Satoshi Konishi

A handover optimization algorithm with mobility robustness for LTE systems

A large number of cells will be deployed to provide high speed services in any places using the Long-Term Evolution (LTE) system. The management of such a large number of cells increases the operating expenditure (OPEX). Self-organizing network (SON) attracts attention as an effective way to reduce OPEX. This paper proposes a self-optimization algorithm for handover (HO) parameters. In conventional studies, the HO optimization techniques are discussed in models with stationary mobility of user equipment (UE). On the other hand, the key feature of the proposed algorithm is the mobility robustness, which means that the HO performance is robust against the change in UE mobility. In order to realize the mobility robustness, the proposed algorithm adaptively adjusts the HO parameters considering cause of HO failures, which changes in UE mobility. We examine the performance of the proposed algorithm through the computer simulations and confirm the mobility robustness. The simulation study demonstrates the following; The HO parameters are initially set to the optimum values for UEs with 3 km/h based on the random walk. Then, when the mobility changes from 3 km/h random walk to 300 km/h linear motion, the HO failure rates increases to 19%. The proposed algorithm reduces such increasing HO failure rates less than 0.2%.

Performance evaluation of handover in LTE-Advanced systems with pico cell Range Expansion

Cell Range Expansion (CRE) combined with Inter-Cell Interference Coordination (ICIC) in Heterogeneous Networks (HetNet) is a promising technique to enhance the utilization of the resource in pico cells. CRE increases the number of the UE sets connected to pico cells (pico UEs) by changing the trigger timing of the handover (HO). However, inappropriate trigger timing of HO increases HO failures and inefficient HOs, thereby degrading HO performance. HO performance directly affects the continuity of the communication service. Hence, not only the utilization of the resource in pico cells but HO performance should be taken into consideration when CRE is adopted. In this paper, we evaluate the effect of CRE on HO performance through system level simulations, and clarify the application range of CRE. As an example, we examine the CRE bias values which keep the HO failure rate to under 1% and the Ping-Pong HO rate to under about 1%. It is shown that the applicable CRE bias values range from 0 dB to 6 dB, which is smaller than the applicable CRE bias values reported by the conventional studies which range from 0 dB to 9 dB. We also show that the CRE bias values over 6 dB result in high HO failure rates over 5%. Therefore, CRE should be adopted with consideration of HO performance in order to keep good HO performance.

Impact of small cell deployments on mobility performance in LTE-Advanced systems

The use of small cell deployments in heterogeneous networks (HetNet) environments is considered a promising strategy to cope with the explosion in mobile traffic. In the deployments, there is a concern that the large number of small cells causes the signalling load on the network nodes to increase due to frequent handovers (HOs) and mobility robustness to be degraded due to increased handover failures (HOFs) and radio link failures (RLFs). To examine these impacts caused by the introduction of small cells, in this paper, we evaluate mobility performance on small cell deployments in LTE-Advanced systems. The simulation results show that the number of HOs occurs at most 3.5 times compared to a macro cell-only deployment even in the case where there are 20 small cells per macro cell and assuming a worst case scenario in which the small cells are located separately and their coverage does not overlap with each other. The results also show that the number of HOFs increases slightly compared to the macro-only case, but these HOFs can be eliminated by applying a handover optimization technique. Contrary to the fact that it is said that we have to deal with frequent HOs and HOFs in small cell deployments, it is concluded that mobility issues have little impact on small cell deployments in HetNet environments.

Development and experiment of linear and non-linear precoding on a real-time multiuser-mimo testbed with limited CSI feedback

This paper presents the development and experimentation of linear precoding (LP) and non-linear precoding (NLP) on a real-time multiuser-multiple input multiple output (MU-MIMO) testbed subject to limited channel state information (CSI) feedback. LP techniques such as zero-forcing and minimum mean square error precoding have already been experimented and the related results are available in the literature. However, they are not capacity-achieving. NLP techniques such as vector perturbation and Tomlinson-Harashima precoding (THP), on the other hand, achieve in theory the capacity of MU-MIMO, but such claims have not been verified in practice, to date. One reason is that theoretical NLP works rely on the assumption of full and perfect CSI at the transmitter as NLP techniques are sensitive to CSI error. The merit of this work is to experimentally confirm (through hardware implementation and pre-field test experimentation) the feasibility of NLP subject to limited CSI feedback using the fading simulator. Experimental results reveal that the testbed achieves less than 2 dB and 3 dB degradation of the block error rate compared to the floating point computer simulations in LP and NLP, respectively.

A proposal of cell selection algorithm for LTE handover optimization

A large number of cells will be deployed to provide high speed services in any places using the Long-Term Evolution (LTE) system. The management of such a large number of cells increases the operating expenditure (OPEX). Self-organizing networks (SON) attracted interest as an effective way to reduce OPEX. One of the main targets in SON is the self-optimization of handover (HO) that realizes mobility robustness. HO optimization algorithms adjust HO parameters between the serving and the reconnected cells based on the HO failure logs and cell selection, which is the procedure used to select a suitable reconnected cell, is very important for HO optimization algorithms. In this paper, we propose a cell selection scheme to enhance the performance of HO optimization. In the proposed scheme, both the uplink and downlink channel quality is considered when selecting a suitable reconnected cell. Through the computer simulation, we can see that the proposed scheme reduces the HO failure rate and the number of HO failures by 3 percentage points and 38%, respectively, compared to the conventional scheme based solely on downlink channel quality.

