Tomoki Murakami

Spatial Domain Resource Sharing for Overlapping Cells in Indoor Environment

As microcell wireless systems become more widespread, intercell interference among the access points will increase due to the limited frequency resource. In the overlapping cell scenario, radio resources should be shared by multiple cells. Although time and frequency resource sharing has been described in many papers, there is no detailed report on dynamic spatial resource sharing among multiple cells for microcell wireless systems. Thus, we present the effectiveness of spatial resource sharing among two access points. We introduce two scenarios based on the zero forcing method; one is the primary-secondary AP scenario and the other is the cooperative AP scenario. To evaluate the transmission performance of spatial resource sharing, channel matrices are measured in an indoor environment. The simulation results using the measured channel matrices show the potential of spatial resource sharing.

Performance evaluation of distributed multi-cell beamforming for MU-MIMO systems

Inter-cell interference (ICI) from nearby cells can substantially degrade the sum-rate in WLAN systems. To improve the sum-rate in actual WLAN systems, some papers have investigated distributed multi-cell beamforming to mitigate ICI by zero forcing beamforming. All most of these approaches consider single-user multiple input multiple output (SU-MIMO) systems. In this paper, we present three techniques for distributed multi-cell beamforming for multi-user MIMO (MU-MIMO) systems with the same total number of transmit and receive antennas. The first technique is distributed multi-cell beamforming using receiver selection. The second one is antenna selection at the receiver. The last one is an iterative technique in which transmitter weight is updated at the transmitter to correspond to receiver weight. The sum-rate of these techniques with imperfect channel state information at transmitter (CSIT) is evaluated. We clarify the effectiveness of these techniques for MU-MIMO systems.

Effectiveness of Relay MIMO Transmission by Measured Outdoor Channel State Information

This paper investigates the effectiveness of the multiple input multiple output (MIMO) transmission with the aid of relay nodes by using measured outdoor channel state information (CSI) for the source, relay, and destination which are used in cooperative transmission. Single/multi-user MIMO (SU/MU-MIMO) systems have attracted much attention as the technology enhances the channel capacity with a limited frequency band. In this paper, cooperative transmission using the relay station is applied to SU/MU-MIMO systems: Relay MIMO transmission. Although the performance of relay MIMO transmission is affected by the actual propagation environment, the performance has not yet been investigated using measured propagation channel data in outdoor scenarios. In this paper, we demonstrate that the channel capacity with the relay MIMO transmission can be improved compared to the conventional MIMO transmission without the relay nodes, especially in multi-user scenarios. Moreover, orthogonal polarization is introduced and its effectiveness is shown in further improving the channel capacity of relay MIMO transmission.

Weighted-combining calibration for implicit feedback beamforming on downlink multiuser MIMO systems

This paper proposes a novel calibration scheme for implicit feedback beamforming on downlink (DL) multiuser (MU) multiple-input multiple-output (MIMO) systems. In the proposed scheme, an access point (AP) calculates calibration coefficients from ratios of DL and uplink (UL) channel state information (CSI) corresponding to multiple stations (STAs), and then combines multiple coefficients with minimum mean square error (MMSE) weights. Although it is difficult to calculate MMSE weights since Gaussian noise terms due to estimated CSI error exist in the denominator of the mean square error, we derive the weights using a linear approximation in the highsignal to noise power ratio (SNR) regime. Simulation results reveal that the proposed scheme obtains signal to interference plus noise power ratio (SINR) gain of more than 6.9 dB over a calibration scheme without combining, at 10% cumulative distribution function (CDF) and SNR of 40 dB. It is found that the proposed scheme improves the calibration accuracy considerably.

Multi site MIMO channel analysis at 4.85GHz in outdoor environment

Multiple-Input Multiple-Output (MIMO) system has been actively investigated to enhance wireless data transmission in outdoor environments. Micro/Macro-cell services such as WLAN/WiMAX have been much attention because they achieve a high throughput. Since proliferation of these services are expected, base stations (BSs) need to be located at various heights and locations in outdoor environments. Although many publications have reported measurements of the channel state information (CSI) in outdoor environments, the relationship between the BS heights or locations and the transmission performance in MIMO systems has not yet been fully investigated. Moreover, when the number of BSs increases per area, inter-cell interference problem occurs. Such an interference deteriorates the communication quality. In this paper, the experiment is conducted in urban area, in Japan, and the CSI was obtained using 4×4 MIMO-OFDM signals. We present signal to noise ratio (SNR), channel capacity and eigenvalues in MIMO channels, when the BS heights and locations are changed. We clarify that eigenvalues are one of important parameters for not only the channel capacity in MIMO channels but also inter-cell interference by the analysis using the measured MIMO channels.

