Compressive channel estimation

In dieser Arbeit untersuchen wir die kompressive Kanalschätzung (KKS), also die Anwendung der Theorie und Methodologie des Compressed Sensing (CS) auf das Problem der Kanalschätzung doppelt selektiver Kanäle in Multicarrier-Systemen. Nach einer kurzen Einführung in die kabellose Kommunikation und einem kleinen Überblick über CS und einigen seiner Varianten betrachten wir die in [1] präsentierte elementare kompressive Kanalschätzmethode. Wir analysieren ihre Leistungsfähigkeit sowie ihre Komplexität, und wir untersuchen die ihr zugrundeliegende Annahme, nämlich die "delay-Doppler sparsity" typischer Kanäle, genauer. Aufbauend auf dieser Analyse stellen wir einige Varianten und Erweiterungen der kompressiven Kanalschätzmethode vor. Zuerst nutzen wir die Tatsache dass typische Kanäle auch als "group sparse" angesehen werden können. Dies ist eine Folge des sogenannten Leck Effekts, welcher die Leistung einer jeden kompressiven Kanalschätzmethode beeinträchtigt und daher eine enorme Herausforderungen für die KKS darstellt. Weiters betrachten wir die Erweiterung der kompressiven Schätzmethode auf Mehrantennensysteme (MIMO). Wir zeigen, dass die einzelnen Querkanäle eines solchen MIMO Systems (in etwa) als "jointly sparse", sogar als "jointly group sparse" angesehen, und daher Methoden des Multichannel CS (MCS) verwendet werden können. Letztens nutzen wir - unter Verwendung der Konzepte des Modified CS (MOD-CS) - die approximative "sequential sparsity" des Kanals zum Kanal-Tracking über mehrere aufeinanderfolgende Symbolblöcke hinweg. Diese Vorgehensweise kann die Leistung zusätzlich steigern, viel wichtiger jedoch, sie kann die Komplexität der Methode reduzieren. Darüber hinaus adaptieren wir die Technik der Basis-Optimierung, welche in [2, 3] vorgestellt wurde, für die verschiedenen Szenarien, und wir präsentieren Simulationsergebnisse, welche die verbesserte Leistung all jener Kanalschätzmethoden demonstrieren, die in dieser Arbeit erklärt werden.In this thesis we investigate compressive channel estimation (CCE), i.e. the application of the theory and methodology of Compressed Sensing (CS) to the problem of estimating doubly selective channels in multicarrier systems. After a brief introduction to wireless communications and a short survey of CS and some of its variations, we review the basic compressive channel estimator that was introduced in [1]. We analyze its performance as well as its computational complexity, and we explore the basic assumption underlying the compressive estimator, namely the delay-Doppler sparsity of typical channels, in more detail. Based on this analysis, we propose several variations and extensions of the conventional compressive channel estimator. First, we make use of the fact that typical channels can be considered group sparse as well. This is due to the so-called leakage effect, which actually impairs the performance of any channel estimator utilizing CS techniques and therefore is one of the main challenges in CCE. Then, we investigate the extension of the compressive estimators to the multi-antenna (MIMO) case. We show that the various cross-channels of a MIMO system can (approximately) be considered jointly sparse, even jointly group sparse, and that therefore the methodology of multichannel CS can be utilized. Last, by using the recently introduced concept of modified CS, we exploit the approximate sequential sparsity of the channel in order to track it over a period of several consecutive symbol blocks. This approach can yield an additional performance gain, but more importantly it can substantially reduce the computational complexity of the method. Additionally, we adapt the basis optimization techniques introduced in [2, 3] to the various settings, and we present simulation results that demonstrate the performance gains that can be achieved by using each of the compressive estimators presented in this thesis

