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

Space time transceiver design over multipath fading channels

Imperial Users onl

Study the Carrier Frequency Offset (CFO) for Wireless OFDM

Due to high proficiency with high bandwidth efficiency, orthogonal frequency division multiplexing (OFDM) has been selected for broadband wireless communication systems. Since OFDM can provide large data rates with sufficient robustness to radio channel impairments, and due to its robustness against the multipath delay spread, OFDM has always been a designated technique for broadband wireless communication mobile systems. Nevertheless, OFDM suffers from Carrier Frequency Offset (CFO). CFO has been recognized as a major disadvantage of OFDM. CFO can lead to the frequency mismatch in transmitter and receiver oscillator. Lack of the synchronization of the local oscillator signal (L.OSC), for down conversion in the receiver with the carrier signal contained in the received signal, can cause the performance of OFDM to degrade. In other words, the orthogonality of the OFDM relies on the condition that the transmitter and receiver operate with exactly the same frequency reference. If this is not the case, the perfect orthogonality of the subcarrier will be lost, which can result in CFO. In this research, the source of creating CFO and the major CFO estimation algorithms have been reviewed and discussed in literature. We then proposed some algorithms and techniques for estimating and compensating for the effect of CFO. We showed that our proposed methods have a better performance with low complexity

Time of Arrival and Angle of Arrival Estimation of LTE Signals for Positioning Applications

With the increase of services that need accurate location of the user, new techniques that cooperate with the Global Navigation Satellite System (GNSS) are necessary. Toward this objective, this thesis presents our research work about the estimation of the time of arrival (TOA) and of the angle of arrival (AOA) exploiting modern cellular signals. In particular, we focus on the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, and in particular uplink and downlink reference signals are exploited to this purposes. The current release of the 3GPP LTE specification supports a UTDOA localization technique based on the Sounding Reference Signal (SRS). In real environments, however, user equipments (UE) are rarely set up to transmit this particular signal. The main original contribution of this thesis consists in a new TOA estimation method based on uplink transmission. In particular, we explore the possibility of performing radio localization exploiting the uplink Demodulation Reference Signal (DM-RS), which is always sent by UEs during data transmission. Real uplink transmissions are modeled in simulations and the performance of known algorithms like SAGE and IAA-APES are evaluated for TOA estimation. A new method to estimate the initial conditions of the SAGE algorithm is proposed and the estimation performance in uplink scenarios is evaluated. The analysis revealed that the proposed method outperforms the non-coherent initial conditions estimation proposed in the literature, when uplink transmission are used. Then, the benefits of our proposal are evaluated and the feasibility of TOA estimation exploiting the DM-RS is demonstrated by means of experiments using real DM-RS signals generated by an LTE module. A second original contribution is given by AOA estimation. In particular, the independence of AOA estimation with respect to uplink and downlink transmission is verified. According to this result, the performance of IAA-APES and SAGE in real-world AOA experiments is evaluated in the downlink scenarios. Based on the overall results, we conclude that the proposed radio localization method, exploiting the uplink Demodulation Reference Signal (DM-RS), can be extended also to joint TOA, AOA using SAGE, for hybrid localization techniques. We can also conclude that the proposed method can be easily extended to downlink transmission exploiting the cell specific reference signal (CRS)

Estudo de formas de onda e conceção de algoritmos para operação conjunta de sistemas de comunicação e radar

