As far as I Know,SAIC mobile stations were designed to demodulate a GMSK carrier and suppress GMSK- modulated interference. These mobile stations are typically designed to suppress one dominant interferer. They can be used to suppress the second sub-channel in an AQPSK modulated signal and therefore can be used as receivers for the VAMOS technique.
However, the SAIC methodology does not exploit the fact that when AQPSK modulation is used, the second sub-channel has a constant 90 degree phase shift with respect to the first sub-channel. Moreover, a SAIC receiver will experience the second sub-channel as the dominant interference and it will lose effectiveness suppressing additional external interferers,hence low SCPIR compared to VAMOS level 1 MS.
VAMOS level 1 MS exploit knowledge of the two TSC’s. When an ordinary QPSK demodulator or a proper modification of a QPSK modulator to deal with AQPSK is used, then joint detection is performed. This type of receiver offers optimal performance whenever the signal is corrupted by Gaussian white noise only. But such a receiver is unable to effectively suppress any external interference.Hence higher SCPIR compared to SAIC level 1 MS and so comes the need of SAIClevel 2 MS.