Inkyu Bang

Self-organizing and self-healing mechanisms in cooperative small-cell networks

Small-cell networks are expected as one of key solutions for high system capacity. However, self-organizing and self-healing mechanisms are necessarily required to deploy and manage an increasing number of small-cell networks. In this paper, we consider a future small-cell network as an intelligent distributed antenna system such as an adaptive array antenna. We propose a self-organizing mechanism and a self-healing mechanism for small-cell networks through cooperative clusters. We evaluate the system performance in terms of resource utilization in both normal and failure cases of a small-cell network. The results show that proposed mechanisms outperform the conventional mechanisms.

A random access scheme based on a special preamble for supporting emergency alarms

In emergency situations such as a disaster and a blackout, machine nodes should report emergency alarms, including the disaster type and disaster area to an M2M server. When many related machine nodes attempt a random access (RA) to report emergency alarms, and this may cause a physical random access channel (PRACH) overload. In this case, the machine nodes hardly access the network within a given delay requirement of emergency alarms. In this paper, we propose a novel random access scheme based on a special preamble for emergency indication in an emergency situation and adaptively reserve more PRACH resources in proportion to the number of machine nodes using the special preamble. The proposed RA scheme outperforms the conventional RA scheme in terms of access delay and can accommodate much more machine nodes in emergency situations, compared with the conventional RA scheme.

Effects of Multiple Antennas and Imperfect Channel Knowledge on Secrecy Multiuser Diversity

In this letter, we investigate the effects of multiple antennas at a legitimate receiver and imperfect channel knowledge on secure user-scaling law in uplink multiuser wiretap networks. We generalize the conventional analysis of the secure user-scaling law for a single antenna system with perfect channel knowledge by considering multiple antennas at the receiver and the channel estimation errors.

Energy-efficient subchannel allocation scheme based on adaptive base station cooperation in downlink cellular networks

An inter-cell interference (ICI) problem between co-channel BSs has been one of main challenging research issues in downlink cellular networks. Fractional frequency reuse (FFR) and BS cooperation are useful techniques to manage the ICI problem. By combining the BS cooperation and FFR techniques, we propose to determine an efficient cooperation mode and the type of subchannels, i.e., reusable subchannels or exclusively used subchannels. The optimization problem to minimize the total power consumption in a cooperative cluster is formulated while satisfying a given constraint in terms of the number of subchannels. When the number of users is 20, our proposed energy-efficient subchannel allocation (EESA) scheme consumes 21%, 25%, 43%, and 43% less power than the conventional resource minimization (RM), fractional frequency reuse (FFR), exclusive (EX), and full cooperation (FCO) scheme respectively. Moreover, our proposed EESA scheme outperforms the other four schemes in terms of outage probability.

Design criteria on a mmWave-based small cell with directional antennas

Due to spectrum shortage in the conventional microwave bands, millimeter wave (mmWave) bands have been attracting great attention as an additional spectrum band for 5G cellular networks. In this paper, we show the feasibility of using a mmWave-based small cell with directional antennas. We investigate the effect of the number of antennas, nant, on the capacity and coverage probability, and propose an antenna clustering scheme (ACS) to utilize the antennas more efficiently. For a given nant, we also investigate the effect of the number of clusters, ncluster, on the capacity and coverage probability. In addition, we propose two design criteria: a capacity-maximization criterion (CapMC) and a coverage-maximization criterion (CovMC), in the mmWave-based small cell. Each design criterion is formulated as a joint optimization problem to find the optimal values of nant and ncluster to maximize each objective function while satisfying system requirements such as capacity and coverage probability. Performance evaluation shows that CapMC can achieve the highest capacity while satisfying a given coverage probability requirement and CovMC can achieve the highest coverage probability while satisfying a given capacity requirement.

