Contributions to Frequency Offset and Time Delay Estimation

University dissertation from Institutionen för systemteknik

Abstract: The demand for reliable high rate and efficient communication is ever increasing. In this thesis we look at two different problems in such systems, and their possible solutions.In recent years orthogonal frequency division multiplexing (OFDM) has gone from a promising data transmission technique to become a mainstream technique used in several current and future standards. The main attractive property of OFDM is that it is inherently resilient to multipath reflections because of its long symbol time. However, this comes at the cost of a relatively high sensitivity to carrier frequency offsets (CFOs).In this thesis we present a technique for CFO estimation in OFDM systems that is based on locating 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 useful for frequency offset tracking or in systems where an estimate is not immediately required, such as in TV or radio broadcasting systems. However, complexity wise the estimator is relatively easy to implement and it does not need any extra redundancy beside a nonmodulated subcarrier. The estimator performance is studied both in a channel with additive white Gaussian noise and in a frequency selective channel environment.A goal for many years has been to be able to implement as much as possible of a radio system in the digital domain, the ultimate goal being so called software defined radio (SDR). One important part of an SDR receiver is the high speed analog-to-digital converter(ADC) and one path to reach this goal is to use a number of parallel, time-interleaved, ADCs. Such ADCs are, however, sensitive to sampling instant offsets, DC offset and gain offset.This thesis also discusses iterative time-delay estimators (TDEs) utilizing adjustable fractional-delay filters. The TDEs could for example be used to estimate and calibrate the relative delay between the ADCs comprising the time interleaved ADC. TDEs using a direct correlator and an average squared difference function are compared. Furthermore, an analysis of the effects of the batch length dependence is presented.