Offset QPSK[Under Modulation >> Digital modulation]
Below is a constellation diagram for QPSK.
Between 01 & 10, and 00 & 11, there is entire 180° phase shift. This abrupt changes can lead certain undersirable effects. Understandable way to minimise abrupt phase shifts, is to delay phase shifts of quarature component by half symbol time resulting overall smoother changes.
Minimum Shift Keying (MSK)[Under Modulation >> Digital modulation]
Here is further variation to Offset QPSK. What if instead of discrete symbols of 1 and 0, we take +ve half sinusoid as 1 and -ve half sinusoid as 0 ? There would be more smoothness to phase shifts. In mathematical terms, this comes to multiplying by sin() function. Below diagram *roughly* illustrate the concept.
Note that this is done for both I and (delayed) Q component.
In CPM (Continuous Phase Modulation) , the phase shifts are done over the complete symbol duration. This way MSK turns out to be under CPM techniques. In addition with summation of I and Q component, it result into frequency changes and so MSK is viewed as "continuous phase frequency shift keying".
Refer tutorial available on compextoreal site and Wikipedia article for more.
© Copyright Samir Amberkar 2010
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