Browsing by Author "Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland"
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Item Algorithms for Power System State Estimation with Phasor Measurement Units(Aalto University, 2021) Kashyap, Neelabh; Werner, Stefan, Prof., NTNU, Norway; Huang, Yih-Fang, Prof., University of Notre Dame, USA; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandPower system state estimation (PSSE) is one of the key components in the suite of computational tools which assist in managing the day to day operations of electric grids. In recent years, two trends have emerged as the world transitions towards smart grids: The first is the introduction of new measurement and protection devices called phasor measurement units (PMUs). The second is the creation of transmission system operators (TSOs) as part of a general move away from vertically integrated monopolies. These TSOs are charged with ensuring the fair and reliable operation of the power system. This thesis proposes new algorithms for PSSE which address the implications of these trends. This thesis introduces a novel approach to state estimation with PMUs where we decouple the estimation of PMU observable and PMU unobservable state variables. We show that this approach has several advantages like improved numerical stability, reduced computational complexity and robustness to time-skew errors. Next, we develop a signal-dependent scheme to choose how many PMU measurements to average over to obtain a more precise estimate of the underlying voltages. This method shows an improvement over the state-of-the-art approach. This thesis also develops methods for using PMUs for multi-area state estimation (MASE). These methods are based on a novel information exchange scheme called clustered gossip and share. We derive analytical results which prove that this approach always results in reduced information exchange when compared to a naive approach to network gossiping. This approach is applied to the traditional measurement-approach, the previously derived reduced-order approach, and also a novel privacy preserving approach for linear power system state estimation. Also presented in this thesis is a novel event-triggered approach to hierarchical multi-area state estimation. The main idea of this algorithm is that the state estimator in each area communicates with neighboring areas only when such an action is informative. This approach is shown to reduce the amount of communication, and also reduces the computational load involved in calculating the state estimation.Item Applications of fast QR-decomposition based adaptive algorithms in wireless systems(Aalto University, 2017) Mohammad, Mohammed Mobien; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandThis thesis presents four contributions: first, it develops new techniques to extend the range of applications of computationally efficient (comparing to recursive least-squares (RLS) algorithm) fast QR-decomposition least-squares (FQRD-LS) algorithms; second, it develops new version of FQRD-LS algorithm for widely-linear (WL) input signal; third, It presents fixed-point analysis of FQRD-LS algorithm; and finally, it applies contant modulus algorithm (CMA) framework to the inverse QR-decomposition recursive least-squares (QRD-RLS) algorithm. The main idea in the new techniques is to make available the adaptive filter coefficients using the internal variables of the FQRD-RLS algorithm. Four applications that result from using these techniques are: system identification, burst-trained equalization, broad-band beamformation, and predistortion. WL adaptive algorithms are well suited for non-circular input signals, which arises for example in adaptive beamforming scenario when number of sources is greater than the number of antennas. In fixed point analysis of FQRD-LS algorithm we present: mathematical expressions for the mean square quantization error (MSQE) of all internal variables of the FQRD-LS algorithms; and derive the conditions that guarantee the stability of FQRD-LS algorithms for the purpose of fixed-point implementation. Finally, we show how to apply the CMA framework toward inverse QRD-RLS algorithm. We show application of CMA based IQRD-RLS algorithm in blind equalization of an optical channel.Item Design and Analysis of Duplexing Modes and Forwarding Protocols for OFDM(A) Relay Links(Aalto University, 2014) Riihonen, Taneli; Werner, Stefan, Dr., Aalto University, Department of Signal Processing and Acoustics, Finland; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandRelaying, i.e., multihop communication via so-called relay nodes, has emerged as an advanced technology for economically realizing long transmission ranges and high data rates in wireless systems. The focus of this thesis is on multihop multiuser systems where signals are modulated with orthogonal frequency-division multiplexing or multiple access, i.e., OFDM(A), and relays are infrastructure-based network nodes. In general, the thesis contributes by investigating how to operate relay links optimally under spectrum, transmit power and processing capability limitations, as well as how to improve signal processing in relays by exploiting other advanced concepts such as multiantenna techniques, spectrum reuse, transmit power adaptation, and new options for multicarrier protocol design. The first theme is the design and analysis of duplexing modes which define how a relay link reuses allocated frequency bands in each hop. Especially, the full-duplex relaying mode is promoted as a feasible, advantageous alternative to conventional half-duplex operation while its inherent loopback self-interference is recognized and dealt with: New techniques are developed and evaluated for the mitigation of self-interference while transmit power