G. Castellini

A continuously adaptive MLSE receiver for mobile communications: algorithm and performance

The paper presents a Euclidean distance maximum likelihood sequence estimation (MLSE) receiver, based on the Viterbi algorithm (VA), suitable for fading and noisy communications channels, as that specified by the Group Special Mobiles (GSM). In a mobile cellular system, the fast varying channel characteristics, due to the fading and Doppler effects, require adaptive methods to update the channel coefficients to the MLSE receiver. The proposed technique continuously estimates the channel characteristics directly within the metric calculation of the VA. At each step of the VA, the sequence associated to the path with the best metric value (minimum-survivor method) among the survivor paths is used to update the channel estimate (employing conventional adaptive algorithms) throughout the entire informative sequence. However, the detection of the transmitted data sequence is performed by the VA only at the end of each burst. The proposed technique allows simpler receiver implementation and the simulation results show a good performance of this adaptive MLSE receiver in typical GSM environments.

Design of a digital MLSE receiver for mobile radio communications

A digital receiver suitable for use in the future GSM pan-European cellular system is described. The receiver uses a maximum-likelihood sequence estimation (MLSE) implemented by means of the Viterbi algorithm to compensate for the heavy selective distortions caused by multipath propagation. The performance of the receiver is evaluated through a channel simulator suitable for mobile communications. The results obtained show the good behavior characteristics for the receiver in different modes of operation. Easy implementation of the device using VLSI technology is expected.<<ETX>>

A within-burst adaptive MLSE receiver for mobile TDMA cellular systems

A simplified structure concerning the implementation of an adaptive digital receiver suitable for the pan-European cellular system Group Special Mobile (GSM) is described. In the GSM system, the performance of the received information is strongly affected by the multipath typical of the mobile communications channel. The proposed maximum-likelihood sequence estimation (MLSE) receiver based on the Viterbi algorithm compensates for selective distortion due to the multipath propagation and Doppler shift also in the case of fast channel variations using an adaptive tracking mode method.<<ETX>>

Computer simulated benchmarks of synthetic aperture techniques for vascular ultrasonography

Abstract Ultrasound investigations and related techniques strongly depend on the imaged structure: in particular our attention is focused on vascular ultrasound imaging. In this work a high-resolution imaging technique for vascular ultrasound diagnosis has been developed, with emphasis on the investigation of the carotid vessel. This application is relevant due to the stroke risk factor associated with pathological situations. Our study envisages the possibility of more accurate measurements of the lesions extent by the application of a synthetic aperture imaging technique. An imaging system based on a standard linear array of ultrasonic transducers has been simulated, exploiting the signal acquisition from multi-offset transducers. The synthetic images are quantitatively compared with in vitro carotid ultrasound real images, obtained with a high resolution B-mode scanner. By an experimental procedure the actual transverse carotid sections have been obtained and the digitized video camera profiles have been used to compute the backscattered field under the weak scattering assumption. The set of synthetic radiofrequency signals has been utilized for the reconstruction of synthetic aperture images and B-mode images as well. Finally the effects due to ultrasound propagation and instrumentation characteristics have been discussed and the implications on the image quality and the carotid contour determination have been analysed.

Performance of Adaptive Receivers in a Mobile Environment

Mobile communications are going to be one of the fastest growing fields in the future, due to the high number of users.

Carotid lesion characterization by synthetic-aperture-imaging techniques with multioffset ultrasonic probes

This paper explores the applications of a high-resolution imaging technique to vascular ultrasound diagnosis, with emphasis on investigation of the carotid vessel. With the present diagnostic systems, it is difficult to measure quantitatively the extension of the lesions and to characterize the tissue; quantitative images require enough spatial resolution and dynamic to reveal fine high-risk pathologies. A broadband synthetic aperture technique with multi-offset probes is developed to improve the lesion characterization by the evaluation of local scattering parameters. This technique works with weak scatterers embedded in a constant velocity medium, large aperture, and isotropic sources and receivers. The features of this technique are: axial and lateral spatial resolution of the order of the wavelength, high dynamic range, quantitative measurements of the size and scattering intensity of the inhomogeneities, and capabilities of investigation of inclined layer. The evaluation of the performances in real condition is carried out by a software simulator in which different experimental situations can be reproduced. Images of simulated anatomic test-objects are presented. The images are obtained with an inversion process of the synthesized ultrasonic signals, collected on the linear aperture by a limited number of finite size transducers.

Neural receiver for CPM signals

Neural networks have been successfully applied in many fields thanks to their learning and generalization capabilities and to the parallel processing and fault tolerance properties. Typical applications concern images processing, pattern recognition and digital signal processing, such as adaptive filtering and channel equalization. The authors propose the use of neural networks as digital receivers for continuous phase modulation (CPM). Simulation results refer to the European GSM digital cellular radio system. The neural receiver performance has been evaluated for coherent detection, considering an additive white Gaussian noise (AWGN) channel and compared with a maximum likelihood sequences estimator (MLSE) receiver based on the Viterbi algorithm. The paper also presents a hardware implementation of the proposed network based on a digital signal processor (DSP) and on a programmable gate array (PGA).<<ETX>>

A multiprocessor structure based on commercial DSP

The authors present a multi-DSP (digital signal processing) system based on commercial chips. The structure is a MIMD (multiple-instruction multiple-data) machine with a data-driven organization. A locally parallel link based on a memory shared between two adjacent nodes assures the asynchronous exchange of data. The system is also characterized by a global serial channel useful for program-loading purposes and noncritical data paths. A mathematical model is used to evaluate the performance of the multi-DSP system. The apparatus is used in ultrasound Doppler parameter estimation but the processing architecture is suitable for general real-time applications.<<ETX>>