Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: Books Puddle, New York, NY, U.S.A.
Condition: New.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: Revaluation Books, Exeter, United Kingdom
US$ 73.00
Quantity: 1 available
Add to basketPaperback. Condition: Brand New. 72 pages. 8.66x5.91x0.17 inches. In Stock.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: preigu, Osnabrück, Germany
Taschenbuch. Condition: Neu. Single and Dual Polarization O-OFDM | Rend N. Alrays (u. a.) | Taschenbuch | Englisch | 2017 | LAP LAMBERT Academic Publishing | EAN 9783330089570 | Verantwortliche Person für die EU: LAP Lambert Academic Publishing, Brivibas Gatve 197, 1039 RIGA, LETTLAND, customerservice[at]vdm-vsg[dot]de | Anbieter: preigu.
Language: French
Published by Editions Notre Savoir, 2024
ISBN 10: 6207348273 ISBN 13: 9786207348275
Seller: moluna, Greven, Germany
Condition: New.
Condition: New.
Language: Portuguese
Published by Edições Nosso Conhecimento, 2024
ISBN 10: 6207348303 ISBN 13: 9786207348305
Seller: moluna, Greven, Germany
Condition: New.
Language: English
Published by LAP LAMBERT Academic Publishing Mai 2017, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware -High speed digital communications has widely used Orthogonal Frequency Division Multiplexing during last three decades. The OFDM implementation initial obstacles are many, such as high speed memory and massive complex computation, due to the Very Large Scale Integrated circuit (VLSI) and Digital Signal Processing (DSP) that marked as an effective technologies . Meantime, the employment of Fast Fourier Transform declines coherent demodulation and sinusoidal generators arrays and makes the cost of implementation of the technology productive. Additional cause for the popularity of OFDM is theoretically proved with its optimal performance. OFDM signals transmitted through multiple subcarriers which mean that OFDM is considered as a multicarrier modulation. OFDM subcarriers have the property of the orthogonality and they operate at different frequencies. OFDM has been applied in wireless and wired communications, such as Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g and IEEE 802.16a . A frequency-selective channel is transformed through OFDM to a parallel set of frequencies . 72 pp. Englisch.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: Majestic Books, Hounslow, United Kingdom
Condition: New. Print on Demand.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: Biblios, Frankfurt am main, HESSE, Germany
Condition: New. PRINT ON DEMAND.
Language: Spanish
Published by Ediciones Nuestro Conocimiento Apr 2024, 2024
ISBN 10: 6207348281 ISBN 13: 9786207348282
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware 64 pp. Spanisch.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: moluna, Greven, Germany
Kartoniert / Broschiert. Condition: New. Dieser Artikel ist ein Print on Demand Artikel und wird nach Ihrer Bestellung fuer Sie gedruckt. Autor/Autorin: Alrays Rend N.Teaching at the University of Technology, Also work as Visitor Lecture in Informatics Institute for Postgraduate Studies, UITC, Baghdad, I got the scientific degree of assistant professor in 2009 and specialized modeli.
Language: French
Published by Editions Notre Savoir Apr 2024, 2024
ISBN 10: 6207348273 ISBN 13: 9786207348275
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware 68 pp. Französisch.
Language: English
Published by LAP LAMBERT Academic Publishing Mai 2017, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -High speed digital communications has widely used Orthogonal Frequency Division Multiplexing during last three decades. The OFDM implementation initial obstacles are many, such as high speed memory and massive complex computation, due to the Very Large Scale Integrated circuit (VLSI) and Digital Signal Processing (DSP) that marked as an effective technologies . Meantime, the employment of Fast Fourier Transform declines coherent demodulation and sinusoidal generators arrays and makes the cost of implementation of the technology productive. Additional cause for the popularity of OFDM is theoretically proved with its optimal performance. OFDM signals transmitted through multiple subcarriers which mean that OFDM is considered as a multicarrier modulation. OFDM subcarriers have the property of the orthogonality and they operate at different frequencies. OFDM has been applied in wireless and wired communications, such as Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g and IEEE 802.16a . A frequency-selective channel is transformed through OFDM to a parallel set of frequencies .VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken 72 pp. Englisch.
