Jihaeng Heo

Optimal mode selection for cognitive radio sensor networks with RF energy harvesting

This paper investigates an optimal mode selection policy for cognitive radio sensor networks powered by RF energy harvesting. The RF energy harvesting enables the sensor node to operate with a potentially perpetual lifetime. We assume that the sensor node harvests RF energy received from the primary network and it cannot carry out RF energy harvesting and opportunistic spectrum access at the same time. Therefore, the sensor node should decide whether to access the spectrum or to harvest RF energy in each time slot to maximize an expected total throughput. We develop the optimal mode selection policy by casting this decision making problem in the framework of partially observable Markov decision process (POMDP). Numerical results show that the developed optimal policy finds a balance between obtaining the immediate throughput and harvesting the RF energy for future use.

Mobile TV White Space with Multi-Region Based Mobility Procedure

Current regulations require that portable unlicensed devices operating in TV white space perform a channel availability query (CAQ) before transmitting whenever they move more than 100m. Since this mobility constraint gives rise to frequent CAQs, the Federal Communications Commission allows a region-based mobility procedure (RMP) to help reduce the number of CAQs. Since the performance of the RMP cannot be precisely evaluated based only on the number of CAQs, in this letter, we formulate an achievable throughput in terms of the average number of available channels, the effective data transmission time, and the TV band database fees. We discover that the achievable throughput for the conventional single RMP is degraded when the radius of the region is not properly determined. In order to alleviate this performance degradation, we propose a multi-RMP using multiple radii in order to obtain multiple available channel lists. Simulation results show that the proposed multi-RMP can mitigate the performance degradation. In addition, the use of multi-RMP enhances the maximum throughput compared to the single RMP.

Effective Small Cell Deployment with Interference and Traffic Consideration

We examine small cell deployment strategies in two- tier cellular networks. Specifically, we propose a joint interference and load deployment (JD) strategy, reflecting a realistic interference environment and traffic load. In this JD, the location of a small base station (s-BS) is sequentially decided in greedy manner by exploiting both the spatial user equipment (UE) distribution and the received signal strength indicator (RSSI). Furthermore, we propose a deterministic deployment (DD), which does not require this RSSI measurement. In DD, the deployed pattern of s-BSs is the same for each macro cell. System level simulation (SLS) results demonstrate that the proposed schemes yield a cell throughput gain of up to 61%, when the UEs are densely placed near a m-BS. Moreover, 5%-tile UE throughput and median UE throughput are improved by up to 134% and 76%, respectively, when UEs are densely distributed far from the m-BS. Furthermore, the SLS results provide intuition on between load and interference in the s-BS deployment as follows: 1) load is more important than interference in the macro edge region, 2) interference is more significant than load when the s-BS is deployed close to the m-BS.

OFDMA with variable tone spaces

Next generation wireless networks such as 3GPP LTE-A demand enhanced system capacity for high-quality multimedia services. Carrier aggregation based on OFDMA has been introduced to achieve this. In CA, each component carrier can be located on a different frequency band, and each user utilizing it moves at a different speed. This results in different channel characteristics for each user. Enhancing the system capacity in this environment requires a multiple access scheme that considers different channel characteristics among users. However, conventional OFDMA using fixed tone space (called OFDMA-FTS) does not consider user-specific channel characteristics, resulting in capacity degradation. In this article, we propose a new OFDMA system with variable tone spaces (OFDMA-VTS) as an approach to enhance system capacity. OFDMA-VTS selects the tone space suitable for each user by considering userspecific channel characteristics in CA. When intersymbol interference is dominant due to a long delay spread, the small tone space is selected to eliminate severe ISI. Conversely, when intercarrier interference due to a large Doppler spread severely degrades the system performance, the large tone space is selected to compensate ICI. In the proposed OFDMA-VTS, each user adaptively selects the optimal tone space needed to minimize the total interference of ISI and ICI, therefore enhancing system capacity.

Spectral Efficiency Analysis of Ultra-Dense Small Cell Networks With Heterogeneous Channel Estimation Capabilities

This letter analyzes a spectral efficiency (SE) performance reflecting heterogeneous channel estimation (CE) capabilities across user equipment (UE) in ultra-dense small cell networks. Specifically, we derive asymptotic and approximated SE performances considering different kinds of CE capabilities. The analysis and simulation results lead us to some new intuitions as follows. First, the heterogeneity of CE capabilities across UEs has a great effect on SE performance in ultra-dense networks. Second, as the cell density increases, SE performance is asymptotically converged to its limit point when heterogeneous CE capabilities are considered, whereas the SE logarithmically increases in the perfect CE case.

Effects of channel estimation errors on ultra-dense small cell networks

In this paper, we investigate the effect of channel estimation (CE) error on the performances for the ultra-dense small cell networks. Specifically, we examine its effect on the distribution of signal to interference and noise ratio (SINR) and trend of spectral efficiency (SE), and outage probability. From these, we found out that the SINR degradation due to the CE errors becomes larger, as the density ratio of small base station (BS) to UE increases. We derived the theoretical lower bound of the outage probability, which is the function of CE correlation coefficient. Finally, numerical results demonstrate that the CE errors diminish the increasing scaling of SE, as the small BS density increases.

A New Link Adaptation Method to Mitigate SINR Mismatch in Ultra-Dense Small Cell LTE Networks

In this paper, we consider a new interference problem caused by idle small cells, which have no associated user equipment in ultra-dense small cell long-term evolution (LTE) networks. Specifically, we investigate the effect of idle small cells on the signal to interference and noise ratio (SINR) of the cell-specific reference signal (CRS) and the data signals. We confirm that CRS interference from idle small cells produces uneven interference pattern across CRS and data signals and eventually causes an SINR mismatch between CRS and data signals as well as between data signals with and without CRS symbols. In addition, these phenomena become severe with cell densification. In order to solve this mismatch problem, we propose a simple link adaptation framework, which utilizes clustered CRS assignment and hybrid SINR measurement. The numerical results show that the proposed method improves the average sum throughput compared with the conventional approaches. Overall, this paper sheds new light on investigating and coping with the interference problem coming from idle small cells in future ultra-dense small cell LTE networks.

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.

Effect of primary power fluctuation on ergodic capacity of a secondary user in a spectrum sharing environment

This paper investigates the effect of the transmit power fluctuation of the primary user on the ergodic capacity of the secondary user in a spectrum sharing environment. Considering an average received-power constraint at the primary user receiver, we derive the closed-form expression for the ergodic capacity of the secondary user and the average received-power assuming Rayleigh fading channels. Numerical results show that the transmit power fluctuation can enhance the ergodic capacity of the secondary user if the secondary user properly control its transmit power so as to fully exploit the transmit power fluctuation.

Directional Information Based Mobility Procedure for Throughput Enhancement in Mobile TVWS

Portable unlicensed devices operating in TV white space must adhere to FCC rules designed to protect licensed devices. These rules decrease the achievable throughput for the portable unlicensed devices because the portable unlicensed devices are required to perform a channel availability query (CAQ) before transmitting. In order to reduce the number of CAQs, the Federal Communications Commission allows a region based mobility procedure (RMP). In this paper, we propose a new-RMP using directional information in order to enhance the achievable throughput. Directional information enables an unlicensed portable device to increase its achievable throughput by considering a uni- directional region that is smaller than an omni-directional region. Simulation results show that the proposed RMPs enhance the achievable throughput compared to the conventional RMPs. In addition, the achievable throughput for the proposed RMP using a navigation system is greatly enhanced compared to the conventional RMPs.