Lars Lundheim

Optimal Pulses Robust to Carrier Frequency Offset for OFDM/QAM Systems

Optimal pulses with minimum ICI power at a given CFO point for OFDM/QAM systems are found analytically. A lower bound of the ICI power is introduced as a measure of the robustness to CFO for a given pulse. Numerical comparison verifies that the optimal pulse is more robust to CFO than previously suggested pulses at the selected CFO point.

New Methods for Blind Fine Estimation of Carrier Frequency Offset in OFDM/OQAM Systems

Carrier frequency offset (CFO) in OFDM/OQAM systems can be estimated based on the conjugate correlation function of the received signals. Traditionally this is done before demodulation. We present new methods based on the subchannel signals. The proposed estimators are robust to multipath effects due to the narrowband property of the subchannels. The performance of the estimators is evaluated by asymptotic analysis and simulation results. Our results show that methods based on estimation in subchannels have better performance than estimators operating before demodulation

Design of robust pulses to carrier frequency offset for OFDM/OQAM system

The high sensitivity to carrier frequency offset of OFDM systems compared to single carrier systems is well known. In this paper we study how this can be remedied for OFDM/OQAM systems by appropriate pulse shaping. Building on earlier methods for minimizing out-of-band energy, a procedure for pulse shape design is derived. The new pulses are optimal with respect to interference in OFDM/OQAM systems. It is also demonstrated that, in this respect, OFDM/OQAM outperforms OFDM/QAM systems using rectangular pulses.

SPC08-4: Blind Carrier Frequency Offset Estimation for OFDM/OQAM Systems Based on Subchannel Signals

New blind carrier frequency offset (CFO) estimation methods based on the correlation function of the subchannel signals are presented for OFDM/OQAM systems. The proposed estimators are robust to multipath effects due to the narrowband property of the subchannels in OFDM systems. The performance of the estimators is evaluated by asymptotic analysis and simulation results. Our results show that methods based on estimation in subchannels have better performance than method based on the signal before demodulation.

Distortion Modeling and Compensation in Step Frequency Radars

Nonideal behavior of ultra wideband (UWE) step frequency radars cause artifacts in the received signal. Several such artifacts are represented by a linear, frequency domain model presented here. The model includes both spurious frequencies, linear distortion, oscillator leakage, and crosstalk between the transmitter and receiver as well as IQ imbalance. A class of compensation methods based on this model using parameter estimation from calibration measurements is suggested. These compensation methods represent a generalization of a well-known approach for equalization of linear distortion in step frequency radars and have been tried out and confirmed by laboratory tests.

A relative rate utility based distributed power allocation algorithm for Cognitive Radio Networks

In an underlay Cognitive Radio Network, multiple secondary users coexist geographically and spectrally with multiple primary users under a constraint on the maximum received interference power at the primary receivers. Given such a setting, one may ask “how to achieve maximum utility benefit at the secondary users given the imposed interference temperature constraint”? In an attempt at answering this question, we introduce a measure of the marginal secondary utility per unit primary interference (termed as relative rate utility) and propose a distributed algorithm that tries to maximize this measure. We present selected computer based Monte-Carlo simulation results, demonstrating the effectiveness of the introduced measure, and the improved performance of the proposed algorithm.

An Efficient Design Methodology for Constant Power Link Adaptation Schemes in Short-Range Scenarios

The energy-efficient design of short-distance wireless communication systems requires the inclusion of circuit energy consumption in the systems total energy budget. We consider the design of constant power link adaptation schemes in which circuit energy consumption is included in the total energy budget and in which the systems instantaneous bit error rate (BER) must never exceed a predefined threshold. We show how-in numerous cases of practical importance-the dimensionality of the associated nonlinear optimization problem can be reduced from N + 1 to 1, where N denotes the number of available transmission schemes. The main contribution-a methodology that significantly simplifies the design of such constant power link adaptation schemes-is then illustrated by an example.

Tool for detecting waveform distortions in inverter-based microgrids: A validation study

This paper presents a validation methodology based on data analysis aimed at identifying the cause of atypical behavior observed in a microgrids data. The data of a standalone PV microgrid in Bhutan is used for the analysis. Voltage waveform distortions observed in the data were analyzed by utilizing the Hilbert-Huang Transform (HHT) and the discrete Fourier transform (DFT). When using HHT, a time-varying frequency, new to electrical systems, is revealed by the microgrid data analysis, while the mathematical derivation hints on amplitude modulation. The distortions appeared in the form of a leakage spectrum composed of multiple numbers of constant frequency components when using a DFT-based frequency power spectrum. The different observations from these methods have raised questions of interpretation of their physical meaning. In an attempt to provide a physical meaning and to understand the cause of the time varying frequency revealed by the HHT, an analytical investigation of the micro-grid controller is performed. The results obtained indicate that modern power electronics introduces new types of distortions that can be traced back to the control of the inverters in the microgrid. The analytically obtained results compared with the analysis of simulated and field measurements hint on the existence of a time-varying frequency in the voltage waveforms of the stand-alone microgrid. The results of this paper will lay the foundation for future studies into more accurate diagnosis tools for microgrids to enable affordable access to electricity in isolated communities.

On the Impact of Fixed Point DSP Implementation on Required Channel Estimator Complexity in Communication Receivers

In this paper we discuss the impact that fixed point implementation of DSP units in a communication system chain has on the required complexity of channel estimators. We have restricted our model to a flat-fading channel and linear FIR estimators, under the assumption that finite word length effects in the DSP operations in the communication chain can be modeled as additional additive noise. We have shown that a small increase in this noise can lead to a considerable increase in the required estimator complexity if a given NMSE performance for the channel estimation must be upheld, in particular at medium- to-high CSNRs.

Second Order Taylor Polyphase Reconstruction of Periodic-Nonuniform Samples in Time-Interleaved ADCs

This paper focuses on a filter bank approach for reconstructing periodic-nonuniform samples using second order Taylor polyphase decomposition for a four-channel time-interleaved ADC (TIADC). The proposed method is based on a parametric linear least squares approach to find the coefficients of synthesis filters used for signal reconstruction of periodic-nonuniform samples. The coefficients of synthesis filters found by linear least squares approach satisfy the general condition for an alias-free filter bank.