This paper investigates methods to achieve high-rate data transmission through metallic barriers using ultrasound. Multiple-input-multiple-output-orthogonal frequency division multiplexing (MIMO-OFDM) is employed in conjunction with interference mitigation techniques to reduce throughput-limiting crosstalk. Several crosstalk mitigation strategies are investigated and their theoretical and practical bit-loaded data rates are determined for the general case of A transmitters and A receivers (A×A MIMO). A physical MIMO acoustic-electric channel array is formed using a 40 mm (1.575 in) thick steel barrier with seven pairs of 4 MHz nominal resonant frequency piezoelectric disk transducers, each with 10 mm (0.394 in) diameter. To investigate the effects of crosstalk, the transducers are closely spaced, and each transmitter-receiver pair is coaxially aligned on opposing sides of the metallic barrier. It is shown that, with the use of crosstalk mitigation techniques, the aggregate multichannel theoretical capacity performance scales linearly with the number of channels. This paper also investigates the use of novel power allocation techniques in a MIMO-OFDM acoustic-electric channel, which show significant throughput performance gains over conventional bit-loading and greedy bit-filling techniques. Finally, this paper presents a study on the effects of transducer misalignment on the multichannel theoretical capacity and achievable data transmission rates using bit-loading techniques.

This paper explores the use of multiple communication channels to transmit data at high rates, without physical penetrations, through thick metallic barriers using ultrasound. Two parallel acoustic-electric channels are formed in close proximity utilizing two pairs of coaxially aligned piezoelectric transducers mounted on, and acoustically coupled to, opposing sides of a metal wall. Each channel employs orthogonal frequency division multiplexing (OFDM) which can achieve a high spectral efficiency in frequency selective channels. The two-transmitter-two-receiver MIMO configuration is studied and analytical expressions of channel capacity are determined for the raw channels as well as for several co-channel interference cancellation techniques and they are verified using a Monte-Carlo simulation. It is shown that excessive crosstalk between the channels can lead to marginal increases or decreases in capacity over the single channel when no interference suppression is used. Several interference cancellation structures, including zero forcing, eigenmode transmission, and minimum mean-square error (MMSE) are investigated to mitigate this effect. It is shown that their aggregate capacity may approximately double that of either channel used independently. It is also determined that, in a relatively static MIMO acoustic-electric channel, a minimal complexity adaptive approach such as the Least Mean Squares (LMS) algorithm may be used while achieving a similar capacity to more complex structures.

Mathematical interest in electrical impedance tomography has been strong since the publication of CalderOn's foundational paper. This paper introduced the idea of applying external voltage patterns to a medium such that, assuming that the medium is sufficiently close to a constant admittivity, the reconstruction can be accomplished directly by inverse Fourier transform. Motivated by CalderOn's method, we have developed a variant of the algorithm which is applicable to the case of measurement on only a part of the boundary and on discrete electrodes. Here we determine voltage or current patterns to apply to the electrodes which optimally approximate CalderOn's special functions in the interior. Furthermore, in three dimensions and higher, CalderOn's method allows each point in Fourier space to be computed in a multiplicity of ways. We show that by making use of the inherent redundancy in our measurements, we can significantly improve the quality of the static images produced by our algorithm.

Electrical impedance tomography (EIT) is a developing imaging modality that is beginning to show promise for detecting and characterizing tumors in the breast. At Rensselaer Polytechnic Institute, we have developed a combined EIT-tomosynthesis system that allows for the coregistered and simultaneous analysis of the breast using EIT and X-ray imaging. A significant challenge in EIT is the design of computationally efficient image reconstruction algorithms which are robust to various forms of model mismatch. Specifically, we have implemented a scaling procedure that is robust to the presence of a thin highly-resistive layer of skin at the boundary of the breast and we have developed an algorithm to detect and exclude from the image reconstruction electrodes that are in poor contact with the breast. In our initial clinical studies, it has been difficult to ensure that all electrodes make adequate contact with the breast, and thus procedures for the use of data sets containing poorly contacting electrodes are particularly important. We also present a novel, efficient method to compute the Jacobian matrix for our linearized image reconstruction algorithm by reducing the computation of the sensitivity for each voxel to a quadratic form. Initial clinical results are presented, showing the potential of our algorithms to detect and localize breast tumors.

