The authors compare the simulated bit error rate (BER) performance and adjacent channel interference (ACT) protection capability of generalized tamed-FM (GTFM) and differentially-coherent quadriphase-shift-keying with square-root of raised-cosine filtering (DCQPSK-SRC) in additive white Gaussian noise (AWGN) and Rayleigh fading. The BER performance is obtained for a data rate of 8 kb/s with channel spacings of both 12.5 kHz and 6.25 kHz. These channel spacings and data rate correspond to those investigated by Levesque, Pizzi and Duncombe (1990) for a proposed standard for narrowband digital land mobile radio (Federal Standard 1024). For simulations in Rayleigh fading we examine RMS Doppler spreads, B/sub RMS/, up to 100 Hz. They show that DCQPSK-SRC is more sensitive to timing errors than GTFM. The adjacent channel interference (ACI) is characterized using the adjacent channel interference protection ratio (ACIPR). GTFM operating at 8 kb/s can achieve an ACIPR of 59 dB in a 12.5 kHz channel with nonlinear amplification. When linearly amplified, DCQPSK-SRC operating at 8 kb/s can achieve an ACIPR of 100 dB in a 12.5 kHz channel and 78 dB in a 6.25 kHz channel.>