Characterizing nonlinearity in multiantenna multibeam transmitters

Abstract. In this thesis, effects of power amplifier (PA) distortion in multiantenna transmitter is studied. Input signal of each PA in the array is modelled by two or multiple tones to characterize the nonlinearity in terms of intermodulation distortion (IMD). In intermodulation, the phase of the nonlinearity depends on the phases of the corresponding input tones. Hence, in beamforming, progressive phase of the nonlinear components over the antenna elements creates a steered beam for the nonlinearity. Measurement setup is created to measure the phase and amplitude of the IMD components in the PA output. The theoretical polynomial relation of the IMD phase dependency on the input tones is validated by measurements. For flexible measurements, the setup is automatized by standard commands for programmable instruments. Second part of the thesis studies the array IMD by simulations in MATLAB. The used PA model is a memoryless polynomial fitted against the measured amplitude-to-amplitude modulation and amplitude-to-phase modulation responses of a real amplifier. The effects of nonlinearity are studied by using two tones to present each independent data stream in the PA inputs. Hence, in multibeam scenario, each data stream is modelled by two tones having individual phase and amplitude depending on the beamforming coefficients of given stream. The simulations are performed in frequency domain by utilizing the concept of spectral convolution to model the intermodulation distortion, and array factor to model the far-field radiation of the linear and nonlinear PA output components. By utilizing the simulator, PA nonlinearity is analyzed in single-beam and multi-beam scenarios by varying the steering angles, allocated stream powers and amplitude distribution over the PAs. It is observed that IMD terms which depend on only one stream are steered to same direction as the linear terms whereas the IMD terms depending on both streams spreads more in space. This has potentially positive impacts on the signal-to-distortion ratio of the streams observed in beamforming directions

CMOS Signal Synthesizers for Emerging RF-to-Optical Applications

The need for clean and powerful signal generation is ubiquitous, with applications spanning the spectrum from RF to mm-Wave, to into and beyond the terahertz-gap. RF applications including mobile telephony and microprocessors have effectively harnessed mixed-signal integration in CMOS to realize robust on-chip signal sources calibrated against adverse ambient conditions. Combined with low cost and high yield, the CMOS component of hand-held devices costs a few cents per part per million parts. This low cost, and integrated digital processing, make CMOS an attractive option for applications like high-resolution imaging and ranging, and the emerging 5-G communication space. RADAR techniques when expanded to optical frequencies can enable micrometers of resolution for 3D imaging. These applications, however, impose upto 100x more exacting specifications on power and spectral purity at much higher frequencies than conventional RF synthesizers. This generation of applications will present unconventional challenges for transistor technologies - whether it is to squeeze performance in the conventionally used spectrum, already wrung dry, or signal generation and system design in the relatively emptier mm-Wave to sub-mmWave spectrum, much of the latter falling in the ``Terahertz Gap". Indeed, transistor scaling and innovative device physics leading to new transistor topologies have yielded higher cut-off frequencies in CMOS, though still lagging well behind SiGe and III-V semiconductors. To avoid multimodule solutions with functionality partitioned across different technologies, CMOS must be pushed out of its comfort zone, and technology scaling has to have accompanying breakthroughs in design approaches not only at the system but also at the block level. In this thesis, while not targeting a specific application, we seek to formulate the obstacles in synthesizing high frequency, high power and low noise signals in CMOS and construct a coherent design methodology to address them. Based on this, three novel prototypes to overcome the limiting factors in each case are presented. The first half of this thesis deals with high frequency signal synthesis and power generation in CMOS. Outside the range of frequencies where the transistor has gain, frequency generation necessitates harmonic extraction either as harmonic oscillators or as frequency multipliers. We augment the traditional maximum oscillation frequency metric (fmax), which only accounts for transistor losses, with passive component loss to derive an effective fmax metric. We then present a methodology for building oscillators at this fmax, the Maximum Gain Ring Oscillator. Next, we explore generating large signals beyond fmax through harmonic extraction in multipliers. Applying concepts of waveform shaping, we demonstrate a Power Mixer that engineers transistor nonlinearity by manipulating the amplitudes and relative phase shifts of different device nodes to maximize performance at a specific harmonic beyond device cut-off. The second half proposes a new architecture for an ultra-low noise phase-locked loop (PLL), the Reference-Sampling PLL. In conventional PLLs, a noisy buffer converts the slow, low-noise sine-wave reference signal to a jittery square-wave clock against which the phase of a noisy voltage-controlled oscillator (VCO) is corrected. We eliminate this reference buffer, and measure phase error by sampling the reference sine-wave with the 50x faster VCO waveform already available on chip, and selecting the relevant sample with voltage proportional to phase error. By avoiding the N-squared multiplication of the high-power reference buffer noise, and directly using voltage-mode phase error to control the VCO, we eliminate several noisy components in the controlling loop for ultra-low integrated jitter for a given power consumption. Further, isolation of the VCO tank from any varying load, unlike other contemporary divider-less PLL architectures, results in an architecture with record performance in the low-noise and low-spur space. We conclude with work that brings together concepts developed for clean, high-power signal generation towards a hybrid CMOS-Optical approach to Frequency-Modulated Continuous-Wave (FMCW) Light-Detection-And-Ranging (LIDAR). Cost-effective tunable lasers are temperature-sensitive and have nonlinear tuning profiles, rendering precise frequency modulations or 'chirps' untenable. Locking them to an electronic reference through an electro-optic PLL, and electronically calibrating the control signal for nonlinearity and ambient sensitivity, can make such chirps possible. Approaches that build on the body of advances in electrical PLLs to control the performance, and ease the specification on the design of optical systems are proposed. Eventually, we seek to leverage the twin advantages of silicon-intensive integration and low-cost high-yield towards developing a single-chip solution that uses on-chip signal processing and phased arrays to generate precise and robust chirps for an electronically-steerable fine LIDAR beam

