding optical fibre in concrete was painstakingly developed and the thermal properties of concrete were investigated using these sensors. Field tests for the structural health monitoring of a road bridge made from a novel concrete material were performed. The phenomenon of shrinkage, creep and cracking in concrete was investigated showing the potential for optical fibre sensors to be used as a viable research tool for the civil engineer.

Στην παρούσα διπλωματική εργασία εξετάζεται η εκμετάλλευση της διαφορετικότητας πολλών χρηστών και της προσαρμοστικής διαμόρφωσης στα OFDMA συστήματα. Οι αλγόριθμοι που εκμεταλλεύονται αυτά τα κέρδη δεν προσδιορίζονται από το πρότυπο WiMAX, και έτσι όλοι οι κατασκευαστές WiMAX να είναι ελεύθεροι να αναπτύξουν τις δικές τους καινοτόμες διαδικασίες. Η ιδέα είναι η ανάπτυξη αλγορίθμων για την κατανομή των υποφορέων στους χρήστες, για τον προσδιορισμό των ποσοτήτων ισχύος σε αυτούς. Η μελέτη εστιάζεται στην κάτω ζεύξη (downlink) του συστήματος OFDMA, δηλαδή στη μετάδοση δεδομένων από το σταθμό βάσης της κυψέλης στους χρήστες – δέκτες. Οι διαθέσιμοι πόροι του συστήματος είναι οι ορθογώνιοι υποφορείς και η συνολική διαθέσιμη ισχύς στο σταθμό βάσης. Οι χρήστες κάνουν εκτίμηση και ανατροφοδότηση της πληροφορίας κατάστασης του καναλιού (CSI-channel state information) σε έναν κεντρικό σταθμό βάσης, όπου υποφορείς και κατανομή ισχύος προσδιορίζονται σύμφωνα με CSI των χρηστών και τη διαδικασία κατανομής των διαθέσιμων πόρων. Μόλις οι υποφορείς για κάθε χρήστη καθοριστούν, ο σταθμός βάσης πρέπει να ενημερώνει τον κάθε χρήστη για το ποιοι υποφορείς έχουν ανατεθεί στον καθένα. Συνήθως, η κατανομή των πόρων πρέπει να γίνεται σε χρονικά διαστήματα της τάξης του χρόνου συνοχής, αν και μπορεί να γίνει πιο συχνά, αν υπάρχουν πολλοί χρήστες που ανταγωνίζονται για τους πόρους που είναι διαθέσιμοι. Στην παρούσα διπλωματική εργασία εξετάζονται διαφορετικά σενάρια κατανομής των πόρων που περιλαμβάνουν αλγορίθμους για την ανάθεση των υποφορέων και το διαμοιρασμό των κατάλληλων ποσοτήτων ισχύος στους υποφορείς. Ανάλογα με το στόχο και τις ανάγκες των χρηστών του συστήματος, καθώς και με το ποσό της διαθέσιμης πληροφορίας καναλιού στο σταθμό βάσης, χρησιμοποιούνται διαφορετικοί αλγόριθμοι. Η κατανομή των διαθέσιμων πόρων συνήθως διατυπώνεται ως πρόβλημα βελτιστοποίησης με περιορισμούς, είτε (1) ελαχιστοποίηση της συνολικής εκπεμπόμενης ισχύoς με περιορισμό στο ρυθμό μετάδοσης δεδομένων στο χρήστη είτε (2) μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων με περιορισμό στην συνολική εκπεμπόμενη ισχύ. Ο αλγόριθμος με περιορισμό αναλογίας των ρυθμών μετάδοσης (Proportional Rate Constraints algorithm - PRC) έχει ως στόχο τη μεγιστοποίηση της συνολικής διεκπεραίωσης με τον περιορισμό ότι ο ρυθμός μετάδοσης δεδομένων σε κάθε χρήστη είναι ανάλογος με ένα σύνολο από προκαθορισμένες παραμέτρους του συστήματος. Ενώ ο στόχος του αλγόριθμου μέγιστου ολικού ρυθμού μετάδοσης (MSR- maximum sum rate algorithm), είναι η μεγιστοποίηση του συνόλου των ρυθμών μετάδοσης όλων των χρηστών, λαμβάνοντας υπόψη έναν περιορισμό της συνολικής εκπεμπόμενης ισχύoς. / In this diploma thesis, multiuser diversity and adaptive modulation in OFDMA systems is considered. Algorithms that take advantage of these gains are not specified by the WiMAX standard, and all WiMAX developer are free to develop their own innovative procedures. The idea is to develop algorithms for determining which users to schedule, how to allocate subcarriers to them, and how to determine the appropriate power levels for each user on each subcarrier. The downlink of a single-cell system is considered in the downlink transmission. The available resources to be distributed among the users of the OFDMA system comprise the subcarriers over which the signals of the users are transmitted and the available power that is allocated among subcarriers. Users estimate and feedback the channel state information (CSI) to a centralized base station, where subcarrier and power allocation are determined according to users’ CSI and the resource-allocation procedure. Once the subcarriers for each user have been determined, the base station must inform each user which subcarriers have been allocated to it. Typically, the resource allocation must be performed on the order of the channel coherence time, although it may be performed more frequently if a lot of users are competing for resources. In this diploma thesis, different resource allocation strategies for the downlink of an OFDMA system are compared. Each algorithm has a different objective. The resource allocation is usually formulated as a constrained optimization problem, to either (1) minimize the total transmit power with a constraint on the user data rate or (2) maximize the total data rate with a constraint on total transmit power. The proportional rate constraints (PRC) algorithm is to maximize the sum throughput, with the additional constraint that each user’s data rate is proportional to a set of predetermined system parameters. While the objective of the maximum sum rate (MSR) algorithm, is to maximize the sum rate of all users, given a total transmit power constraint.