José A. B. Gerald

Visual neuroprosthesis: a non invasive system for stimulating the cortex

This paper describes a complete visual neuroprosthesis wireless system designed to restore useful visual sense to profoundly blind people. This visual neuroprosthesis performs intracortical microstimulation through one or more arrays of microelectrodes implanted into the primary visual cortex. The whole system is composed by a primary unit located outside the body and a secondary unit, implanted inside the body. The primary unit comprises a neuromorphic encoder, a forward transmitter, and a backward receiver. The developed neuromorphic encoder generates the spikes to stimulate the cortex by approximating the spatio-temporal receptive fields characteristic response of ganglion cells. Power and stimuli information are carried to inside the cranium by means of a low-coupling transformer, which establishes a wireless inductive link between the two units. The secondary unit comprises a forward receiver, microelectrode stimulation circuitry and a backward transmitter that is used to monitor the implant. Address event representation is used for communicating spike events. Data is modulated with binary frequency-shift keying and differential binary phase-shift keying in the forward and in the backward directions, respectively. A prototype of the proposed system was developed and tested. Experimental results show that the spikes to stimulate the visual cortex are accurately generated and that the efficiency of the inductive link is relatively high, about 28% in average for 1 cm intercoil distance providing a power of about 50 milliwatts to the secondary implanted unit. Application specific integrated circuits were designed for this secondary unit, showing that, with current technology, it is possible to implement such a unit, respecting the power constraints.

Dealing With Unknown Impedance and Impulsive Noise in the Power-Line Communications Channel

Power-line communication (PLC) allows establishing digital communications without adding any new wires. It will turn ones house or neighborhood grid into a smart grid. PLC has some issues, namely, high noise at low frequencies and varying characteristic impedance. This paper addresses these issues to improve the signal-to-noise ratio by increasing the signal or reducing the noise. PLC MODEMs are subject to legislations that limit the signals in the line. The radiated signal is proportional to the current, but not to the input current, since the current forms a standing wave along the line. However, better performance can be achieved if the input current is measured. The receiver circuit of the transmitting MODEM can be used to estimate the input impedance. This paper presents a technique to use this new information to achieve better performance and to follow legislation changes in the band above 30 MHz. A study of the viability of using impulsive noise reduction techniques to further increase performance is also presented. The short noise pulses result in high correlation between the noises in different carriers. Impulse position detection should result in an increase in capacity.

New Normalized LMS Algorithms Based on the Kalman Filter

While the LMS algorithm and its normalized version (NLMS), have been thoroughly used and studied. Connections between the Kalman filter and the RLS algorithm have been established however, the connection between the Kalman filter and the LMS algorithm has not received much attention. By linking these two algorithms, a new normalized Kalman based LMS (KLMS) algorithm can be derived that has some advantages to the classical one. Their stability is guaranteed since they are a special case of the Kalman filter. More, they suggests a new way to control the step size, that results in good convergence properties for a large range of input signal powers, that occur in many applications. They prevent high measurement noise sensitivity that may occur in the NLMS algorithm for low order filters, like the ones used in OFDM equalization systems. In these paper, different algorithms based on the correlation form, information form and simplified versions of these are presented. The simplified form maintain the good convergence properties of the KLMS with slightly lower computational complexity.

Modeling and optimization of the access impedance of power line channels

Digital networks can be established using the same set of wires that is used to distribute the power signal through our homes, the power-line channel (PLC). This networks have no new wires. However, the PLC channel has highly varying characteristics that need to be taken into account, namely the equivalent transmission line characteristic impedance and input impedance. The impedance seen at the input of the power-line channel varies with frequency, time and from line to line. This means that for the same voltage signal the injected current will vary. The current flowing in the channel is the main source of radiated electromagnetic interference (EMI), and imposes limits on the injected signal. The amplitude of the current in the line is not equal to the input current. The current peaks in the line can be minimized by minimizing the current of the positive traveling wave and it will be shown that this can be accomplished if the transmitter is coupled through a matched resistor (to the line impedance) or adequate signal processing. The signal processing method allows to easily change the access impedance. At the output of the channel the signal to noise ratio is independent of charge resistance. A system with input impedance estimation and minimization of the positive wave amplitude was simulated.

