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Papers by Marcos Hernandez

Research paper thumbnail of In vitro Antiplasmodial Investigation of Medicinal Plants from El Salvador

uated. From the lipophilic extract of C. tenuifolia five active flavones were obtained. 4Ј,5-Dihy... more uated. From the lipophilic extract of C. tenuifolia five active flavones were obtained. 4Ј,5-Dihydroxy-7-methoxyflavone [genkwanin] and 5-hydroxy-4Ј,7-dimethoxyflavone [apigenin 4Ј,7-dimethylether] exhibited the strongest antiplasmodial activity against a chloroquine-sensitive strain (poW) and a chloroquine-resistant strain (Dd2) of Plasmodium falciparum (IC 50 values: 17.1Ð28.5 µm). Furthermore octadeca-9,12-dienoic acid [linoleic acid] {IC 50 values of 21.8 µm (poW) and 31.1 µm (Dd2)} and octadeca-9,12,15-trienoic acid (α-linolenic acid) were isolated.

Research paper thumbnail of Uplink performance enhancement for WCDMA systems through adaptive antenna and multiuser detection

Vehicular Technology, IEEE Conference, 2000

Multipath fading and multiple access interference (MAI) are the major constraints on the performa... more Multipath fading and multiple access interference (MAI) are the major constraints on the performance of DS/CDMA systems. Antenna arrays and multiuser detectors have been considered to increase the system capacity through mitigation of such impairments. Multiuser detectors have the capability of eliminating MAI, the near-far problem and providing a significant capacity increase. In order to gain from the enhancements of both: multi-user detection and adaptive antenna, we propose in this paper to combine both schemes for an asynchronous WCDMA system. The performance of the combined scheme is evaluated for the uplink over the Gaussian channel and the UMTS frequency selective Rayleigh fading channel. We make a comparison of the most promising multiuser structures and the recursive least square algorithm to control the beamformer

Research paper thumbnail of The Airborne Multi-angle Imaging SpectroRadiometer (AirMISR): instrument description and first results

IEEE Transactions on Geoscience and Remote Sensing, 1998

An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assi... more An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assist in validation of the Earth Observing System (EOS) MISR experiment. Unlike the EOS MISR, which contains nine individual cameras pointed at discrete look angles, AirMISR utilizes a single camera in a pivoting gimbal mount. The AirMISR camera has been fabricated from MISR brassboard and engineering model components and, thus, has similar radiometric and spectral response as the MISR cameras. This paper provides a description of the AirMISR instrument and summarizes the results of engineering flights conducted during 1997

Research paper thumbnail of Design and performance of a low-complexity iterative multiuser joint decoder based on Viterbi decoding and parallel interference cancellation

Abstract We propose a low-complexity multiuser joint parallel interference cancellation (PIC) dec... more Abstract We propose a low-complexity multiuser joint parallel interference cancellation (PIC) decoder for direct-sequence CDMA. An estimate of the multiple-access interference (MAI) signal is formed by weighting the hard decisions produced by hard-output Viterbi decoders. Such MAI interference estimate is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, we optimize the feedback weights at each iteration. Then, we consider ...

Research paper thumbnail of Low-complexity turbo equalization and multiuser decoding for TD-CDMA

IEEE Transactions on Wireless Communications, 2004

The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) dec... more The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) decoder and turbo decision feedback equalizer for code division multiple access (CDMA). In their scheme, an estimate of the interference signal (both multiple-access interference and intersymbol interference) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, they optimize the feedback weights at each iteration. Then, they consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. The authors show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and they propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of third-generation systems, contemplating a combination of time-division multiple access and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, soft-input soft-output decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.

Research paper thumbnail of In vitro Antiplasmodial Investigation of Medicinal Plants from El Salvador

uated. From the lipophilic extract of C. tenuifolia five active flavones were obtained. 4Ј,5-Dihy... more uated. From the lipophilic extract of C. tenuifolia five active flavones were obtained. 4Ј,5-Dihydroxy-7-methoxyflavone [genkwanin] and 5-hydroxy-4Ј,7-dimethoxyflavone [apigenin 4Ј,7-dimethylether] exhibited the strongest antiplasmodial activity against a chloroquine-sensitive strain (poW) and a chloroquine-resistant strain (Dd2) of Plasmodium falciparum (IC 50 values: 17.1Ð28.5 µm). Furthermore octadeca-9,12-dienoic acid [linoleic acid] {IC 50 values of 21.8 µm (poW) and 31.1 µm (Dd2)} and octadeca-9,12,15-trienoic acid (α-linolenic acid) were isolated.

Research paper thumbnail of Uplink performance enhancement for WCDMA systems through adaptive antenna and multiuser detection

Vehicular Technology, IEEE Conference, 2000

Multipath fading and multiple access interference (MAI) are the major constraints on the performa... more Multipath fading and multiple access interference (MAI) are the major constraints on the performance of DS/CDMA systems. Antenna arrays and multiuser detectors have been considered to increase the system capacity through mitigation of such impairments. Multiuser detectors have the capability of eliminating MAI, the near-far problem and providing a significant capacity increase. In order to gain from the enhancements of both: multi-user detection and adaptive antenna, we propose in this paper to combine both schemes for an asynchronous WCDMA system. The performance of the combined scheme is evaluated for the uplink over the Gaussian channel and the UMTS frequency selective Rayleigh fading channel. We make a comparison of the most promising multiuser structures and the recursive least square algorithm to control the beamformer

Research paper thumbnail of The Airborne Multi-angle Imaging SpectroRadiometer (AirMISR): instrument description and first results

IEEE Transactions on Geoscience and Remote Sensing, 1998

An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assi... more An Airborne Multi-angle Imaging SpectroRadiometer (AirMISR) instrument has been developed to assist in validation of the Earth Observing System (EOS) MISR experiment. Unlike the EOS MISR, which contains nine individual cameras pointed at discrete look angles, AirMISR utilizes a single camera in a pivoting gimbal mount. The AirMISR camera has been fabricated from MISR brassboard and engineering model components and, thus, has similar radiometric and spectral response as the MISR cameras. This paper provides a description of the AirMISR instrument and summarizes the results of engineering flights conducted during 1997

Research paper thumbnail of Design and performance of a low-complexity iterative multiuser joint decoder based on Viterbi decoding and parallel interference cancellation

Abstract We propose a low-complexity multiuser joint parallel interference cancellation (PIC) dec... more Abstract We propose a low-complexity multiuser joint parallel interference cancellation (PIC) decoder for direct-sequence CDMA. An estimate of the multiple-access interference (MAI) signal is formed by weighting the hard decisions produced by hard-output Viterbi decoders. Such MAI interference estimate is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, we optimize the feedback weights at each iteration. Then, we consider ...

Research paper thumbnail of Low-complexity turbo equalization and multiuser decoding for TD-CDMA

IEEE Transactions on Wireless Communications, 2004

The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) dec... more The authors propose a low complexity multiuser joint parallel interference cancellation (PIC) decoder and turbo decision feedback equalizer for code division multiple access (CDMA). In their scheme, an estimate of the interference signal (both multiple-access interference and intersymbol interference) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, they optimize the feedback weights at each iteration. Then, they consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. The authors show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and they propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of third-generation systems, contemplating a combination of time-division multiple access and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, soft-input soft-output decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.