On The Performance Limits Of Cache-aided MIMO Networks

The mobile data traffic has been growing rapidly and video contents contribute more than 60%.An important feature of video is that the same contents can be requested by many users.Wireless caching is to prefetch the popular contents at the wireless edge such as local base stations(BSs)or mobile users during the off-peak time and thus can reduce the peak data traffic and improve user perceived experience.The goal of this thesis is to investigate the performance limits of cache-aided multiple-input multiple-output(MIMO)networks in order to characterize the essential properties of the network and provide guidelines for the design and optimization in the practical systems.Coded caching is an important technique for analyzing the fundamental limits of cache-aided networks.By carefully file splitting and placement,it can exploit the broadcast and su-perposition nature of wireless medium and hence bring more opportunities of transmitter cooperation and multicast transmission,which,however,increases the complexity of the network topology.The complicated network topology,as well as the spatial multiplexing potential induced by MIMO,makes the analysis extremely challenging.In this thesis,we adopt degrees of freedoms(DoF)and the normalized delivery time(NDT)for all user de-mands as performance metrics and follow the specific-to-general and simple-to-complex principle to analyze the performance limits of several wireless networks equipped with both multiple antennas and caching,including broadcast channel,interference network,cellular network,and fog radio access network(Fog-RAN).The main contributions and finds of this thesis are summarized as follows.1.We propose to treat the content delivery under the receive-side decentralized coded caching in a MIMO broadcast channel as the transmission problem with general message sets where all possible messages,intended for different user sets,are present and delivered concurrently.We derive the inner and outer bounds of DoF region of the MIMO broadcast channel with general message sets.For any given set of coded messages,we obtain its minimum NDT based on the above DoF results.It is shown that the obtained minimum NDT is optimal at certain antenna configurations and is within a bounded gap to the optimum at other antenna configurations.2.We propose a novel cooperative transmitter/receiver coded caching strategy with adjustable file splitting ratios in a 3 × 3 cache-aided MIMO interference network.The interference network topology is changed to hybrid forms of MIMO broadcast channel,MIMO X channel,and MIMO multicast channels.We analyze the DoF of these new channel models by using linear interference management schemes with finite symbol extensions.Based on the collective use of these DoF results,we obtain an achievable NDT of the network.The obtained NDT is for arbitrary antenna numbers and any feasible cache sizes.By using the cut-set bound,we derive a lower bound of the minimum NDT and show that the multiplicative gap between the achievable NDT and the lower bound is a constant.3.We design a new prefetching scheme to satisfy all possible user demands in a cellular network with partial connectivity.According to the overlapped degree of cached contents among BSs,we propose transmission schemes with no BS cooperation,partial BS cooperation,and full BS cooperation,respectively,for different cache sizes.Our achievable results indicate that the reciprocal of per-user DoF of the cellular network decreases piecewise linearly with the normalized cache size at each BS,and the gain of BS caching is more significant for the small cache region.Under the given cache placement scheme,we show that the achievable DoF is optimal in part of the cache size region,and within an additive gap of(?)to the optimum in the remaining cache size region.4.We investigate the caching and delivery strategies in a MIMO Fog-RAN and focus on the small cache size scenario where the accumulated cache memories of all edge nodes(ENs)are not enough to cache the whole database.We propose a novel caching scheme tailored for the small cache size,which offers the freedom to adjust the duplication order of the cached contents.In the fronthaul delivery,we use multiple antennas to multiplex multiple messages while in the access delivery,transmitter cooperation induced by the duplicated contents among ENs is utilized to cancel interference.By jointly optimizing caching and delivery,we obtain an achievable minimum NDT.It is shown that the achievable NDT is within an additive gap of 1 to the theoretical optimum.By analyzing the performance limits of the above typical channels and general networks,it can be found that the MIMO gain and caching gain are cumulative in wireless networks.Furthermore,the cache placement depends on antenna configurations while the deployment of caching also changes the MIMO transmission strategies.