Secrecy multiuser diversity for distributed antenna systems from the perspective of user-scaling law

In this paper, we introduce the notions of a user-scaling function and a user-scaling exponent and multiuser diversity (UEMD) pair. Our notions for the UEMD pair could be utilized as a basic tool for analyzing user-scaling laws. We derive a secure user-scaling law in terms of SNR, the number of eavesdroppers, the number of distributed antennas in distributed antenna systems, and design parameter λ. Through analytical and numerical results, we characterize the effect of the number of distributed antennas on the user-scaling behavior in terms of achievable secrecy rate. Compared with the conventional results based on a single antenna system, our results indicate that installing a small number of distributed antennas could provide a significant performance gain in terms of the achievable secrecy rate.

Design of a multi-variable QoE function based on the remaining battery energy

Recently, study on designing a new metric to express quality of experience (QoE) has attracted attention since user satisfaction may be different according to the types of served traffic. The utility function for various traffic has been studied for many years and most of the previous work focused on the utility functions with respect to a single performance metric such as throughput. However, multiple performance metrics may affect user satisfaction according to user properties or situations. One of critical factors that affect the sensitivity of user satisfaction is the remaining battery energy. In this paper, we investigate the characteristics of the user satisfaction with respect to throughput and energy efficiency considering the remaining battery energy. Based on the characteristics, we design a multi-variable QoE function based on the unified utility, especially, considering users remaining battery energy and verify the feasibility of the proposed multi-variable QoE function through numerical results.

Effect of control and data frame overheads on the capacity scaling of a hierarchical cooperation scheme

The hierarchical cooperation (HC) scheme is known to exhibit linear scaling from the viewpoint of information theory under the assumption of no control and data frame overhead. However, if we consider the control and data frame overheads of the HC scheme in a more practical ad-hoc network environment, the scaling law of the aggregate throughput is expected to be far from linear scaling. In this paper, we observe the effect of the control and data frame overheads on the capacity scaling of the HC scheme. From the results of performance evaluation, we find that the data frame overhead only affects the degradation of the aggregate throughput in proportion to the data frame overhead ratio. Moreover, the control overhead affects the trend of the aggregate throughput. The slope of the aggregate throughput converges to near zero as the total number of nodes, N, increases, and the aggregate throughput becomes saturated for any given number of stages, h. The significant performance degradation in the aggregate throughput is inevitable due to the control and data frame overheads. Therefore, it is not always good to use as many stages as possible, and, thus, there exists an optimal number of stages according to the given system parameters.

Opportunistic user selection with adaptive jamming for secure communication in heterogeneous networks

In this paper, we propose an opportunistic user selection criterion to obtain a multiuser diversity gain for secure communication. At the same time, we also propose a strategy for utilizing a small base station in heterogeneous networks as a helper or a jammer adaptively. Our approach is to maximize the secrecy achievable rate based on an analytically derived lower bound instead of the original objective function due to its complexity. Based on a simple proposed user selection criterion considering both macro and small base stations, we propose two jamming strategies according to knowledge of eavesdroppers channel: adaptive jamming and probabilistic jamming strategies. Through simulations in two scenarios, it is shown that our proposed user selection and jamming schemes provide a good performance gain in terms of secrecy achievable rate.

Aggregate throughput maximization in a hierarchical cooperation scheme under consideration of packet arrival rate, control and data overhead

The conventional hierarchical cooperation (HC) scheme proposed by Ozgur et al. [1] is a novel communication scheme that can achieve linear capacity scaling, O(N) in wireless ad-hoc network from the viewpoint of information theory without considering control and data overhead. In this paper, we investigate the effects of practical parameters such as packet arrival rate, control overhead and data overhead on the HC scheme. We formulate an aggregate throughput optimization problem in terms of cluster size m and solve it by well-known algorithms. Through the performance evaluations, we observe three main effects of the practical parameters. First, large data overhead ratio degrades the aggregate throughput. 31% performance degradation occurs when we consider data overhead ratio α = 0.45. Second, large control overhead also degrades the aggregate throughput and affects to determine the cluster size m for maximum aggregate throughput. Third, the packet arrival rate λ plays a key factor for system stability and we obtain a sufficient condition for system stability.