control is proposed as a countermeasure against residual distortion. Finally, the fundamental criteria to prefer one mode over the other are determined which not only serves as a comparison between the modes but allows for opportunistic mode switching during operation. The second theme is the design and analysis of forwarding protocols which specify signal processing done between reception and retransmission in a relay. The study identifies a comprehensive set of design options for OFDM(A) relaying protocols and evaluates their combinations. Similar analysis is conducted for evaluating the duplexing modes and multiuser scheduling. Furthermore, a new analytical framework is developed for studying the channel delay spread characteristics of non-regenerative multihop OFDM links in conjunction with inter-carrier and inter-symbol interference as well as time synchronization error. Throughout the thesis, the design of relaying modes and protocols is conducted together with statistical performance analysis which facilitates discerning comparisons between existing and proposed concepts. Especially, all the contributions are established by means of closed-form expressions derived for the considered systems. The results cover all essential system variations, including amplify-and-forward and decode-and-forward protocols as well as downlink and uplink relaying reflecting their differences in infrastructure-based relaying.Item Development of digital predistorters for broadband power amplifiers in OFDM systems using the simplicial canonical piecewise linear function(Aalto University, 2014) Cheong, Mei Yen; Werner, Stefan, Dr., Aalto University, Department of Signal Processing and Acoustics, Finland; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandPower amplifiers (PAs) are inherently nonlinear devices. Linearity of a PA can be achieved by backing off the PA to its linear region at the expense of power efficiency loss. For signals with high envelope fluctuation such OFDM system, large backoff is required, causing significant loss in power efficiency. Thus, backoff is not a favourable solution. Digital predistorters (PDs) are widely employed for linearizing PAs that are driven to the nonlinear regions. In broadband systems where PAs exhibit memory effects, the PDs are also required to compensate the memory effects. This thesis deals with the development of digital PDs for broadband PAs in OFDM systems using the Simplicial Canonical Piecewise Linear (SCPWL) function. The SCPWL function offers a few advantages over polynomial models. It imposes a saturation after the last breakpoint, making it suitable for modelling nonlinearities of PA and PD. The breakpoints of the function can be freely placed to allow optimum fitting of a given nonlinearity. It is suitable for modeling strong nonlinearities. Analysis of the SCPWL spectra property shows that the function models infinite order of intermodulation distortion, even with small number of breakpoints. The accuracy of the model can be improved by increasing the number of breakpoints. The original real-valued SCPWL function is extended to include memory structure and complex-valued coefficients, resulting in the proposed baseband SCPWL model with memory. The model is adopted in the development of the Hammerstein-SCPWL PD and memory-SCPWL PD. Vector projection methods are developed for static SCPWL PDs identification. Adaptive algorithms employing the indirect and direct learning architectures are developed for identifying the Hammerstein-SCPWL PD and memory-SCPWL PD. By exploiting the properties of the SCPWL function, the algorithms are simplified. A modified Wiener model estimator is employed to circumvent the non-convex cost function problem of block models. This further reduces the complexity of the Hammerstein PD algorithms. The thesis also analyses the effects of measurement noise on indirect learning SCPWL filter. Due to its linear basis function, the SCPWL filter coefficients do not suffer the coefficient bias effects which are observed in polynomial models. The performance of the proposed SCPWL PDs are compared with state-of-the-art polynomial-based PDs by simulations and measurements.Item Limited feedback and channel estimation for closed-loop MIMO systems(Aalto University, 2016) Schober, Karol; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandMultiple-input-multiple-output (MIMO) techniques have been one of the key features in the globally-endorsed 3GPP LTE cellular standard, and are also expected to be a key feature to compensate for the increased path-loss in the mm-wave spectrum. A MIMO transmitter requires knowledge of MIMO channel in order to harvest beamforming gain. Therefore, in frequency division duplex (FDD) system, or the time division duplex (TDD) system with uncalibrated uplink and downlink chains, the transmitter has to rely on limited feedback from a receiver. Feedback constraints are a consequence of the limited feedback link capacity.In this thesis, we firstly describe the MIMO precoding space as the Stiefel manifold, together with its MIMO-relevant quotient spaces; the Grassmannian and the Flag manifolds. Further, we introduce an alternative closed-form solution for geodesic construction on the Grassmannian manifold, and we introduce a framework for geodesic construction on manifolds, where a closed-form solution does not exist, i.e. the Stiefel manifold and the Flag manifold. Secondly, we discuss channel estimation on the spatially precoded user-specific reference symbols, with TDD-based Eigenbeamforming and FDD-based closed-loop LTE precoding. When the spatial precoder is varied across the allocated frequency band, the corresponding effective channel changes its frequency correlation properties and/or suffers from discontinuities. Herein, both issues are tackled by