Statistical channel modeling for short range line-of-sight terahertz communication

Underutilized spectrum constitutes a major concern in wireless communications especially in the presence of legacy systems and the prolific need for high-capacity applications as well as consumer expectations. From this perspective, Terahertz frequencies provide a new paradigm shift in wireless communications since they have been left unexplored until recently. Such a vast frequency spectrum region extending all the way up to visible light and beyond points out significant opportunities from dramatic data rates on the order of tens of Gbps to a variety of inherent security and privacy mechanisms, and techniques that are not available in the traditional systems. Thus, in this paper, we investigate statistical parameters for short-range line- of-sight channels of Terahertz communication. Short-range measurement campaign within the interval of [3cm, 20cm] are carried out between 275GHz to 325GHz range. Path loss model is examined for different frequencies and distances to provide the insight regarding the effect of the operating frequency. Measurement results are provided with relevant discussions and future directions

Sub-THz Ray Tracing Simulation and Experimental Validation for Indoor Scenarios

Sub-terahertz (THz) communication is envisioned as one of the key components for 6G because of the abundantly available spectrum resource. Accurate and efficient channel models are prerequisites for developing sub-THz communicationsystems. Due to the sparsity and more ray optics propagation characteristics of the sub-THz channel, deterministic Ray-Tracing (RT) has attracted much attention for sub-THz channel modeling, which shows the potential of reducing the simulation complexity yet maintaining the accuracy. This paper presents an implementation of RT for sub-THz channel modeling and demonstrates its performance based on sub-THz channel measurements. A virtual massive multiple-inputmultiple-output (MIMO) channel operating at 100 GHz anda double-directional 300 GHz channel are considered in the RT implementation, where the RT achieves a high similarity compared to the channel measurements in terms of channel impulse response and power angular spectrum. Besides, thenear-field and spatial non-stationary properties of the sub-THz massive MIMO channel and the dominant multipaths of the 300 GHz channel are accurately reconstructed in the RT simulation. This work can provide insights into deterministic sub-THz channel modeling research from the implementation,evaluation, and challenges perspectives