The soft-input–soft-output (SISO) MSK demodulator
is a key component in the iterative receiver for coded MSK
systems. To reduce the complexity of the optimal SISO MSK
demodulator using the BCJR algorithm, a low complexity suboptimal
MSK demodulator was developed by K. R. Narayanan et al.
However, our investigation on the symmetric information rate of
MSK with the suboptimal demodulator shows that it incurs a
significant information rate loss compared with the BCJR demodulator.
To avoid the rate loss, a low complexity SISO MSK
demodulator is derived based on a new factor graph for MSK,
which is found to have comparable complexity to the suboptimal
demodulator. Moreover, the proposed algorithm is equivalent to
the BCJR algorithm and thus achieves optimal performance.
Simulation results for LDPC coded MSK systems are provided to
demonstrate the effectiveness of the proposed algorithm.