Summary: A High--Performance ReducedComplexity GMSK
Naofal AlDhahir \Lambda and Gary Saulnier y
A four--state adaptive MLSE demodulator for GMSK modulation (BT=0:3) on AWGN channels is
analyzed and simulated. This demodulator uses the linear representation of GMSK signals [6, 5] and
achieves near--optimum BER performance. The channel impulse response used in the MLSE demodulator
is initialized to the highest--energy component in the linear representation and then adapted in a decision--
directed mode to offset any performance losses incurred by initially ignoring other lower--energy (and
time--varying) components in the linear representation. The number of MLSE states is reduced to two, at
about 0:1 dB performance loss, by implementing a whitening matched filter which concentrates most of
the GMSK pulse energy in its two leading samples.
Gaussian minimum shift keying (GMSK) has been adopted as the digital modulation scheme for
the European global system for mobile communications (GSM) standard due to its spectral efficiency
and constant--envelope property [7, 8]. These two characteristics result in superior performance in the
presence of adjacent channel interference (ACI) and non--linear amplifiers.
Since a GMSK signal is obtained from an MSK signal by prefiltering it with a narrow--band Gaus
sian filter, it can likewise be interpreted as a special case of continuous--phase frequency shift keying
(CPFSK) with a modulation index of 0:5 or as filtered offset quadrature phase shift keying (OQPSK).