Kojiro Hamabe

Adaptive Power Level Setting of Femtocell Base Stations for Mitigating Interference with Macrocells

Femtocells are expected to increase network capacity, extend macrocell coverage, and introduce new services. Because femtocells share the same frequency band with macrocells in many cases, the femtocell base station (BS) must mitigate the interference with macrocells as well as ensure coverage in customer premises. However, conventional femtocell BS transmit power setting have not adequately accounted for the interference with neighboring macrocell mobile stations (MSs), leading to small femtocell user throughput. In this paper, we describe an adaptive femtocell BS transmit power level setting scheme to mitigate the interference with macrocell MSs. The proposed scheme estimates the path loss between the femtocell BS and the neighboring macrocell MSs on the basis of the received power levels from the neighboring macrocell MSs, thereby it mitigates the interference with macrocell MSs while maintaining good indoor coverage for femtocell MSs. According to the system level simulations of downlink LTE, femtocell user throughput of the proposed scheme at the edge of the macrocells is 36 % to 73 % greater than that of the conventional transmit power level setting scheme.

Enhancement of site selection diversity transmit power control in CDMA cellular systems

In CDMA cellular systems, site selection diversity transmit (SSDT) power control is employed during soft handover to reduce interference in the downlink. In SSDT, mobile stations select a cell from active cells during soft handover based on the received power of common pilot signals. The selected cells transmit data signals, and the other cells suspend transmission of data signals. We introduce multiple-SSDT as a natural extension of SSDT, and three approaches to enhance the performance of SSDT are presented. With the enhancements, it is possible to eliminate residual frame errors, and to optimize the measurement period of common pilot signals. Also it is possible to reduce interference with efficient transmission of control signals.

Forward-link power control utilizing neighboring-cell pilot power for DS-CDMA cellular systems

This paper proposes for DS-CDMA cellular systems a forward-link power control scheme utilizing neighboring-cell pilot power in order to improve the quality of forward traffic channels and to increase traffic capacity. In this scheme, each mobile station measures pilot powers received from neighboring cells. Transmission power of the forward traffic channel is then controlled by base stations according to the pilot powers measured at the respective mobile station, and uniform service quality is provided to all channels. Simulation results show that the proposed scheme reduces outage and forced termination probabilities with a pilot power error of 1.5 dB, and increases capacity for the forced termination probability of 1% by 5% and 60% in the case of soft handover and hard handover.

Dynamic Neighbor Cell List Management for Handover Optimization in LTE

Self-optimization of the neighbor cell list (NCL) is expected to improve handover performance and reduce the need for site surveys. 3GPP Long Term Evolution (LTE) has introduced automatic neighbor relation (ANR), which enables a base station to manage neighbor cells on the basis of measurements made by mobiles. Because the radio coverage changes during network operations, it is essential to immediately update the NCL to improve handover performance, especially when the number of measured neighbor cells exceeds the upper limit of NCL size. In this paper, we propose a dynamic NCL management scheme to enhance NCL convergence and alleviate missing neighbor problems. The proposed scheme gives higher priority to newly detected neighbor cells over existing cells and ensures fast and accurate NCL updates after radio coverage changes. According to the LTE network simulations, the proposed scheme provides 70% faster recovery of the average radio link failure rate due to the missing neighbors compared with the non-prioritized scheme. It was also confirmed that the duration of missing neighbors is reduced by 39% on average.

Uplink power control method for LTE femtocells based on resource usage aggregation

In LTE, uplink (UL) transmit power control is important to mitigate the UL aggregate interference from all femtocells to the macrocell, which is increased as the number of femtocells is increased. We propose an UL power control method for LTE femtocells adaptively setting target received power by using the aggregation of the resource usage of femtocells. With this method, when the aggregate resource usage is small, the UL throughput of femtocells is improved by increasing the UL transmit power. When the aggregate resource usage is large, the UL throughput of macrocells is maintained by decreasing the UL transmit power. With LTE system level simulations, it is shown that while in the full buffer traffic condition the decline in the UL throughput of macrocells due to introducing femtocells was suppressed, in the FTP traffic condition the UL throughput of femtocells was improved by 24% at the largest in comparison with the method using the number of active HeNBs.

Inter-eNB Coordination-Free Algorithm for Mobility Robustness Optimization in LTE HetNet

Mobility Robustness Optimization (MRO), i.e. self-optimization of handover (HO) parameters, is a key driver for expanding Heterogeneous Networks (HetNet) while suppressing the need for drive tests. 3GPP Long Term Evolution (LTE) has introduced a scheme for MRO that enables the eNB, i.e. a base station, to detect HO failures while coordinating with other eNBs via the X2-interface (X2-Intf). However, neighboring eNBs do not always have X2-Intf because of technical constraints. In this paper, we propose an inter-eNB coordination-free algorithm for MRO. The algorithm adjusts the Cell Individual Offset (CIO) on the basis of ping-pong HOs and HO failures that are detected by monitoring HO related signaling via interfaces other than X2-Intf. LTE HetNet simulations confirmed that the algorithm was resistant to the detection error of HO failures and its HO performance was comparable to that of an algorithm using X2-Intf. Moreover, the proposed algorithm reduced the total number of HO failures from pico cells by 33% while keeping the ping-pong HO rate at an acceptable level, even with quasi-optimal default settings. The HO performance from the macro cells always had good quality, irrespective of whether the proposed algorithm was used or not.