Wireless technology is in a rapid chase of higher data rates to satisfy the growingrndemands of services and devices which will all interconnect. One of the envisionedrntechnologies to fulfill this is Terahertz wireless communication owing tornwide bandwidth and possible bitrates into the Tbps. Challenges however exist withrnTerahertz source generation. One such candidate based on Schottky diode carrierrnupconversion may be a low cost solution, nonetheless phase noise presents a bottleneckrnin such an upconversion scheme for Phase Shift Keying (PSK) and QuadraturernAmplitude Modulation (QAM).rnIn this study, high bitrate communication was investigated with respect to phasernnoise and Local Oscillator (LO) (Terahertz (THz)) obtained by frequency multiplication.rnTerahertz transceivers with both Zero-Intermediate Frequency (IF) and doublernconversion architectures were designed. The transceivers were evaluated with simulationrnusing PSK and QAM modulation schemes and examined for the optimalrnfrequency range. Due to the excellent properties of graphene as well as future relevancernGraphene Field Effect Transistor (GFET) and Schottky Barrier Diode (SBD)rnwere the primary device technologies.rnHigh bitrate communication was possible at 209GHz with double conversion architecture.rnFive Gbps was obtained with Binary Phase Shift Keying (BPSK), 10Gbpsrnwith Quadrature Phase Shift Keying (QPSK), 15Gbps with 8 Phase Shift Keyingrn(8PSK), 20Gbps with 16-ary Quadrature Amplitude Modulation (16QAM) and 30Gbpsrnwith 64-ary Quadrature Amplitude Modulation (64QAM) and had Error VectorrnMagnitude (EVM) of 40.9%, 49.1%, 12.9%, 28.7% and 13.1% respectively. A 73 mrnGFET was employed as the Radio Frequency (RF) transistor technology. The 185.9GHzrnLO was designed with Schottky diode multiplication and had peak phase noise ofrn-101dBc=Hz. The frequency multiplication process was found to be limited to thernlower THz range of less than 375GHz, for satisfactory communication performance.rnActive frequency multipliers degrade the phase noise for high order PSK. Howeverrna x2 subharmonic mixer can be used, if the LO power is limiting the conversionrnefficiency. Single stage Schottky multipliers with multiple harmonics and filteringrnwas found to produce high power but also high phase noise as result of thernButterworth filter. As a result the single stage multipliers are suitable for Pulse AmplitudernModulation (PAM) only. A frequency threshold for the SBD THz LO beyondrnwhich spectrally efficient modulation is not optimal is contributed in this research.rnIn addition the GFET was demonstrated in its viability, by its successful use in allrnTHz analog circuits in the In Phase - Quadrature (IQ) (de-) modulator. Demonstrated by the high bit rates achieved in this work, Schottky multiplied THzrncarriers and graphene Field Effect Transistor (FET)s make a cost effective route for 6G wireless as well as Local Area Network (LAN) and Personal Area Network (PAN)rnwithin the lower THz range.