Language: English
Published by LAP LAMBERT Academic Publishing, 2017
ISBN 10: 3330089571 ISBN 13: 9783330089570
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - High speed digital communications has widely used Orthogonal Frequency Division Multiplexing during last three decades. The OFDM implementation initial obstacles are many, such as high speed memory and massive complex computation, due to the Very Large Scale Integrated circuit (VLSI) and Digital Signal Processing (DSP) that marked as an effective technologies . Meantime, the employment of Fast Fourier Transform declines coherent demodulation and sinusoidal generators arrays and makes the cost of implementation of the technology productive. Additional cause for the popularity of OFDM is theoretically proved with its optimal performance. OFDM signals transmitted through multiple subcarriers which mean that OFDM is considered as a multicarrier modulation. OFDM subcarriers have the property of the orthogonality and they operate at different frequencies. OFDM has been applied in wireless and wired communications, such as Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g and IEEE 802.16a . A frequency-selective channel is transformed through OFDM to a parallel set of frequencies .
Language: Spanish
Published by Ediciones Nuestro Conocimiento Apr 2024, 2024
ISBN 10: 6207348281 ISBN 13: 9786207348282
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -Las comunicaciones digitales de alta velocidad han utilizado ampliamente la multiplexación por división de frecuencias ortogonales durante las tres últimas décadas. Los obstáculos iniciales para la implementación de OFDM son muchos, como la memoria de alta velocidad y el cálculo complejo masivo, debido a los circuitos integrados a muy gran escala (VLSI) y el procesamiento digital de señales (DSP), que se han convertido en tecnologías eficaces. Mientras tanto, el empleo de la Transformada Rápida de Fourier disminuye la demodulación coherente y las matrices de generadores sinusoidales y hace que el coste de implementación de la tecnología sea productivo. Otra causa de la popularidad de OFDM se demuestra teóricamente con su rendimiento óptimo. Las señales OFDM se transmiten a través de múltiples subportadoras, lo que significa que OFDM se considera una modulación multiportadora. Las subportadoras OFDM tienen la propiedad de la ortogonalidad y operan a diferentes frecuencias. La OFDM se ha aplicado en comunicaciones inalámbricas y por cable, como la radiodifusión de audio digital (DAB), IEEE (Instituto de Ingenieros Eléctricos y Electrónicos) 802.11a/g e IEEE 802.16a . Un canal selectivo en frecuencia se transforma mediante OFDM en un conjunto paralelo de frecuencias.VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken Spanisch.
Language: Spanish
Published by Ediciones Nuestro Conocimiento
ISBN 10: 6207348281 ISBN 13: 9786207348282
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - Las comunicaciones digitales de alta velocidad han utilizado ampliamente la multiplexación por división de frecuencias ortogonales durante las tres últimas décadas. Los obstáculos iniciales para la implementación de OFDM son muchos, como la memoria de alta velocidad y el cálculo complejo masivo, debido a los circuitos integrados a muy gran escala (VLSI) y el procesamiento digital de señales (DSP), que se han convertido en tecnologías eficaces. Mientras tanto, el empleo de la Transformada Rápida de Fourier disminuye la demodulación coherente y las matrices de generadores sinusoidales y hace que el coste de implementación de la tecnología sea productivo. Otra causa de la popularidad de OFDM se demuestra teóricamente con su rendimiento óptimo. Las señales OFDM se transmiten a través de múltiples subportadoras, lo que significa que OFDM se considera una modulación multiportadora. Las subportadoras OFDM tienen la propiedad de la ortogonalidad y operan a diferentes frecuencias. La OFDM se ha aplicado en comunicaciones inalámbricas y por cable, como la radiodifusión de audio digital (DAB), IEEE (Instituto de Ingenieros Eléctricos y Electrónicos) 802.11a/g e IEEE 802.16a . Un canal selectivo en frecuencia se transforma mediante OFDM en un conjunto paralelo de frecuencias.
Language: Portuguese
Published by Edições Nosso Conhecimento Apr 2024, 2024
ISBN 10: 6207348303 ISBN 13: 9786207348305
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware 68 pp. Portugiesisch.
