Equivalence of MSK and Offset QPSK

Hi everyone,

i'm currently trying to create a offset QPSK modulation using an MSK with a half-sine matched filter. This is based on the IEEE paper https://ieeexplore.ieee.org/document/4102435. I generated an OQPSK and MSK signal with MATLAB and plotted the angle of the signal. Both can be seen below. There is no frequency error or noise. They spectrum for both is the same and the amplitude of the symbol stays constant, so this is indeed a OQPSK signal. Just the resulting bits are different.

I can demodulated the OQPSK using the OQPSK demodulation function from the Communications Toolbox as well as the with the symbol order based on the phase seen in the picture. So the OQPSK is correct. I can also demodulate the MSK with a normal FSK demodulater and the ouput is the same as with the OQPSK. But when applying any kinf of OQPSK demodulation to the MSK signal the resulting sequence is different as well as the phase (picture).

So is a MSK with Half-Sine Filter really equivalent to an OQPSK and both can be received with the same receiver?

This is the phase of the MSK signal

And this is the phase of the OQPSK signal

Best Regards,

Axel

Answer 1

[UDAYA PEDDIRAJU]

Dear Axel,

MSK with a half-sine filter and OQPSK share similar spectral properties and constant envelope characteristics, but they are not fully equivalent.

1. Signal Characteristics: Both maintain a constant amplitude and have similar spectra, but differ in bit mapping and phase trajectory.
2. Phase Continuity: MSK is a type of CPFSK with a smooth phase trajectory, akin to OQPSK's offset phase transitions.
3. Demodulation: They require different demodulation techniques due to differences in phase mapping. An MSK signal cannot be accurately demodulated with an OQPSK demodulator, and vice versa, without adjustments.

while they appear similar, MSK with a half-sine filter and OQPSK are not interchangeable in terms of demodulation.