AbstractAlthough the continuous phase FSK (CPFSK) scheme has been used for narrow‐band data transmission, its error rate performance is rather poor. This paper describes a coding method for improving the error rate performance of the narrow‐band CPFSK scheme with modulation indices of 1/3 and 1/4. the proposed method examines the structure of the signal space for the given modulation scheme and avoids using the signal metric of small value in the coding. Hence, the objective of this method is not error correction, but maximization of the minimum signal metric. the application of this method in narrow‐band CPFSK system yields better performance than the MSK scheme in terms of both bandwidth and error