Language: German
Published by Verlag Unser Wissen Apr 2024, 2024
ISBN 10: 6207348265 ISBN 13: 9786207348268
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware 68 pp. Deutsch.
Language: Italian
Published by Edizioni Sapienza Apr 2024, 2024
ISBN 10: 620734829X ISBN 13: 9786207348299
Seller: BuchWeltWeit Ludwig Meier e.K., Bergisch Gladbach, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - it takes 3-4 days longer - Neuware 64 pp. Italienisch.
Language: French
Published by Editions Notre Savoir Apr 2024, 2024
ISBN 10: 6207348273 ISBN 13: 9786207348275
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -Les communications numériques à grande vitesse ont largement utilisé le multiplexage par répartition orthogonale de la fréquence au cours des trois dernières décennies. Les obstacles initiaux à la mise en ¿uvre de l'OFDM sont nombreux, tels que la mémoire à grande vitesse et les calculs complexes massifs, en raison des circuits intégrés à très grande échelle (VLSI) et du traitement numérique du signal (DSP) qui se sont imposés comme des technologies efficaces. En même temps, l'utilisation de la transformée de Fourier rapide diminue la démodulation cohérente et les réseaux de générateurs sinusoïdaux et rend le coût de mise en ¿uvre de la technologie productif. La popularité de l'OFDM est également prouvée théoriquement par ses performances optimales. Les signaux OFDM sont transmis par plusieurs sous-porteuses, ce qui signifie que l'OFDM est considéré comme une modulation multiporteuse. Les sous-porteuses OFDM ont la propriété d'être orthogonales et fonctionnent à des fréquences différentes. L'OFDM a été appliqué aux communications sans fil et câblées, telles que la diffusion audio numérique (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g et IEEE 802.16a. Un canal à fréquence sélective est transformé par l'OFDM en un ensemble parallèle de fréquences.VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken Französisch.
Language: French
Published by Editions Notre Savoir, 2024
ISBN 10: 6207348273 ISBN 13: 9786207348275
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - Les communications numériques à grande vitesse ont largement utilisé le multiplexage par répartition orthogonale de la fréquence au cours des trois dernières décennies. Les obstacles initiaux à la mise en oeuvre de l'OFDM sont nombreux, tels que la mémoire à grande vitesse et les calculs complexes massifs, en raison des circuits intégrés à très grande échelle (VLSI) et du traitement numérique du signal (DSP) qui se sont imposés comme des technologies efficaces. En même temps, l'utilisation de la transformée de Fourier rapide diminue la démodulation cohérente et les réseaux de générateurs sinusoïdaux et rend le coût de mise en oeuvre de la technologie productif. La popularité de l'OFDM est également prouvée théoriquement par ses performances optimales. Les signaux OFDM sont transmis par plusieurs sous-porteuses, ce qui signifie que l'OFDM est considéré comme une modulation multiporteuse. Les sous-porteuses OFDM ont la propriété d'être orthogonales et fonctionnent à des fréquences différentes. L'OFDM a été appliqué aux communications sans fil et câblées, telles que la diffusion audio numérique (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g et IEEE 802.16a. Un canal à fréquence sélective est transformé par l'OFDM en un ensemble parallèle de fréquences.
Seller: moluna, Greven, Germany
Condition: New. Dieser Artikel ist ein Print on Demand Artikel und wird nach Ihrer Bestellung fuer Sie gedruckt. Autor/Autorin: Alrays Rend N.Lehre an der University of Technology, arbeiten auch als Gastvorlesung in Informatik Institut fuer Postgraduate Studies, UITC, Bagdad, bekam ich den wissenschaftlichen Grad des Assistenzprofessors im Jahr 2009 und spezi.
