Effects of channel fading on a direct sequence spread spectrum signal at 2 GHz.
Item
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Title
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Effects of channel fading on a direct sequence spread spectrum signal at 2 GHz.
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Identifier
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AAI9510663
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identifier
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9510663
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Creator
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Ghassemzadeh, Saeed Seyed.
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Contributor
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Adviser: Donald L. Schilling
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Date
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1994
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Language
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English
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Publisher
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City University of New York.
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Subject
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Engineering, Electronics and Electrical
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Abstract
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This dissertation considers the most critical parameter in a Personal Communication System; Multipath Fading, and its impact on system performance.;It is shown that the received power loss due to fading (the fade margin before despreader) increases dramatically as the signalling bandwidth decreases below 11 MHz. In particular, comparing the fade margins of 1 MHz and 15 MHz bandwidth signals reveal a difference in fade margin of approximately 10 dB.;Further, it is shown that the multipath observed in practice had almost all of its energy concentrated in a time interval of less than 1 {dollar}\mu{dollar}sec (for high antenna heights) with the largest, most significant signal components about 100-200 ns apart. For low antenna heights the excess delay is concentrated in a time interval of less than 500 n sec with main signal components 40 nsec apart.;The suburban experiments performed to evaluate the fade margins after, despreading, reveal an increase in fade margin of 15 dB with probability 0.001, for a one finger receiver. The variations tend to decrease less when the chip rate is increased. Since, a wider band spread spectrum system can resolve most of the multipath returns, it can use a coherent RAKE receiver to lower its fade margin by up to 14 dB. This improvement cannot be achieved by narrowband CDMA systems which typically have bandwidths less than 10 MHz.;It is also shown that in a particular environment, if a distant multipath signal is present, and if this reflection is delayed by more than 1{dollar}\mu{dollar}s compared to the main return, then a narrowband spread spectrum system can use RAKE to collect this added power. It should be noted, however, that a narrowband system must collect this extra energy since its main signal rays suffer from severe multipath fading independent of the other delayed signals. However, locations having a main and a distant multipath signal, which exceeded 1 {dollar}\mu{dollar}sec delay and which was of amplitude comparable to that of the primary return, were found to be confined to a very low probability of existence within a prescribed cell.;Indoor measurements reveal that there are 5 dominant returns when a 24 Mchips/s system is used. The typical excess delay was 350 nsec and individual components were often 40 nsec apart. It was shown that a coherent RAKE receiver with five fingers could improve the fade margin by approximately 9 dB. Again this improvement cannot be achieved by a narrowband system since such a system could not distinguish between the multipath returns.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
