Page 146 - ITU Journal, Future and evolving technologies - Volume 1 (2020), Issue 1, Inaugural issue

ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1

partial and full overlapping through available resources for rural/isolated areas and capacity enhancement for
can also be employed while designing new generation temporarily crowded places (such as stadiums/concert
NOMA techniques [30,96]. The waveform-domain NOMA venues) [77]. FAP-based networks are expected to be an
concept provides an important lexibility by increasing important part of 6G not only for achieving deployment
the resource allocation possibilities in 6G networks [78]. lexibility but also for having better wireless propagation
Another lexibility aspect that can arise with 6G is the provided by a high probability of Line of Sight (LOS)
use of an alternate waveform domain rather than the communications [41,63].
conventional time-frequency lattice employed by 5G and
older generations.
In addition to the aerial and terrestrial networks, the
integration of space (satellite) networks is another as-
In addition to the waveform itself, there is a large number pect of the lexible heterogeneous networks [54]. Space
of new generation modulation options in the litera- networks are also a promising solution for rural area
ture [97] and only a small set of them have appeared in communications [31]. They are employed for wireless
the 5G standards. 6G can be enriched with the lexibility backhaul communications in the previous cellular net-
provided by these options, particularly Index Modulation works. However, space networks can also serve aerial
(IM) based solutions [11]. This concept can even be user equipment such as drones and UAVs to increase
extended to multiple domains to provide an additional coverage lexibility in 6G systems [72]. Moreover, under-
degree of freedom [98]. Moreover, modulation tech- sea network integration with the other networks will be
niques are adaptively designed considering the other useful while serving naval platforms.
key enablers such as Non-Orthogonal Multiple Access
(NOMA) [99] and Recon igurable Intelligent Surface
(RIS) [45] for 6G. Although, the integration of different networks is en-
sured, the cell structures of these networks are changing.
Since the con iguration of the PHY parameters is, to a Cell-less or cell-free networks are one of the poten-
large extent, controlled by the Medium Access Control tial 6G concepts considering the network architecture
(MAC) layer, it is imperative to develop the lexibility and richness [105, 106]. User equipment connects to the
adaptation capabilities of both layers simultaneously. network via multiple small cells in the cell-less networks.
Two important issues that require lexibility in PHY and Cell-centric design is transformed into the user-centric
MAC would be the “waveform parameter assignment” or system. Hence, it provides both handover-free com-
“numerology scheduling” paradigm under the context of munications and zero inter-cell interference. Cell-less
5G multi-numerology systems [25, 100], where the MAC networks may exploit a new dimension of network
layer is responsible for assignment of parameters of the Multi-Input Multi-Output (MIMO) lexibility in 6G. As
PHY signal. Similarly, adaptive guard utilization methods another network MIMO example, advanced coordinated
have been developed for the MAC layer [101–103] to and centralized networks [107] are addressed together
control the new type of interferences in 5G systems. with NOMA schemes for 6G communications [108, 109].
On this basis, it is expected that highly intelligent UE These networks are called multi-cell NOMA. Flexibility
capabilities, and con igurable network parameters, and comes with the number of the cells and architecture
lexible and ef icient MAC designs will play a key role in richness while exploiting other dimensions with NOMA.
6G networks due to the expected increased diversity in
service types and consequently requirements. From the network virtualization perspective, network
slices are used in 5G to customize and optimize the
Example lexibility perspectives for ultra- lexible PHY and network for service types or any other requirement
MAC technologies of potential 6G key enablers are given sets [110–112]. Hence, the overall performance is
in Table 3. increased by meeting different requirement sets with
virtually privatized networks. Network slicing brings
3.3 Ultra-Flexible Heterogeneous Networks an important lexibility in 5G since it enables different
network options under the same umbrella. The number
Flying Access Points (FAPs) provide enhanced lexibility of network slices can increase for 6G and there may be
for network deployment by allowing dynamic (3-D) network slices for each user equipment. This user-centric
positioning of the nodes or even optimized trajectory network slicing architecture can provide full lexibility in
planning for different objective functions [47,55,59]. The the network layer.
push in this direction occurred around the turn of the
century [104], and was further empowered by projects,
such as: 1) Google Loon project, 2) Facebook Aquila The number of examples for the lexibility aspects
project, 3) ABSOLUTE project, 4) Matternet project, and of promising 6G heterogeneous networks can be in-
5) Thales Stratobus project. The integration of FAPs creased with particular technologies and concepts such
with the terrestrial network can be leveraged to provide as blockchain systems [26,33] and quantum communica-
coverage in disaster/emergency scenarios, connectivity tions [29] in the future.

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