Language: Italian
Published by Edizioni Sapienza Apr 2024, 2024
ISBN 10: 620734829X ISBN 13: 9786207348299
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -Negli ultimi trent'anni le comunicazioni digitali ad alta velocità hanno fatto largo uso dell'Orthogonal Frequency Division Multiplexing. Gli ostacoli iniziali all'implementazione dell'OFDM sono molti, come la memoria ad alta velocità e l'enorme complessità di calcolo, a causa dei circuiti integrati su larghissima scala (VLSI) e dell'elaborazione digitale del segnale (DSP) che si sono imposti come tecnologie efficaci. Nel frattempo, l'impiego della trasformata veloce di Fourier riduce la demodulazione coerente e gli array di generatori sinusoidali e rende produttivo il costo di implementazione della tecnologia. Un'ulteriore causa della popolarità dell'OFDM è dimostrata teoricamente dalle sue prestazioni ottimali. I segnali OFDM sono trasmessi attraverso più sottoportanti, il che significa che l'OFDM è considerato una modulazione multicarrier. Le sottoportanti OFDM hanno la proprietà dell'ortogonalità e operano a frequenze diverse. L'OFDM è stato applicato nelle comunicazioni wireless e cablate, come ad esempio la trasmissione audio digitale (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g e IEEE 802.16a . Un canale selettivo in frequenza viene trasformato tramite OFDM in un insieme parallelo di frequenze.VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken Italienisch.
Language: German
Published by Verlag Unser Wissen Apr 2024, 2024
ISBN 10: 6207348265 ISBN 13: 9786207348268
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -Die digitale Hochgeschwindigkeitskommunikation hat in den letzten drei Jahrzehnten das Orthogonal Frequency Division Multiplexing weit verbreitet. Die OFDM-Implementierung anfänglichen Hindernisse sind viele, wie High-Speed-Speicher und massive komplexe Berechnung, aufgrund der Very Large Scale Integrated Circuit (VLSI) und Digital Signal Processing (DSP), die als eine effektive Technologien markiert. In der Zwischenzeit verringert die Verwendung der Fast-Fourier-Transformation kohärente Demodulation und sinusförmige Generatoren und macht die Kosten für die Implementierung der Technologie produktiv. Ein weiterer Grund für die Beliebtheit von OFDM ist die theoretisch belegte optimale Leistung. OFDM-Signale werden über mehrere Unterträger übertragen, was bedeutet, dass OFDM als eine Mehrträgermodulation betrachtet wird. OFDM-Unterträger haben die Eigenschaft der Orthogonalität und arbeiten auf unterschiedlichen Frequenzen. OFDM wurde in der drahtlosen und drahtgebundenen Kommunikation eingesetzt, z. B. bei Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g und IEEE 802.16a. Ein frequenzselektiver Kanal wird durch OFDM in einen parallelen Satz von Frequenzen umgewandelt.VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken 68 pp. Deutsch.
Language: Portuguese
Published by Edições Nosso Conhecimento Apr 2024, 2024
ISBN 10: 6207348303 ISBN 13: 9786207348305
Seller: buchversandmimpf2000, Emtmannsberg, BAYE, Germany
Taschenbuch. Condition: Neu. This item is printed on demand - Print on Demand Titel. Neuware -As comunicações digitais de alta velocidade têm utilizado amplamente a multiplexagem por divisão de frequência ortogonal nas últimas três décadas. Os obstáculos iniciais à implementação do OFDM são muitos, tais como a memória de alta velocidade e a computação maciça e complexa, devido aos circuitos integrados de grande escala (VLSI) e ao processamento digital de sinais (DSP), que se tornaram tecnologias eficazes. Entretanto, o emprego da Transformada Rápida de Fourier diminui a desmodulação coerente e as matrizes de geradores sinusoidais e torna o custo de implementação da tecnologia produtivo. Uma causa adicional para a popularidade do OFDM é teoricamente comprovada pelo seu ótimo desempenho. Os sinais OFDM são transmitidos através de múltiplas subportadoras, o que significa que a OFDM é considerada uma modulação multiportadoras. As subportadoras OFDM têm a propriedade da ortogonalidade e funcionam em frequências diferentes. A OFDM tem sido aplicada em comunicações sem fios e com fios, tais como Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g e IEEE 802.16a . Um canal seletivo em frequência é transformado através de OFDM para um conjunto paralelo de frequências.VDM Verlag, Dudweiler Landstraße 99, 66123 Saarbrücken 68 pp. Portugiesisch.
