Selasa, 07 Mei 2013

DIGITAL MODULATION ; WHY DIGITAL MODULATION


1. 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.
1.1 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 communication is over air or wire, analog or digital.
Figure 1. The Fundamental Trade-off
1.2 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)
Figure 2. Trends in the Industry
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