Ecole d'ingénieur et centre de recherche en Sciences du numérique

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

Zhang, Jingjing

Thesis

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 networks). 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 quality. 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.

Document Bibtex

Titre:Interplay between caching, feedback and topology in the wireless broadcast channel
Type:Thèse
Langue:English
Ville:
Date:
Département:Systèmes de Communication
Eurecom ref:5174
Copyright: © TELECOM ParisTech. Personal use of this material is permitted. The definitive version of this paper was published in Thesis and is available at :
Bibtex: @phdthesis{EURECOM+5174, year = {2017}, title = {{I}nterplay between caching, feedback and topology in the wireless broadcast channel}, author = {{Z}hang, {J}ingjing}, school = {{T}hesis}, month = {04}, url = {http://www.eurecom.fr/publication/5174} }
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