CFO estimation in OFDM systems under timing and channel length uncertainties with model averaging

In this letter, we investigate the problem of CFO estimation in OFDM systems when the timing offset and channel length are not exactly known. Instead of explicitly estimating the timing offset and channel length, we employ a multi-model approach, where the timing offset and channel length can take multiple values with certain probabilities. The effect of multimodel is directly incorporated into the CFO estimator. Results show that the proposed estimator outperforms the estimator selecting only the most probable model and the method taking the maximal model. © 2006 IEEE.published_or_final_versio

Direction of Arrival Estimation for Radio Positioning: a Hardware Implementation Perspective

Nowadays multiple antenna wireless systems have gained considerable attention due to their capability to increase performance. Advances in theory have introduced several new schemes that rely on multiple antennas and aim to increase data rate, diversity gain, or to provide multiuser capabilities, beamforming and direction finding (DF) features. In this respect, it has been shown that a multiple antenna receiver can be potentially used to perform radio localization by using the direction of arrival (DoA) estimation technique. In this field, the literature is extensive and gathers the results of almost four decades of research activities. Among the most cited techniques that have been developed, we find the so called high-resolution algorithms, such as multiple signal classification (MUSIC), or estimation of signal parameters via rotational invariance (ESPRIT). Theoretical analysis as well as simulation results have demonstrated their excellent performance to the point that they are usually considered as reference for the comparison with other algorithms. However, such a performance is not necessarily obtained in a real system due to the presence of non idealities. These can be divided into two categories: the impairments due to the antenna array, and the impairments due to the multiple radio frequency (RF) and acquisition front-ends (FEs). The former are strongly influenced by the manufacturing accuracy and, depending on the required DoA resolution, have to be taken into account. Several works address these issues in the literature. The multiple FE non idealities, instead, are usually not considered in the DoA estimation literature, even if they can have a detrimental effect on the performance. This has motivated the research work in this thesis that addresses the problem of DoA estimation from a practical implementation perspective, emphasizing the impact of the hardware impairments on the final performance. This work is substantiated by measurements done on a state-of-the-art hardware platform that have pointed out the presence of non idealities such as DC offsets, phase noise (PN), carrier frequency offsets (CFOs), and phase offsets (POs) among receivers. Particularly, the hardware platform will be herein described and examined to understand what non idealities can affect the DoA estimation performance. This analysis will bring to identify which features a DF system should have to reach certain performance. Another important issue is the number of antenna elements. In fact, it is usually limited by practical considerations, such as size, costs, and also complexity. However, the most cited DoA estimation algorithms need a high number of antenna elements, and this does not yield them suitable to be implemented in a real system. Motivated by this consideration, the final part of this work will describe a novel DoA estimation algorithm that can be used when multipath propagation occurs. This algorithm does not need a high number of antenna elements to be implemented, and it shows good performance despite its low implementation/computational complexity

Blind Estimation of Carrier Frequency Offset for OFDM Systems using Maximum Likelihood Technique

A Multicarrier Communication system such as an Orthogonal Frequency Division Multiplexing OFDM has been shown to be an impressive approach to combat multipath fading in wireless communications. OFDM is a modulation scheme that allows digital data to be efficiently and reliably transmitted over a radio channel, even in multipath environments. OFDM transmits data by using a large number of narrow bandwidth carriers. These carriers are regularly spaced in frequency, forming a block of spectrum. The frequency spacing and time synchronization of the carriers is chosen in such a way that the carriers are orthogonal, meaning that they do not cause interference to each other. In spite of the success and effectiveness of the OFDM systems, it suffers from a well-known drawback of high sensitivity to Carrier Frequency Offset (CFO). The presence of the CFO in the received carrier will lose orthogonality among the carriers and causes a reduction of desired signal amplitude in the output decision variable and introduces Inter Carrier Interference (ICI). It then brings up an increase of Bit Error Rate (BER). This makes the problem of estimating the CFO an attractive and necessary research problem. In this thesis Blind Modified ML CFO estimation technique based on data symbol repetition is discussed to estimate the offset parameter

Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Advanced Signal Processing Algorithms for GNSS/OFDM Receiver

The recent years have shows a growing interest in urban and indoor positioning with the development of applications such as car navigation, pedestrian navigation, local search and advertising and others location-based-services (LBS). However, in urban and indoor environment the classical mean of positioning, the Global Positioning Satellite System (GNSS) has limited availability, accuracy, continuity and integrity due to signal blockage by building, intense multipath conditions and interferences from the other signals, abundant in metropolitan areas. Even some improvements of GNSS can reduce these issues (high-sensitivity receiver, assisted-GNSS, multi-constellation GNSS…), they do not permit to reach sufficient performance in deep urban and indoor environments. However, some alternatives to GNSS allow complementing it in difficult environments. They are, for example, additional sensors (accelerometers, gyrometers, magnetometers, odometers, laser, and video), radiofrequency systems dedicated to positioning (pseudolites, RFID, UWB) or signals of opportunity (SoO). SoO are telecommunication signals (as mobile phone, TV, radio, Wi-Fi) that are used opportunely to provide a positioning service. Even if these signals are not designed for such application, they have the advantages to be many and varied in urban and indoor environments. In addition they allow, by definition, a good integration of communication and positioning services. Among all the SoO available, this thesis focuses on the one based on the Orthogonal Frequency Division Multiplexing (OFDM) modulation. This choice is motivated by the important popularity of this modulation, that has been chosen in several actual and future telecommunication and broadcasting standards (Wi-Fi, WiMAX, LTE, DVB-T/H/SH, DAB, T-DMB, ISDB-T, MediaFLO…). Among this standard using the OFDM modulation, the European standard for digital television called “Digital Video Broadcasting – Terrestrial” (DVB-T) has been selected to be studied in this thesis. The choice is motivated by the relatively simple definition of this standard, allowing reuse of the work for other OFDM standards, and also because it is already operational in Europe, allowing tests on real signals. A method to obtain ranging measurements based on timing synchronization using DVB-T signals has been developed. This method uses delay lock loops (DLL) and takes into account the specificity of the terrestrial propagation channel (many multipathes, direct signal sometimes absent, quick variation of received power…). The performance of the method has been determinate theoretically and validated by simulation, in an ideal case (i.e.; with a Gaussian propagation channel). This theoretical study has proven than the ranging error standard deviation has an order of magnitude of 1 meter, for signal to noise ratio of about -20 dB, a SNR 40 dB under the demodulation threshold of the TV signal. The performance in a realistic propagation channel has been determined on real signal. For that purpose a test bench has been developed. It allows to receive and record TV signals on two synchronized antennas and it includes and GPS receiver to record a reference position and provide a GPS time reference to the test bench. Tests on real signals have been realized in several environments (sub-urban, urban and indoor) using 1 emitter synchronized on GPS time and 2 emitters in a signal frequency network (SFN). The results of these tests on real signals showed a precision of the ranging estimation of about 10 meters with a better performance in rural environment and an improvement of the ranging estimate using antenna diversity. Finally, the thesis proves the feasibility of positioning with signal using the OFDM modulation, with a technique that can be easily tailored to other OFDM signal than DVB-T

System design and validation of multi-band OFDM wireless communications with multiple antennas

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