Portland State University. Department of Electrical and Computer Engineering
Date of Award
Doctor of Philosophy (Ph.D.) in Electrical and Computer Engineering
Electrical and Computer Engineering
1 online resource (xxi, 324 pages)
Antenna arrays, Modulation (Electronics), Signal processing
This dissertation investigates the use an array of active nonlinear elements, with particular emphasis on controlling distortion products generated by nonlinear elements in space rather than using conventional ways such as transmission lines, waveguides, and power dividers and combiners. The nonlinear elements are made of assemblies of antennas and electronic switches, called modulated scatterers (MSs). These so-called MSs elements are utilized in a wide variety of applications such as radio frequency identification (RFID) systems, microwave imaging, Internet-of-Things sensors, etc. However, no research work has been reported in the literature regarding exploiting and controlling several distortion products generated by MSs at the same time according to the best of authors' knowledge. To facilitate controlling distortion products which means suppressing or enhancing distortion products in space, we present a nonlinear array with elements that are MSs instead of conventional antennas. MSs are switched ON-OFF at different times by modulation signals having the same frequency. The time delay of the switching process between array elements represents a relative phase shift difference in the frequency domain. Thus, the presented structure is called the phase-modulated scattering array (PMSA). The PMSA has a similar layout of phased arrays, but it does not have a feeding network and is fed by an external source called the illuminating source. Because our system does not need a feeding network and phase shifters, it is potentially easier to implement with low cost. Two different signals which are the illuminating (incident) and modulation signals interact inside switches to generate a huge number of distortion products due to the nonlinearity of switches and the periodic nature of the presented system. Distortion products then leave the presented PMSA to space again (i.e., scattering distortion products). The PMSA is able to treat distortion products and achieve beamforming functions.
The operation mechanism of the PMSA is explained by developing two different mathematical models. Communication signal processing perspectives are the basis of the first mathematical model developed to show the spatial characteristics of distortion products generated by our presented PMSA. Its root is originated from a mathematical model of the widely-used polyphase multipath technique in RF communication circuits. However, the adopted technique is suitable only for communication circuits with a single output and parameters prescribed in advance. Thus, the model is further developed to circumvent all the problems mentioned above and to be able to detect the spatial characteristics of distortion products at any point in space. Static impacts of the measurement environment, real radiation patterns of actual antennas utilized in prototypes, and phase and gain errors among paths have been taken into account as well. In the model, every single scatterer is represented by a single separate path. Furthermore, the modified model is extended to include single, two, and multi tones modulation signals. Simulation results have been obtained before and after the modification for a different number of paths and modulation signals with different tones. Results show that the modified model can quantify spatial characteristics of distortion products at any point in space where specific distortion products are enhanced, and others are canceled. Because distortion products are independent in their nature (i.e., each single distortion product has different frequency and phase), they have independent radiation patterns (scattered beams). Therefore, the second mathematical model based on phased antenna array perspectives is developed. The relationship between the two models states that a distortion product which is enhanced at a certain point in space has a maximum scattered beam at that point. Also, the second mathematical model being similar to mathematical models of phased arrays considers effects of all distortion products resulting from single, two, and multi tones modulation signals, and it states that each single distortion component has its particular scattered beam.
Next, sub-models for some properties and applications of the presented PMSA such as a diffraction grating-like behavior, nonreciprocity, beamforming, a tool for distortion product analysis of phased arrays and multi-input multi-output (MIMO systems), a reconfigurable-spatial harmonic generator, and a direction finding technique are derived depending on the two main mathematical models. All parts are simulated and results validate all proposed functionalities.
Single antennas, antenna arrays, electronic switches (modulators), and a 4-to-8 phase transformer kit using only resistors have been designed, simulated, fabricated, assembled, and tested.
Eventually, different structures of the presented PMSAs working at 432MHz and 2.3GHz are tested inside the anechoic chamber. Both frequencies are downconverted to the band 2-22kHz. Modulation signals used in the experimental setups are single and two tones. Data are measured using the commercial software SigView running on a laptop and a spectrum analyzer. Both spatial characteristics and scattered beams of distortion products are measured. Comparisons have been made between measured received responses of scattered signals and theoretical results. They are in good agreement although limitations and challenges are encountered with each round of measurement. Measured results confirm practically that as a number of scatterers increases, more distortion products are controlled at the same time. The distortion product rejection ratio DPRR is more than 15dB for all measured distortion products supposed to be canceled. Directions of scattered beams are found at expected locations with errors less than 3%. Furthermore, directions of illuminating signals or distances separating between PMSA elements are varied to change directions of scattered beams when prescribed values of parameters governing the overall performance are being broken. In other words, the beamforming functionality has been validated practically. Different elements of 8*1-PMSA are turned-off at measurements in order to find fault tolerances of the presented system. Measured results show that when two elements are failed simultaneously, responses can be accepted to some extent.
Alkhafaji, Nasr Nomas Hussein, "UHF and Microwave Phase-Modulated Scattering Array" (2019). Dissertations and Theses. Paper 4998.