Design of a non-data-aided differential-QAM modem for real-time speech communication: a software-defined-radio PC-based approach

In this paper, a non-data-aided differential-QAM (NDA-DQAM) system is proposed to solve the phase ambiguity problem which arises in continuous speech transmission. Moreover, the system design and test are done by the software-defined radio (SDR) approach which is based only on the PC soundcard and Matlabreg software for a minimum-cost implementation of the NDA-DQAM system. In so doing, first, the speech signal is converted into bit stream by the adaptive delta modulation. Then using two differential angles mapped by Gray-coded dibits, a passband 16-DQAM waveform can be generated. It is shown that the 16-DQAM system incurs only about 0.5-dB performance degradation as compared to the coherent 16-QAM system. With the SDR approach, all the system parameters and TX/RX algorithms are embodied in high-level Matlab programs. A free-running T/2-spaced receiver structure is especially used with interpolation-based timing recovery and Costas carrier synchronization. Finally, experimental results are included to show how real-time speech transmission can be readily achieved. Hence this paper also suggest an efficient and cost-effective approach to teach a design-oriented communication lab course