Auxiliary Noise Power Scheduling Algorithm for Active Noise Control with Online Secondary Path Modeling and Sudden Changes

In active noise control (ANC), online secondary path modeling can be achieved by adding a small auxiliary noise signal. If the secondary path changes are slow, then this signal can be low, but keeping the stability of the ANC system with sudden secondary path changes requires higher values for this signal, so auxiliary noise power scheduling is required. The proposed algorithm deals well with sudden (and strong) changes, due to the fast convergence of the secondary path model. It is compared with other similar algorithms in the literature. The ability to deal with different physical conditions without changing the algorithms parameters is also compared. The proposed algorithm increases the auxiliary noise to levels close to the acoustic noise when no cancelation is done and reduces it to lower levels when noise cancelation is being performed. In addition variants of the LMS that improve performance are proposed.

A New Type of Normalized LMS Algorithm Based on the Kalman Filter

While the LMS algorithm and its normalized version (NLMS), have been thoroughly used and studied, and connections between the Kalman filter and the RLS algorithm have bean established, the connection between the Kalman filter and the LMS algorithm has not received much attention. By linking these two algorithms, a new normalized LMS algorithm can be derived that has some advantages to the classical one. Firstly, its stability is guaranteed since it is a special case of the Kalman filter. Secondly, it suggests a new way to control the step size that results in optimum convergence for a large range of input signal powers, that occur in many applications. Finally, it prevents measurement noise amplification that occurs in the NLMS algorithm for low order filters, like the ones used in OFDM equalization systems.

RF-link for cortical neuroprosthesis

This paper describes an RF-link system suitable for intracortical neuroprosthesis. The wireless coupling minimizes the risk of infection and improves the patient health level. Data and power are transmitted by means of an RF low-coupling transformer. Binary FSK modulation is used in the forward link (from outside to implanted prosthesis) and BPSK is used in the backward link (implanted prosthesis to outside). Experimental results are presented, and important concerns regarding the magnetic coupling are addressed.

New PN even balanced sequences for spread-spectrum systems

A new class of pseudonoise even balanced (PN-EB) binary spreading sequences is derived from existing classical odd-length families of maximum-length sequences, such as those proposed by Gold, by appending or inserting one extra-zero element (chip) to the original sequences. The incentive to generate large families of PN-EB spreading sequences is motivated by analyzing the spreading effect of these sequences from a natural sampling point of view. From this analysis a new definition for PG is established, from which it becomes clear that very high processing gains (PGs) can be achieved in band-limited direct-sequence spread-spectrum (DSSS) applications by using spreading sequences with zero mean, given that certain conditions regarding spectral aliasing are met. To obtain large families of even balanced (i.e., equal number of ones and zeros) sequences, two design criteria are proposed, namely the ranging criterion (RC) and the generating ranging criterion (GRC). PN-EB sequences in the polynomial range are derived using these criteria, and it is shown that they exhibit secondary autocorrelation and cross-correlation peaks comparable to the sequences they are derived from. The methods proposed not only facilitate the generation of large numbers of new PN-EB spreading sequences required for CDMA applications, but simultaneously offer high processing gains and good despreading characteristics in multiuser SS scenarios with band-limited noise and interference spectra. Simulation results are presented to confirm the respective claims made.

IIR echo canceller using a dual error criterion and incorporating a lattice structure

Abstract A new recursive echo cancellor (EC) for data transmission is proposed, where the output-error criterion and the equation-error criterion are used simultaneously. The combination of these two error criteria allows a fast initial convergence and leads to error surfaces with a single (global) minimum corresponding to unbiased parameter estimates, provided that the EC structure is not underspecified. The proposed EC includes an all-pole lattice structure, which greatly simplifies the control of the poles position to ensure stability. The transversal-to-lattice conversion is used to update the reflection coefficients, but the total amount of signal processing is reduced, because those coefficients need not be updated at every iteration. The results of a computer simulation show that the new echo canceller, when compared to commonly used conventional structures, can give a better performance.

An adaptive IIR echo canceller combining output-error and equation-error criteria

An echo canceller structure which permits the combined use of the equation error and the output error criteria in the adaptation of the coefficients is presented. A method is described for the smooth transition between the two criteria in such a way that the advantages of both are retained and their drawbacks are eliminated. Further improvement is obtained by using lattice structures in the realization of some of the cancellers blocks. Computer simulation results are presented which confirm the superior performance of the echo canceller with respect to other realizations.<<ETX>>