We present the joint maximum likelihood (ML) symbol-time and carrier-frequency offset estimator in orthogonal frequency-division multiplexing (OFDM) systems. Redundant information contained within the cyclic prefix enables this estimation without additional pilots. Simulations show that the frequency estimator may be used in a tracking mode and the time estimator in an acquisition mode.

/pdf/ml-estimation-of-time-and-frequency-offset-in-ofdm-systems-22x9rvpv87.pdf

ML estimation of time and frequency offset in OFDM systems

The design and optimization of multicarrier communications systems often involve a maximization of the total throughput subject to system resource constraints. The optimization problem is numerically difficult to solve when the problem does not have a convexity structure. This paper makes progress toward solving optimization problems of this type by showing that under a certain condition called the time-sharing condition, the duality gap of the optimization problem is always zero, regardless of the convexity of the objective function. Further, we show that the time-sharing condition is satisfied for practical multiuser spectrum optimization problems in multicarrier systems in the limit as the number of carriers goes to infinity. This result leads to efficient numerical algorithms that solve the nonconvex problem in the dual domain. We show that the recently proposed optimal spectrum balancing algorithm for digital subscriber lines can be interpreted as a dual algorithm. This new interpretation gives rise to more efficient dual update methods. It also suggests ways in which the dual objective may be evaluated approximately, further improving the numerical efficiency of the algorithm. We propose a low-complexity iterative spectrum balancing algorithm based on these ideas, and show that the new algorithm achieves near-optimal performance in many practical situations

/pdf/dual-methods-for-nonconvex-spectrum-optimization-of-45uov3hj65.pdf

Dual methods for nonconvex spectrum optimization of multicarrier systems

We present and analyze low-rank channel estimators for orthogonal frequency-division multiplexing (OFDM) systems using the frequency correlation of the channel. Low-rank approximations based on the discrete Fourier transform (DFT) have been proposed, but these suffer from poor performance when the channel is not sample spaced. We apply the theory of optimal rank-reduction to linear minimum mean-squared error (LMMSE) estimators and show that these estimators, when using a fixed design, are robust to changes in channel correlation and signal-to-noise ratio (SNR). The performance is presented in terms of uncoded symbol-error rate (SER) for a system using 16-quadrature amplitude modulation (QAM).

/pdf/ofdm-channel-estimation-by-singular-value-decomposition-2q9twaj1cb.pdf

OFDM channel estimation by singular value decomposition

Channel estimation techniques for OFDM systems based on a pilot arrangement are investigated. Channel estimation based on a comb type pilot arrangement is studied through different algorithms for both estimating the channel at pilot frequencies and interpolating the channel. Channel estimation at pilot frequencies is based on LS and LMS methods while channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to the time domain by means of IDFT (inverse discrete Fourier transform), zero padding and going back to the frequency domain by DFT (discrete Fourier transform). In addition, channel estimation based on a block type pilot arrangement is performed by sending pilots in every sub-channel and using this estimation for a specific number of following symbols. We have also implemented a decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rates with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multipath Rayleigh fading and AR based fading channels as channel models.

/pdf/channel-estimation-techniques-based-on-pilot-arrangement-in-usn3udu2w0.pdf

Channel estimation techniques based on pilot arrangement in OFDM systems

The QRS complex is the most striking waveform within the electrocardiogram (ECG). Since it reflects the electrical activity within the heart during the ventricular contraction, the time of its occurrence as well as its shape provide much information about the current state of the heart. Due to its characteristic shape it serves as the basis for the automated determination of the heart rate, as an entry point for classification schemes of the cardiac cycle, and often it is also used in ECG data compression algorithms. In that sense, QRS detection provides the fundamentals for almost all automated ECG analysis algorithms. Software QRS detection has been a research topic for more than 30 years. The evolution of these algorithms clearly reflects the great advances in computer technology. Within the last decade many new approaches to QRS detection have been proposed; for example, algorithms from the field of artificial neural networks genetic algorithms wavelet transforms, filter banks as well as heuristic methods mostly based on nonlinear transforms. The authors provide an overview of these recent developments as well as of formerly proposed algorithms.

/pdf/the-principles-of-software-qrs-detection-1vgts9f7g6.pdf

The principles of software QRS detection

The densification of mobile networks in order to meet increased capacity demands is ongoing, needed and costly. A few papers have been published based on the insight that the fixed broadband networks offer a multitude of sites, for instance our homes, for potential small cell deployment providing backhaul capacity and power without site costs. However, in order to reach economical large-scale benefits, we explore the case when radio systems are deployed in coexistence with DSL. In this paper, we establish the feasibility of such a concept under constraints invoked by state-of-the-art and emerging systems (3GPP, VDSL2 and G.fast) and make statements about the required architecture. We also point out that the enthusiasm of previously published results should be lowered a notch.

LTE over copper — Potential and limitations

This p aper describes an a rrangement that uses AD and DA conversion and a computer in order to preshape a transmitted ultrasonic wave. Normally the ultrasonic transducer is excited by a very short e lectrical pulse. Using a DAC, the ultrasonic transducer can be excited by any desired waveform e g the digitally determined inverse filter, chirp signals etc. The desired excitation waveform is stored in a high speed memory. It is shifted out via the DAC to an amplifier and thence to the transducer. The ADC, which is triggered by the DAC equipment, converts one sample per sweep into 12 bits. Therefore, actual sample rate is low. But the virtual sample rate limited by the sample and hold circuit can reach 15 MHz. The ADC and DAC are connected to a microcomputer system that determines the desired excitation waveform. Various waveforms can be stored and it is then possible to change the excitation waveforms interactively during clinical examinations.

Digital Preshaping of Ultrasonic Signals: Equipment and Applications

A recent paper1 discusses prediction and interpolation for data compression of ECG. This communication points out that both methods can be viewed as linear filtering. They are therefore equivalent and give the same result in terms of the amount of data compression achieved.

/pdf/comments-on-compression-of-the-ecg-by-prediction-or-k175v6cjwu.pdf

Comments on "Compression of the ECG by Prediction or Interpolation and Entropy Encoding"

Solved Problem: Symbol arrival time and carrier frequency offset estimation is addressed in an OFDM system using symbol extension with a cyclic prefix and employing pulse shaping. Originality of the work: Previous work has presented a method that without the use of pilots exploits the inherent correlation in the cyclically extended OFDM symbol in Gaussian channels. This contribution investigates what accuracy is achievable with this method in systems using pulse shaping. New results: The performance of the joint Maximum Likelihood estimator for a time and frequency offset is evaluated for different pulse shapes.

http://documents.irevues.inist.fr/bitstream/handle/2042/12795/262_402.pdf;sequence=1

Synchronization in OFDM Systems Sensitivity to the Choice of Pulse Shape

In this paper we discuss three signal models and respective Maximum Likelihood estimators of time and frequency osets. We consider implementation structures for these estimators. Furthermore, we di ...

http://www.sm.luth.se/csee/sp/research/conference/bbb97c.pdf

Per Ola Börjesson

Papers

LTE over copper — Potential and limitations

Digital Preshaping of Ultrasonic Signals: Equipment and Applications

Comments on "Compression of the ECG by Prediction or Interpolation and Entropy Encoding"

Synchronization in OFDM Systems Sensitivity to the Choice of Pulse Shape

On synchronization in an OFDM based UMTS proposal