Doctor of Philosophy

dissertationThe demand for high speed communication has been increasing in the past two decades. Multicarrier communication technology has been suggested to address this demand. Orthogonal frequency-division multiplexing (OFDM) is the most widely used multicarrier technique. However, OFDM has a number of disadvantages in time-varying channels, multiple access, and cognitive radios. On the other hand, filterbank multicarrier (FBMC) communication has been suggested as an alternative to OFDM that can overcome the disadvantages of OFDM. In this dissertation, we investigate the application of filtered multitone (FMT), a subset of FBMC modulation methods, to slow fading and fast fading channels. We investigate the FMT transmitter and receiver in continuous and discrete time domains. An efficient implementation of FMT systems is derived and the conditions for perfect reconstruction in an FBMC communication system are presented. We derive equations for FMT in slow fading channels that allow evaluation of FMT when applied to mobile wireless communication systems. We consider using fractionally spaced per tone channel equalizers with different number of taps. The numerical results are presented to investigate the performance of these equalizers. The numerical results show that single-tap equalizers suffice for typical wireless channels. The equalizer design study is advanced by introducing adaptive equalizers which use channel estimation. We derive equations for a minimum mean square error (MMSE) channel estimator and improve the channel estimation by considering the finite duration of channel impulse response. The results of optimum equalizers (when channel is known perfectly) are compared with those of the adaptive equalizers, and it is found that a loss of 1 dB or less incurs. We also introduce a new form of FMT which is specially designed to handle doubly dispersive channels. This method is called FMT-dd (FMT for doubly dispersive channels). The proposed FMT-dd is applied to two common methods of data symbol orientation in the time-frequency space grid; namely, rectangular and hexagonal lattices. The performance of these methods along with OFDM and the conventional FMT are compared and a significant improvement in performance is observed. The FMT-dd design is applied to real-world underwater acoustic (UWA) communication channels. The experimental results from an at-sea experiment (ACOMM10) show that this new design provides a significant gain over OFDM. The feasibility of implementing a MIMO system for multicarrier UWA communication channels is studied through computer simulations. Our study emphasizes the bandwidth efficiency of multicarrier MIMO communications .We show that the value of MIMO to UWA communication is very limited

Channel estimation with TCH codes for machine-type communications

TCH codes possess several properties that allow us to use them efficiently in various applications. One of these applications is channel estimation and, in this dissertation, it is studied the performance of TCH codes to estimate the channel in an Orthogonal Frequency Division Multiplexing system, regarding Machine-Type Communications. Bit error rate performance results were obtained by executing simulations that allowed the evaluation of the impact of using two different pilot techniques, such as data multiplexed and implicit pilots, different pilot power levels and different modulations, QPSK and 64-QAM. Pilots based on TCH codes are also compared with other conventional pilots. Results show that TCH codes have a very positive and reliable performance. Joint timing synchronization and channel estimation is also performed using different sparse based approaches, such as Orthogonal Matching Pursuit, L1- regularized and Iterative Reweighted L1. TCH codes are compared against different sequence types, namely Zadoff-Chu sequences and pseudorandom codewords, and variations in the pilot size, the channel length and the observation window size are executed in order to understand their effects. Results ultimately illustrate that TCH codes can be effectively used in joint channel estimation and synchronization, managing to withstand worst simulation conditions better than its counterparts. It is also proven that compressed sensing can successfully be utilized in joint synchronization and channel estimation, an area where its use has not been very explored.Os códigos TCH possuem várias propriedades que nos permitem usá-los eficientemente em diversas aplicações. Uma delas é a estimação de canal e nesta dissertação é estudado o desempenho dos códigos TCH em estimação de canal num sistema OFDM, tendo em conta as comunicações Machine-Type. Resultados que ilustram a taxa de erro de bit foram obtidos através de simulações que permitem avaliar o impacto de usar diferentes técnicas de pilotos, nomeadamente multiplexados e implícitos, diferentes valores de potência para os pilotos e diferentes modulações, QPSK e 64-QAM. Também é feita a comparação entre os pilotos TCH e pilotos convencionais. Os resultados mostram que os pilotos TCH tem um desempenho muito positivo e confiável, dentro dos parâmetros testados. Também é efetuado o estudo de sincronização e estimação de canal conjunta usando métodos esparsos como o OMP, o L1-regularized e o Iterative Reweighted L1. Os códigos TCH são comparados com outros tipos de sequências, tais como as sequências Zadoff-Chu e os códigos pseudo-aleatórios. São consideradas variações no tamanho dos pilotos, no comprimento do canal e no tamanho da janela de observação para perceber quais são os seus efeitos no desempenho. Os resultados demonstram que os códigos TCH podem ser utilizados com sucesso em estimação de canal e sincronização conjunta e conseguem aguentar condições adversas de simulação melhor que os outros pilotos utilizados. Também é provado que compressed sensing pode ser utilizado com sucesso em sincronização e estimação conjunta, que é uma área onde o seu uso ainda não foi explorado aprofundadamente