Successive Optimization Transmission for High and Low SNR Stations in Wireless LAN Systems

Multiuser (MU) - multiple input multiple output (MIMO) techniques are attractive for increasing the downlink spectrum efficiency while requiring few antennas at each user station (STA). As practical precoding methods for multi-user MIMO systems, block diagonalization (BD) and successive optimization (SO) algorithms have been investigated using the metric of channel capacity. However, the throughput of SO algorithms has not been evaluated for wireless LAN systems. This paper focuses on the two-STA scenario where a close STA lies in the very near field of the access point (AP) while the distant STA lies farther from the AP; it proposes MU-MIMO transmission based on an SO algorithm that uses only the channel state information (CSI) of the close STA. The proposed method determines the transmission weight for the distant STA so as to nullify the close STA and reduces the inter-user interference at the distant STA by decreasing the transmission power for the close STA. Thus, the distant STA does not have to implement the CSI feedback function. Simulations compare zero forcing (ZF), BD, and proposed SO algorithms and confirm the conditions wherein the proposed method outperforms single user MIMO and multiuser MIMO based on ZF and BD algorithms.

User Selection for Multiuser MIMO Systems Based on Block Diagonalization in Wide-Range SNR Environment

Multiuser (MU) - multiple input multiple output (MIMO) techniques are attractive to increase the downlink spectrum efficiency since they allow a small number of antennas at each user station (STA). In MU-MIMO systems, the transmission performance is affected by the STA combination that is spatially multiplexed in the same frequency and same time slot. Thus, the AP needs to determine the optimum STA subset and the number of spatially multiplexed STAs using the channel state information. In this paper, we propose a user selection scheme that uses an iterative algorithm to determine the number and the optimal combination of multiplexed STAs. The proposed scheme chooses STAs one by one so as to improve the throughput of all STAs in the subset compared to that in single-user MIMO transmission. Thus, all STAs have a fair chance at being selected even when their SNRs vary widely. Furthermore, the iterative algorithm has lower calculation complexity than the exhaustive search approach. The index for employed for user selection is also simplified by using the signal-space vectors of the channel matrices. Computer simulations confirm that the effectiveness of the proposed user selection scheme in a wide-range of SNR values. The results show that the user selection proposal almost matches the performance of exhaustive search.

Performance of adaptive movable access point system in the presence of positioning error

This paper proposes an adaptive movable access point (AMAP) system to enhance the system capacity in dense wireless-device environments. In the proposed approach, the positions of access points (APs) are adaptively moved in accordance with clustered user distribution, which is sure to be effective in ununiform user distribution due to dense device environments. In order to derive the appropriate positions of APs, we utilize k-means method and estimate the users position which is essential for k-means method by means of receive signal strength indicator (RSSI) or time of arrival (ToA). The effectiveness of the proposed systems is evaluated in the presence of the positioning error obtained from the experimental results concerning RSSI and ToA and is demonstrated in comparison with the fixed position of AP with equal intervals.

Experimental Results of Network-Assisted Interference Suppression Scheme Using Adaptive Beam-Tilt Switching

This paper introduces a network-assisted interference suppression scheme using beam-tilt switching per frame for wireless local area network systems and its effectiveness in an actual indoor environment. In the proposed scheme, two access points simultaneously transmit to their own desired station by adjusting angle of beam-tilt including transmit power assisted from network server for the improvement of system throughput. In the conventional researches, it is widely known that beam-tilt is effective for ICI suppression in the outdoor scenario. However, the indoor effectiveness of beam-tilt for ICI suppression has not yet been indicated from the experimental evaluation. Thus, this paper indicates the effectiveness of the proposed scheme by analyzing multiple-input multiple-output channel matrices from experimental measurements in an office environment. The experimental results clearly show that the proposed scheme offers higher system throughput than the conventional scheme using just transmit power control.

Antenna decision method for downlink multiuser MIMO systems with receive antenna allocation

This paper proposes an antenna decision method for downlink multiuser MIMO (MU-MIMO) with receive antenna allocation per subcarrier. When receive antenna allocation is used, each receiver requires the information of the antenna allocation before receiving the data; for this an overhead signal is used. The proposed method decreases the overhead by deciding an antenna with comparison of received power at each antenna of the receiver. Computer simulations show that the proposed method offers highly accurate antenna decision and high channel capacity with one OFDM symbol.