Realization of the future 5G systems requires the design of novel mm-wave base station antenna systems that are capable of generating multiple beams with low mutual interference, while serving multiple users simultaneously using the same frequency band. Besides, small wavelengths and high packaging densities of front-ends lead to overheating of such systems, which prevents safe and reliable operation. Since the strict cost and energy requirements of the first phase 5G systems favor the use of low complexity beamforming architectures, computationally efficient signal processing techniques and fully passive cooling strategies, it is a major challenge for the antenna community to design multibeam antenna topologies and front-ends with enhanced spatial multiplexing, limited inter-beam interference, acceptable implementation complexity, suitable processing burden and natural-only/radiative cooling.
Traditionally, array design has been performed based on satisfying the given criteria solely on the radiation patterns (gain, side lobe level (SLL), beamwidth etc.). However, in addition to the electromagnetic aspects, multibeam antenna synthesis and performance evaluation in 5G systems at mm-waves must combine different disciplines, including but not limited to, signal processing, front-end circuitry design, thermal management, channel & propagation and medium access control aspects.
Considering the interdisciplinary nature of the problem, the main objective of this
research is to develop, evaluate and verify innovative multibeam array techniques and solutions for 5G base station antennas, not yet used nor proposed for mobile communications. The research topics include the investigation of (i) new array topologies, compatible with IC passive cooling, including sparse, space tapered arrays and optimised subarrays, meeting key requirements of 3-D multi-user coverage with frequency re-use and power-efficient side-lobe control, (ii) adaptive multiple beam forming strategies and digital signal processing algorithms, tailored to these new topologies, and (iii) lowcost/competitive and sufficiently generic implementation of the above array topologies and multi beam generation concepts to serve multiple users with the same antenna(s) with best spectrum and power efficiencies.
This doctoral thesis consists of three parts. Part I focuses on the system-driven aspects which cover the system modeling (including the link budget and precoding), propagation in mm-wave channels and statistical assessment of the Quality of Service (QoS). Although separate comprehensive studies exist both in the field of propagation/system modeling and antennas/beamforming, the link between the two disciplines is still weak. In this part, the aim of the study is to bridge the gap between the two domains and to identify the trade-offs between the complexity of beamforming, the QoS and the computational cost of precoding in the 5G multibeam base station arrays for various use cases. Based on the system model developed, a novel quantitative relation between the antenna SLLs/pattern nulls and the statistical QoS is established in a line-of-sight (LoS) dominated mm-wave propagation scenario. Moreover, the potential of using smart (low in-sector side-lobe) array layouts (with simple beam steering) in obtaining sufficiently high and robust QoS, while achieving the optimally low processing costs is highlighted. For a possible pure non-line-of-sight (NLoS) scenario, the system advantages (in terms of the beamforming complexity and the interference level) of creating a single, directive beam towards the strongest multipath component of a user are explained via ray-tracing based propagation simulations. The insightful system observations from Part I lead to several fundamental research questions: Could we simplify the multiple beamformingarchitecture while keeping a satisfying QoS? Are there any efficient yet effective alternative interference suppression methods to further improve the QoS? How should we deal with the large heat generation at the base station? These questions, together with the research objectives, form the basis for the studies performed in the remaining parts.
Part II of the thesis focuses on the electromagnetism-driven aspects which include innovative, low-complexity subarray based multibeam architectures and new array optimization strategies for effective SLL suppression. The currently proposed multibeam 5G base stations in the literature for beamforming complexity reduction use either a hybrid array of phased subarrays, which limits the field-of-view significantly, or employ a fully-connected analog structure, which increases the hardware requirements remarkably. Therefore, in the first half of this part, the aim is to design low-complexity hybrid (or hybrid-like) multiple beamforming topologies with a wide angular coverage. For this purpose, two new subarray based multiple beamforming concepts are proposed: (i) a hybrid array of active multiport subarrays with several digitally controlled Butler Matrix beams and (ii) an array of cosecant subarrays with a fixed cosecant shaped beam in elevation and digital beam forming in azimuth. Using the active (but not phased) multiport subarrays, the angular sector coverage is widened as compared to that of a hybrid array of phased subarrays, the system complexity is decreased as compared to that of a hybrid structure with a fully-connected analog network, and the effort in digital signal processing is reduced greatly. The cosecant subarray beamforming, on the other hand, is shown to be extremely efficient in serving multiple simultaneous co-frequency users in the case of a fairness-motivated LoS communication thanks to its low complexity and power equalization capability. Another critical issue with the currently proposed 5G antennas is the large inter-user interference caused by the high average SLL of the regular, periodic arrays. Therefore, in the second half of Part II, the aim is to develop computationally and power-efficient SLL suppression techniques that are compatible with the 5G’s multibeam nature in a wide angular sector. To achieve this, two novel techniques (based on iterative parameter perturbations) are proposed: (i) a phase-only control technique and (ii) a position-only control technique. The phase-only technique provides peak SLL minimization and simultaneous pattern nulling, which is more effective than the available phase tapering methods in the literature. The position-only technique, on the other hand, yields uniform-amplitude, (fully-aperiodic and quasi-modular) irregular planar phased arrays with simultaneous multibeam optimization. The latter technique combines interference-awareness (via multibeam SLL minimization in a predefined cell sector) and thermal-awareness (via uniform amplitudes and minimum element spacing constraint) for the first time in an efficient and easy-to-solve optimization algorithm.
Part III of the thesis concentrates on the thermal-driven aspects which cover the ther mal system modeling of electronics, passive cooling at the base stations and the role of antenna researchers in array cooling. The major aim here is to form a novel connection between the antenna system design and thermal management, which is not yet widely discussed in the literature. In this part, an efficient thermal system model is developed to perform the thermal simulations. To effectively address the challenge of thermal management at the base stations, fanless CPU heatsinks are exploited for the first time for fully-passive and low-cost cooling of the active integrated antennas. To reduce the size of the heatsinks and ease the thermal problem, novel planar antenna design methodologies are also proposed. In the case of having a low thermal conductivity board, using a sparse irregular antenna array with a large inter-element spacing (such as a sunflower array) is suggested. Alternatively, for the densely packed arrays, increasing the equivalent substrate conductivity by using thick ground planes and simultaneously enlarging the substrate dimensions is proven to be useful.
The performed research presents the first ever irregular/sparse and subarray based antennas with wide scan multibeam capability, low temperature, high efficiency power amplifiers and low level of side lobes. The developed antenna arrays and beam generation concepts could have also an impact over a broad range of applications where they should help overcome the capacity problem by use of multiple adaptive antennas, improve reliability and reduce interference.