📄 qam basic.htm
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<H1>Quadrature Amplitude Modulation</H1></CENTER>
<HR>
<B>Quadrature amplitude modulation</B>, or <B>QAM</B>, is a big name for a
relatively simply technique. It is simply a combination of amplitude modulation
and phase shift keying.
<P>Let's just dive right in. We'll use a signal that is transmitting at 3600
bps, or 3 bits per baud. This means that we can represent 8 binary combinations.
<P>We'll use 2 measures of amplitude, 1 and 2, just we did before. We'll also
use 4 possible phase shifts, like we did before. Combining the two, we have 8
possible waves that we can send. How convenient. :)
<P>First step is to generate a table to show us which waves correspond to which
binary combination. This can basically be done at random, although modem
manufacturers have agreed on standards.
<P>
<CENTER>
<TABLE border=1>
<TBODY>
<TR align=middle>
<TD><B>Bit value</B></TD>
<TD><B>Amplitude</B></TD>
<TD><B>Phase shift</B></TD></TR>
<TR align=middle>
<TD>000</TD>
<TD>1</TD>
<TD>None</TD></TR>
<TR align=middle>
<TD>001</TD>
<TD>2</TD>
<TD>None</TD></TR>
<TR align=middle>
<TD>010</TD>
<TD>1</TD>
<TD>1/4</TD></TR>
<TR align=middle>
<TD>011</TD>
<TD>2</TD>
<TD>1/4</TD></TR>
<TR align=middle>
<TD>100</TD>
<TD>1</TD>
<TD>1/2</TD></TR>
<TR align=middle>
<TD>101</TD>
<TD>2</TD>
<TD>1/2</TD></TR>
<TR align=middle>
<TD>110</TD>
<TD>1</TD>
<TD>3/4</TD></TR>
<TR align=middle>
<TD>111</TD>
<TD>2</TD>
<TD>3/4</TD></TR></TBODY></TABLE></CENTER>
<P>Let's encode a big bit stream:
<P>
<CENTER>001010100011101000011110 </CENTER>
<P>First, we break it up into 3-bit triads:
<P>
<CENTER>001-010-100-011-101-000-011-110 </CENTER>
<P>Now all we have to do is figure out what the resulting signal should look
like. Remember that we shift each wave <I>relative to the wave before it</I>!
<P>
<CENTER><IMG src="QAM basic_files/qam.gif"> </CENTER>
<P>(These values <I>are</I> correct.)
<P>This is the technique most often used. You can imagine how many different
amplitudes and phase shifts are needed for 28.8 kbps, which uses 24 bits per
baud! </P></BODY></HTML>
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