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Post-Cellular Wireless Networks

Periodic Reporting for period 2 - POSTCELL (Post-Cellular Wireless Networks)

Reporting period: 2018-04-01 to 2019-09-30

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. This entails both the definition of a new architecture for such networks, and the characterization of the ensuing performance.

The art of wireless communications is arguably one of the biggest technological revolutions in history, and a crowning achievement of modern engineering. Its impact on the functioning of contemporary society cannot be overstated. With much of the earth’s population already using mobile communication devices, one could think that the surge in wireless traffic is bound to subside. Nothing could be further from the truth. The growth of wireless traffic is relentless, and it is actually gaining new momentum on account of fresh mechanisms: smart devices, cloud computing, and machine-to-machine communication. As a result, the volume of wireless traffic is poised to increase to truly staggering levels and, to face this challenge, wireless networks need to enter a new stage.

In the three decades since their inception, 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) soon to be deployed. Despite their substantial differences and incompatible signaling formats, subsequent generations have all conformed to the same architecture that underpinned 1G, namely the cellular architecture (See Fig. 1). The idea of organizing wireless systems into cells, proposed in the 1940s, lay dormant for years until such time when processors and other necessary elements had been developed; then, it blossomed into 1G and the rest is history. 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. Wireline infrastructure then connects all these base stations. A given user transmits to the base station in its cell in an uplink channel while receiving from that base station in a downlink channel.

The cellular architecture has served us well and it has remained unchallenged, to the point that base station sites have witnessed generational transitions as hardware upgrades. However, the time is approaching when this architecture will have become exhausted. To face the unfolding perfect storm, wireless networks will need to leap forward and enter a new stage. And, unlike in previous generational transitions, a more transformational change that transcends the cellular architecture will be required. There is relatively broad consensus on two ways to tackle this transformation:
1) A dramatic scaling of the number of antennas per base station, from the current handful to tens or possibly even hundreds. In this vision, base stations would swell to host vast arrays of tiny antennas occupying entire rooftops, walls, or building façades.
2) Extreme densification, a vision whereby base stations themselves would become tiny and exceedingly abundant.

While seemingly exclusive, we posit that the two ideas above are complementary in that neither by itself can resolve the conundrum. Many antennas per base cannot guarantee indoor coverage and extreme densification alone cannot ensure contiguous coverage; the former suffers from holes, the latter from gaps. A driver of POSTCELL is that, with a proper architecture, these can become two sides of a coin we term massification:
1) Scaling the number of antennas per base station amounts to localized massification; lots of tiny antennas stacked at certain places.
2) Extreme densification can be regarded as distributed massification; lots of tiny scattered antennas.
Reconciling these ideas and enabling a truly phenomenal degree of massification calls for an entirely new architecture (see Fig. 1) where cells are transcended and base stations are deconstructed. Devising this new architec
POSTCELL is organized into three tracks. Tracks 1 and 2 have been running in parallel since the beginning of the project whereas Track 3 is now getting started.

One of the first actions in the project was to identify and characterize the various phase transitions that take place in a process of distributed massification and that inform of important phenomena:
1) As the communication range contracts, there comes a point when the propagation exponent experiences a sudden shift from values in the range 3.5-4.5 to values only slightly over 2. This shift radically improves the strength of the signal received from the closest transmitter, relative to the rest.
2) Another transition, related but occurring further down the massification process, shifts the propagation exponent for the entire tier of nearby transmitters besides the closest. This equalizes the advantage in strength gained by the closest transmitter in the previous transition, radically altering the signal ratios and thus the decision of which transmitters to connect with.
3) A further phase transition occurs in places where the density of antenna-radio units becomes comparable to the device density; at that point, the likelihood of idle antenna-radio units becomes significant and the number of such units connecting with a given device starts to surge.

Because of the aforedescribed transitions, the network is not scale-independent across different densification phases. Recent results suggested that, in standard cellular networks, beyond a certain point densification would cease to help. Similar findings also exist for ad-hoc networks. Our results, however, indicate that this is not fundamental and that a properly designed post-cellular architecture can sustain densification gains well beyond what those early results indicated.

