A two-stage MSK-type detector for low-BT GMSK signals

As a very bandwidth-efficient modulation format, Gaussian minimum shift keying (GMSK) with a low bandwidth-time (BT) product value is important for bandwidth-limited satellite communications. The MSK-type linear detector, which works well for GMSK signals with BT values greater than 0.25, has a large performance degradation when detecting low-BT GMSK signals when compared with the optimum Viterbi equalizer. After decomposing the differential precoded low-BT GMSK signal into a linear component and a nonlinear component with significant signal energy, we propose a two-stage MSK-type detector for low-BT GMSK signals in a coded system. Tentative decisions are generated in the first stage and then used to cancel the interfering nonlinear component in the received signal. The second detection stage deals only with the linear component and a very small amount of the nonlinear component due to the decision errors in the first stage. The detector design methodology is presented. Simulations have been performed using BT=0.143 GMSK example systems and the two-stage MSK-type detector shows significant improvement over the conventional one-stage linear detector in coded systems.