jasnoch
Joined: 31 Dec 2005
Posts: 19
Helped: 2
Location: Rio Rancho, New Mexico
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 14 Mar 2006 21:56 Re: QPSK and pi/4QPSK???? |
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| tags: pi/4 qpsk pi 4 qpsk |
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Hey,
QPSK modulation represents symbols by a constellation of four phase angles of an RF carrier. There are two bits per symbol. So for example 00 = 45 degrees, 01 = 135 degrees, 10 = 225 degrees and 11= 315 degrees. The phase is free to change from any position to any other position depending on the data transmitted. This is not good since when a phase change of 180 degrees occurs there is a large and fast amplitude change of the carrier. This greatly increases the bandwidth of the channel as well as putting big demands on system lineararity and dynamic range.
Pi/4 QPSK gets around this to some extent by limiting phase change from one symbol to another to four possibilties. Namely +/- 45 degrees or +/- 135 degrees. Hence avoiding the 180 degree phase change. As you can see there are now eight positions in the pi/4 QPSK constellation as opposed to four positions in the QPSK constellation but only only four positions of the pi/4 QPSK constellation are allowable at any time since only phase changes of +/-45 degrees or +/-135 degrees are allowed.
There are many variations of QPSK designed to avoid large phase changes such as offset QPSK, Feher's QPSK to name a few. But that's another subject.
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