It has been known for some time that many tumors have a significantly different conductivity and permittivity from surrounding normal tissue. This high "contrast" in tissue electrical properties, occurring between a few kilohertz and several megahertz, may permit differentiating malignant from benign tissues. Here we show the ability of electrical impedance spectroscopy (EIS) to roughly localize and clearly distinguish cancers from normal tissues and benign lesions. Localization of these lesions is confirmed by simultaneous, in register digital breast tomosynthesis (DBT) mammography or 3-D mammograms.

This paper studies the channel estimation, detection and performance analysis of mobile wireless communication systems employing space-time block codes (STBC), where the channels exhibit flat Rayleigh fading with a significant amount of Doppler spread. Pilot-symbol-assisted modulation (PSAM) is developed to estimate the channels. The performance of the channel estimation is evaluated. Maximum-likelihood (ML) detection is first formulated. A linear detector, called the symmetric linear (SL) detector, is proposed. An interference cancellation (IC) detector is also developed to improve the performance of the SL detector. We derive the closed-form analytical expressions for the bit-error rate (BER) or symbol-error rate (SER) of pilot-sequence-assisted STBC with binary phase-shift keying (PSK), quaternary PSK, 16-quadrature amplitude modulation (QAM) and 64-QAM, when the SL and IC detectors are used. Some important design issues in the pilot-sequence-assisted STBC systems, including the selection of pilot spacing and interpolator size, are discussed. Simulation results are presented that verify our analytical results. The tradeoffs between the performance and complexity for ML, SL, and IC detection are also discussed

The self-cancellation scheme (SCS) has been previously proposed as an intercarrier interference (ICI) mitigation technique for OFDM systems. In this paper, we propose a more general approach called the general self-cancellation scheme (GSCS). In this technique, windowing is used to reduce the ICI resulting from the frequency shifts. After describing the system, we use a numerical method to calculate the bit error rate of the system in presence of a fixed frequency shift. We also study the performance of the system when ICI is generated as a result of channel fading. The results demonstrate that the proposed system performs better than the conventional and SCS systems, both in the presence of an oscillator frequency offset and when ICI is created as a result of channel fading.

We obtain an analytical result for the symbol-error rate of a wireless orthogonal frequency-division multiplexing system employing Fischer's bit-loading algorithm. First we assume that the subchannel coefficients are statistically independent, then we extend the results to the case with dependent subchannel coefficients. Throughout the analysis, we assume that the number of subchannels is large, and that subchannels have Rayleigh-distributed coefficients. The comparison of analytical and simulated results shows that the approximation error is small, especially at low noise and high bit rate.

A novel reception method for OFDM systems, based on the use of multiple local oscillators at the receiver is proposed. In this method the received signal is down-converted using multiple oscillators with slightly different frequencies. The down-converted signals are then combined before the OFDM demodulation takes place. The signal-to-inference ratio and bit error rate performance of the proposed system has been obtained and compared to those of the existing systems. It has been shown that a higher frequency offsets the proposed method can provide better performance.

In this paper, we investigate the effect of varying the processing gain in place of power control in a CDMA packet radio network. The classic CDMA protocol is modified to accommodate the variable processing gain, allowing for variable packet durations due to variable processing gain. The performance of the variable processing gain system is measured in terms of the network throughput and is compared to the performance of conventional fixed rate CDMA systems in which perfect power control is assumed. Results are first generated for a terrestrial CDMA network and extended to a low Earth orbit satellite (LEOS) communication topology. In the interference limited CDMA network, we have demonstrated that processing gain control can be used in place of power control and, depending on the details of the implementation, can achieve the same or nearly the same throughput performance.

