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A new system enhancement method is proposed for the EIA/TIA-136 system offering both channel operational range extension and improved performance within the current operational range. The research presented investigates the application of the narrowband adaptive multi-rate (NB-AMR) speech codec and the wideband AMR (WB-AMR) codec, both originally designed for the 200 kHz GSM channel, in the TDMA (TIA/EIA-136) 30 kHz system. In particular, we investigate adaptively allocating bits between NB/WB speech coding and error control within the limited channel bandwidth. Four modes out of seventeen have been carefully chosen for the new TDMA/AMR system. Switching between codec rates as channel conditions change produces range extension below a C/I of 15 dB while also improving performance in the existing operational range above 15 dB. Time slot formats are unchanged so that the method is completely compatible with existing 136 systems.