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5965-7160E (1) - Agilent Digital Modulation in...

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Agilent Digital Modulation in Communications Systems An Introduction Application Note 1298
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2 This application note introduces the concepts of digital modulation used in many communications systems today. Emphasis is placed on explaining the tradeoffs that are made to optimize efficiencies in system design. Most communications systems fall into one of three categories: bandwidth efficient, power efficient, or cost efficient. Bandwidth efficiency describes the ability of a modulation scheme to accommodate data within a limited bandwidth. Power efficiency describes the ability of the system to reliably send information at the lowest practical power level. In most systems, there is a high priority on band- width efficiency. The parameter to be optimized depends on the demands of the particular system, as can be seen in the following two examples. For designers of digital terrestrial microwave radios, their highest priority is good bandwidth efficiency with low bit-error-rate. They have plenty of power available and are not concerned with power efficiency. They are not especially con- cerned with receiver cost or complexity because they do not have to build large numbers of them. On the other hand, designers of hand-held cellular phones put a high priority on power efficiency because these phones need to run on a battery. Cost is also a high priority because cellular phones must be low-cost to encourage more users. Accord- ingly, these systems sacrifice some bandwidth efficiency to get power and cost efficiency. Every time one of these efficiency parameters (bandwidth, power, or cost) is increased, another one decreases, becomes more complex, or does not perform well in a poor environment. Cost is a dom- inant system priority. Low-cost radios will always be in demand. In the past, it was possible to make a radio low-cost by sacrificing power and band- width efficiency. This is no longer possible. The radio spectrum is very valuable and operators who do not use the spectrum efficiently could lose their existing licenses or lose out in the competition for new ones. These are the tradeoffs that must be considered in digital RF communications design. This application note covers the reasons for the move to digital modulation; how information is modulated onto in-phase ( I ) and quadrature ( Q ) signals; different types of digital modulation; filtering techniques to conserve bandwidth; ways of looking at digitally modulated signals; multiplexing techniques used to share the transmission channel; how a digital transmitter and receiver work; measurements on digital RF communications systems; an overview table with key specifications for the major digital communications systems; and a glossary of terms used in digital RF communi- cations.
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