Abstract
A new modem circuit technique is proposed for multilevel signal transmission to utilize frequency bandwidth efficiently in high-speed digital carrier transmission systems. With this technique a multilevel amplitude and phase-keying (APK) signal is generated as the sum of the outputs of two multiphase modulators and is detected by tandem phase demodulators. This superposed modem has a simple circuit configuration. It is constructed from phase-shift-keying (PSK) modems. It appears advantageous for realizing high-baud-rate multilevel transmission of severalhundred Megabauds (MBd). Experimental results show the feasibility of a 16-level modem with performance degradation of only about 1 dB from the theoretical predictions.
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