Performance analysis of wireless edge caching in future wireless networks

Abstract

The demand for higher capacity and lower latency in wireless mobile networks is constantly on the rise, motivating the development of next generation mobile systems. Accordingly, various fifth generation (5G) and beyond solutions such as i) network densification, ii) moving from the congested frequency range 1 (FR1, < 7.125 GHz) bands to FR2 bands (> 24.25 GHz), commonly known as millimetre wave (mmWave) spectrum, iii) massive multiple input multiple output (mMIMO), vi) integrated access and backhaul (IAB) architecture, and v) in-band full duplex (IBFD) radio have been proposed. However, while the above solutions are beneficial for access links, they do little to alleviate the burden on backhaul links. In fact, the backhaul network has become a new bottleneck with respect to both capacity and latency requirement due to the fact that the substantial amount of redundant and repeated requests generated over networks by the billions of connected devices are inevitably forwarded to the core networks through backhaul links. Consequently, in this thesis, caching technique is investigated for the wireless domain to seek potential solutions for the aforementioned issues. In particular, wireless edge caching (WEC) in conjunction with the aforesaid solutions provided for access is investigated in a holistic framework to achieve the common goals of improving the quality of service (QoS) of networks and quality of experience (QoE) for users. The work is divided into the following four main parts. In the first part, we focus on the design of content caching placement algorithms through the joint optimisation of content placement and delivery phases with respect to the average success probability (ASP) of file delivery for FR2-FR1 hybrid heterogeneous networks (HetNets). To simplify the non-convex nature of the problems and obtain design insights, we consider two different scenarios: i) noise-limited and ii) interference-limited, and then separately derive the ASP of file delivery for both environments using stochastic geometric tools. Finally, we propose optimal caching placement algorithms by maximising the derived ASP of file delivery for both the scenarios. Numerical results show the feasibility of WEC in such hybrid networks and demonstrate the superiority of the proposed caching placement scheme over other commonly used caching placement schemes, namely uniform caching (UC), most-popular content caching (MC), and random caching (RC). In the second part, we develop a unified analytical framework involving both backhaul and access for a cache-enabled hybrid network, which includes a tier of storage-equipped FR2 small cells with hybrid beamforming overlaid with a tier of mMIMO-aided traditional FR1 macro cells. In particular, under the limited backhaul constraint, we give a holistic performance analysis of the cache-enabled hybrid network on the average latency, throughput, and ASP of file delivery with respect to two open-access user association policies: i) location-based and ii) content-based. Numerical results demonstrate that there exists an optimal small base station (SBS) density that provides the best latency and throughput performance. Also, we show that the latency under content-based user association is less than that of location-based user association, and although the difference in the average rates under two user associations is not obvious, content-based user association can extricate more backhaul capacity and thus reduce installation cost significantly. In the third part, we shift our attention to the study of WEC for a Third Generation Partnership Project (3GPP) inspired IAB network operating at FR2 bands. Traditionally, both access and backhaul are allocated fixed spectrum. However, as caching alleviates the backhaul traffic, a large amount of spectrum resource that can be moved to access links but occupied by the backhaul limits the capacity of access links and henceforth constrains the overall network performance. Motivated by this, the objective of this part is to analyse the joint impact of spectrum resource partitioning and WEC for the IAB framework under the wide-band FR2 channel assumption in terms of three key performance metrics, namely the ASP, throughput, and latency of file delivery. Numerical results demonstrate that there exists an optimal spectrum partition for IAB with respect to different network parameter settings. Furthermore, there exists a tradeoff in the selection of the optimal partitioning factor with respect to the ASP/throughput and latency performance for varying IAB node densities. Finally, in order to further improve the spectral efficiency and latency for retrieving non-cached contents, the fourth part develops an analytical framework for cache-enabled IBFD-aided FR2-IAB networks with the wide-band FR2 channel model, using which we analyse the ASP, throughput, and latency of file delivery considering the content delivery phase to give several design insights with respect to residual self-interference (SI) cancellation, antenna number, target data rate, and IAB node density. Finial results demonstrate the precedency of using WEC as a cost-effective measure not only for improving the IBFD-aided FR2-IAB network’s performance but also for alleviating the level of residual SI cancellation required for the deployment of practical IBFD-aided FR2-IAB networks.