The focus of this thesis is the processing of signals and design of algorithms that can be used to enable radar functions in communications systems. Orthogonal frequency division multiplexing (OFDM) is a popular multicarrier modulation waveform in communication systems. As a wideband signal, OFDM improves resolution and enables spectral efficiency in radar systems, while also improving detection performance thanks to its inherent frequency diversity. This thesis aims to use multicarrier waveforms for radar systems, to enable the simultaneous operation of radar and communication functions on the same device. The thesis is divided in two parts. The first part, studies the adaptation and application of other multicarrier waveforms to radar functions. At the present time many studies have been carried out to jointly use the OFDM signal for communication and radar functions, but other waveforms have shown to be possible candidates for communication applications. Therefore, studies on the evaluation of the application of these same signals to radar functions are necessary. In this thesis, to demonstrate that other multicarrier waveforms can overcome the OFDM waveform in radar/communication (RadCom) systems, we propose the adaptation of the filter bank multicarrier (FBMC), generalized frequency division multiplexing (GFDM) and universal filtering multicarrier (UFMC) waveforms for radar functions. These alternative waveforms were compared performance-wise regarding achievable target parameter estimation performance, amount of residual background noise in the radar image, impact of intersystem interference and flexibility of parameterization. In the second part of the thesis, signal processing techniques are explored to solve some of the limitations of the use of multicarrier waveforms for RadCom systems. Radar systems based on OFDM are promising candidates for future intelligent transport networks. Exploring the dual functionality enabled by OFDM, we presents cooperative methods for high-resolution delay-Doppler and direction-of-arrival estimation. High-resolution parameter estimation is an important requirement for automotive radar systems, especially in multi-target scenarios that require reliable target separation performance. By exploring the cooperation between vehicles, the studies presented in this thesis also enable the distributed tracking of targets. The result is a highly accurate multi-target tracking across the entire cooperative vehicle network, leading to improvements in transport reliability and safety.O foco desta tese é o processamento de sinais e desenvolvimento de algoritmos que podem ser utilizados para a habilitar a função de radar nos sistemas de comunicação. OFDM (Orthogonal Frequency Division Multiplexing) é uma forma de onda com modulação multi-portadora, popular em sistemas de comunicação. Para sistemas de radar, O OFDM melhora a resolução e fornece eficiência espectral, além disso sua diversidade de frequências melhora o desempenho na detecção do radar. Essa tese tem como objetivo utilizar formas de onda multi-portadoras para sistemas de radar, possibilitando a operação simultânea de funções de radar e de comunicação num mesmo dispositivo. A tese esta dividida em duas partes. Na primeira parte da tese são realizados estudos da adaptabilidade de outras formas de onda multi-portadora para funções de radar. Nos dias atuais, muitos estudos sobre o uso do sinal OFDM para funções de comunicação e radar vêm sendo realizados, no entanto, outras formas de onda mostram-se possíveis candidatas a aplicações em sistemas de comunicação, e assim, avaliações para funções de sistema de radar se tornam necessárias. Nesta tese, com a intenção de demonstrar que formas de onda multi-portadoras alternativas podem superar o OFDM nos sistemas de Radar/comunicação (RadCom), propomos a adaptação das seguintes formas de onda: FBMC (Filter Bank Multicarrier); GFDM (Generalized Frequency Division Multiplexing); e UFMC (Universal Filtering Multicarrier) para funções de radar. Também produzimos uma análise de desempenho dessas formas de onda sobre o aspecto da estimativa de parâmetros-alvo, ruído de fundo, interferência entre sistemas e parametrização do sistema. Na segunda parte da tese serão explorados técnicas de processamento de sinal de forma a solucionar algumas das limitações do uso de formas de ondas multi-portadora para sistemas RadCom. Os sistemas de radar baseados no OFDM são candidatos promissores para futuras redes de transporte inteligentes, porque combinam funções de estimativa de alvo com funções de rede de comunicação em um único sistema. Explorando a funcionalidade dupla habilitada pelo OFDM, nesta tese, apresentamos métodos cooperativos de alta resolução para estimar o posição, velocidade e direção dos alvos. A estimativa de parâmetros de alta resolução é um requisito importante para sistemas de radar automotivo, especialmente em cenários de múltiplos alvos que exigem melhor desempenho de separação de alvos. Ao explorar a cooperação entre veículos, os estudos apresentados nesta tese também permitem o rastreamento distribuído de alvos. O resultado é um rastreamento multi-alvo altamente preciso em toda a rede de veículos cooperativos, levando a melhorias na confiabilidade e segurança do transporte.Programa Doutoral em Telecomunicaçõe

Carrier frequency offset estimation for orthogonal frequency division multiplexing systems

