Huu Phu Bui

Channel Extrapolation Techniques for E-SDM System in Time-Varying Fading Environments

Multiple-input multiple-output (MIMO) systems using eigenbeam space division multiplexing (E-SDM) perform well and have increased capacities compared with those using conventional space division multiplexing (SDM). However, channel state information (CSI) is required at a transmitter, and the performance of E-SDM systems depends much on the accuracy of the CSI at a transmitter and a receiver. In time-varying fading environments, the channel change between the transmit weight determination time and the actual data transmission time causes the system performance to degrade. To compensate for the channel error, a linear extrapolation method has been proposed for a time division duplexing system. Unfortunately, the system performance still deteriorates as the maximum Doppler frequency increases. Here, two new techniques of channel extrapolation are proposed. One is second order extrapolation, and the other is exponential extrapolation. Also, we propose maximum Doppler frequency estimation methods for exponential extrapolation. Simulation results for 4tx 4rx MIMO systems showed that using the proposed techniques, E-SDM system performs better in a higher Doppler frequency region.

Channel characteristics and performance of MIMO E-SDM systems in an indoor time-varying fading environment

Multiple-input multiple-output (MIMO) systems employ advanced signal processing techniques. However, the performance is affected by propagation environments and antenna characteristics. The main contributions of the paper are to investigate Doppler spectrum based on measured data in a typical meeting room and to evaluate the performance of MIMO systems based on an eigenbeam-space division multiplexing (E-SDM) technique in an indoor time-varying fading environment, which has various distributions of scatterers, line-of-sight wave existence, and mutual coupling effect among antennas. We confirm that due to the mutual coupling among antennas, patterns of antenna elements are changed and different from an omnidirectional one of a single antenna. Results based on the measured channel data in our measurement campaigns show that received power, channel autocorrelation, and Doppler spectrum are dependent not only on the direction of terminal motion but also on the antenna configuration. Even in the obstructed-line-of-sight environment, observed Doppler spectrum is quite different from the theoretical U-shaped Jakes one. In addition, it has been also shown that a channel change during the time interval between the transmit weight matrix determination and the actual data transmission can degrade the performance of MIMO E-SDM systems.

Considerations on a multi-user MIMO system using channel prediction based on an AR model

We consider a multi-user multiple-input multiple-output (MIMO) system using block diagonalization and eigenbeam-space division multiplexing to suppress inter-user interference and inter-stream interference. In time-varying environments, the performance seriously degrades due to mistracking of beamforming. A channel prediction scheme can eliminate the effect of the time-varying environments. In this paper, using indoor measurement data, we evaluate the bit error rate (BER) performance of a multi-user MIMO system with channel prediction based on an autoregressive model. It is shown that the proposed channel prediction technique can improve the BER performance significantly.

Multiuser MIMO E-SDM Systems: Performance Evaluation and Improvement in Time-Varying Fading Environments

In the paper, performance of multiuser multiple-input multiple-output eigenbeam-space division multiplexing (E-SDM) systems in the downlink transmission is evaluated in both uncorrelated and correlated time-varying fading environments based on computer-generated data. In the ideal case, using the block diagonalization (BD) scheme, inter-user interference can be completely eliminated at each user; and using the E-SDM technique for each user, optimal resource can be allocated, and spatially orthogonal substreams can be obtained. In realistic environments, however, due to the dynamic nature of the channel and the processing delay at both the transmitter and the receiver, the channel change during the delay may cause existence of inter-user interference even if the BD scheme is used. In addition, the change may also result in large inter-substream interference and no longer lead the allocated data resource for each user to the optimal condition. As a result, system performance may be degraded seriously. To overcome the problem, we propose a method of channel extrapolation to compensate for the channel change. Applying our proposed method, simulation results show that much better system performance can be obtained than the conventional case.

Doppler spectrum and performance of E-SDM systems in indoor time-varying MIMO channels

MIMO measurement campaigns were conducted in indoor environments. Based on the measured data, this paper examines the channel auto-correlation and Doppler spectrum in a case where a receiver is moving with respect to a fixed transmitter in the propagation environments with static scatterers. The bit error rate (BER) performance of 4 times 4 MIMO eigenbeam-space division multiplexing(E-SDM) system was evaluated in time-varying fading environments.

Multi-user MIMO system with channel prediction for time-varying environments

The performance of a downlink multi-user MIMO system seriously degrades in time-varying environments because MIMO channels change during the time interval between transmission parameter determination and actual MIMO transmission. Channel prediction techniques have been proposed to improve the performance in dynamic environments. In this paper, based on indoor measurement data, we show the effect of the linear channel extrapolation, and evaluate the bit error rate performance of the multi-user MIMO system with the channel prediction technique.

On the performance of MIMO E-SDM systems with channel prediction in indoor time-varying fading environments

Multiple Input Multiple Output (MIMO) systems using an eigenbeam-space division multiplexing (E-SDM) technique have been recently considered high-rate wireless communications. In the E- SDM technique, channel state information (CSI) is required at a transmitter in order to form orthogonal transmission beams and allocate transmit data resources adaptively. In the ideal case, received signals can be detected without inter-substream interference, therefore very high performance is obtained.

Performance of MIMO-OFDM E-SDM systems using circular arrays

In the paper, performance of multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) eigenbeam-space division multiplexing (E-SDM) systems using uniform circular antenna arrays is evaluated. We conducted a MIMO channel measurement campaign at a 5.2 GHz frequency band in an indoor fading environment. Simulation results based on the measured data have shown that differently from the linear array, the BER performance of the circular array is almost the same for the array orientation. It is also shown that the performance can be degraded due to a channel change during the time delay between the transmit weight matrix determination and the actual data transmission. To compensate for the channel change, we have proposed two methods of channel extrapolation. Applying the proposed methods, robust performance can be obtained.