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Ultra-dense multi-tier cellular networks have recently drawn the attention of researchers due to their potential efficiency in dealing with high-data rate demands in upcoming 5G cellular networks. These networks consist of multi-tier base stations including micro base stations with very high-system capacity and short inter-site distances, overlooked by central macro base stations. In this way, network densification is achieved in the same area as that of traditional mobile networks, which offers much higher system capacity and bandwidth reuse. This paper utilizes a well-known analytical tool, stochastic geometry for modeling and analyzing interference in ultra-dense multi-tier cellular networks. Primarily, we have studied different factors affecting the system capacity including the network densification, cell load, and multi-tier interference. The role of the ergodic channel capacity is also discussed. Moreover, the effects of channel interference, system bandwidth, and the network densification on the spectral and energy efficiencies of the network are observed. Finally, the results show that the network densification and the cell load have a profound impact on system performance as well as spectral and energy efficiencies of the networks.INDEX TERMS System capacity, ultra-dense multi-tier networks (UDMN), spectral efficiency, energy efficiency, stochastic geometry, 5G.

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Caching close to users in a radio access network (RAN) has been identified as a promising method to reduce a backhaul traffic load and minimize latency in 5G and beyond. In this paper, we investigate a novel community detection inspired by a proactive caching scheme for device-to-device (D2D) enabled networks. The proposed scheme builds on the idea that content generated/accessed by influential users is more probable to become popular and thus can be exploited for pro-caching. We use a Clustering Coefficient based Genetic Algorithm (CC-GA) for community detection to discover a group of cellular users present in close vicinity. We then use an Eigenvector Centrality measure to identify the influential users with respect to the community structure, and the content associated to it is then used for pro-active caching using D2D communications. The numerical results show that, compared to reactive caching, where historically popular content is cached, depending on cache size, load and number of requests, up to 30% more users can be satisfied using a proposed scheme while achieving significant reduction in backhaul traffic load.

In this paper, we propose network-assisted device-to-device (D2D) communication in licensed and unlicensed spectrum interoperable networks, to improve D2D users' throughput while alleviating the spectrum scarcity issue of cellular networks. The idea of licensed and unlicensed spectrum interoperability is based on the findings of the IEEE 1932.1 working group. Conventionally, D2D users were only able to communicate by using either cellular or non-cellular networks and no interoperability mechanism was available. The proposed scheme brings in many benefits including but not limited to higher D2D users' throughput, alleviation in spectrum scarcity issue of cellular networks, and better network management. However, ensuring quality-of-service (QoS) in this dynamic environment is a challenging task. To this end, we analyze the QoS using a well-known analytical tool "Effective Capacity (EC)" for eNodeB-assisted as well as WiFi-assisted D2D communication. Moreover, we also see the impact of neighboring cells' load and full-duplex transceiver at eNodeB and WiFi access point on the EC of D2D users. Simulation results show that EC increases with a decrease in neighboring cell's load and decreases when more stringent QoS constraints are imposed. Results also show that the maximum sustainable source rate at the transmitter's queue increases with an increase in maximum allowed packet delay but converges to a maximum value soon after that. INDEX TERMS Licensed-unlicensed spectrum interoperatability, D2D communication, effective capacity, quality-of-service.