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Post-Cellular Wireless Networks

Periodic Reporting for period 4 - POSTCELL (Post-Cellular Wireless Networks)

Reporting period: 2021-04-01 to 2022-09-30

POSTCELL aims at laying the foundation for future generations of wireless networks as they move past the current cell-centric paradigm and enter the post-cellular era.

Cellular systems have undergone four generational transitions, stretching from the 1st Generation (1G) of analog telephony in the 1980s to the advanced and fully digital 5th Generation (5G). Despite their major differences, subsequent generations have all conformed to the same architecture that underpinned 1G, namely the cellular architecture (See Fig. 1). A cellular network tessellates a territory into cells, each centered on a site where a so-called base station houses all the necessary equipment to radio-communicate with users.

The cellular architecture has served us well and has remained unchallenged, to the point that base stations have witnessed generational transitions as hardware upgrades. However, the time is approaching when this architecture will have become exhausted and, unlike in previous transitions, a more transformational change that transcends the cellular architecture will be required. There is broad consensus on two ways to tackle this transformation:
- A dramatic scaling of the number of antennas per base station.
- Extreme densification.

While seemingly exclusive, we posit that the two ideas above are complementary, and a driver of POSTCELL was that, with a proper architecture, these can become two sides of a coin we term massification:
- Scaling the number of antennas per base station amounts to localized massification.
- Extreme densification can be regarded as distributed massification.
Reconciling these ideas and enabling a truly phenomenal degree of massification calls for an entirely new architecture where cells are transcended and base stations are deconstructed. Precisely, some of the base station functionalities should be pushed towards the antennas while others should be pulled away from them. On one hand, as antennas multiply, radios need to also multiply, and each should attach to one antenna yielding antenna-radio units that become the basic building blocks of the infrastructure. On the other hand, the signal processing needs to move up to a higher plane such that all these units can work together and the network can be smarter.
This deconstruction of the base station, hitherto the cornerstone of cellular networks, is the starting point for POSTCELL.

The research conducted under the umbrella of the action has led to a number of powerful conclusions:
1) Future wireless networks will necessitate a variable degree of massification, taking the form of dense distributed antenna-radio units in urban areas while taking the form of localized antenna-radio units in suburban and rural areas.
2) In post-cellular wireless networks, every user is to be served by a subset of antenna-radio units, and results have been derived on how to determine these subsets depending on the type of deployment. Concepts aligned with this idea are already discussed for 6G networks, as the improvements in coverage, capacity, and power efficiency, are major.
3) Power control functionalities, relevant in cellular networks, acquire even higher importance in post-cellular networks. A specific form of power control has been propounded and is getting considerable traction.
4) For the downlink, linear transmission schemes are preferred as their nonlinear brethren become rather unwieldy.
5) For the uplink, the classical nonlinear approach of successive interference cancellation is unfeasible, but a parallel interference cancellation scheme has been proposed that outperforms linear solutions.
6) Pilot contamination, a long-standing concern in cellular networks, has been shown not to be a significant impediment in postcellular networks.
POSTCELL was organized into three tracks. Tracks 1 and 2 run in parallel for the first three years whereas Track 3 started in year 2 and run through the end.
The 1-year extension kindly granted to the action enabled completing all of the tasks therein.

Track 1 dealt with infrastructure massification. Within it, Task 1.1 was concerned with the study of various phase transitions that arise in a process of massification, as well as with how localized massification is limited by footprint constraints and electromagnetic laws.
Task 1.2 addressed the integration of distributed and localized massification, concepts that had for the most part been separately considered to date.
Finally, Task 1.3 dealt with the association between antenna-radio units and devices.

Within Track 2, Task 2.1 was devoted to the association of devices while Task 2.2 to adapting schemes, in principle devised for peer-to-peer connection, for the communication within device clusters formed as devised in Task 2.1.
Task 2.3 dealt with the interplay between device data caching, in-cluster communication, and over-the-air-interface traffic from the infrastructure.

In Track 3, Task 3.1 was geared to the design of communication procedures between virtual base stations and device clusters. Subsequently, Task 3.2 was where the findings of the distinct activities converged in order to quantify how the system capacity scales with massification and whether there are points where such scaling exhibits relevant inflexions.
Finally, Task 3.3 wrapped up this track and the entire project, cross-referencing the findings with research external to the project so as to present a comprehensive summary of the state-of-art in post-cellular networks as an output of the action.

The dissemination of results continues, and will extend well into 2023. At the end of it, on the order of 50 first-tier publications will have been produced.
A large number of invited lectures, keynotes, and presentations, detailed elsewhere in the report, have further helped disseminate the findings. These activities too will extend well into 2023.
The ability of a cellular network to serve its customers is chiefly characterized by its capacity, broadly defined as the number of bits that can be reliably communicated per units of time and area. Dissecting this quantity, three factors are identified:
1) The cell spectral efficiency, measured in bits/s/Hz per cell. It captures all aspects related to communication, including modulation, coding, multiaccess, fading mitigation, interference management, power control, and multiantenna transmission/reception. The research efforts over the years have steadily improved it: while in 2G it was on the order of 0.1 in 5G systems it can exceed 1.
2) The bandwidth. Regulators have progressively allocated new bands over the years.
3) The cell area, which has shrunk from macrocell diameters of several Km in 1G to below 1 Km in 5G.
All in all, the capacity of cellular networks has increased by two orders of magnitude since their inception, making it possible to keep pace with the rise in demand. However, some of the pillars that have sustained this growth are faltering. First, the improvement in cell spectral efficiency is all but exhausted as the fundamental limits imposed by the laws of physics and information theory are approached. Second, the amount of bandwidth that is of prime quality from a wireless perspective is finite: too low frequencies require excessively large antennas, too high frequencies suffer from poor propagation.

The findings of POSTCELL enable transcending the limits that were choking the cell spectral efficiency, altogether providing a new multiplier to the capacity.
Fig. 2
Fig. 1