Cellular networks have become the cornerstone of modern communications in the past decades. Starting with the first deployments by the end of 1970s until nowadays, cellular networks have connected every place on earth, offering services to billions of people. Initially planned for voice communication only, over time they have enabled support for various applications, some of which are characterized by stringent traffic requirements, ranging from high data rates, extremely low latencies, up to an almost 100% reliability. Of course, this was not an abrupt process, but it took years until this became possible, and several generations of cellular networks, each of which brought significant improvements to the user experience.
The current generation of cellular networks, 5G, deployed first in 2019, is experiencing rapid development. 5G networks arose from the need to support data-intensive applications, such as virtual reality, augmented reality, extended reality, high-quality live video streaming, and online gaming, which was not possible with the previous generations of cellular systems, on the one hand, and providing access to a huge number of machine-type devices, on the other. 5 The research community backed up by industrial players have already started working on 6G networks, planned to be fully operational by 2030.
With the ever-increasing number of users running these bandwidth-hungry applications, even the operation of 5G is jeopardized, given the scarcity of radio spectrum. Therefore, it is of paramount importance to properly allocate the network resources so that as many users can run cutting-edge applications on their smartphones with a satisfying level of service. This is one of the aspects considered in this habilitation.
Further, applications related to immersive communications require, besides radio resources, computation resources as well to obtain results quickly. This fact, together with the rapid development in the area of mobile edge computing has enabled the rise of in-network computing feature in 5G, where data-intensive tasks can be executed on edge clouds, which are usually collocated with base stations. Hence, understanding the performance limits of edge clouds has become particularly important. Besides understanding the achievable performance, optimizing different aspects of edge clouds is important too. To that end, as part of this habilitation, we solve different optimization problems taking into account various resource limitations.
The dynamic nature of cellular communications requires providing support for mobile users when leaving the area of coverage of one base station and entering the area of another base station, a process known as handover. Therefore, it is also important to investigate mobility management in cellular networks, and in particular in 5G. More importantly, considering the joint problem of performing handovers and resource allocation to users within several base stations while they are moving around is challenging. This should be done while providing performance guarantees to the users and considering different types of fairness. This is to be followed by solving optimization problems, whose results should lead to considerable improvements compared to the results obtained using the current 3GPP-based standards. This is the third aspect covered in this habilitation.
Another way of improving the performance of wireless communication systems is by predicting correctly the channel conditions. The common way used nowadays is by transmitting pilot symbols. However, frequent pilot transmissions lead to an increase in overhead signaling, increasing the latency, making it detrimental for time- and reliability-sensitive applications. A representative example are robots in factory environments. Given the nature of the traffic in this use case, reducing the signaling overhead is of paramount importance. A way to achieve this is by using depth images as part of a machine learning system for predicting the channel state information without feedback and pilot signals. Such a system should be capable of predicting even the channel conditions in the next slots. This approach in optimizing the cellular network operation is the fourth aspect considered in this habilitation.
Although the aforementioned topics are related more to the Radio Access Network part of the cellular network, we have also considered optimization aspects that cover the Core Network too, encompassing this way the entire cellular architecture. In addition, we have also focused on minimizing energy consumption, as a contribution to sustainability, which is one of the key pillars of next generation cellular networks.
In this habilitation, we follow two broad approaches that lead to performance improvements of different aspects in the operation of cellular networks. In the first, we formulate constrained optimization problems, and solve them using the respective techniques, depending on the type of the optimization problem at hand (convex optimization problems, integer linear programs, mixed-integer linear programs), including the tools from Machine Learning, such as Reinforcement Learning. The second approach consists in exploiting one or more of the features of the problem to obtain an optimal solution, without using the traditional techniques in formulating and solving (constrained) optimization problems.
In all the problems considered in this habilitation, we compare our approaches against state of the art for different scenarios, running simulations mostly on publicly-available datasets, and show the advantages our approaches offer in terms of the optimality of the results and reduced complexity. The results obtained here can be used by cellular operators to optimize various aspects of cellular network operation as well as to provide different types of fairness.