Normally GMSK with BT = 0.5 should give the same performance as BPSK. For Eb/N0 = 4, and 8 samples per symbol , the corresponding SNR for the simulation is -5.03 dB.
The theoretical BER for Eb/No = 4 dB of BPSK is 0.0125 and that works for the BPSK case. If I switch to GMSK, the BER is around 0.025, which is about twice of what I expect.
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