PERSEUS offers a breakthrough for cell-free networks

At a crowded festival or much-anticipated sporting event, your videos just won't send despite your many desperate attempts? On a high-speed train, your videoconference that seemed stable a few kilometers ago suddenly freezes? Rest assured, you are not alone. All of us experience these scenarios in the age of 5G, which nonetheless promises ultra-fast speeds and low latency. So why do these network fluctuations persist? The problem lies in the very architecture of our cellular networks.

Based on fixed geographical cells, today's networks rely on transmit-receive antennas that serve each user according to their position. This infrastructure reaches its limits when users move or when connection density increases: insufficient throughput, repeated outages, increased latency. These limits are becoming increasingly problematic with the rise of connected objects (IoT) - watches, medical sensors, autonomous cars... - and the growing demand for quality of service.

To meet these needs, research is exploring new infrastructures, including cell-free networks. Rather than organizing connections around fixed cells, cell-free networks place users at the center, offering homogeneous connectivity through collaboration between multiple antennas. This paradigm shift comes with its technical challenges, which the PERSEUS project sets out to address. Two IMT Atlantique professors, Catherine Douillard and Charbel Abdel Nour, are co-sponsors for Institut Mines-Télécom of this project dedicated to the technological building blocks of the PEPR Networks of the Future.

A new user-centered approach

In conventional networks, each geographical cell has at its center a base station with an antenna, which serves all users within the well-defined boundaries of the cell. When a user moves, he changes cell and another antenna takes over to serve him. This transfer, known as handover, relies on an exchange of information between base stations to ensure that the connection is not interrupted.

With this process, “if a user is very far away, on the edge of two cells, he often suffers a degradation in signal quality”, explains Charbel Abdel Nour, a researcher specializing in channel coding and resource allocation techniques. The same applies when many users try to connect simultaneously. “In both cases, you need more time-frequency resources to receive the throughput you need."

With cell-free networks, these limitations disappear, as the user is no longer connected to a single antenna, but can be served by several antennas simultaneously. By combining them, they provide an optimal signal at lower power. “This collaboration ensures uniform, continuous coverage, with no break in connection,” continues Abdel Nour. Catherine Douillard, an expert in the physical layers of communication systems, underlines another key advantage: “Cell-free networks enable optimized resource management, whether in terms of spectrum, power or time, while reducing energy impact.” This efficiency is decisive in meeting the future needs of a growing number of connected objects and users.

The challenge of distributed MIMO

To make cell-free networks a reality, PERSEUS relies on distributed MIMO: a key technology in which several antennas work together to concentrate signal power and limit interference to users. However, this collaboration requires an additional exchange of signals between antennas. Finding the best compromise between the quantity of signaling information exchanged and the level of interference acceptable for the targeted quality of service therefore represents a major challenge.

« Nous travaillons sur des méthodes avancées de correction en réception et d’optimisation des ressources pour minimiser les effets indésirables », expose Charbel Abdel Nour. « Nous étudions notamment des techniques de modulation du signal, très robustes vis-à-vis des décalages en temps ou en fréquence de l’onde reçue, afin de pouvoir servir plusieurs utilisateurs simultanément. Ces techniques permettent d’augmenter la capacité du réseau sans en augmenter la consommation électrique. »

“We are working on advanced reception correction and resource optimization methods to minimize undesirable effects,” explains Charbel Abdel Nour. “In particular, we are studying signal modulation techniques that are highly robust to time or frequency shifts in the received wave, so as to be able to serve several users simultaneously. These techniques make it possible to increase network capacity without increasing power consumption."

Unlike other PEPR projects exploring millimeter or terahertz frequencies, PERSEUS focuses on frequency bands below 7 GHz. At very high frequencies, many antennas can be grouped together on a single site, but at lower frequencies, they need to be spread over several sites. “Hence the interest in cell-free, which consists in getting the distributed antennas to work together so that they point the power to a single location”, argues Catherine Douillard.

Waveforms for synchronization

However, the implementation of cell-free networks is still largely theoretical, and raises major synchronization challenges. “In a distributed system, when several antennas serve a user, their signals often arrive at different times, due to their respective distances or the presence of obstacles on the propagation channel,” explains Charbel Abdel Nour. These variations make it difficult to receive and decode data, especially when a large number of antennas are working together. The same applies to synchronizing a large number of users, each of whom transmits a wave with its own delay.

To solve this problem, conventional networks rely on constant exchanges of information between antennas and users to precisely synchronize signals. Much of the data exchanged is used to synchronize signals rather than to transmit truly useful information, reducing the overall efficiency of the network. “And the more transmissions there are, the more data needs to be exchanged for synchronization,” adds Catherine Douillard.

PERSEUS is therefore exploring techniques to reduce these exchanges as much as possible, including innovative waveforms enabling operation in relaxed synchronization - the receiver is able to synchronize even with small shifts in signal arrival - or even in asynchronism, with large shifts. “These solutions limit unnecessary information exchanges between antennas and users, saving both bandwidth and energy”, adds the researcher.

Technological solutions between exploration and adaptation

Finally, PERSEUS also relies on complementary technologies such as Reconfigurable Intelligent Surfaces (RIS), which help shape the radio channel to improve connectivity. These surfaces act as adjustable mirrors that intelligently reflect radio signals and reroute them to users in areas with poor coverage. “In a stadium, for example, the connection between the relay antenna and certain users may be obstructed by walls or bleachers. RIS then redirects the signals in a targeted way, improving coverage and quality of service”, illustrates Charbel Abdel Nour. These technologies could improve network efficiency while reducing energy consumption. However, they are still the subject of a great deal of research, particularly into manufacturing methods based on the use of metamaterials.

All the technologies used in conventional networks are thus adapted and optimized to meet the new challenges of cell-free networks. “All the solutions we have been working on for years - such as waveforms, multi-user management and error-correcting codes - find a direct application in PERSEUS, but in a framework that requires them to be reinvented,” concludes Charbel Abdel Nour.

« Lay the foundations for standardizable technological innovations »

Launched in July 2023, the PERSEUS project is scheduled to last 56 months. It brings together academic and industrial partners, including CNAM, Inria, CEA and several CNRS laboratories, who are contributing to various technical aspects, including waveforms, resource allocation and electronic components.

For their part, Catherine Douillard and Charbel Abdel Nour have set themselves the clear ambition of registering a series of patents based on innovations developed within the PERSEUS framework, notably around waveforms. Their work is part of a tradition of applied research within their team, recognized for its major contributions to telecommunications, such as the invention of turbocodes. “Our aim with PERSEUS is to demonstrate the advantages of cell-free networks in terms of quality of service and energy efficiency, while laying the foundations for standardizable technological innovations,” sums up the researcher. Signal measurement campaigns in a distributed MIMO environment have already been carried out in Lille, to assess the potential of this technology under real-life conditions.