QPSK:

• Quadrature Phase Shift Keying is referred as QPSK.
• In QPSK maximum phase shift is limited to about 90 degree.
• In QPSK, first input bit stream is split into two bit streams referred as odd and even.These streams are applied simultaneously to the mixers.
• Power spectral density(PSD) of QPSK modulated spectrum is same as OQPSK.
• QPSK doesn't prevents phase transition from the origin by shifting one stream by a bit period.
• Null Bandwidth is 1.0 X Data rate.
• Amplitude variations are of the order of 30dB.

OQPSK:

• Offset Quadrature Phase Shift Keying is referred as OQPSK.
• In OQPSK maximum phase shift is about +/- 90 degree.
• In OQPSK, after splitting the bit stream into odd and even, one bit stream is made offset by 1 bit period with respect to the other.
• Power spectral density(PSD) of OQPSK modulated spectrum is same as QPSK.
• OQPSK prevents phase transition from the origin by shifting one stream by a bit period and allowing only one bit to change between the transitions.
• Null Bandwidth is 1.0 X Data rate.
• Amplitude variation are of the order of 3 dB.