Yoshitaka Hara

Spatial Scheduling With Interference Cancellation in Multiuser MIMO Systems

This paper proposes a novel downlink spatial scheduling algorithm in multiuser multiple-input-multiple-output (MIMO) systems, which selects a good combination of terminals and base stations (BSs) transmit beams so that the BSs beams nullify interstream interference at the selected terminals. In the derivation process, we reveal the new property that the optimization problem of downlink spatial scheduling is equivalent to that of uplink scheduling under the BSs zero-forcing beamforming. Using this property, an efficient downlink scheduling algorithm is presented, applying the principle of an uplink scheduling algorithm. Numerical results show that the presented spatial scheduling achieves a much higher system throughput than a multiuser MIMO system without spatial scheduling or with conventional spatial scheduling by linear processing. We also present a realistic control structure to achieve the uplink and downlink spatial scheduling in time-division duplex systems.

Antenna Array Calibration Using Frequency Selection in OFDMA/TDD Systems

This paper proposes a new antenna array calibration using frequency selection in OFDMA/TDD systems. In the proposed method, subbands or frequencies of good channel conditions are initially selected for accurate channel measurement. Next, the relative calibration is performed at the selected subbands, in which mismatch of analog gains in multiple antennas is compensated by use of measured channel parameters on uplink and downlink. Third, the calibration parameters are interpolated in frequency domain for whole bandwidth. The proposed method keeps more accurate channel reciprocity for whole bandwidth than the conventional method which does not use the frequency selection. The proposed method is effective in exploiting channel reciprocity at both base station and terminals keeping feasible feedback amount and low-cost operation.

Weight control scheme for MIMO system with multiple transmit and receive beamforming

This paper presents a weight control scheme for multiple transmit and receive beamforming in multi-input multi-output (MIMO) channels. First, we describe the theoretical conditions of the transmit and receive weights to achieve maximum channel capacity in the presence of interference and noise. Second, we propose a weight control scheme for a MIMO system for efficient transmission. The proposed method uses a round trip control signal before starting communications to determine suitable transmit and receive weights. The numerical results show that the proposed method achieves a high throughput in both Rayleigh and Rician channels.

Pilot-Based CSI Feedback in TDD/MIMO Systems with Cochannel Interference

This paper proposes a pilot-based channel state information (CSI) feedback in time-division duplex (TDD) / multi-input multi-output (MIMO) system, which enables the transmitter to perform transmit beamforming considering the receivers interference. In the proposed method, the receiver sends reverse-link pilot signals spatially prefiltered by linear precoder dependent on the receivers interference characteristics. The transmitter exploits responses of the reverse-link pilot signals to perform transmit beamforming considering the receivers interference. The transmitter also predicts the receivers output signal-to-interference-plus-noise power ratio (SINR) for each data stream. From simulation results, it is found that the transmitter achieves efficient transmission performance maintaining small control errors based on the proposed CSI feedback.

OFDMA/TDD/MIMO System with Spatial Scheduling

In this paper, we propose OFDMA/TDD/MIMO system with spatial scheduling, showing frame structure and control procedures to achieve the proposed system. The frame structure includes all functions required for spatial scheduling in a subband of 200 kHz and in a frame time of 2.5 ms. We show that the frame structure can be a realistic solution to achieve spatial scheduling on subband basis. From the presented frame structure, about 60% of total radio resources are expected to be available for data symbol transmission. Furthermore, we estimate that data rate up to 570 Mbps can be supported on MAC layer for both uplink and downlink, under total bandwidth of 100 MHz.

Pilot-Based Channel Quality Reporting for OFDMA/TDD Systems with Cochannel Interference

This paper proposes a new uplink signalling to report the terminals channel quality in multiple subbands to the base station (BS) using channel reciprocity in orthogonal frequency division multiple access / time division duplex (OFDMA/TDD) systems. In the proposed method, the terminal transmits uplink pilot signals in the subbands with different transmit power which is inversely proportional to the subband- based interference-plus-noise power. The BS can obtain knowledge of the terminals received signal-to-interference-plus-noise power ratio (SINR) on subband basis, measuring the pilot signal power. In performance evaluation, accuracy of the channel quality reporting and amount of the control signalling are examined. From numerical results, it is found that the proposed method becomes effective as the number of subbands and terminals for channel quality reporting increases.

Downlink Spatial Scheduling with Mutual Interference Cancellation in Multiuser MIMO Systems

This paper presents downlink spatial scheduling which nullifies interference among multiplexed signals perfectly in multiuser MIMO systems. Under the base stations zero-forcing transmit beamforming, each terminal can receive packet of interest without interference from the other multiplexed packets. The scheduling algorithm successively selects appropriate terminals for packet transmission in the presence of already selected packets. We show that the downlink spatial scheduler is equivalent to the virtual uplink spatial scheduler in terms of received signal characteristics. Applying the uplink scheduling concept to downlink, the downlink spatial scheduler achieves much higher system throughput than the system without spatial scheduling. Also, it is shown that the spatial scheduler has similar system throughput in uplink and downlink

Spatial Scheduling Using Partial CSI Reporting in Multiuser MIMO Systems

This paper proposes a new partial channel state information (CSI) reporting for spatial scheduling in TDD/MIMO systems. In the proposed method, a terminal transmits pilot signals using transmit beams which have large channel gains between the base station (BS) and the terminal. Then, the BS can obtain partial CSI through responses of the pilot signals. Furthermore, adaptive allocation of pilot signals is proposed, in which pilot signals for CSI reporting are adaptively allocated to terminals depending on the number of terminals. We evaluate system throughput of spatial scheduling under the partial CSI reporting from multiple terminals. Numerical results show that the proposed method reduces uplink radio resources for CSI reporting effectively, keeping high system throughput of spatial scheduling.

Frequency-averaged MMSE channel estimator for multicarrier transmission schemes

A new frequency-averaged MMSE estimator is proposed for multicarrier transmission schemes. The proposed estimator employs a new criterion of minimizing the mean square errors (MSE) over subcarriers, which provides an accurate channel estimation using a few pilot OFDM symbols without a priori channel knowledge. The proposed estimator has low complexity, since it uses high correlated adjacent subcarriers and neglects low correlated distant subcarriers. In the estimation process, intersymbol and intersubcarrier interference due to excess delay paths beyond the guard interval are considered to achieve good performance. The simulation results showed that the frequency-averaged MMSE estimator can effectively reduce estimation errors and keep better performance than the conventional robust estimator in any channel conditions.

Radio resource management and power control for W-CDMA uplink with high data rate packet transmission

In W-CDMA uplink, immediate accommodation of high data rate packet causes power control error and makes active users signal quality deteriorate in the beginning of a frame. To avoid the deterioration, we propose a new radio resource management (RRM), which accommodates high data rate traffic gradually in several frames. The proposed RRM reduces the signal quality deterioration in the beginning of a frame. We also propose an effective power control scheme, where a power increase command is sent to all users before a new high data rate packet is transmitted. Simulation results show that joint utilization of the proposed two methods is effective to keep signal quality good for all users.