Optical Beamforming Networks for Next Generation Mobile Communication Systems

Τσώκος, Χρήστος; Tsokos, Christos

dc.contributor.author	Τσώκος, Χρήστος	el
dc.contributor.author	Tsokos, Christos	en
dc.date.accessioned	2023-02-15T10:10:01Z
dc.date.available	2023-02-15T10:10:01Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/57142
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.24840
dc.rights	Αναφορά Δημιουργού-Μη Εμπορική Χρήση-Όχι Παράγωγα Έργα 3.0 Ελλάδα	*
dc.rights.uri	http://creativecommons.org/licenses/by-nc-nd/3.0/gr/	*
dc.subject	Optical beamforming networks	en
dc.subject	Photonic integration circuit technology	en
dc.subject	5G networks	en
dc.subject	Multi-element array antennas	en
dc.subject	Microwave photonics	en
dc.subject	Οπτικά δίκτυα σχηματοποίησης	el
dc.subject	Φωτονική ολοκλήρωση	el
dc.subject	Δίκτυα 5ης Γενεάς	el
dc.subject	Στοιχειοκεραίες	el
dc.subject	Μικροκυματική φωτονική	el
dc.title	Optical Beamforming Networks for Next Generation Mobile Communication Systems	en
heal.type	doctoralThesis
heal.classification	Microwave Photonics	en
heal.language	en
heal.access	campus
heal.recordProvider	ntua	el
heal.publicationDate	2022-10-11
heal.abstract	The 5G vision for a fully mobile and connected society defines a set of performance requirements for next-generation wireless networks, which is very broad and demanding and is still unfulfilled. Compared to the current generation, different system performance indicators are required for the 5G communication systems, including peak data rates on the gigabit per second level, end-to-end latencies in the order of a few milliseconds, very high traffic volume density, and improved spectral, energy, and cost efficiencies. With these requirements on the table and in particular, because of the huge growth of mobile data demand, it has become apparent to the research community that the very limited spectrum resources available in the sub 6-GHz band can no longer meet the system needs. Therefore, high-speed mobile communication systems at frequencies above 6 GHz became a promising path. However, as the operating frequency shifts deeper into the millimetre band, the system performance starts to deteriorate due to the severe free-space loss and blockage effects experienced by the electromagnetic signals at these bands. To overcome this hurdle, high-gain antenna arrays with hundreds or even thousands of antenna elements can be deployed, forming very directional beams which greatly enhance the signal-to-noise ratio. However, these beams provide only limited spatial coverage, which is not compliant with a dynamic environment with multiple users. Within this context, beamforming, a well-known functionality in modern wireless networks thanks to its ability to steer a single or multiple wireless beams to a user or group of users that should be served at each point of time, is expected to become even more important. Both in the case of single- and the case of multi-beam operation, beamforming networks apply proper excitation signals to each antenna element with time delays that correspond to the intended beam directions. In current beamforming networks operating in the sub 6-GHz band, adjustment of these delays is realized in the electronic domain using digital electronic platforms, analog microwave components, or hybrid implementations. However, at higher operating frequencies the fractional bandwidth of the signals gets higher, and the number of the AEs larger, these electronic solutions start having significant limitations in terms of time delay accuracy across the signal bandwidth, insertion loss across the same bandwidth, physical size and weight, electromagnetic interference, power consumption (especially in the case of digital solutions) and cost. In order to overcome this problem, solutions based on microwave photonics (MWP) technology have been proposed as a powerful alternative for the processing of microwave or mm-wave signals and the implementation of high-performance beamforming networks in the optical domain. Within this framework, the present dissertation deals with the design, implementation, system integration, and performance evaluation of optical beamforming networks (OBFNs) for multi-element array antennas. Following a holistic and systematic approach, we start by analysing the theory that dictates the operation of such complex systems and we continue with the design, implementation, system integration as well as performance evaluation of four different OBFNs. In particular, the theoretical analysis starts from the fundamental theory of a single antenna element and continues with the theory that dictates the operation of multi-element antenna arrays. Focusing on multi-element antenna arrays following a linear geometry, the theory of the array factor is introduced. Subsequently, the concept of beamforming is introduced and how single-beam , multi-beam, and multicast operations can be theoretically achieved. At the physical layer, the BFNs can be based on either true time delay elements or phase shifters. The main difference between these two approaches is the beam squint effect which is present only in phase shifters based BFNs and leads to a frequency dependent radiation pattern. In addition to their ability to control the steering angles of the wireless beams, the BFNs can also control the power of the sidelobes thanks to a technique known as amplitude tapering which is a very important feature since it reduces the emission/reception from/to unwanted directions. Before introducing the concept of the OBFNs, a thorough analysis of the principle operation of the MWP systems is provided. The key building blocks of a typical MWP link are the laser source, the optical modulators, and the photodiode. After describing their operation and their key parameters, the key figure of merits of an MWP system, namely the link gain, noise figure, and spurious-free dynamic range, are defined. Having a solid theoretical background, the