additional transmitter signal processing. The rest of the thesis is dedicated to feedback design for single-user (SU) MIMO, multi-user (MU) MIMO and Coordinated multi-point (CoMP). We develop a subspace packing algorithm called ECA which is analogous to the max-min algorithm placing points equidistantly within the wrap-around two-dimensional rectangle. Straight lines are substituted by geodesics. Further, we provide improvements to LTE double-codebook design for both uniform linear arrays and cross-polarized arrays. A generalized eigenvector MU-MIMO technique benefits from knowledge of the normalized covariance matrix at the transmitter. We separately quantize the normalized eigenvalues and corresponding eigenvectors, and study the optimal split of feedback bits between them. On the other hand, LTE does not support feedback of eigenvalues, and MU-MIMO performance may be improved only by improving the knowledge of eigenvectors at the transmitter. Set in an LTE context, we propose a low-complexity successive refinement technique using the additional m-th best feedback and geodesical interpolation at the transmitter. Finally, we discuss feedback for CoMP and introduce a concept of flexible layer arrangement for coherent and non-coherent SU-MIMO joint-transmission CoMP.Item Multi-Antenna Techniques for Next Generation Cellular Communications(Aalto University, 2016) Ugurlu, Umut; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandFuture cellular communications are expected to offer substantial improvements for the pre- existing mobile services with higher data rates and lower latency as well as pioneer new types of applications that must comply with strict demands from a wider range of user types. All of these tasks require utmost efficiency in the use of spectral resources. Deploying multiple antennas introduces an additional signal dimension to wireless data transmissions, which provides a significant alternative solution against the plateauing capacity issue of the limited available spectrum. Multi-antenna techniques and the associated key enabling technologies possess unquestionable potential to play a key role in the evolution of next generation cellular systems. Spectral efficiency can be improved on downlink by concurrently serving multiple users with high-rate data connections on shared resources. In this thesis optimized multi-user multi-input multi-output (MIMO) transmissions are investigated on downlink from both filter design and resource allocation/assignment points of view. Regarding filter design, a joint baseband processing method is proposed specifically for high signal-to-noise ratio (SNR) conditions, where the necessary signaling overhead can be compensated for. Regarding resource scheduling, greedy- and genetic-based algorithms are proposed that demand lower complexity with large number of resource blocks relative to prior implementations. Channel estimation techniques are investigated for massive MIMO technology. In case of channel reciprocity, this thesis proposes an overhead reduction scheme for the signaling of user channel state information (CSI) feedback during a relative antenna calibration. In addition, a multi-cell coordination method is proposed for subspace-based blind estimators on uplink, which can be implicitly translated to downlink CSI in the presence of ideal reciprocity. Regarding non-reciprocal channels, a novel estimation technique is proposed based on reconstructing full downlink CSI from a select number of dominant propagation paths. The proposed method offers drastic compressions in user feedback reports and requires much simpler downlink training processes. Full-duplex technology can provide up to twice the spectral efficiency of conventional resource divisions. This thesis considers a full-duplex two-hop link with a MIMO relay and investigates mitigation techniques against the inherent loop-interference. Spatial-domain suppression schemes are developed for the optimization of full-duplex MIMO relaying in a coverage extension scenario on downlink. The proposed methods are demonstrated to generate data rates that closely approximate their global bounds.Item OFDM under Oscillator Phase Noise : Contributions to Analysis and Estimation Methods(Aalto University, 2016) Jacob Mathecken, Pramod; Riihonen, Taneli, Dr.; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandMost modern transmitters and receivers involve an analog front-end unit and a digital back-end unit. The digital back-end is responsible for information processing which involves thefollowing: redundancy removal from information; information representation; improvinginformation resilience; and information correction. The analog front-end is responsible forinformation transmission and reception. The information processing algorithms developedand implemented in the digital back-end assume a linear and noiseless analog front-end which,in reality, is not the case. This renders some of the information processing algorithms to be lesseffective in practice. The focus of this thesis is on orthogonal frequency-division multiplexing(OFDM) systems under the influence of oscillator phase noise. OFDM is an efficientinformation representation technique used in many communication systems. On the otherhand, phase noise is one type of undesired noise that occurs in the oscillator device used in theanalog front-end. It arises due to the imperfect task of frequency conversion, performed by theoscillator device, between baseband and radio frequency. This thesis contributes to the areas of analysis and estimation in OFDM systems under theinfluence of oscillator phase noise. With regard to analysis, this thesis contributes by derivingthe channel capacity assuming a Gaussian input alphabet. The aim here is to show bothquantitatively and qualitatively the