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - Die digitale Hochgeschwindigkeitskommunikation hat in den letzten drei Jahrzehnten das Orthogonal Frequency Division Multiplexing weit verbreitet. Die OFDM-Implementierung anfänglichen Hindernisse sind viele, wie High-Speed-Speicher und massive komplexe Berechnung, aufgrund der Very Large Scale Integrated Circuit (VLSI) und Digital Signal Processing (DSP), die als eine effektive Technologien markiert. In der Zwischenzeit verringert die Verwendung der Fast-Fourier-Transformation kohärente Demodulation und sinusförmige Generatoren und macht die Kosten für die Implementierung der Technologie produktiv. Ein weiterer Grund für die Beliebtheit von OFDM ist die theoretisch belegte optimale Leistung. OFDM-Signale werden über mehrere Unterträger übertragen, was bedeutet, dass OFDM als eine Mehrträgermodulation betrachtet wird. OFDM-Unterträger haben die Eigenschaft der Orthogonalität und arbeiten auf unterschiedlichen Frequenzen. OFDM wurde in der drahtlosen und drahtgebundenen Kommunikation eingesetzt, z. B. bei Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g und IEEE 802.16a. Ein frequenzselektiver Kanal wird durch OFDM in einen parallelen Satz von Frequenzen umgewandelt.
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - Negli ultimi trent'anni le comunicazioni digitali ad alta velocità hanno fatto largo uso dell'Orthogonal Frequency Division Multiplexing. Gli ostacoli iniziali all'implementazione dell'OFDM sono molti, come la memoria ad alta velocità e l'enorme complessità di calcolo, a causa dei circuiti integrati su larghissima scala (VLSI) e dell'elaborazione digitale del segnale (DSP) che si sono imposti come tecnologie efficaci. Nel frattempo, l'impiego della trasformata veloce di Fourier riduce la demodulazione coerente e gli array di generatori sinusoidali e rende produttivo il costo di implementazione della tecnologia. Un'ulteriore causa della popolarità dell'OFDM è dimostrata teoricamente dalle sue prestazioni ottimali. I segnali OFDM sono trasmessi attraverso più sottoportanti, il che significa che l'OFDM è considerato una modulazione multicarrier. Le sottoportanti OFDM hanno la proprietà dell'ortogonalità e operano a frequenze diverse. L'OFDM è stato applicato nelle comunicazioni wireless e cablate, come ad esempio la trasmissione audio digitale (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g e IEEE 802.16a . Un canale selettivo in frequenza viene trasformato tramite OFDM in un insieme parallelo di frequenze.
Language: Portuguese
Published by Edições Nosso Conhecimento, 2024
ISBN 10: 6207348303 ISBN 13: 9786207348305
Seller: AHA-BUCH GmbH, Einbeck, Germany
Taschenbuch. Condition: Neu. nach der Bestellung gedruckt Neuware - Printed after ordering - As comunicações digitais de alta velocidade têm utilizado amplamente a multiplexagem por divisão de frequência ortogonal nas últimas três décadas. Os obstáculos iniciais à implementação do OFDM são muitos, tais como a memória de alta velocidade e a computação maciça e complexa, devido aos circuitos integrados de grande escala (VLSI) e ao processamento digital de sinais (DSP), que se tornaram tecnologias eficazes. Entretanto, o emprego da Transformada Rápida de Fourier diminui a desmodulação coerente e as matrizes de geradores sinusoidais e torna o custo de implementação da tecnologia produtivo. Uma causa adicional para a popularidade do OFDM é teoricamente comprovada pelo seu ótimo desempenho. Os sinais OFDM são transmitidos através de múltiplas subportadoras, o que significa que a OFDM é considerada uma modulação multiportadoras. As subportadoras OFDM têm a propriedade da ortogonalidade e funcionam em frequências diferentes. A OFDM tem sido aplicada em comunicações sem fios e com fios, tais como Digital Audio Broadcast (DAB), IEEE (Institute of Electrical and Electronics Engineers) 802.11a/g e IEEE 802.16a . Um canal seletivo em frequência é transformado através de OFDM para um conjunto paralelo de frequências.