Tatsuhiko Iwakuni

Inter-user interference suppression in time varying channel with null-space expansion for multiuser massive MIMO

This paper proposes a novel Inter-User Interference (IUI) suppression scheme in time varying channels with null-space expansion for multiuser massive MIMO systems. The channel time variation deteriorates the accuracy of the channel state information at the transmitter (CSIT) and causes IUI on multiuser MIMO downlink transmission. In massive MIMO, the degrees of freedom (DoFs) of the base station antenna array are generally used to obtain the beamforming gain and capacity enhancement. The proposed null-space expansion scheme exploits the excess DoFs to perform additional null-steering, which extend the null-space dimension and thus suppress the IUI caused by the time varying channel. Computer simulations show that the proposed scheme has superior IUI suppression performance to existing channel prediction scheme; our scheme can improve the SIR by over 50dB.

First Eigenmode Transmission by High Efficient CSI Estimation for Multiuser Massive MIMO Using Millimeter Wave Bands.

Drastic improvements in transmission rate and system capacity are required towards 5th generation mobile communications (5G). One promising approach, utilizing the millimeter wave band for its rich spectrum resources, suffers area coverage shortfalls due to its large propagation loss. Fortunately, massive multiple-input multiple-output (MIMO) can offset this shortfall as well as offer high order spatial multiplexing gain. Multiuser MIMO is also effective in further enhancing system capacity by multiplexing spatially de-correlated users. However, the transmission performance of multiuser MIMO is strongly degraded by channel time variation, which causes inter-user interference since null steering must be performed at the transmitter. This paper first addresses the effectiveness of multiuser massive MIMO transmission that exploits the first eigenmode for each user. In Line-of-Sight (LoS) dominant channel environments, the first eigenmode is chiefly formed by the LoS component, which is highly correlated with user movement. Therefore, the first eigenmode provided by a large antenna array can improve the robustness against the channel time variation. In addition, we propose a simplified beamforming scheme based on high efficient channel state information (CSI) estimation that extracts the LoS component. We also show that this approximate beamforming can achieve throughput performance comparable to that of the rigorous first eigenmode transmission. Our proposed multiuser massive MIMO scheme can open the door for practical millimeter wave communication with enhanced system capacity.

Correlation-based user scheduling and multi-planar parallelogram array for Massive antenna systems

This paper proposes a simplified user scheduling method suitable for our proposed antenna arrangement and an antenna configuration with multi-planar antennas, for Massive MIMO. Massive MIMO has two key problems: the heavy overhead of feeding back the channel state information (CSI) for the very large number of transmission/reception antenna element pairs and the huge computation complexity imposed by the very large scale matrices. We previously proposed a parallelogram planar array (PPA) arrangement for Massive MIMO to realize channel spatial correlation reduction in LOS environments. Assuming that the proposed array is to cover omnidirectional areas, this paper proposes a user scheduling method that can easily enhance outage capacity performance in a simple manner and an antenna configuration that offers more efficient user accommodation. Simulations show that the proposed method can quadruple the system capacity over the uniform circular array antenna system.

Experimental Verification of Null-Space Expansion for Multiuser Massive MIMO Using Measured Channel State Information

This paper presents experimental results of our proposed null-space expansion scheme for multiuser massive MIMO in time varying channel.　Multiuser MIMO transmission with proposed scheme can suppress inter-user interference (IUI) caused by outdated channel state information (CSI).　The excess degrees of freedom (DoFs) of massive MIMO is exploited to perform additional null-steering using past estimated CSI.　The signal-to-interference ratio (SIR) performance achieved by a null-space expansion scheme that uses measured CSI is experimentally evaluated through CSI measurements.　It is confirmed that the proposed scheme shows SIR performance superior to the conventional channel prediction scheme.　In addition, IUI can be stably suppressed even in high mobility environments by further increasing the null-space dimension.