design, implementation, system integration, and performance evaluation of a 2×4 OBFN based on bulk micro-optics elements are reported. More specifically, the 2×4 TTD-OBFN is developed using commercial off-the-shelf optical modulators, optical delay lines (ODLs), optical attenuators, optical couplers, and photodiodes. It can support single-beam, multi-beam, and multicast beamforming with continuous tuning of the beam angles and feed an array antenna with four elements. The operation of the proposed OBFN is based on a number of ODLs for introducing time delays to the copies of the signals that correspond to different wireless beams and different antenna elements. As a first step, through extensive simulation studies, its principle of operation is validated and then, by using signals with high-order quadrature amplitude modulation formats at 15 GHz, the OBFN is evaluated for all three modes of operation (single-beam, multi-beam, and multicast). Leveraging the advancements in photonic integration technology, a 1×4 OBFN photonic integrated circuit (PIC) based on the hybrid integration of InP components in a Si3N4 platform is presented. The OBFN-PIC consists of a hybrid external cavity laser, two InP phase modulators, a Si3N4 SSBFC optical filter, four Si3N4 OTTDLs based on tuneable eight micro-ring resonators, and four InP photodetectors and can process microwave signals with central frequencies up to 40 GHz and impose continuously tuneable time delays over wide operating bandwidths. Apart from measuring its key figure of merits, the prototype is extensively tested using modulated signals at 5 GHz and 10 GHz and evaluated in terms of bit-error ratios, exhibiting error-free operation for all the experimental scenarios. This is the very first time that a fully functional and self-contained OBFN is evaluated within system experiments in lab settings. Especially, for application in 5G and beyond 5G mobile wireless communication networks, OBFNs that can support multi-beam operation is of utmost importance. The most promising architecture is the multi-beam optical beamforming network based on Blass matrix. However, no details have been reported until now to explain in depth the optical Blass matrix concept, describe the methods for its design, and give an insight into its expected beamforming performance. For the first time, a solid background for the design and operation of a multi-beam optical beamforming network based on Blass matrix is provided via a mathematical analysis and extensive simulation studies. The analysis starts from the processing steps that are required for the microwave photonics signals at the input and output of the Blass matrix and a simple design and an algorithm for the configuration of the matrix, taking into account the properties of the Mach-Zehnder Interferometers (MZIs) as tunable optical couplers is proposed. The multi-beam capability of the beamforming network is validated and the impact of the beam squinting effect which is inherent to this design is evaluated as a function of the symbol rate, modulation format and pulse shaping of the signals. Furthermore, assuming operation with quadrature amplitude modulation (QAM) signals at 28.5 GHz, it is proven through error-vector magnitude (EVM) calculations that the beam squinting effect is not critical in typical cases, where the symbol rate remains below 3 Gbaud. Moreover, the additional frequency dependence of the proposed beamforming network due to inevitable asymmetries of the MZIs and length variations of the waveguides inside the Blass matrix, and the additional impact of imperfections is investigated with respect to the couplers inside the MZIs and the phase shifters inside the Blass matrix. In all cases, the impact of the asymmetries and the imperfections remain negligible for realistic fabrication and operation conditions. Within the framework of the European-funded project, ICT-HAMLET, the first-ever fully integrated optical beamforming networks based on the Blass-matrix architecture were designed, fabricated, and evaluated. A 2×2 and an 8×8 optical beamforming networks have been fabricated based on the hybrid integration of the InP, PolyBoard and TriPleX photonic platforms. The optical beam forming networks are based on PZT-based phase actuators for fast and low-power configuration. Both prototypes are extensively characterized in lab settings and the main individual building blocks are evaluated. However, due to the complexity of the integration of these systems, their system characterization could not be assessed. Nevertheless, the lessons learned from these testing campaigns have been used as the starting points for the design and fabrication of the next generation of optical beamforming networks based on the Blass-matrix which is currently being developed by the successor of HAMLET, the European-funded project ICT-TERAWAY. As demonstrated in this dissertation, thanks to the advancements in photonic integrated circuit technology, MWP enables the realization of key functionalities in microwave systems that either are complex or even not directly possible in the RF domain. Among the several systems that MWP can radically transform, mobile wireless communication systems stand on top of this list, and it is expected that MWP will enable the commercial deployment of the 5G mobile communication networks as well as the next generations in beyond 5G era.	en
heal.advisorName	Αβραμόπουλος, Ηρακλής	el
heal.committeeMemberName	Αβραμόπουλος, Ηρακλής	el
heal.committeeMemberName	Ουζούνογλου, Νικόλαος	el
heal.committeeMemberName	Πλέρος, Νικόλαος	el
heal.committeeMemberName	Παναγόπουλος, Αθανάσιος	el
heal.committeeMemberName	Κοζύρης, Νεκτάριος	el
heal.committeeMemberName	Βαρβαρίγος, Εμμανουήλ	el
heal.committeeMemberName	Σούντρης, Δημήτριος	el
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Ηλεκτρολόγων Μηχανικών και Μηχανικών Υπολογιστών	el
heal.academicPublisherID	ntua
heal.numberOfPages	197 σ.	el
heal.fullTextAvailability	false