Joint Millimeter-Wave Communication and Radar for Automotive Applications

DTRT13-G-UTC58Automotive joint communication and radar (JCR) waveforms with fully digital baseband generation and processing can now be realized at the millimeter-wave (mmWave) band. Prior work has developed a mmWave wireless local area network (WLAN)-based automotive JCR that exploits the WLAN preamble for radars. The performance of target velocity estimation, however, was limited. In this paper, we propose an adaptive virtual JCR waveform design for automotive applications at the mmWave band. The proposed system transmits a few non-uniformly placed preambles to construct several receive virtual preambles for enhancing velocity estimation accuracy, at the cost of only a small reduction in the communication data rate. We evaluate JCR performance trade-offs using the Cramer- Rao Bound (CRB) metric for radar estimation and a novel distortion minimum mean square error (MMSE) metric for data communication. Additionally, we develop three different MMSE-based optimization problems for the adaptive JCR waveform design. Simulations show that an optimal virtual (non-uniform) waveform achieves a significant performance improvement as compared to a uniform waveform. For a radar CRB constrained optimization, the optimal radar range of operation and the optimal communication distortion MMSE (DMMSE) are improved. For a communication DMMSE constrained optimization with a high DMMSE constraint, the optimal radar CRB is enhanced. For a weighted MMSE average optimization, the advantage of the virtual waveform over the uniform waveform is increased with decreased communication weighting. Comparison of MMSE-based optimization with traditional virtual preamble count-based optimization indicated that the conventional solution converges to the MMSE- based one only for a small number of targets and a high signal-to-noise ratio

Development and verification of semi-blind receiver structures for broadband wireless communication systems

The increasingly high demands for high data rate wireless communication services require spectrum- and energy-efficient solutions. In this thesis, a number of energy-efficient semi-blind receiver structures are proposed to perform Doppler spread estimation, channel estimation and equalisation for broadband wireless orthogonal frequency division multiplexing (OFDM) systems. A real-time wireless communication testbed is developed to verify the proposed semi-blind receiver structures. In the first contribution, a semi-blind Doppler spread estimation and Kalman filtering based channel estimation approach is proposed for wireless OFDM systems. A short sequence of reference data is carefully designed and applied as pilot symbols for Doppler spread estimation and channel estimation initialisation of the Kalman filter. Then the estimates of inter-carrier interference (ICI) caused by Doppler spread are gathered into the equivalent channel model and compensated for through channel equalisation, which dramatically reduces the computational complexity. The simulation results show that the proposed approach outperforms the conventional pilot aided Doppler spread and channel estimation schemes. In the second contribution, a semi-blind Doppler spread estimation and independent component analysis (ICA) based equalisation scheme aided by non-redundant precoding is proposed for wireless multiple-input multiple-output (MIMO) OFDM systems. A number of reference data sequences are selected from a pool of orthogonal sequences for two purposes. First, the reference data sequences are superimposed in the source data sequences through non-redundant linear precoding to enable the Doppler spread estimation by minimising the sum cross-correlation between the compensated signals and the rest of the orthogonal sequences in the pool. Second, the same reference data sequences are applied to eliminate the phase and permutation ambiguity in the ICA equalised signals. Simulation results show that the proposed semi-blind MIMO OFDM system can achieve a bit error rate (BER) performance which is close to the ideal case with perfect channel state information (CSI). In the third contribution, a real-time wireless communication testbed is developed with a vector signal generator, a vector signal analyser and a pair of antennas, to verify the effectiveness of the proposed receiver structures over the air in different environments such as Reverberation chamber and office area. Measurement results show a good match with simulation results. Also, a pilot is employed for three purposes at a semi-blind receiver: time synchronisation, Doppler spread estimation and Kalman filtering initialisation, which is an extension of the work in the first contribution