Digital Modulation in Communications Systems

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 examples.

For designers of digital terrestrial microwave radios, their highest priority is good bandwidth efficiency with a 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, 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?
2. Using I/Q Modulation to Convey Information
3. Digital Modulation Types and Relative Efficiencies
   
4. Filtering
5. Different Ways of Looking at a Digitally Modulated Signal Time and Frequency Domain View
   
6. Sharing the Channel
   
7.  How Digital Transmitters and Receivers Work
   
8. Measurements on Digital RF Communications Systems
   
9. Summary
10. Overview of Communications Systems
    
11. Glossary of Terms

 Why Digital Modulation?

The move to digital modulation provides more information capacity, compatibility with digital data services, higher data security, better quality communications, and quicker system availability. Developers of communications systems face these constraints:

• available bandwidth
• permissible power
• inherent noise level of the system 

The RF spectrum must be shared, yet every day there are more users for that spectrum as demand for communications services increases. Digital modulation schemes have greater capacity to convey large amounts of information than analog modulation schemes. 

Trading off simplicity and bandwidth

There is a fundamental tradeoff in communication systems. Simple hardware can be used in transmitters and receivers to communicate information. However, this uses a lot of spectrum which limits the number of users. Alternatively, more complex transmitters and receivers can be used 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 the communication is over air or wire, analog or digital.

Industry trends

Over the past few years, a major transition has occurred from simple analog Amplitude Modulation (AM) and Frequency/Phase Modulation (FM/PM) to new digital modulation techniques. Examples of digital modulation include

• QPSK (Quadrature Phase Shift Keying)
• FSK (Frequency Shift Keying)
• MSK (Minimum Shift Keying)
• QAM (Quadrature Amplitude Modulation)

Another layer of complexity in many new systems is multiplexing. Two principal types of multiplexing (or “multiple access”) are TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). These are two different ways to add diversity to signals allowing different signals to be separated from one another.