Have you been at a crowded festival or major sports event and experienced difficulty sending videos, despite many desperate attempts? Have you been in a video conference on a high-speed train when the image suddenly freezes after being stable a few miles back? Rest assured, you are not alone. In the age of 5G, we all have similar experiences, despite the promise of ultra-fast networks and low latency. So, why do these fluctuations persist? The problem lies in the very architecture of our cell 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, self-driving cars, etc. – 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-leaders for Institut Mines-Télécom of this project dedicated to the technological building blocks of the Networks of the Future PEPR.
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, they change cell and another antenna takes over to serve them. 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, they often suffer 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.
“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 asynchrony, 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”, explains 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.
