Sungrae Cho

Energy-Efficient Repulsive Cell Activation for Heterogeneous Cellular Networks

In this paper, we consider a two-tier heterogeneous cellular network (HCN) where macrocells and distributed low power cells, namely daughtercells, are operated in a common spectrum. Due to the ad-hoc nature of daughtercell BS deployments such as pico and femto cells, the mutual interference varies and obviously the coverage probability behaves differently in terms of transmit powers and densities of macrocells and daughtercells. In this paper, we employ repulsive cell activation in the interfering daughtercell network and see the impact of a minimum separation distance between the daughtercell BSs in terms of coverage under open access and power efficiency. The control of the minimum separation distance plays a role in balancing cell load effectively according to changing user density and is justified for the coexistence of low power daughtercells. The optimal minimum separation distance in terms of user density and target per-tier user throughput requirements is found by a numerical search based on a simple bisection method. Numerical results show the benefit of cell repulsion in terms of increased user density support and less area power consumption.

QoS Provisioning Relay Selection in Random Relay Networks

In this paper, we propose an analytical framework for determining the outage probability of random and best relay selection schemes given a Poisson field of relay nodes and the presence of path loss and fading. For relay selection, relays geographically close to the source and destination are preferred to others. This selection guideline ensures a target quality of service (QoS) and reduces the signaling overhead and the relay selection delay. A spatial region called the QoS region is obtained for the random relay selection and is shown to shrink as the distance between the source and the destination increases and the interfering node density increases. When the QoS region for random relay selection is not large enough and cannot probabilistically ensure a reliable relay therein, the best relay selection is employed since the required relay node density and selection range for a desired QoS can be reduced for the best relay selection. The gain of the best relay selection with respect to the random relay selection is quantified in terms of relay node density reduction and coverage extension due to selection diversity.

Coverage and Load Balancing in Heterogeneous Cellular Networks with Minimum Cell Separation

In this paper, we consider a downlink heterogeneous cellular network (HCN) where K tiers operate in a common spectrum and differ in terms of transmit power, target data rate, and base station (BS) density. We employ a repulsive cell activation (or planning) in the HCN by ensuring a minimum separation distance between interfering BSs in each tier. We consider a modified Matern hardcore process (MHP) for rendering a minimum separation distance between the BSs, which is realized by outweighing random BS distribution for closed and open access networks. Repulsive cell activation not only improves the coverage probability but also plays a role in balancing per-cell loads effectively according to varying user density. Assuming a finite BS capacity in terms of a limited number of per-cell users, we point out the importance of a relative BS density control between the tiers, and propose a tier-wise density and power control by introducing a load factor for configuring a HCN distributively while satisfying per-tier user throughput constraints given user density.

Relay Cooperation with Guard Zone to Combat Interference from an Underlaid Network

In this paper, we investigate the impact of relay cooperation for maintaining coverage area against aggregate interference from incumbent underlaid interferers. We employ a guard zone for uplink so that non-urgent interferers are inhibited and urgent ones are admitted with a controlled access probability as long as the primary receiver can tolerate. Numerical results show the outage probability of the primary relay network with the guard zone and that a desired quality-of-service (QoS) determines the access probability of urgent interferers depending on interfering node density.

Pattern-Identified Online Task Scheduling in Multitier Edge Computing for Industrial IoT Services

In smart manufacturing, production machinery and auxiliary devices, referred to as industrial Internet of things (IIoT), are connected to a unified networking infrastructure for management and command deliveries in a precise production process. However, providing autonomous, reliable, and real-time offloaded services for such a production is an open challenge since these IIoT devices are assumed lightweight embedded platforms with limited computing performance. In this paper, we propose a pattern-identified online task scheduling (PIOTS) mechanism for the networking infrastructure, where multitier edge computing is provided, in order to handle the offloaded tasks in real time. First, historical IIoT task patterns in every timeslot are used to train a self-organizing map (SOM), which represents the features of the task patterns within defined dimensions. Consequently, offline task scheduling among edge computing-enabled entities is performed on the set of all SOM neurons using the Hungarian method to determine the expected optimal task assignments. In real-time context, whenever a task arrives at the infrastructure, the expected optimal assignment for the task is scheduled to the appropriate edge computing-enabled entity. Numerical simulation results show that the proposed PIOTS mechanism overcomes existing solutions in terms of computation performance and service capability.

Spatially efficient distributed relay selection for random relay networks

In the presence of a large number of relays, some effective relays close to the source and destination nodes need to be regionally elected and the relays far from them shall be disregarded for relay selection process because of the likelihood of higher outage probability. In this paper, we propose an opportunistic feedback mechanism combining a geographical selection region where the relay whose effective channel gain is above a pre-determined threshold can only feed back, and thereby establish a relation between required relay-node density and the level of feedback threshold. It is shown that required relay-node density and selection region for a desired quality-of-service (QoS) depend on the feedback threshold.

Distributed Relay Selection for QoS Provisioning in Regenerative Relay Networks

In relay communications, the relay is in cooperation without doubt at a price of coordination signaling such as synchronization and associated channel state information (CSI) acquisition. Distributed relay selection based on average channel gain information available at the relays is preferred from the perspective of the cost of coordination signaling only if the link reliability can be maintained. In this paper, we propose a simple distributed scheme for relay selection such that a set of relays that satisfies QoS constraints is only activated and one of them is selected randomly from an \emph{opportunistic} set of QoS relays. The proposed scheme avoids unnecessary power consumption caused by non-beneficial relay cooperation and performs comparable to the centralized best relay selection while it minimizes signaling overheads.