More precisely, the main research achievements thus far can be summarized as follows.
- We have put forth closed-form expressions that characterize the key performance measure of networks reliant on localized massification. These expressions are anticipated to be of great use to the research community, as they allow testing and calibrating the large-scale system simulators that are otherwise used to study such networks. In some cases, such simulations can now be altogether skirted. These results are already accepted for publication, to appear throughout 2019, and have also been featured in a topical blog devoted to the matter.
- We have characterized the spatial distribution of users in networks featuring localized massification. These results are also accepted for publication, to appear later in 2019.
- We have devised two uplink power control policies for networks featuring distributed massification. One of these policies is fully distributed, and hence implementationally very attractive, while the other one is centralized and machine-learning-based, implementationally more costly but superior in performance. Both policies represent the state of the art on this subject, and will be presented at international conferences in 2019.
- We have formulated a new receiver type for the uplink of distributed massive networks. This receiver structure represents the state-of-the-art in terms of performance versus complexity, and its description and initial evaluation have been submitted to a first conference.
The ability of a cellular network to serve its customers is chiefly characterized by its system capacity, broadly defined as the number of bits that can be reliably communicated per units of time and area. Dissecting this quantity, three factors can be identified:
1) The cell spectral efficiency, measured in bits/s/Hz per cell.
2) The bandwidth, in Hz.
3) The cell area, in units of area per cell.
It is worth dwelling briefly on what these items represent, and on how they have evolved over the successive cellular generations to date.
1) The cell spectral efficiency captures all the aspects related to communication, including modulation, coding, multiaccess, fading mitigation, interference management, power control, and more recently multiantenna transmission/reception. The research efforts over the years have steadily improved the cell spectral efficiency: while in 2G it was on the order of 0.1 bits/s/Hz per cell, in 4G systems it can exceed 1 bit/s/Hz per cell.
2) The bandwidth represents the right to transmit signals in a certain range of frequencies. Regulators have progressively allocated additional bands over the years.
3) The cell area determines the degree to which the bandwidth is reused spatially. Cell sizes have shrunk progressively, from macrocell diameters of several Km in 1G and 2G to below 1 Km in 4G. In addition, networks have been augmented by sprinkling high-traffic zones with small microcells.
All in all, the system 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, just as the need for capacity is surging, some of the pillars that have sustained this growth are faltering. Firstly, growth is stalling in terms of cell spectral efficiency, whose improvement is all but exhausted as the fundamental limits imposed by the laws of physics and information theory are approached. Things are highly optimized already and can hardly yield more than incremental gains henceforth. Secondly, the amount of bandwidth that is of prime quality from a wireless perspective is finite: too low frequencies require excessively large antennas and too high frequencies suffer from poor propagation. The new paradigms of millimeter-wave, Terahertz, or visible-light transmission, will be welcome but they will serve as complementary measures rather than replacements.

There is some consensus on two possible ways to proceed:
⎯ A dramatic scaling of the number of antennas per base station, from the current handful to tens or possibly even hundreds. In this vision, base stations would swell to host vast arrays of tiny antennas occupying entire rooftops, walls, or building façades.
⎯ Extreme densification, a vision whereby base stations themselves would become tiny and exceedingly abundant.
Although these two ideas may seem mutually exclusive, and are for the most part regarded as alternatives and studied separately, we postulate that they are actually complementary in that neither by itself can resolve the wireless conundrum. Many antennas per base station cannot guarantee indoor coverage, for instance, while extreme densification alone cannot ensure the contiguous coverage that mobile users have grown accustomed to. Put differently, the former suffers from holes and the latter from gaps; one falls short in term of depth and the other one in terms of breadth.
One of the drivers of POSTCELL is that, with the proper architecture, the two ideas above can become two sides of the same coin, a coin we term massification:
⎯ Scaling the number of antennas per base station amounts to localized massification; lots of tiny antennas stacked at certain places.
⎯ Extreme densification can be regarded as distributed massification; lots of tiny antennas geographically scattered.
Reconciling these ideas and enabling a truly phenomenal degree of massification calls for an entirely new architecture where cells are surpassed.

The first step to const
Fig. 1
Fig. 2