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I/Q offset modulation
Attached Figure 7 Synthesized Constellation
QPSK modulation method
QPSK modulation I and Q bit streams are transformed at the same time, namely the two symbol
clocks are provided synchronously. OQPSK modulation is another solution to solve the problem of
the vector crossing zero point and has been adopted in narrowband CDMA systems. OQPSK is
called Offset-QPSK and is the improved QPSK. It is also called offset quadrature phase shift keying.
It has the same phase relation as QPSK. It also divides the input code stream into two channels and
then performs quadrature modulation. The difference is that it shifts the code streams of the in-phase
and quadrature branches by a half symbol period Tb/2 in time. Due to the offset of half symbol period
of the two branches, only the polarity of one branch may be inverted at a time and the polarity invert of
the symbols in the two branches will not occur at the same time. In addition, other implementation
methods have the same effect as QPSK.
Attached Figure 8 QPSK Modulation Method
Constant envelope digital modulation MSK (minimum shift keying)
FSK modulation does not have as great envelope change as QAM and PSK, but FSK has an
abrupt phase change when the sign changes, which can also cause amplitude abruption and
spectrum regeneration. The minimum shift keying (MSK) is a constant envelope digital modulation
technique developed on the basis of FSK. The minimum shift keying (MSK) is a continuous phase
FSK modulation CPFSK. The continuous phase ensures that the signals have small modulation
bandwidths. The frequency offset Δf of the MSK modulation and its frequency difference 2Δf satisfy
the following equation:
Tb
f
2
1
2
It can be seen from the equation that the frequency offset Δf is equal to one quarter of the
symbol rate l/Tb, namely the frequency difference 2Δf is equal to half the symbol rate. This special
selection is called minimum shift keying (MSK).
