DESIGN, ANALYSIS AND CHARACTERIZATION OF TUNABLE BANDPASS FILTERS FOR 5G AND BEYOND

Abstract

As compared to the current 4G mobile technology, a higher bandwidth and a lower latency will be provided by 5G and beyond technologies, which will be deployed in 2020s, providing connectivity to billions of devices and enabling bandwidth with more than hundreds of megabits per second (Mb/s) and latency less than 1 millisecond (ms). The 5G wireless communication systems will therefore require broadband, multiband and tunable radio-frequency (RF) bandpass filters (BPFs). Current research activities in the RF tunable filter field is to increase levels of reconfiguration in the filter transfer function in respect of centre frequency, bandwidth and filtering type (e.g., controllable single/multiband operations or switchable bandpass/bandstop responses). Hence, new developments in implementation of efficient bandpass filters without much compromise in filter performance are highly desirable. A lot of research has been done on the realization of BPFs. Researchers across the globe have investigated various performance issues of filter design and implementation using several concepts such as coupling matrix synthesis, transmission zeros concept, multiplemode resonator, intrinsic switching concept, substrate-integrated waveguide filters, multiband filters, defected ground structures and spoof surface plasmon polaritons (SSPPs) based structures, etc. Recently, researchers have verified that highly confined surface electromagnetic (EM) waves, namely spoof surface plasmon polaritons or designer surface plasmon polaritons, can be supported by textured metal surfaces with subwavelength scale grooves or dimples. These artificially engineered material-based structures (also known as plasmonic metamaterial) can be used in the implementation of RF through THz integrated circuits with a broad operational bandwidth as compared to that bandwidth achievable in devices implemented by means of ordinary transmission lines and stubs. Plasmonic metamaterial-based structures exhibit passband and stopband and therefore they are filtering structures by nature. An important advantage of these engineered materials is the fact that their dispersion characteristics and cut-off frequencies can be controlled by the geometrical parameters of unit element. Plasmonic metamaterial-based transmission line structures must be designed as per the impedance requirements at the design frequencies because frequency-dependent characteristic impedance is exhibited by the above-mentioned structures. So as to increase the degrees of freedom and fulfil impedance and phase requirements at more than two frequencies, the number of elements of the artificial lines should be vi increased for multiband functionality. At two arbitrary frequencies, dual-band components exhibit certain functionality; applying the same concept, the required functionality for triband, quad-band and, in general, to n-band can be fulfilled at three, four and n controllable frequencies, respectively. Conventional distributed RF components cannot be called multiband components as the values of operating frequencies do not correspond to the system requirements. However, a periodic response and the required functionality are exhibited at the design frequency and odd harmonics by these components. They are hence termed as singleband components as their functionality can be fulfilled at a single-design frequency. Compared to the conventional lines, a larger number of free parameters, by dispersion and impedance engineering, assist in the design of dual-band and multiband components. The impedance and phase requirements (for the transmission lines and stubs of the circuit) at the two design frequencies, f1 and f2, should be fulfilled in order to realize a dual-band operation. The dispersion diagram can be tailored using artificial line structure to fix the phases to the requisite values at the two operating frequencies. Thus, this research work focuses on the theory, circuit models and implementations of plasmonic metamaterial-based transmission line structures. Many applications of plasmonic metamaterial-based transmission line structures are based on the superior controllability of the dispersion characteristics and characteristic impedance of these lines. The attention is also focused towards the development of state of the art in the BPF design, analysis and implementation so that its performance enhancement can be achieved at RF/microwave frequencies for which traditional techniques are inadequate. The broad objective of this thesis is design, analysis and characterization of tunable bandpass filters for 5G and beyond. The research contribution and outcome have been described in six chapters, which are discussed below: Chapter 1 highlights the importance of the topic and motivation behind the work. Further, it gives an idea of current state of the art and challenges and major research gaps. The proposed methodology, formulation of the problem of the thesis and the significance of this work are also discussed. In Chapter 2, the concept and theory of SSPPs is discussed in detail and EM analysis of SSPP structure and its applications are elaborated. The state-of-the-art literature surveys have also been presented in this chapter. In Chapter 3, the SSPP unit cell structures have been described. This chapter provides the novelty in unit cells .On the basis of the series resonant circuit theory, a new unit cell is designed by introducing inductive element (L) in series with capacitive arm (C). Each vii of these elements (L and C) is realized from a short section of transmission line having a length smaller than quarter wavelength. The analysis of unit cell using circuit model is also discussed. Chapter 4 primarily focuses on mode converters that are used to convert quasi transverse electro-magnetic (QTEM) mode of conventional circuitry to transverse magnetic (TM) mode of spoof surface plasmon polariton (SSPP) structures. This chapter also discussed the alternatives to conventional transmission lines for the design and implementation of radio-frequency (RF) components having superior characteristics. These lines are referred to as SSPPs transmission lines and this resulting term is as extensive as the number of strategies that can be envisaged for improving the size, performance or functionality of ordinary lines. In Chapter 5 broadband BPFs, multiband filters and reconfigurable aspects are discussed. The detailed studies on different configurations of BPFs have been done. In this chapter, broadband SSPPs BPFs are developed using different approaches. The broad passband is splitted into two bands using upper and lower coupled structures. The proposed dual-band filter design includes combination of two technologies: coupling coefficient synthesis and SSPPs structure. Reconfigurability aspects of SSPPs coupled structure using different ways are also discussed. Furthermore, by demonstrating the controlling of SSPPs on the hybrid waveguide through varying the structure geometry and physical properties of the dielectric medium, it can be inferred that SSPPs could be highly controllable and their bandwidth on the hybrid waveguide could be conveniently adapted as per the network requirement. By using varactor-based SSPPs BPF and ring resonator, tunability aspects are also demonstrated. The conclusions drawn from the work and the suggestions for future work are presented in the Chapter 6. The publications based on the present work and the references made in the text are provided at the end of the each chapter