Seong-Lyun Kim

Joint subcarrier and power allocation in uplink OFDMA systems

In this letter, we focus on joint subcarrier and power allocation in the uplink of an OFDMA system. Our goal is to maximize the rate-sum capacity in the uplink. For the purpose, we formulate an optimization problem subject to subcarrier and power constraints and draw necessary conditions for optimality, from which we derive joint subcarrier and power allocation algorithms. Simulation results show that our proposed scheme enhances the system capacity, providing almost near optimal solutions with low computational burden.

Temporal Spectrum Sharing Based on Primary User Activity Prediction

In this paper we investigate the opportunistic spectrum access in temporal domain where a secondary user shares a radio channel with a primary user during the OFF period of the primary user. We consider practical ON/OFF traffic models whose bursty natures are not properly described by a Markovian assumption. An optimal strategy to determine the transmission power of the secondary user is proposed, which can be adapted to any source traffic model of the primary user. This strategy will maximize the spectrum utilization of the secondary user while keeping interference violations to the primary user below a threshold. Numerical results show that the transmission power of the secondary user depends on the probability distribution of the primary traffic as well as the elapsed time of the OFF period.

Hierarchically Modulated Network Coding for Asymmetric Two-Way Relay Systems

Network coding has been known as a spectrally efficient technique in wireless networks. However, when it is applied to a two-way relay channel (TWRC), it suffers from performance degradation caused by the asymmetric position of the relay. In this paper, we suggest remedying this problem by using hierarchical modulation at the source node. We investigate how hierarchical modulation can be incorporated and optimized with network coding. Our results are encouraging in that hierarchically modulated network coding (HMNC) significantly improves end-to-end bit-error probability and spectral efficiency in asymmetric relay channels, as compared with direct transmission (DT), bidirectional network coding (BNC), and coded bidirectional relay (CBR).

Tractable Resource Management With Uplink Decoupled Millimeter-Wave Overlay in Ultra-Dense Cellular Networks

The forthcoming 5G cellular network is expected to overlay millimeter-wave (mmW) transmissions with the incumbent micro-wave (μW) architecture. The overall mm-μW resource management should, therefore, harmonize with each other. This paper aims at maximizing the overall downlink (DL) rate with a minimum uplink (UL) rate constraint, and concludes: mmW tends to focus more on DL transmissions while μW has high priority for complementing UL, under time-division duplex (TDD) mmW operations. Such UL dedication of μW results from the limited use of mmW UL bandwidth due to excessive power consumption and/or high peak-to-average power ratio (PAPR) at mobile users. To further relieve this UL bottleneck, we propose mmW UL decoupling that allows each legacy μW base station (BS) to receive mmW signals. Its impact on mm-μW resource management is provided in a tractable way by virtue of a novel closed-form mm-μW spectral efficiency (SE) derivation. In an ultra-dense cellular network (UDN), our derivation verifies mmW (or μW) SE is a logarithmic function of BS-to-user density ratio. This strikingly simple yet practically valid analysis is enabled by exploiting stochastic geometry in conjunction with real three-dimensional (3-D) building blockage statistics in Seoul, South Korea.

Iterative and greedy resource allocation in an uplink OFDMA system

We suggest an iterative power and greedy subcarrier allocation algorithm to improve rate-sum capacity in an uplink OFDMA system. For the downlink, it has been accepted as an optimal solution that each subcarrier is allocated to the user with the best channel condition and power is allocated by water-filling over subcarriers. However, it is not true in an uplink OFDMA system which has distributive power constraints. We formulate the optimization problem having constraints power and subcarriers in uplink, then draw two necessary conditions for optimality. Using the conditions, we propose a greedy subcarrier allocation algorithm based a marginal rate junction and iterative power allocation algorithm based on water-filling. Simulation results show that the enhanced system capacity is achieved by our proposed scheme.

Random power control in wireless ad hoc networks

In this letter, we present stochastic analysis of two power control schemes for wireless ad hoc networks: random-vs. fixed power controls. Using numerical examples, we show that randomizing transmit power has positive effect of reducing high interferences to the other nodes, and improves network connectivity, in high-density networks. However, the fixed power control is more favorable in low-density environments. In this letter, we derive a formula of such cross-over (density) point at which superiority of each power control is switched.

Asymptotic behavior of ultra-dense cellular networks and its economic impact

This paper investigates the relationship between base station (BS) density and average spectral efficiency (SE) in the downlink of a cellular network. This relationship has been well known for sparse deployment, i.e. when the number of BSs is small compared to the number of users. In this case the SE is independent of BS density. As BS density grows, on the other hand, it has previously been shown that increasing the BS density increases the SE, but no tractable form for the SE-BS density relationship has yet been derived. In this paper we derive such a closed-form result that reveals the SE is asymptotically a logarithmic function of BS density as the density grows. Further, we study the impact of this result on the network operators profit when user demand varies, and derive the profit maximizing BS density and the optimal amount of spectrum to be utilized in closed forms. In addition, we provide deployment planning guidelines that will aid the operator in his decision if he should invest in densifying his network or in acquiring more spectrum.

Feasibility of cognitive machine-to-machine communication using cellular bands

We evaluate the feasibility of cognitive machine-to-machine communication on cellular bands from engineering and business perspectives. We propose a hierarchical network structure for cognitive M2M communication, where cluster headers gather M2M traffic using cognitive radio and forward it to the cellular networks. This structure can resolve the congestion problem that arises in conventional M2M systems. We obtain the optimal network parameters that minimize congestion at the radio access network. In addition, we investigate the business value of cognitive M2M on cellular bands. Taking into account the network usage fee, service fee, and hardware production costs, we model the profit structure of M2M services and derive the condition under which the CR type of M2M communication is superior to conventional M2M communication. We find that the optimal network design parameters (i.e., the number of cluster headers and cluster size) for business value would be different from the parameters expected from an engineering point of view. We believe that cognitive M2M communication can be a good solution for dealing with excessive M2M traffic in cellular networks, in terms of technical feasibility and business opportunity.

Cross-layer multiservice opportunistic scheduling for wireless networks

We consider opportunistic communication in multiservice wireless data networks using centralized controllers. Opportunistic scheduling (OS) is an emerging cross-layer media access control design approach to multiuser communication over fading wireless channels. OS exploits channel variations to maximize wireless throughput. We present a general background of OS schemes and then propose an OS scheme making an optimum trade-off between throughput and fairness, while maintaining stability in different channel environments and guaranteeing minimum service to multiple queues active concurrently at a given user

Multi-robot path finding with wireless multihop communications

Path finding by multiple robots has been studied by many researchers. In particular, when the robot is able to communicate with other colleague robots, path finding will be solved collaboratively among the robots. The main motivation of this article is to apply wireless multihop communications to the collaborative path-finding problem. For this purpose, we propose a cooperative path finding algorithm (CPA) and report its performance by real implementation on ZigBee-based micro-robots as well as MATLAB simulations. There has been some controversy over the use of wireless multihop communications in commercial wireless networks for technical as well as non-technical reasons. However, our study highlights a new potential of multihop communications in inter-robot information exchange. We show the effects of wireless communications on the operation of cooperative multi-robots and find the benefit of multihop communications.