Improved decision boundaries of narrowband M-ary CPFSK

Improved decision boundaries of narrowband Mary CPFSK with LDI, DC, and CDDD receivers are reported at modulation index 1/M. A simple method to determine each improved decision boundary by considering only the two closest symbols to that boundary is discussed. Numerical results presented for quaternary and octonary signals indicate that improved boundaries are almost identical to optimal boundaries and perform significantly better than the standard boundaries.