Terahertz communications has been foreseen as a key enabler to the sixth generation  (6G) of wireless communications systems. However, the design of spectrally-efficient waveforms and modulation schemes is an ongoing challenge in this regime. In this paper, we propose and experimentally demonstrate the transmission of M-ary carrierless amplitude and phase (CAP) modulated signals using a 253 GHz photo-mixing-based terahertz wireless communications system combined with the optical transmission over a 10-km standard single mode fiber (SSMF). Experimental results show that the CAP modulation technique has a capability to support the high-speed transmission of terahertz signals over a wide range of data rates from 4 Gbit/s to 96 Gbit/s based solely on optical intensity modulation at the transmitter side and terahertz envelope detection at the receiver side. Consequently, M-ary CAP can potentially be adopted as a low-complexity waveform and modulation contender to simplify the transceivers architectures for the sixth generation (6G) terahertz communications systems, without sacrificing the high throughput targeted by these systems.

In this work, we present a new Nyquist pulse that shows high tolerance to the timing-jitter, which is one of the key factors that ceil the end-to-end bit error rate performance of terahertz communications systems. As a proof-of-concept, an experiment is conducted using a 300 GHz photonics-based terahertz communications link in order to demonstrate the performance of the proposed waveform.

In this work, we establish the context of pulse shaping for terahertz communications networks that comply with the technical specifications defined in the IEEE 802.15.3d Standard. Importantly, we propose a waveform that full complies with the spectral emission mask of this Standard, which has been developed for terahertz communications.