Modulation Techniques (Part Two) - FSK, QPSK, QAM

Modulation Techniques (Part Two)

Although, this article refers to telephone modems and analogue audio modulation, the same principles apply to satellite modems. This material has been used for the tutorial as it explains the principles well.

By Eugene Blanchard, Edited by Joshua Drake, Bill Randolph, Phuong Ma
Copyright © 2001 by Commandprompt, Inc.
Copyright © 2001 by Eugene Blanchard.
Copyright © 2000 by Eugene Blanchard.

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Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

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Modulation Used in Modems

• FSK - Frequency Shifted Keying
  
• QPSK - Quadrature Phase Shifted Keying
  
• QAM - Quadrature Amplitude Modulation

FSK - Frequency Shift Keying

Frequency Shift Keying (or FSK) is the frequency modulation of a carrier that represents digital intelligence. For Simplex or Half Duplex operation, a single carrier is used - communication can only be transmitted in one direction at a time. A Mark or 1 is represented by Freq A, and a Space or 0 is represented by Freq B. FSK is not really used in satellite communications because of the inefficient use of bandwidth and frequencies.

QPSK - Quadrature Phase Shift Keying

Quadrature Phase Shift Keying employs shifting the phase of the carrier plus an encoding technique. QPSK is used in almost all modems. The digital information is encoded using 4 (Quad) level differential PSK.

The data is encoded as follows:

Quadrature Amplitude Modulation refers to QPSK with Amplitude Modulation. Basically, it is a mix of phase modulation and amplitude modulation. QAM phase modulates the carrier and also modulates the amplitude of the carrier.

Phase Modulated and Amplitude Modulated Carrier:

There are two types, 8-QAM and 16-QAM:

16-QAM has 12 phase angles, 4 of which have 2 amplitude values!

This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)

Distribution of substantively modified versions of this document is prohibited without the explicit permission of the copyright holder.

Distribution of the work or derivative of the work in any standard (paper) book form is prohibited unless prior permission is obtained from the copyright holder.' to the license reference or copy.