Contributions to Frequency Offset and Time Delay Estimation

The interest in reliable high rate and efficient communication is ever increasing. In this particular project we look at two diverse issues in such devices, as well as their feasible solutions. In recent times orthogonal frequency division multiplexing (OFDM) moved from a promising data transmission technique to become a popular technique utilized in a number of present and future standards. The key attractive property of OFDM is that it is inherently resilient to multipath reflections due to its long symbol time. However, this comes at the expense of a relatively high sensitivity to carrier frequency offsets (CFOs). Within this thesis we present a method for CFO estimation in orthogonal frequency division multiplexing OFDM systems that draws on choosing the spectral minimas within so-called null or virtual subcarriers embedded in the spectrum. The spectral minimas are found iteratively over a number of symbols and is therefore mainly ideal for frequency offset tracking or in systems where an estimate is not immediately required, for instance in Radio or television broadcasting systems. However, complexity wise the estimator is comparatively simple to apply and it doesn’t require any extra redundancy beside a nonmodulated subcarrier. The estimator efficiency is analyzed both in a channel with additive white Gaussian noise and in a frequency selective channel environment. An objective for several years has been to be able to implement wherever possible of a radio system in the digital domain, the final objective being so called software defined radio (SDR). One significant component of an SDR receiver is the high-speed analog-to-digital converter(ADC) and one way to achieve this objective is to apply numerous parallel, time-interleaved, ADCs. Such ADCs are, however, sensitive to sampling instant offsets, DC offset and gain offset. This project report also talks about iterative time-delay estimators (TDEs) making use of adjustable fractional-delay filters. The TDEs could for instance be utilized to estimate and calibrate the relative delay between the ADCs including the time interleaved ADC. TDEs using a direct correlator and an average squared difference function are compared….


1 Introduction
1.1 Applications
1.1.1 Multicarrier Modulation
1.1.2 Time-Interleaved Analog to Digital Converters
1.2 Publications
1.3 Outline of the Thesis

2 Introduction to OFDMCFO Estimation
2.1 Introduction
2.2 System Model
2.3 PreviousWork
2.3.1 Maximum Likelihood Estimation
2.3.2 Time Domain Estimators
2.3.3 Frequency Domain Estimators
3 CFO Estimation Using Null Subcarriers
3.1 Introduction
3.2 CFO Estimation Using Null Subcarriers
3.2.1 Proposed Estimator
3.3 Complexity
3.4 Simulations
3.4.1 Convergence
3.4.2 Performance
3.4.3 Multipath Environment
3.5 Conclusions
4 Introduction to Time Delay Estimation
4.1 Introduction
4.2 Time Delay Estimation
4.2.1 Cost Functions
4.2.2 Interpolation
4.2.3 Maximization/Minimization Methods
4.2.4 Fundamental Performance Limits
5 Time Delay Estimation Using Adjustable FD Filters
5.1 Introduction
5.2 Locating the Minimum/Maximum
5.2.1 Steepest Descent
5.2.2 Newton-Raphson
5.2.3 Recursive Gauss-Newton
5.3 Interpolation Methods
5.3.1 First Order Linear Interpolation
5.3.2 Interpolation Using FIR FD Filters
5.3.3 Interpolation Using All-pass IIR FD Filters
5.4 Error Analysis
5.4.1 Estimator Offset
5.4.2 Estimator Variance
5.5 Complexity….

Source: Linköping University

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