Orthogonal frequency division multiplexing (OFDM) is an attractive modulation scheme used in wideband communications because it essentially transforms the frequency selective channel into a flat fading channel. Furthermore, the combination of multiple-input multiple-output (MIMO) signal processing and OFDM seems to be an ideal solution for supporting reliable high data rate transmission for future wireless communication systems. However, despite the great advantages OFDM systems offer, such systems present challenges of their own. One of the most important challenges is carrier frequency offset (CFO) estimation, which is crucial in building reliable wireless communication systems. In this thesis, we consider CFO estimation for the downlink and uplink OFDM systems. For the downlink channel, we focus on blind schemes where the cost functions are designed such that they exploit implicit properties associated with the transmitted signal where no training signal is required. By taking the unconditional maximum likelihood approach, we propose a virtual subcarrier based blind scheme for MIMO-OFDM systems in the presence of spatial correlation. We conclude that the presence of spatial correlation does not impact the CFO estimation significantly. We also propose a CFO estimator for OFDM systems with constant modulus signaling and extend it to MIMO-OFDM systems employing orthogonal space-time block coding. The curve fitting method is used which gives a closed-form expression for CFO estimation. Therefore, the proposed scheme provides an excellent trade-off between complexity and performance as compared to prominent existing estimation schemes. Furthermore, we design a blind CFO estimation scheme for differentially modulated OFDM systems based on the finite alphabet constraint. It can achieve better performance at high signal-to-noise ratios (SNRs) at the expense of some additional computational complexity as compared to the schemes based on the constant modulus constraint. The constrained Cramer-Rao lower bound (CRLB) is also derived for the blind estimation scheme. As for the uplink channel, which is a more challenging problem, we propose two training aided schemes. One is based on a scalar extended Kalman filter (EKF) and the other one is on the variable projection (VP) algorithm. For both schemes, we assume that the system uses an arbitrary subcarrier assignment scheme, which is more involved than the other two schemes, namely block and interleaved subcarrier assignment scheme. In the first scheme, to apply the scalar EKF algorithm, we represent the measurement equation as a function of a scalar state, i.e., each user's CFO, in lieu of a state vector which consists of both CFO and channel coefficients by replacing the unknown channel coefficients with a nonlinear function of CFO. This proposed scheme can achieve the CRLB at high SNR for two users with a complexity lower than that of the alternating-projection method. In the second scheme, the VP algorithm is used for CFO estimation which is followed with a robust minimum mean square error (MMSE) estimator for channel estimation. In the VP algorithm, the nonlinear least square cost function is optimized numerically by updating the CFOs and channel coefficients separately at each iteration. We demonstrate that this proposed scheme is superior to the existing methods in terms of convergence speed, computational complexity and estimation performance

Carrier frequency offset recovery for zero-IF OFDM receivers

As trends in broadband wireless communications applications demand faster development cycles, smaller sizes, lower costs, and ever increasing data rates, engineers continually seek new ways to harness evolving technology. The zero intermediate frequency receiver architecture has now become popular as it has both economic and size advantages over the traditional superheterodyne architecture. Orthogonal Frequency Division Multiplexing (OFDM) is a popular multi-carrier modulation technique with the ability to provide high data rates over echo ladened channels. It has excellent robustness to impairments caused by multipath, which includes frequency selective fading. Unfortunately, OFDM is very sensitive to the carrier frequency offset (CFO) that is introduced by the downconversion process. The objective of this thesis is to develop and to analyze an algorithm for blind CFO recovery suitable for use with a practical zero-Intermediate Frequency (zero-IF) OFDM telecommunications system. A blind CFO recovery algorithm based upon characteristics of the received signal's power spectrum is proposed. The algorithm's error performance is mathematically analyzed, and the theoretical results are verified with simulations. Simulation shows that the performance of the proposed algorithm agrees with the mathematical analysis. A number of other CFO recovery techniques are compared to the proposed algorithm. The proposed algorithm performs well in comparison and does not suffer from many of the disadvantages of existing blind CFO recovery techniques. Most notably, its performance is not significantly degraded by noisy, frequency selective channels

Subspace based carrier frequency offset estimations for OFDM systems

Master'sMASTER OF ENGINEERIN