degradation in performance of the OFDM system in thepresence of phase noise. The analysis is conducted for phase noise processes that occur in bothfree-running and phase-locked loop based oscillators and also extended to include the effect ofcarrier frequency offset. With regard to estimation, two new phase noise estimation algorithmsare proposed in this thesis. In particular, these algorithms restrict the search space to a specific subset, where the desired phase noise parameter of interest is shown to lie. For example, in the first estimation method, possible subspaces in which the desired phase noise spectral vector may lie are used in the estimation step. In the second method, the geometry of the desired phase noise spectral vector is used in the estimation step. Specifically, this geometry corresponds to a non-convex set described by a set of quadratic forms that involve permutation matrices. By restricting the search space to this set, the accuracy of phase noise estimation can be improved.Item Self-interference Mitigation in Full-duplex Relays:Blind Adaptive Techniques and Optimal Linear Filter Design(Aalto University, 2018) Antonio Rodríguez, Emilio; Werner, Stefan, Prof., Norwegian University of Science and Technology, Norway; López Valcarce, Roberto, Prof., University of Vigo, Spain; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandWireless communication systems need to satisfy the increasing demand for high data rates, low latency and ubiquitous connectivity. Besides, efficient resource management and a high spectral efficiency are also paramount. Multi-hop communication via relays is an attractive and cost-effective solution for cooperative communication and coverage extension. Full-duplex technology, i.e., simultaneous transmission and reception in the same frequency band, offers twice the spectral efficiency than its half-duplex counterpart, where transmission and reception take place in different time slots. Full-duplex technology makes possible for devices to operate directly on the waveform samples without synchronizing to the network, thus allowing to reduce the end-to-end latency and posing itself as an ideal solution for wireless systems. However, the implementation of a full-duplex device has several practical issues. Of utmost importance is the self-interference, which causes the transmitted signal to interfere with the received signal, leading to a severe drop of the signal-to-noise ratio. To fully take advantage of the full-duplex technology, self-interference must be mitigated. This dissertation addresses the issue of self-interference mitigation for different relay protocols and configurations. The first theme deals with adaptive blind algorithms for self-interference cancellation. Assuming no available training information or synchronization with the network, we propose algorithms that yield residual self-interference below noise level. The second theme considers the optimization of a full-duplex relay link subjected to limited dynamic range, i.e., impairments at transmission and reception sides. Considering available channel state information, we design optimal linear filters at each node under the maximum signal-to-noise ratio and the minimum mean square error criteria.Item Spectral and Power Efficient Modulation Methods in Presence of Nonlinear Distortions(Aalto University, 2017) Kiviranta, Markku; Mämmelä, Aarne, Res. Prof., VTT Technical Research Centre, Finland; Signaalinkäsittelyn ja akustiikan laitos; Department of Signal Processing and Acoustics; Sähkötekniikan korkeakoulu; School of Electrical Engineering; Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, FinlandThis thesis presents a state-of-the-art review and brings new contributions to the waveform design for 5G systems in the presence of analog nonlinearities. A system model for the typical analog impairments at each stage of the transmitter and the receiver with a channel is first presented. The highlighted nonlinearities are power amplifier (PA), phase noise, and DC offset. In this thesis, the further development of the orthogonal frequency division multiplexing (OFDM) is considered since the evolutionary approach from the 4G to the 5G is assumed to be more mature for technology demonstrations. In particular, the constant envelope OFDM (CE-OFDM) where phase modulation creates a constant envelope is studied, and the results indicate that CE-OFDM has an advantage compared to OFDM when the PA efficiency is taken into account in the simulations. Since the phase noise transforms just into an additive noise term after the phase detector, CE-OFDM is very resistant to phase noise unlike OFDM. CE-OFDM can be seen as an interesting candidate for the 5G scenarios when the power efficiency is a more important parameter than the spectral efficiency. This thesis presents also new findings and results for the constant envelope tamed frequency modulation (TFM) and trellis-coded modulation (TCM) that is a traditional combined coding and modulation technique for spectral and power limited systems. In phase or frequency modulation spectral spreading and detection performance are controlled using a modulation index. In a simple analog implementation of TFM, the modulation index deviates during the transmission and generates time varying phase jitter. In this work a modulation index estimator in conjunction with per-survivor processing (PSP) carrier phase estimation is presented for the TFM which has also very small spectral out-of-band radiation compared to other known CPM methods. A novel scheme based on multiple TCM, PSP and Reed–Solomon (RS) coding is proposed and shown to prevent the trellis-code to become a catastrophic code in the presence of phase noise. In general, the TCM and CPM modulations can complement the future adaptive OFDM based 5G systems.