Yasuaki Yuda

Interference Mitigation Using Coordinated Backhaul Timing Allocation for LTE-Advanced Relay Systems

This paper proposes an inter-cell co-channel interference (CCI) mitigation method with the mixed deployment of evolved NodeBs (eNBs) and relay nodes (RNs) for Long Term Evolution (LTE)-Advanced systems studied in the 3rd Generation Partnership Project (3GPP). Supporting RNs for LTE-Advanced can increase coverage in areas without service, and also improve system capacity. Although RNs can improve system capacity by reusing identical frequency bands in macro and relay cells, this also causes additional CCI. To deal with this issue, we investigated three methods of mitigating CCI by differentiating the backhaul resource allocation timing among eNBs and RNs. Then, we evaluated these methods with a system level simulator. This confirmed improvement of both the received signal-to-interference ratio plus noise power patio (SINR) at user equipment (UE) and system capacity.

Impact of Channel Estimation Error on LTE-Advanced Uplink Using CoMP Joint Reception in a Heterogeneous Network

In this paper, we investigate the impact of channel estimation error on LTE-Advanced (LTE-A) uplink using coordinated multipoint (CoMP) joint reception (JR) in a heterogeneous network (HetNet). The uplink CoMP capacity in bits/sec/Hz/m2 in principle increases in proportional to the number/density of CoMP reception points (RPs). However, the data capacity is limited due to reference signal (RS) capacity for channel estimation in practical situations. In this paper, we model channel estimation error of LTE-A uplink assuming the use of de-modulation RS (DMRS) introduced in LTE-A with taking into account the property of DMRS. System-level simulation is carried out to evaluate the impact of channel estimation error on CoMP operation.

An interference coordination method based on geometric mean of user throughput for heterogeneous networks

This paper proposes an inter-cell interference coordination (ICIC)-parameter-adjustment method for wireless heterogeneous cellular networks (HetNet), in which low-power nodes (such as pico base stations (BSs)) are overlaid with conventional high-power macro BS deployment. The fractional radio resource reuse (resource partitioning) scheme adopted for HetNet promises to mitigate interference from high-power BSs to low-power nodes. However, the ratio of protected resources that maximizes the system throughput may depend on the distribution of users or BSs. Thus, this ratio cannot be determined in advance. In this study, we consider both cell association and optimization of the resource-partitioning parameter in HetNet. The proposed resource-partitioning parameter-adjustment method alleviates the above mentioned problem, and improves network performance in terms of the geometric mean of user throughputs. The strong performance of the proposed method is verified by system level simulations of various cell distributions. The proposed method does not require manual adjustment of the resource-partitioning parameter.

Study of Leakage-Based Precoding Scheme That Supports Coordinated Multi-Point Operation for LTE

This paper investigates leakage-based precoding schemes that support coordinated multi-point (CoMP) operation for Long-Term Evolution (LTE) systems studied in the 3rd Generation Partnership Project. CoMP operation, which improves cooperative transmission among cells and reduces or eliminates intercell interference, enables performance improvement in cell edge and average throughputs. Although leakage-based precoding has been investigated for multiuser multiple-input multiple-output (MIMO) systems, its extension to CoMP requires further analysis of the near-far effect under multicell deployment scenarios. To this end, we propose to incorporate the channel between a victim user equipment (UE) and the cell that serves the victim UE into the leakage-based precoding calculation. We investigated optimal linear scaling of the covariance matrix involving this channel. To determine the benefits of the investigated algorithm, multicell system level evaluations were carried out. This confirmed the improved performance of the systems average and cell edge throughputs.

A Study on Link Adaptation Scheme with Multiple Code Words for Spectral Efficiency Improvement on OFDM-MIMO Systems

In this paper, we propose the stream multiplexing scheme to achieve the high spectral efficiency on OFDM-MIMO system considering (a) the effects of the inter-stream interference suppression techniques, (b) the influence of the quality measurement in the receiver to select the modulation and coding scheme (MCS) and (c) the influence of the signaling overheads to the spectral efficiency. Two kinds of schemes for stream multiplexing, namely Multiple CodeWord (MCW) and Single CodeWord (SCW) are investigated and compared. In the simulations, we assume real channel estimation for SINR measurement for MCS selection and we compare the schemes with the spectral efficiency taking into account overheads. In consequence, it is presented that MCW can achieve the higher efficiency than SCW in the middle to high SINR region. Moreover, in 4 × 4 antenna configuration, a 2CW-MCW scheme where 2 codewords mapped onto 4 streams according to SINR of each streams is proposed. The scheme gives higher throughput by improving the MCS selection in the low SINR region due to increase of data length per a codeword and by reducing the signaling overhead due to the decrease of the number of codewords. As a result, the proposed scheme achieves spectral efficiency improvement in whole SINR region.

Distributed transmission power control method based on soft FFR for cellular downlink

We propose a distributed transmission power control method based on soft fractional frequency reuse (SFR) for cellular downlink orthogonal frequency division multiple access (OFDMA). The proposed method adaptively controls the transmission power density in the protected and non-protected bands of SFR depending on the user distribution within a cell in order to maximize the geometric mean of the user throughput (in other words, log sum of user throughput). Thus, it is based on the proportional fair (PF) criteria. The proposed method assumes a decentralized approach, in which complicated inter-base station (BS) cooperation is not required. Since the optimal power allocation to the protected and non-protected bands is dependent on the resource allocation of the protected and non-protected bands to the respective users, the proposed method jointly optimizes the power allocation and resource allocation by using an iterative algorithm. Computer simulation results show the effectiveness of the proposed method compared to conventional universal frequency reuse (UFR) and non-adaptive SFR.

Decentralized user association for (p, α)-proportional fair-based system throughput maximization in cellular networks

This paper proposes a new user association method to maximize the downlink system throughput in a cellular network, where the system throughput is defined based on (p, α)-proportional fairness. The proposed method assumes a fully decentralized approach, which is practical in a real system as complicated inter-base station (BS) cooperation is not required. In the proposed method, each BS periodically and individually broadcasts supplemental information regarding the bandwidth allocation to newly connected users. Assisted by this information, each user calculates the expected throughput that will be obtained by connecting the respective BSs. Each user terminal feeds back the metric for user association to the temporally best BS, which represents a relative increase in throughput through re-association to that BS. Based on the reported metrics from multiple users, each BS individually updates the user association. The proposed method is especially effective in heterogeneous cellular networks where low transmission-power pico BSs are overlaid onto a high transmission-power macro BS for traffic offloading. Computer simulation results show that the proposed method maximizes the system throughput from the viewpoint of the given (p, α)-proportional fairness.

A low-complexity beamforming algorithm for 3G macro-cellular system to reduce interference from high data rate users

A beamforming algorithm for a macro cell base station (BS) is proposed. The method has the following three features. Firstly, each direction of arrival (DOA) of waves from mobile terminals to the BS is calculated by the modified Fourier beamformer using a real number correlation vector (RV-Fourier), which greatly reduces complexity. Secondly each DOA is estimated accurately even if there are high power interference waves, applying the technique to subtract the other users correlation vector. Lastly, nulls in interference directions are formed using the modified directionally constrained minimization of power (DCMP) method, which does not need the derivation of the inverse correlation matrix. In comparison with the conventional recursive least-squares (RLS) method, the proposed method requires a much smaller computational load (about 10-20%). Simulation results show that the tracking performance of the proposed method excels RLS when the terminals are moving fast.