# 8 QAM

• The amplitude gets changed in 8-QAM , and it is not a constant value.
• 3 bits encoded forming tribits and producing 8 different output phase.
• N = 3, M = 2N = 23 = 8
• Therefore 8 output phases will be produced.

8-QAM transmitter: • The difference between 8-QAM and 8-PSK transmitter is the emission of invertor between C channel and Q product modulator.
• Input given to bit splitter where it is converted to parallel 3 channel output.
• It is inphase, quadrature control.
• The bit rate of 3 channel is fb/3.
• I channel bit enters 2 to 4 level convertor.
• Q channel enters 2 to 4 level convertor. Where it converts parallel input digital to analog convertors(DAC).
• With 2 input bits 4 output combinations are possible.

Algorithm:

• I,Q – polarity

Logic 1 – +1V

Logic 0 – -1V

• C – magnitude

Logic 1 – 1.307V

Logic 0 – 0.541V

 I C 0 0 -0.541V 0 1 -1.307 V 1 0 +0.541V 1 1 +1.307V
 Q C 0 0 -0.541V 0 1 -1.307 V 1 0 +0.541V 1 1 +1.307V

Adjucent phase change by 1 bit and is called grade code (or) maximum distance code.

Bandwidth consideration:

Input data rate =fb.

Data rate in I,Q channel = fb /3.

Therefore fundamental frequency,

fa = (fb /3) / 2

 fa = fb /6

The output of BM = (X sin wat) (sin wct)

 wa = 2∏fbt/6
 wc = 2∏fc

= {X sin 2∏fbt/6 } {sin wc = 2∏fc }

= X/2 {cos 2∏ (fc  – fb/6 )t} -{cos 2∏ (fc  + fb/6 )t}

Therefore bandwidth,

BW= (fc + fb/6 ) – (fc  – fb/6 )

=2fb/6

 BW = fb/3
 Q I C 0 0 0 -112.5 0 0 1 -157.5 0 1 0 -67.5 0 1 1 -22.5 1 0 0 112.5 1 0 1 157.5 1 1 0 67.5 1 1 1 22.5