Interplay between caching, feedback and topology in the wireless broadcast channel

Zhang, Jingjing

3eme prix du Prix de Thèse TELECOM ParisTech 2018

Current PHY-based communication solutions do not successfully scale in large
networks, because as the number of users increases, these solutions cannot
fully separate all users' signals. This problem in turn can gradually leave each
user (i.e., each receiver) with small communication rates, and it comes at a
time when wireless data trac is expected to increase dramatically. Recently
a new type of technology was proposed, in the form of an upper-layer solution,
that relied on caching network-coded data at the receiving devices. The
proposed solution  termed as 'coded caching'  was motivated by the fact
that wireless trac is heavily predictable, and it was based on pre-caching content
from an existing library of many popular les. This promising approach
naturally came with its own limitations though, and the corresponding performance
would still remain limited. These two approaches (advanced PHY-based
techniques, and coded-caching which used caching at the receivers) had been
treated independently; after all, one uses feedback on the PHY layer, the other
uses receiver-side memory on MAC.
With this as a starting point, part of what we show here is that there are
interesting interplays between the two, which can potentially allow for a combination
that bypasses each approach's limitations. More generally, the main
work of this thesis on advanced wireless caching, is about exploring the deep
connections between caching and fundamental primitives of wireless networks,
such as feedback and topology.
The rst part of the work explores the synergistic gains between coded
caching and delayed CSIT feedback (i.e., Channel State Information at the
Transmitters). Here we consider the K-user cache-aided wireless MISO broadcast
channel (BC) with random fading and delayed CSIT, and identify the
optimal cache-aided degrees-of-freedom (DoF) performance within a factor of
4. To achieve this, we propose a new scheme that combines basic codedcaching
with MAT-type schemes, and which eciently exploits the prospectivehindsight
similarities between these two methods. This delivers a powerful
synergy between coded caching and delayed feedback, in the sense that the
total synergistic DoF-gain can be much larger than the sum of the individual
gains from delayed CSIT and from coded caching. The derived performance
interestingly reveals  for the rst time  substantial DoF gains from coded
caching when the (normalized) cache size 
 (fraction of the library stored at each receiving device) is very small. Specically, a microscopic 
  e􀀀G can
come within a factor of G from the interference-free optimal. For example,
storing at each device only a thousandth of what is deemed as 'popular' content
  10􀀀3), we approach the interference-free optimal within a factor of
ln(103)  7 (per user DoF of 1=7), for any number of users. This result carries
an additional practical ramication as it reveals how to use coded caching to
essentially buer CSI, thus partially ameliorating the burden of having to acquire
real-time CSIT (considered by many as the main bottleneck in wireless
The second part of the work explores an interesting interplay (and tradeo
) between caching and current CSIT feedback quality. For the same setting
as before, but now with an additional ability to incorporate imperfect-quality
current CSIT, this part explores the link between feedback-quality and the
size of the caches. This is guided by the intuition that the bigger the caches,
the more side information the receivers have in their caches, the less interference
one needs to handle (because of the side information), and the lesser
the feedback that is potentially needed to steer interference. Conversely, more
feedback may diminish the utility of memory because the better the feedback
is, the more pronounced the broadcast element should be, leading to more
separated streams of information, and thus fewer commonly-heard (not separated)
streams which are the basis of cache-aided multi-casting. This work
provides another new scheme that utilizes current and delayed feedback, to
again come within a factor of at most 4 from optimal. Furthermore we describe
the savings in current CSIT that we can have due to coded caching,
while we also show how much caching can allow us to entirely remove current
CSIT without degrading performance, basically describing the memory cost
for buering CSI.
The third part of the thesis explores feedback-aided coded caching for the
same MISO BC, but with emphasis on very small caches, focusing on the case
where the cumulative cache size is smaller than the library size. Here the work
proposes new schemes that boost the impact of small caches, dealing with the
additional challenge of having some of the library content entirely uncached,
which in turn forces us to dynamically change the caching redundancy to
compensate for this. Our proposed scheme is near-optimal, and this part of
the thesis identies the optimal (small) cache-aided DoF performance within
a factor of 4.
The fourth part of the thesis explores  for the same K-user BC setting
 the connection between coded caching and current-quality CSIT, but now
without delayed CSIT. Here we show how caching, when combined with a ratesplitting
broadcast approach, can improve performance and reduce the need for
CSIT feedback, in the sense that the cache-aided interference-free optimal DoF
performance (directly associated to perfect-CSIT and local-caching gains), can
in fact be achieved with reduced-quality CSIT. These CSIT savings can be
traced back to an inherent relationship between caching, performance, and CSIT; caching improves performance by leveraging multi-casting of common
information, which automatically reduces the need for CSIT, by virtue of the
fact that common information is not a cause of interference. At the same time
though, too much multicasting of common information can be detrimental,
as it does not utilize existing CSIT. For this, we designed simple schemes
that build on the Maddah-Ali and Niesen coded caching scheme, by properly
balancing multicast and broadcast opportunities, and by combing caching with
rate-splitting communication schemes that are specically designed to operate
under imperfect-quality CSIT. The observed achievable CSIT savings here are
more pronounced for smaller libraries and for smaller numbers of users. In
the end, this part allows us to quantify the intuition that, in the presence of
coded-caching, there is no reason to improve CSIT beyond a certain threshold
The fth part of the thesis explores wireless coded caching from a topological
perspective. The fact that coded caching employs multicasting (i.e.,
communicating a common message to many receiving users at the same time),
causes performance to deteriorate when the links have unequal capacities. Such
uneven topologies, where some users have weaker channels than others, introduce
the problem that any multicast transmission that is meant for at least
one weak user, could conceivably have to be sent at a lower rate, thus 'slowing
down' the rest of the strong users as well. With this as motivation, we explore
coded caching in a SISO BC setting where some users have higher link capacities
than others (all users have the same cache size). Focusing on a binary
and xed topological model where strong links have a xed normalized capacity
1, and where weak links have reduced normalized capacity, we identify
 as a function of the cache size and the topology  the optimal throughput
performance, within a factor of at most 8. The transmission scheme that
achieves this performance, employs a simple form of interference enhancement,
and exploits the property that weak links attenuate interference, thus allowing
for multicasting rates to remain high even when involving weak users. This
approach ameliorates the negative eects of uneven topology in multicasting,
now allowing all users to achieve the optimal performance associated to maximal
capacity, even if the capacity of the weaker users decreases down to a
certain threshold capacity. This leads to the interesting conclusion that for
coded multicasting, the weak users need not bring down the performance of
all users, but on the contrary to a certain extent, the strong users can lift the
performance of the weak users without any penalties on their own performance.
At the end of the thesis we also present a result that does not involve
caching. This part explores the DoF limits of the (two user) SISO X channel
with imperfect-quality CSIT, and it shows that the same DoF-optimal performance
 previously associated to perfect-quality current CSIT  can in
fact be achieved with current CSIT that is of imperfect quality. The work also
shows that the DoF performance previously associated to perfect-quality delayed
CSIT, can in fact be achieved in the presence of imperfect-quality delayed CSIT. These follow from the presented sum-DoF lower bound that bridges the
gap  as a function of the quality of delayed CSIT  between the cases of
having no feedback and having delayed feedback, and then another bound that
bridges the DoF gap  as a function of the quality of current CSIT  between
delayed and perfect current CSIT. The bounds are based on novel precoding
schemes that are presented here and which employ imperfect-quality current
and/or delayed feedback to align interference in space and in time.

Systèmes de Communication
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