This paper analyzes the throughput for an adaptive packet radio network (PRN). The proposed adaptive OFDM spread spectrum (OFDM-SS) system can trade off system processing gain (i.e. user data rate) for transmission quality under changing channel conditions. Classic throughput analysis for fixed and random access schemes is extended for this proposed adaptive PRN. The focus is on pure-ALOHA and nonpersistent CSMA (NP-CSMA) systems. The paper introduces a method to analyze network throughput with variable packet durations due to variable user data rate. The large improvement from the adaptive system compared to the conventional fixed rate counterpart is demonstrated by various mobile user population profiles and radio propagation path loss models.

A four-state adaptive maximum-likelihood sequence estimation (MLSE) Gaussian minimum-shift keying (GMSK) demodulator for modulation (BT=0.3) on AWGN channels is analyzed and simulated. This demodulator uses the linear representation of GMSK signals and achieves near-optimum BER performance. The channel-impulse response used in the MLSE demodulator is initialized to the highest energy component in the linear representation, and then adapted in a decision-directed mode to offset any performance losses incurred by initially ignoring other lower energy (and time-varying) components in the linear representation. The number of MLSE states is reduced to two, at about 0.1-dB performance loss, by implementing a whitening matched filter which concentrates most of the GMSK pulse energy in its two leading samples.

Two systems that use orthogonal frequency division multiplexing (OFDM) as a spread spectrum technique are described and their performance is evaluated in the presence of tone and pulsed Gaussian interference. One system obtains frequency diversity by sending the same data bit simultaneously on all the carriers while a second system obtains both time and frequency diversity by sequentially sending a given data bit on different carriers until it is sent on all possible carriers. Interference excision, in which some of the transform bins are removed in the detection process, is used in the receivers. Simulated BER performance in AWGN and interference is presented for systems that use a number of different block transforms and the modulated lapped transform (MLT) and it is shown that the MLT provides much better performance due to its ability to confine the interference energy to a few transform bins.

In order to mitigate narrow-band interference in spread spectrum communications systems, novel communications receivers incorporating transform domain filtering techniques are designed. In this paper, lapped transforms are used to transform the received data signal to the transform domain wherein adaptive excision is performed. Transform domain detection algorithms, which yield bit decisions based on the remaining signal energy, are analyzed and, together with excision, are employed on a block-by-block basis to suppress single-tone and narrow-band Gaussian interference. System performance is analytically quantified in terms of the overall system bit-error rate (BER). Subsequent results are presented for a variety of channel conditions and compared to those obtained using excision algorithms based on orthonormal block transforms (Medley 1995). These results demonstrate the improved performance and increased robustness with respect to jammer frequency and bandwidth of lapped transform domain excision techniques relative to similar algorithms based on nonweighted block transforms.

This paper presents a novel receiver for direct sequence spread-spectrum signals over channels containing interference and multipath. The receiver employs an adaptive correlator that jointly detects the transmitted data, removes interference, and compensates for multipath. The optimum correlation vector is derived by determining the Wiener vector that minimizes the mean squared error (MSE) between the transmitted data bit and the correlator output. For an additive white Gaussian noise (AWGN) channel, the optimal correlation vector is the spreading sequence used by the transmitter. For interference and multipath channels, the optimal correlation vector will suppress the interference and combine the multipath while optimizing the detection of the transmitted data bit. The paper presents analytical and simulation results which illustrate the bit-error rate (BER) performance of the receiver in multipath and narrowband interference. Additionally, simulation results are presented illustrating the convergence performance of the receiver when the tap weights are adjusted using either the least mean square (LMS) or recursive least squares (RLS) adaptive algorithms.

