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This paper investigates the throughput performance of a CSMA/CA based network of land mobile radios (LMR) over a Rayleigh fading FM channel. The throughput calculations are based on the medium access (MAC)/physical (PHY) cross-layer characteristics for the distributed digital radios (DDR). From PHY-layer perspective, the effects of additive white Gaussian noise (AWGN), Rayleigh fading and push-to-talk (PTT) delays are considered while the MAC-layer perspective includes the performance of carrier sense multiple access with collision avoidance (CSMA/CA) for a half duplex narrowband FM (NBFM) channel. The throughput performance was simulated for channel data rates of 12000 bps in presence of additive white Gaussian noise (AWGN) and Rayleigh fading. Typical values for PTT delays for two of the most commonly used LMR terminals were calculated experimentally. The results show that in addition to fading and number of DDR nodes in a network, the push-to-talk (PTT) delays associated with LMR radios significantly influence the achievable throughput of the system.