Hybrid transmission of SC-QAM and OFDM-QAM signals in 220GHz terahertz band based on BP DSM and two-stage all-analog frequency conversion

To address the problems of complex architecture, significant signal interference, and weak anti-noise performance of high-order quadrature amplitude modulation (QAM) signals in the terahertz (THz) band single-carrier (SC) and orthogonal frequency division multiplexing (OFDM) hybrid transmission system, this paper proposes a THz band SC-OFDM hybrid transmission scheme based on band-pass delta-sigma modulation (BP DSM) and two-stage all-analog frequency conversion. At the transmitter, this scheme generates two channels of high-order QAM signals modulated by SC and OFDM respectively through software programming; after interpolation filtering, oversampling and digital up-conversion, the signals are fed into two parallel 1-bit discrete-time BP DSM modulators and quantized into on-off keying (OOK) signals with only ±1 levels. The two channels of OOK signals are converted into THz quadrature phase shift keying (QPSK) signals via digital-to-analog conversion (DAC) and two-stage all-analog up-conversion. The resultant THz QPSK signals are then transmitted over a 1-meter wireless link. At the receiver, the received THz QPSK signal is processed through two-stage all-analog down-conversion, analog-to-digital conversion (ADC) and a series of digital signal processing (DSP) procedures, so as to recover the original two channels of high-order QAM signals. Experimental results show that when the up-conversion frequency of BP DSM is set to 1/4 of the DAC sampling rate, the bit error rates (BER) of the SC 512QAM and OFDM 512QAM signals are as low as 9.65×10^-4 and 1.69×10^-3, respectively, both meeting the threshold of hard-decision forward error correction (HD-FEC) of 3.8×10^-3. When the QAM modulation order of the OFDM signal is increased from 64 to 1024, the BER of the SC 512QAM signal remains unchanged, which verifies that the transmission of the two channels of QAM signals is mutually independent. The proposed scheme enhances the anti-interference capability of the system by adopting the BP DSM technology at the software level, and reduces the bandwidth requirements of the system for DAC and ADC by employing the two-stage all-analog frequency conversion technology at the hardware level, thereby providing a feasible technical approach for the high-flexibility and low-complexity applications of THz communication.