Abstract — A solid-state 216-GHz communication link is presented. It utilizes a novel mixer-less transmitter technique to achieve efficient complex modulation using frequency multipliers (which are inherently nonlinear). The transmitter operates with coherent power combining of two frequency multiplier chains to achieve precise linear amplitude and phase modulation despite the strong nonlinearity of the multipliers and the saturated power amplifiers. A 16-QAM modulation signal is demonstrated with a transmitted output power of 100 mW. I. INTRODUCTION he interests in millimeter-wave (mmW) and THz semiconductor technologies are witnessing very rapid development [1]–[3]. In addition, interest has been growing in using high-mmW and THz frequencies for communications [4] – [7] and imaging [8]. Power combining has been utilized to obtain significant output power [9]-[10]. Operating frequencies in the 0.2 – 1 THz range often exceed the cutoff (ft) and the maximum frequency of oscillation (fmax) of most semiconductor devices. As such, the availability of many active components (such as low-noise amplifiers, power amplifiers, and active mixers) is limited, and one is left with passive components. As a result, the attainable solid-state output power available at this frequency range is limited. Many transmitters have relied on the use of frequency multipliers and/or passive mixers to generate and modulate signals. Most of these blocks are either intrinsically nonlinear (e.g. frequency multipliers) or are operated nonlinearly (e.g. saturated amplifiers) to obtain the highest power. Thus, most of the communication links demonstrated in this frequency regime, so far, have relied on simple modulations such as on-off keying (OOK), or low order phase modulations such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK). However, the spectral efficiency of such modulation schemes is limited compared to higher order modulations (such as 32QAM) that have varying-amplitudes. Reports of higher order modulations are very few [6], and [7] (where a 670 GHz 64QAM link utilizing InP devices was demonstrated); and they usually require some output power back-off. Supporting communications links with varying-amplitude modulations at the high frequency regime introduces several problems. First, power generation and amplification are difficult. Second, there are strong nonlinearities present, in most cases, caused by the use of multipliers and the desire to obtain maximum power from amplifiers and mixers. Third, setting up a frequency plan to eliminate unwanted signals (such as image rejection) requires low-loss high-order filters that are difficult to realize. Thus, many of the demonstrations are done with double-side band signals. This paper presents a novel mixer-less technique for generating general-purpose modulations (such as nQAM with varying amplitude/phase) in the presence of highly nonlinear components (such as frequency multipliers, and amplifiers operating in highly saturated mode). The new transmitter architecture is termed direct quadrature transmitter (DQ- transmitter). It was inspired by the linear amplification using nonlinear components (LINC) architecture technique [11]-[12], and our earlier work on continuous phase modulation (CPM) where full power (no back off) was used with a 16-level signal [13]. To the best of our knowledge, this is the first demonstration of a variable amplitude/phase 16QAM modulation at frequencies above 200 GHz with significant output power. The single side band signal is obtained without the use of filters. The improvement in output power is the result of avoiding the use of mixers (which have limited convergence efficiency) and filters. Additionally, there is no need to back- off on power since the architecture works efficiently even with strong nonlinearities in the transmit chain. II. DQ-TRANSMITTER ARCHITECTURE The operation of the DQ-Transmitter relies on a simple concept. Consider Fig. 1 where two equal-amplitude vectors, a (= |a|a), and b (= |b|b), are added to produce their vector- sum C (= |C|c). By adjusting the angles a, and b, one can produce a vector C with any arbitrary angle c, and any arbitrary amplitude |C|; provided that |C| ≤ |a| + |b| = 2|a|. Fig. 1 Example of construction of an arbitrary vector C (= |C|c) using two equal amplitude vectors a (= |a|a), and b (= |b|b). Efficient Linear Transmission of Complex Waveforms at 216 GHz Using Nonlinear Multiplier Chains Ali Darwish 1 , Joe Qiu 1 , Edward Viveiros 1 , H. Alfred Hung 1 , and Jeffrey Hesler 2 1 Army Research Laboratory, Adelphi, MD 20783 2 University of Virginia, Charlottesville, VA, 22902 T U.S. Government work not protected by U.S. Copyright