Yosub Park

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.

Latency of Cellular-Based V2X: Perspectives on TTI-Proportional Latency and TTI-Independent Latency

Vehicle-to-everything (V2X) is a form of wireless communication that is extremely sensitive to latency, because the latency is directly related to driving safety. The V2X systems developed so far have been based on the LTE system. However, the conventional LTE system is not able to support the latency requirements of latency-aware V2X. Fortunately, the state-of-the-art cellular technology standard includes the development of latency reduction schemes, such as shortened transmission time intervals (TTI) and self-contained subframes. This paper verifies and analyzes the latency of cellular-based V2X with shortened TTI, which is one of the most efficient latency reduction schemes. To verify the feasibility of V2X service, we divide the V2X latency into two types of latency, TTI-independent latency and TTI-proportional latency. Moreover, using system-level simulations considering additional overhead from shortened TTI, we evaluate the latency of cellular-based V2X systems. Based on this feasibility verification, we then propose cellular-based V2X system design principles in terms of shortened TTI with only one OFDM symbol and while sustaining radio resource control connection.

Energy-efficient spectram access for ultra low power sensor networks

This paper investigates a dynamic spectrum access (DSA) scheme for ultra low power sensor networks (ULPSN) in an open spectrum where multiple systems coexist and interfere with each other. Low transmission power and simple communication protocol are significant obstacles for ULPSN to operate in the open spectrum. The DSA has been researched to solve this problem by assuming that coexisting systems can perfectly detect each other. However, this assumption is limited for ULPSN because detection of ULPSN in coexisting system is difficult due to sensitivity limitation of spectrum sensing. As a result, available spectrum is more scarce for ULP system than other coexisting systems. Thus, we derive a new Markov chain model of open spectrum access. Based on the Markov chain mode, we design a DSA scheme to maximize the energy efficiency and investigate the channel access and switch policy of the proposed DSA within the framework of partially observable Markov decision process (POMDP). The simulation results demonstrate that the proposed DSA scheme yields improved energy efficiency and lifetime compared to a random channel selection for ULPSN.

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.

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.

Effect of Unpredictable Interference on MU-MIMO Systems in HetNet

In this paper, we investigate the outage probability of a multiuser multiple-input multiple-output (MU-MIMO) system in the heterogeneous network (HetNet) in the presence of unpredictable interference (UI) from coexisting ad hoc systems. In order to achieve multiuser diversity gain, the MU-MIMO system requires feedback carrying the signal-to-interference-plus-noise ratio (SINR) from all active users to the base station. However, the coexisting ad hoc systems in HetNet can unpredictably initiate their data transmission during the SINR feedback duration. This unpredictable behavior by the coexisting ad hoc systems causes a mismatch between the measured SINR and the instantaneous SINR. In order to investigate the performance of the MU-MIMO system in HetNets, we first categorize the interferences into predictable interference and UI. Based on this categorization, we analyze the outage probability of an MU-MIMO system using a max-SINR scheduler. The theoretical analyses show that the MU-MIMO system cannot achieve the maximum diversity gain, since the UI from the coexisting systems causes beam selection mismatch. Additionally, we show that reducing the interval between the measurement and data transmission prevents performance degradation by reducing the effect of the unpredictable behavior of the coexisting ad hoc systems.

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.

Subband-Wise Filtering in FBMC to Improve Time and Frequency Efficiency for Short Burst Transmission

This paper proposes a subband-wise filtering with different filter orders in filter bank multi-carrier (FBMC) system. The objective of the proposed scheme is to improve the time and frequency efficiency of the FBMC for short burst transmission in future communication systems. The proposed scheme divides the band into two subbands, which are at the inner and the outer of the band, and uses the high filter order to the outer subband and the low filter order to the inner subband. Numerical results show that the proposed scheme has the high time and frequency efficiency when transmitting short burst. Moreover, the proposed scheme also has the nearly same bit error rate performance compared to the conventional FBMC.

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.

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.