On determining soft output of the cellular text telephone modem (CTM) demodulator

Using soft output of a demodulator is essential to improve performance of the following channel decoder, and performance of the overall communication system. In this study, aspects on determining the soft output of the cellular text telephone modem (CTM) demodulator are described. The optimal soft value of individual demodulated bits, which is explicitly related to correlator output for symbols, is derived for an AWGN channel. When additional information from the cellular receiver, such as the bad (speech) frame indicator, is available at the CTM receiver, it can be used to properly manipulate the obtained soft output of the CTM demodulator. By employing the CTM receiver optimized in this way for text transmission via the GSM-based PCS1900, a performance gain of up to 1-2 dB in channel signal-to-noise ratio can be obtained, in comparison with the example solution by the 3rd Generation Partnership Project (3GPP).