Minsoo Na

Optimization of femtocell network configuration under interference constraints

Femto BS (Base Station) is emerging as a key technology to secure the coverage and capacity in indoor environments. However, since the existing macrocell network is overlaid on femtocell networks utilizing the same set of frequency channels, femtocell networks can originate severe co-channel interference to the macrocell network unless the femtocell network is carefully configured. Therefore, according to a desired network-wide objective, we optimize the femtocell network with constraints such that the service connectivity with a femto BS is secured in the target indoor area while the signal emitted out of the building, playing as interference to the outdoor users, should be controlled with an appropriate strength in order not to interrupt the communication between macro BS and outdoor users. Each optimization problem is formulated as a mixed integer programming, and as the results, we obtain not only the transmit power and operational frequency channel of each femto BS, but also the optimal femto BS-to-user association pair at each geographical position.

An Energy-Efficient Data Reporting Scheme Based on Spectrum Sensing in Wireless Sensor Networks

Wireless sensor networks (WSNs) are widely used in a variety of fields including environmental and industrial monitoring. However, the battery-powered sensor devices in WSNs can suffer from a lack of energy resources problem because of their limited battery capacity. Although the energy harvesting (EH) sensor devices recently have become an attractive alternative to address the limited battery capacity problem, a small number of EH sensor devices can be deployed because of their high cost. To address this problem, we propose an energy-efficient data reporting scheme based on spectrum sensing. In the proposed scheme, each sensor device senses the spectrum over the air and then determines the transmission time of the access signal and the number of repetitions of access signal transmission. Also, the sink node resolves the collisions by detecting the variation of the received signal power. The simulation results show that the proposed scheme increases the total energy efficiency of the sensor devices and moreover it decreases the access failure probability of the sensor devices.

Sensing-based adaptive data reporting scheme in wireless sensor networks

Wireless sensor networks (WSNs) typically have a massive number of the sensor devices. The battery-powered (BP) sensor devices are the conventional type that has low transmission power due to the limited battery lifetime. The energy harvesting (EH) sensor devices can recharge their battery and therefore may transmit data with high transmission power. However, due to the low transmission power of the BP sensor device, the data reporting process can suffer from high access failure probability. To address this problem, we propose a sensing-based adaptive data reporting scheme. According to the spectrum sensing results, each sensor device determines the time and duration of data reporting. The sink node resolves the collisions by detecting the variation of the received signal power. The simulation results show that the proposed scheme decreases the access failure probability and increases the total number of successful reportings per frame.

Dual Layer Small Cell On/Off Control for Ultra-Dense Small Cell Networks

Ultra-dense small cell networks have been proposed as a way of addressing the explosive growth of data traffic. However, this approach often results in empty cells which only end up causing interference instead of serving UEs because they do not have any associated UEs. In this paper, we investigate the empty cell problem using system-level simulation and propose a dual layer on/off strategy for solving the empty cell problem. Systemlevel simulation results verify that the proposed scheme achieves performance enhancement in terms of capacity compared with conventional ultra- dense small cell networks.

Aligned Reverse Frame Structure for Interference Mitigation in Dynamic TDD Systems

The dynamic time division duplex (TDD) system has been proposed as a way to meet today’s asymmetrically and dynamically changing traffic demand. However, this approach causes cross-link interference, because neighboring base stations and user elements transmit in opposite directions. In this paper, we investigate and analyze the characteristics of cross-link interference in dynamic TDD systems. Based on this observation, we propose an aligned reverse frame structure to utilize and cancel the cross-link interference. Mathematical analysis and numerical results verify that the proposed scheme achieves performance enhancement in terms of capacity compared with conventional dynamic TDD systems.

A Novel SCMA System for Coexistence of Active Users and Inactive Users

This letter proposes a new sparse code multiple access (SCMA) system for handling the coexistence of active user equipments (UEs) and inactive UEs, which share the same frequency/time resources for an uplink grant-free scenario. SCMA systems use a message passing algorithm, which considers all UEs to be active UEs. Therefore, inactive UEs cause significant decoding problem in SCMA systems. In order to solve the coexistence problem, we propose code words and a novel SCMA receiver which can distinguish the signal of active/inactive UEs. The numerical results show that the proposed SCMA receiver achieves a remarkable enhancement in false alarm probability, miss detection probability, and bit-error rate.

Smart Small Cell Wake-Up Field Trial: Enhancing End-User Throughput and Network Energy Performance

Field trial measurements of small cell sleep mode with three different wake-up solutions are presented in this paper. A small heterogeneous LTE test network consisting of one macro base station and four small cells was deployed in Bundang, South Korea. Two UEs, one stationary and one moving, were used in the field trial. The studied solutions were (1) uplink interference based stand-alone small cell activation; (2) macro- assisted load-based small cell activation; and (3) macro-assisted load-based and timing advance-based small cell activation. Solution 1 was evaluated in a scenario where the same frequency band is used in the macro and small cell layers and the results show that Solution 1 provides an average reduction in energy consumption of 10% as well as a user throughput increase of 24% compared to a reference case without small cell sleep mode. For solutions 2 and 3 different frequency bands were used for the small cells and the macro layer and here the energy reduction gains were 16% and 23% respectively while the increase in user throughput was 28% and 51% respectively.