Hyun-Ho Choi

Hierarchical Interference Alignment for Downlink Heterogeneous Networks

This paper focuses on interference issues arising in the downlink of a heterogeneous network (HetNet), where small cells are deployed within a macrocell. Interference scenario in a HetNet varies based on the type of small cell access modes, which can be classified as either closed subscriber group (CSG) or open subscriber group (OSG) modes. For these two types of modes, we propose hierarchical interference alignment (HIA) schemes, which successively determine beamforming matrices for small cell and macro base stations (BSs) by considering a HetNet environment in which the macro BS and small cell BSs have different numbers of transmit antennas. Unlike prior work on interference alignment (IA) for homogeneous networks, the proposed HIA schemes compute the beamforming matrices in closed-form and reduce the feedforward overhead through a hierarchical approach. By providing a tight outer bound of the degrees-of-freedom (DoF), we also investigate the optimality of the proposed HIA schemes with respect to the number of antennas without any time expansion. Furthermore, we propose a new optimization process to maximize the sum-rate performance of each cell while satisfying the IA conditions. The simulation results show that the proposed HIA schemes provide an additional DoF compared to the conventional interference coordination schemes using a time domain-based resource partitioning. Under multi-cell interference environments, the proposed schemes offer an approximately 100% improvement in throughput gain compared to the conventional coordinated beamforming schemes when the interference from coordinated BSs is significantly stronger than the remaining interference from uncoordinated BSs.

Exact Outage Probability of Relay Selection in Decode-and-Forward Based Cooperative Multicast Systems

In this letter, we present the outage performance of decode-and-forward based cooperative multicast systems with the best relay selection (CM-RS) over Rayleigh fading channels. Especially, we provide an exact and closed-form expression for the outage probability of CM-RS and its diversity order. Through numerical results, the analysis is verified, and the outage performance of CM-RS is compared with that of a single-hop multicast system.

Distributed Transmit Power Control for Maximizing End-to-End Throughput in Wireless Multi-hop Networks

Wireless multi-hop networks have a solidarity property, in which each multi-hop link interferes mutually and so an increase in one link’s rate results in a decrease of the other links’ rate. In a multi-hop link, the end-to-end throughput between a source and destination is restricted by the lowest link rate, so the max-min fair allocation on the link rates is an optimal strategy to maximize the end-to-end throughput. In this paper, we verify that if the wireless links have a solidarity property, the max-min fair allocation has all link rates equal, so we propose a transmit power control (TPC) algorithm that decides the transmit power of multi-hop nodes to equalize all link rates. The proposed algorithm operates in a distributed manner, where each node averages the recognized link rates around itself, allocates its transmit power to achieve this average rate, and iterates this operation until all link rates become equal. Intensive simulation shows that the proposed TPC algorithm enables all link rates to converge on the same value, and thus maximizes the multi-hop end-to-end throughput while decreasing the power consumption of multi-hop nodes.

Carrier Sense Multiple Access with Collision Resolution

We propose a distributed medium access control (MAC) protocol called carrier sense multiple access with collision resolution (CSMA/CR) by extending the wireless CSMA/CD protocol. The proposed CSMA/CR protocol identifies a collision by additional sensing after data transmission starts. If one station detects a collision, it transmits a jam signal to stop the other stations transmissions and then retransmits the data without backoff, which resolves the next collision that may occur. The throughput analysis shows that the proposed CSMA/CR protocol outperforms other conventional CSMA-based MAC protocols.

On the design of user pairing algorithms in full duplexing wireless cellular networks.

In this paper, we consider a full duplexing (FD) wireless cellular network where a central base station (BS) works in the FD mode while the downlink (DL) and uplink (UL) users work in the time division duplexing (TDD) mode. Since this FD system induces the inter-user interference from UL user to DL user, the main challenge for maximizing the system performances is user pairing that makes a pair of DL user and UL user to use the same radio resource simultaneously. We formulate two optimization problems for user pairing to maximize the cell throughput or minimize the outage probability, and then propose two suboptimal user pairing algorithms with low complexity. The first algorithm is designed in a way where the DL user with a better signal quality has a higher priority to choose its UL user for the throughput maximization. On the contrary, the second algorithm gives the DL user with a worse signal quality a higher priority to choose its UL user for the outage minimization. Simulation results show that the FD system using the proposed user pairing algorithms achieves the optimal performances and significantly outperforms the conventional TDD system in terms of the cell throughput and the outage probability.

