Introduction
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 bandwidth efficiency. The parameter to be optimized depends on the demands of the particular system, as can be seen in the following two xamples.
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 concerned 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. Accordingly, 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, or becomes more complex or does not perform well in a poor environment. Cost is a dominant 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 bandwidth 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 communications.
These concepts form the building blocks of any communications system. If you understand the building blocks, then you will be able to understand how any communications system, present or future, works.
Table of contents
1. Why digital modulation?
1.1 Trading off simplicity and bandwidth
1.2 Industry trends
2. Using I/Q modulation (amplitude and phase control) to convey information
2.1 Transmitting information
2.2 Signal characteristics that can be modified
2.3 Polar display - magnitude and phase represented together
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 use I and Q?
3. Digital Modulation types and relative efficiencies
3.1 Applications
3.1.1 Bit rate and symbol rate
3.1.2 Spectrum (bandwidth) requirements
3.1.3 Symbol clock
3.2 Phase Shift Keying (PSK)
3.3 Frequency Shift Keying (FSK)
3.4 Minimum Shift Keying (MSK)
3.5 Quadrature Amplitude Modulation (QAM)
3.6 Theoretical bandwidth efficiency limits
3.7 Spectral efficiency examples in practical radios
3.8 I/Q offset modulation
3.9 Differential modulation
3.10 Constant amplitude modulation
4. Filtering
4.1 Nyquist or raised 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 impulse response) filter
4.7 Spectral efficiency versus power consumption
5. Different ways of looking at a digitally modulated signal
5.1 Power and frequency view
5.2 Constellation diagrams
5.3 Eye diagrams
5.4 Trellis diagrams
6. 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
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 phase error
8.8 I/Q phase 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
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 bandwidth efficiency. The parameter to be optimized depends on the demands of the particular system, as can be seen in the following two xamples.
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 concerned 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. Accordingly, 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, or becomes more complex or does not perform well in a poor environment. Cost is a dominant 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 bandwidth 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 communications.
These concepts form the building blocks of any communications system. If you understand the building blocks, then you will be able to understand how any communications system, present or future, works.
Table of contents
1. Why digital modulation?
1.1 Trading off simplicity and bandwidth
1.2 Industry trends
2. Using I/Q modulation (amplitude and phase control) to convey information
2.1 Transmitting information
2.2 Signal characteristics that can be modified
2.3 Polar display - magnitude and phase represented together
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 use I and Q?
3. Digital Modulation types and relative efficiencies
3.1 Applications
3.1.1 Bit rate and symbol rate
3.1.2 Spectrum (bandwidth) requirements
3.1.3 Symbol clock
3.2 Phase Shift Keying (PSK)
3.3 Frequency Shift Keying (FSK)
3.4 Minimum Shift Keying (MSK)
3.5 Quadrature Amplitude Modulation (QAM)
3.6 Theoretical bandwidth efficiency limits
3.7 Spectral efficiency examples in practical radios
3.8 I/Q offset modulation
3.9 Differential modulation
3.10 Constant amplitude modulation
4. Filtering
4.1 Nyquist or raised 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 impulse response) filter
4.7 Spectral efficiency versus power consumption
5. Different ways of looking at a digitally modulated signal
5.1 Power and frequency view
5.2 Constellation diagrams
5.3 Eye diagrams
5.4 Trellis diagrams
6. 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
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 phase error
8.8 I/Q phase 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
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