Jaechan Lim

Effects of time-to-trigger parameter on handover performance in SON-based LTE systems

In this paper, we evaluate handover (HO) performance when we apply various time-to-trigger (TTT) methods in self-organizing network (SON)-based long term evolution (LTE) systems. Although TTT can mitigate the wasteful ping-pong HO effect, it also can cause undesirable radio link failure (RLF) due to delayed HO. The optimal HO timings that produce the lowest ping-pong rate within allowable RLF rate vary depending on user equipment (UE) speeds and neighboring cell configurations. To achieve efficient HO timings, we propose and investigate two methods: “adaptive” and “grouping.” In the “adaptive method,” we select the adaptive TTT value for each UE speed based on RLF rate of 2%. The “grouping method” classifies UE speeds into three ranges and assigns the proper TTT value to each range. To apply the LTE specification more effectively, we suggest the criteria of grouping, and propose the proper TTT value for each range. We consider HO in two neighboring cell configurations: from a macro cell to either a macro cell or a pico cell. Simulation results show that the HO performance of the “adaptive method” is greatly improved compared to that of applying fixed TTT values. The results also show that the performance of the “grouping method”, using proposed criteria and proper TTT values, is comparable to that of the “adaptive method.”

Cost Reference Particle Filtering Approach to High-Bandwidth Tilt Estimation

In this paper, cost reference particle filter (CRPF) approach in estimating 1-D “tilt” of a vehicle attitude is proposed. CRPF has a couple of advantageous features compared to standard particle filtering; particularly, it does not require noise statistics in its application. H_ filter (HF) has common features as that of CRPF. The extended HF (EHF) is employed, which uses the approximate linearization of the nonlinear measurement function as the extended Kalman filter is extended. The performance of both approaches is investigated and compared in this paper. Low-cost “accelerometer” and “gyroscope” sensors are cooperatively employed instead of inclinometer in measuring the tilt. Simulation results show that CRPF outperforms EHF in estimating the tilt due to its robustness against the nonlinearity of the measurement equation, whereas EHF outperforms CRPF in estimating the tilt rate whose measurement equation is linear. Notably, an efficient CRPF outperforms EHF in tracking the tilt with just ten particles.

Inter-Carrier Interference Estimation in OFDM Systems With Unknown Noise Distributions

There are a number of approaches to estimating carrier frequency offset (CFO) that causes inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems: self-cancelation method, the extended Kalman filter (EKF), particle filter, Hinfin filter (HF), etc. In particular, the HF is of interest because prior statistical noise information is not necessarily required in its application. Cost reference particle filter (CRPF), newly developed in the particle filtering framework, has the same feature as HF; it also does not require the prior noise information of the state and the measurement equation. In this letter, we compare and analyze the performances of two similar methods. The simulation results show that CRPF outperforms HF, particularly when the bit energy to noise ratio of the measurement is low. Therefore, CRPF is very effective and robust, especially when the noise statistics are unknown with a low bit energy to noise ratio.

Gaussian Particle Filtering Approach for Carrier Frequency Offset Estimation in OFDM Systems

We propose Gaussian particle filtering (PF) approach for estimating carrier frequency offset (CFO) in OFDM systems. PF is more powerful especially for nonlinear problems where classical approaches (e.g., maximum likelihood estimators) may not show optimal performance. Standard PF undergoes the particle impoverishment (PI) problem resulting from resampling process for this static parameter (i.e., CFO) estimation. Gaussian PF (GPF) avoids the PI problem because resampling process is not needed in the algorithm. We show that GPF outperforms current approaches in this nonlinear estimation problem.

Management of neighbor cell lists and physical cell identifiers in self-organizing heterogeneous networks

In this paper, we propose self-organizing schemes for the initial configuration of the neighbor cell list (NCL), maintenance of the NCL, and physical cell identifier (PCI) allocation in heterogeneous networks such as long term evolution systems where lower transmission power nodes are additionally deployed in macrocell networks. Accurate NCL maintenance is required for efficient PCI allocation and for avoiding handover delay and redundantly in creased system overhead. Proposed self-organizing schemes for the initial NCL configuration and PCI allocation are based on evolved universal terrestrial radio access network NodeB (eNB) scanning that measures reference signal to interference and noise ratio and reference symbol received power, respectively, transmitted from adjacent eNBs. On the other hand, the maintenance of the NCL is managed by adding or removing cells based on periodic user equipment measurements. We provide performance analysis of the proposed schemes under various scenarios in the respects of NCL detection probability, NCL false alarm rate, handover delay area ratio, PCI conflict ratio, etc.

Frequency-selective and nonlinear channel estimation with unknown noise statistics

We propose cost reference particle filter (CRPF) and extended game theory-based H¿ filter approaches to the problem of estimating frequency-selective and slowly varying nonlinear channels with unknown noise statistics. The proposed approaches have a common advantageous feature that the noise information is not required in their applications. The simulation results justify that both approaches are effective, and that CRPF is more robust against highly nonlinear and drastically varying channels.

Self-optimization of single femto-cell coverage using handover events in LTE systems

In LTE systems, low-power required small-coverage-cell is additionally deployed on a macro-cell to enhance the system performance in the respects of coverage and capacity improvement. In this paper, we propose a self-optimization algorithm for a single femto-cell coverage. The objective function to be minimized for optimal coverage adjustment is the sum of camp on (CO) and unnecessary hand over (UHO) areas that is defined shortly. CO problem results in inefficient usage of base station with low received signal strength, and consequently, causes the low data rate. UHO incurs the wasteful usage of limited resources and overhead increase due to HO processes. We verify the validity of the proposed algorithm by comparing the performance with the result of theoretical analysis. Furthermore, we develop the proposed algorithm to increase the system throughput.

Game theory based H infinity filter approach for estimating the number of competing terminals in IEEE 802.11 networks

The performance of the IEEE 802.11 protocol based on the distributed coordination function (DCF) depends on the number of competing terminals. We propose game theory-based extended H∞ filter approach to the problem of estimating the number of competing terminals. The proposed approach has an advantageous feature that the noise information of the dynamic state system and the measurement equation is not required in its applications as opposed to the case of the Kalman filter. Simulation results show that H∞ filter outperforms the Kalman filter in estimating the number of competing terminals in both saturated and non-saturated conditions. In particular, the performance of the proposed approach is more satisfactory in non-saturated condition.

Deployment of small cells with biased density in heterogeneous networks

Heterogeneous network systems were proposed to cope with explosively increased data traffic in wireless communications networks. In heterogeneous networks, small cells are additionally deployed to offload the data traffic of a macrocell, which results in highly improved data traffic capacity. Locational property of small cells affects the capacity of small cell networks significantly. In this paper, we propose a scheme for small cell deployment where cells are placed in a biased manner, i.e. spatially non-uniform deployment. Furthermore, we control the density of small cell distribution, and obtain improved network capacity as well as keeping the minimum-distance-constraint between macrocells and small cells. Computer simulation results show that the proposed approach increases the network throughput significantly compared to various existing schemes.

Performance analysis of VoIP according to relay capability in OFDMA-based systems

We consider a relay-assisted wireless communication system where relays are given the capability of scheduling function. We evaluate and compare the performances of voice over internet protocol (VoIP) in orthogonal frequency division multiple access (OFDMA) systems where two different scheduling capabilities of relays are employed in each system. In the system with relays that have the scheduling capability, channel-resource allocation is not performed efficiently, which results in degraded performance of the system compared to the system with the relays that do not have the scheduling capability as shown in the simulation result.