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Digital Modulation in Communications Systems—An Introduction
Application Note 1298
This application note introduces the concepts of digital modulation ud 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 nd 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 en 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 becau 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 becau the phones need to run on a battery. Cost is also a high priority becau cellular phones must be low-cost to encourage more urs. Accord-in
gly, the systems sacrifice some bandwidth efficiency to get power and cost efficiency. Every time one of the efficiency parameters (bandwidth, power, or cost) is incread, another one decreas, 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 u the spectrum efficiently could lo their existing licens or lo out in the competition for new ones. The are the tradeoffs that must be considered in digital RF communications design. This application note covers
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•the reasons for the move to digital modulation;•how information is modulated onto in-pha (I) and quadrature (Q) signals;
•different types of digital modulation;
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•filtering techniques to conrve bandwidth; •ways of looking at digitally modulated signals;•multiplexing techniques ud 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 ud in digital RF communi-cations.
The concepts form the building blocks of any communications system. If you understand the building blocks, then you will be able to under-stand how any communications system, prent or future, works.
Introduction
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12 12 12 13 14 14 15 15 16 17 18 19 20 21 22 22 23 23 24 25 26 27 28 29 29 30 311. Why Digital Modulation?
1.1 Trading off simplicity and bandwidth
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1.2 Industry trends
2. Using I/Q Modulation (Amplitude and Pha Control) to Convey Information
2.1 Transmitting information
2.2 Signal characteristics that can be modified
2.3 Polar display—magnitude and pha reprented
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2.4 Signal changes or modifications in polar form
2.5 I/Q formats
2.6 I and Q in a radio transmitter
2.7 I and Q in a radio receiver
2.8 Why u I and Q?
3. Digital Modulation Types and Relative Efficiencies
3.1 Applications
3.1.1 Bit rate and symbol rateatretochoana eilti
3.1.2 Spectrum (bandwidth) requirements
3.1.3 Symbol clock
3.2 Pha Shift Keying (PSK)
3.3 Frequency Shift Keying
3.4 Minimum Shift Keying (MSK)fsad
3.5 Quadrature Amplitude Modulation (QAM)
3.6 Theoretical bandwidth efficiency limits
3.7 Spectral efficiency examples in practical radios
3.8 I/Q offt modulation
3.9 Differential modulation
3.10 Constant amplitude modulation
4. Filtering
4.1 Nyquist or raid cosine filter
4.2 Transmitter-receiver matched filters
4.3 Gaussian filter
4.4 Filter bandwidth parameter alpha
4.5 Filter bandwidth effects
4.6 Chebyshev equiripple FIR (finite impul respon) filter
托福报名如何缴费4.7 Spectral efficiency versus power consumption
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5. Different Ways of Looking at a Digitally Modulated Signal Time and Frequency Domain View
5.1 Power and frequency view
5.2 Constellation diagrams
5.3 Eye diagrams
5.4 Trellis diagrams
Table of Contents
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466. Sharing the Channel
6.1 Multiplexing—frequency
6.2 Multiplexing—time
6.3 Multiplexing—code
6.4 Multiplexing—geography
6.5 Combining multiplexing modes
6.6 Penetration versus efficiency
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7. How Digital Transmitters and Receivers Work
7.1 A digital communications transmitter
7.2 A digital communications receiver
8. Measurements on Digital RF Communications Systems 8.1 Power measurements
8.1.1 Adjacent Channel Power
8.2 Frequency measurements
8.2.1 Occupied bandwidth
8.3 Timing measurements
8.4 Modulation accuracy
8.5 Understanding Error Vector Magnitude (EVM)
8.6 Troubleshooting with error vector measurements
8.7 Magnitude versus pha error
8.8 I/Q pha error versus time
8.9 Error Vector Magnitude versus time
8.10 Error spectrum (EVM versus frequency)
9. Summary
10. Overview of Communications Systems
11. Glossary of Terms
Table of Contents (continued)
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The move to digital modulation provides more information capacity, compatibility with digital data rvices, higher data curity, better quality communications, and quicker system availability. Developers of communications systems face the constraints:
•available bandwidth
•permissible power
•inherent noi level of the system
The RF spectrum must be shared, yet every day there are more urs for that spectrum as demand for communications rvices increas. Digital modulation schemes have greater capacity to con-vey large amounts of information than analog mod-ulation schemes. 1.1 Trading off simplicity and bandwidth
There is a fundamental tradeoff in communication systems. Simple hardware can be ud in transmit-ters and receivers to communicate information. However, this us a lot of spectrum which limits the number of urs. Alternatively, more complex transmitters and receivers can be ud to transmit the same information over less bandwidth. The transition to more and more spectrally efficient transmission techniques requires more and more complex hardware. Complex hardware is difficult to design, test, and build. This tradeoff exists whether communication is over air or wire, analog or digital.
Figure 1. The Fundamental Tradeoff
1. Why Digital Modulation?
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