An adaptive correlator is presented as the data detector in a direct sequence spread spectrum communication system. The adaptive correlator has the same structure as an least-mean-square (LMS) adaptive filter, but the tapped delay line of the filter is shifted multiple times between each adaptation. This method implements an LMMSE estimator of the transmitted data based on the received samples. The tap weight behavior is predicted for such an estimator in the presence of single tone interference and multipath propagation, and the resulting receiver is simulated to verify convergence. A decision directed receiver using this detector is simulated and is shown to be effective at removing both interference and multipath effects. An efficient serial method for implementing the receiver is also presented.>

The author investigates the use of transform-domain adaptive filtering for the suppression of narrowband jammers in a direct-sequence (DS) spread-spectrum receiver. An input signal consisting of the DS signal, white Gaussian noise, and a narrowband jammer is transformed into the frequency domain where adaptive filtering acts to suppress the jammer. Weight leakage is used to allow jammer suppression while preserving the desired DS signal. Bit error rate (BER) results obtained by computer simulation are presented to illustrate performance for a single-tone jammer and for a jammer consisting of a second DS signal with a lower chip rate. These results are compared to those for a transform-domain exciser. Expressions for the converged tap weights are derived and the results are verified via the simulation. Finally, the convergence properties of the adaptive suppressor are illustrated.>

An all-digital demodulator/detector which is suitable for both analog FM and digital phase/frequency modulations is presented. The system uses complex sampling, which employs a single A/D (analog/digital) converter to sample the signal at an intermediate frequency (IF) and produce baseband in-phase (I) and quadrature phase (Q) signals, and a simplified technique for reducing the effect of the I/Q timing misalignment usually associated with this approach. The system also includes two detectors which operate simultaneously to provide noncoherent and differentially coherent detection, as well as automatic gain control (AGC) and automatic frequency control (AFC). The flexibility afforded by the two concurrent detectors in this all-digital system is shown to make it suitable for a wide range of applications. The theory behind the demodulator/detector system is described, and an implementation using a 1.25- mu m bulk CMOS VLSI process is presented. Methods are shown for extending and improving the I/Q sampling misalignment correction technique, as well as for reducing the A/D sampling rate for a given IF frequency. Simulation and experimental results illustrate system performance for both analog and digital modulations.>

It is well known that least-mean-squared (LMS) adaptive filtering can be used to suppress narrow-band jammers in a direct sequence spread spectrum receiver. To date, much of the work in this area has concentrated on the use of transversal adaptive filter structures. This paper demonstrates the performance of an adaptive lattice filter in this application. The reflection coefficients of the filter are adjusted using a gradient descent algorithm in an effort to minimize the mean-squared error and, in the process, suppress narrow-band interference. Experimental bit-error-rate performance curves are presented for 7- and 31- chip spreading sequences with a single tone jammer.

This paper documents the laboratory and theoretical performance of a pilot-aided digital radio system. The technique considered transmits a midband pilot tone to improve the receiver data detection performance in a multipath fading channel and is referred to as the tone calibrated technique (TCT). We report on the performance of a 860 MHz prototype system carrying 2.4 kbit/s data under Rician fading conditions. Both experimental and analytical results show that the error floor experienced with nonpilot-aided transmission methods is effectively removed by the TCT scheme, resulting in significant performance gains at high signal-to-noise ratio (SNR) values. The paper also examines the TCT system performance under typical operating conditions and presents a new analysis of the TCT theoretical error probability.

This paper describes the structure of a digital implementation of the Widrow-Hoff LMS algorithm which uses a burst processing technique to obtain some hardware simplification. This adaptive system is used to suppress narrow-band interference in a direct-sequence spread-spectrum communication system. Several different narrow-band interferers are considered, and probability of error results are presented for all cases. While, in general, the results show significant improvement in performance when the LMS algorithm is used, certain disadvantages are also present and are discussed in this paper.

Analytical results have shown that adaptive filtering can be a powerful tool for the rejection of narrow-band interference in a spreadspectrum receiver. However, the complexity of adaptive filtering hardware has hindered the experimental verification of these results. This paper describes a new adaptive filter architecture for implementing the Widrow-Hoff LMS algorithm while using only two multipliers regardless of filter order. This hardware simplification is achieved through the use of a burst processing technique. A 16-tap version of this adaptive filter constructed using charge-transfer devices (CTD's) is used to suppress a single tone jammer in a direct sequence spread-spectrum receiver. Probability of error measurements demonstrating the effectiveness of the adaptive filter for suppressing the single tone jammer along with simulation results for the optimal Weiner-Hopf filter are presented and discussed.