MIMO-OFDM Throughput Performances on MIMO Antenna Configurations Using LTE-Based Testbed with 100 MHz Bandwidth

This paper presents throughput performance of 22 multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) using a testbed in line with the Long Term Evolution (LTE) numerology. Then, this paper clarifies the relation among the throughput, modulation schemes and antenna polarization. The field experiments clearly demonstrate that a measured throughput exceeding 200 Mbps can be obtained in a coverage area more than 50% and a throughput of 456.0 Mbps is achieved for a limited number of users even when a maximum Doppler frequency is higher than 125 Hz. In addition, it is found that the single polarization antenna configuration is suitable for 16QAM while the dual polarization antenna configuration is suitable for 64QAM. It is concluded from analysis of the eigenvalue ratio and signal to noise power ratio that the aforementioned trade-off is caused by sensitivity to the eigenvalue ratio depending on the modulation scheme and distribution of the eigenvalue ratio depending on the MIMO antenna configuration.

Shared Remote Radio Head architecture to realize semi-dynamic clustering in CoMP cellular networks

In conventional cellular systems, transmission rate degrades at cell-edge because of inter-cell interference and pathloss. This problem is called “cell-edge problem”. To solve this problem, Coordinated Multi-Point (CoMP) technique has been proposed recently. CoMP can convert inter-cell interference signals from neighbor base stations (BSs) to desired signals. However, CoMP requires accurate synchronization among cooperative BSs as well as Channel State Information (CSI) between target user and all cooperative BSs. For practical realization of CoMP, new BS architecture in which a BS unit is connected to multiple Remote Radio Heads (RRHs) located apart through optical fiber has been proposed. Even using this architecture, however, CoMP can only be realized within the predefined connected RRHs (CoMP cluster). Therefore, cluster-edge users cannot experience throughput improvement by means of CoMP. This paper proposes a novel BS architecture called shared RRH network, in which each RRH is additionally connected to multiple BS units. As flexible clustering is made possible by this architecture, semi-dynamic clustering using geometrically overlapped cluster patterns allocated with orthogonal resources can be achieved which alleviates the cluster-edge problem. Simulation with parameters based on the 3GPP is setup for performance evaluation of the proposed system. Numerical results show that semi-dynamic CoMP using shared RRHs can significantly improve the system performance both at cell-inner and cell-edge compared with the conventional RRH systems, which confirms the effectiveness of the proposed network.

Cell association method for multiband heterogeneous networks

In traditional heterogeneous cellular networks (Het-Net), base stations (BS) basically associate users based on received signal power. However, in the case of multiband HetNet, in which macro BSs and smallcell BSs use different frequency bands, this conventional cell association method is not effective because there is no interference between macro BSs and smallcell BSs. Additionally, there are huge differences in coverage and available bandwidth. These differences cause inefficient association which causes the imbalance between achievable rate and traffic demand. In order to overcome this problem, we propose a novel cell association method based on the combinatorial optimization. The proposed method considers achievable rates, traffic demands and the number of users belonging to each BSs simultaneously. Numerical simulation results show that the proposed association method achieves system rate gain twice as high as the conventional one.

Optimization of picocell locations and its parameters in heterogeneous networks with hotspots

This work analyzes the optimal pico base station (BS) deployment in heterogeneous cellular networks (HetNet) with hotspots. Most of conventional works for HetNet focused on interference coordination and effective cell association methods, however, the problem of pico BS deployment (cell planning) for HetNet with hotspots has not been analyzed so much. In this paper, we extend the previously proposed optimization problem in terms of network parameters (spectrum splitting ratio and SINR bias value) to the optimal pico BS locations to maximize the system rate. Furthermore, the average user and outage user rates are evaluated numerically to show the effectiveness of the proposed optimization method. Numerical results show that the optimized pico BS locations can improve both the average and outage user rates in HetNet with hotspots.

A study on the autonomous network synchronization scheme for mesh wireless network

In a mesh wireless network system that consists of plural wireless nodes, it is important that all wireless nodes share the same time. For example, in a TDD wireless system, the frequency resources utilization over the network can be improved by acquiring TDD frame synchronization. This paper proposes a novel scheme that autonomously realizes the synchronization in a whole wireless network without centralized control. The proposed scheme just needs information from adjacent wireless nodes and other mechanisms are not necessary. In addition, this scheme maintains synchronization in the link connected-area, if the network topology changes or node failure occurs in a local area. The proposed scheme responds flexibly to changing the network topology. The scheme is applicable in the case where many nodes join in and out frequently in the network, such as in nomadic wireless access. This paper discusses the procedures of the proposed synchronization scheme and presents examples of establishment network-wide synchronization and recovery from link failure.