Theortical BER for uncoded AWGN channels for all cases, the function assumes the use of a Gray-coded signal constellation. QAM order M must be at least 4. When k=log2(M) is odd, a rectangular constellation of size M=I×J is used. berlb_CPFSK: returns a lower bound (LB) on the BER of uncoded CPFSK modulation over an AWGN channel. modindex is the modulation index, a positive real number. kmin is the number of paths.