Carrier sensing multiple access with collision resolution (CSMA/CR) protocol for next-generation wireless LAN

This paper proposes a distributed medium access control (MAC) protocol called carrier sensing multiple access with collision resolution (CSMA/CR) based on a fully connected single hop network considering next-generation wireless LAN environments with a high density of stations. The proposed CSMA/CR follows the typical CSMA/CA protocol and identifies a collision by additional sensing after data transmission starts without a ready-to-send (RTS) and clear-to-send (CTS) exchange. If one station detects a collision, it transmits a jam signal to stop the other stations transmissions and has access priority for the retransmission without backoff, which prevents the next collision that may occur. The simulation results show that the proposed CSMA/CR has a low overhead due to the detection of a collision without an RTS/CTS exchange and always achieves the best throughput regardless of the number of access stations by using the collision resolution scheme.

Device-to-device communication assisted interference mitigation for next generation cellular networks

Advanced interference management schemes have become crucial for achieving the required cell edge spectral efficiency targets and to provide ubiquity of user experience throughout the network, actively discussed for 4G and beyond 4G cellular standards. On the other hands, it requires the significant channel state information (CSI) feedback overhead on consumer units so as to promote the cell-edge performance. In this paper, we explore the interplay between interference management, device-to-device (D2D) communication, and CSI feedback. To show this, a novel interference management method, D2D communication assisted interference alignment (DIA) is proposed, which is inspired by subspace intersection property. We believe that the proposed technology can be well applied to consumer mobile communication devices over cellular networks such as smart phones, tablets, etc.

Distributed transmit power control for maximizing end-to-end throughput in wireless multi-hop networks

Wireless multi-hop network has a solidarity property in which each link consisting of the multi-hop interferes mutually and so the increase of one links rate results in the decrease of the other links rate. In the multi-hop link, the end-to-end throughput between source and destination is restricted by the lowest link rate, so the max-min fair allocation on the link rates is an optimal strategy to maximize the end-to-end throughput. In this paper, we verify that if the wireless links have a solidarity property, the max-min fair allocation has all link rates equal, so we propose a transmit power control (TPC) algorithm that decides the transmit power of multi-hop nodes to make all link rates be equal. The proposed algorithm operates in a distributed manner where each node averages the recognized link rates around itself, allocates its transmit power to achieve this average rate, and iterates this operation until all link rates become equal. Intensive simulation shows that the proposed TPC algorithm enables all link rates to converge on the same value and so maximizes the multi-hop end-to-end throughput while decreasing the power consumption of multi-hop nodes.

Analysis of Tradeoff Between Energy Consumption and Activation Delay in Power Management Mechanisms

Power management mechanisms (PMMs) inherently have a tradeoff between the energy consumption and delay. In this letter, we analyze an active-idle state transition mechanism for power-saving in mobile stations by considering the general traffic pattern of smartphones, which consists of uplink request packets and downlink bursty data. We derive closed-form solutions of both the energy consumption and activation delay, and characterize their tradeoff relationship. The results show that the tradeoff curve is linear and its slope is not affected by the PMMs operational parameters or the traffic arrival rate.

A Bioinspired Fair Resource-Allocation Algorithm for TDMA-Based Distributed Sensor Networks for IoT

Many studies on distributed resource-allocation algorithms have been conducted recently because of the increasing number of network nodes and the rapidly changing network environments in the Internet of Things (IoT). In this paper, we propose the multihop DESYNC algorithm, which is a bioinspired Time Division Multiple Access- (TDMA-) based distributed resource-allocation scheme for distributed sensor networks. We define a detailed frame structure for the proposed multihop DESYNC algorithm and a firing message, which acts as a reference for resource allocation. In addition, operating procedures for resource allocation and collision detection avoidance under multihop DESYNC are explained. Simulations show that multihop DESYNC effectively resolves the hidden-node problem and that it fairly shares resources among nearby nodes in multihop networks. Moreover, it achieves better performance than the CSMA/CA algorithm in terms of channel reuse gain and average throughput.