Myeongcheol Shin

An eigen-based MIMO multiuser scheduler with partial feedback information

An eigen-based MIMO multiuser scheduler which is robust to spatial channel correlation is proposed. The proposed scheduler uses the principal right singular vector for reduced feedback information and exploits the eigen-mode transmission using a precoder matrix, whose columns are the principal right singular vectors of selected users. Since the proposed scheduler uses only each principal eigen channel of the user, it is hot affected by spatial channel correlation even if all spatial channels are equal. Moreover, since the distribution of a singular value is concentrated on the principal singular value under the spatially correlated environments, more channel gain can be obtained. By computer simulations, it is shown that the proposed scheduler has better performance for both capacity and fairness under the moderate spatial channel correlation than the existing schedulers.

A new training symbol structure to enhance the performance of channel estimation for MIMO-OFDM systems

In MIMO-OFDM systems, a conventional channel estimation technique using comb type training symbol structure has high mean squared error (MSE) at edge subcarriers. To reduce the MSE at those subcarriers, we propose the cyclic comb type training structure and the weighted-averaging method. In the proposed technique, all kinds of comb type training symbols are transmitted cyclically at each transmitter. At the receiver, channel frequency responses estimated using each training symbol are averaged with weight which is obtained from the corresponding MSE. By computer simulations, we have shown that the proposed cyclic comb type training symbols gives higher SNR gain than the conventional training structure.

An Efficient Multimode Quantized Precoding Technique for MIMO Wireless Systems

Multimode quantized precoding (QP) can provide full diversity gain or high capacity gain by adapting the number of substreams, as well as the precoding matrix, according to the instantaneous channel condition with low-rate feedback. Conventional multimode QP (MM-QP), however, does not consider the adaptive rate allocation among substreams; thus, it cannot have the additional gain by adaptive modulation. Furthermore, it is computationally complex since exhaustive matrix inversions are required to determine the optimal mode. In this paper, we propose an efficient MM-QP system that improves the performance of a conventional system in terms of error rate and has a lower computational complexity than the conventional system. First, we define the rate-partitioning vector as the mode and control the rate among substreams and the number of substreams according to the channel instantaneous condition. Second, to reduce the computational complexity for the receiver to determine the optimal mode, the simplified mode-selection technique using estimates of the modal metric is proposed. In the proposed mode-selection technique, the optimal mode can be obtained by several multiplication and division operations. Finally, the mode-reduction technique eliminating the less-frequently used modes is proposed, which leads to a significant reduction of the feedback information with negligible performance loss. In numerical experiments, it was verified that the proposed MM-QP system gives a better error-rate performance than the conventional system, with much less computational complexity for the same amount of feedback information.

Performance Analysis of Maximum Likelihood Detection for MIMO Systems

The performance of ML detection for the given channel is analyzed in spatially multiplexed MIMO system. In order to obtain the vector symbol error rate, we define error vectors which represent the geometrical relation between lattice points. The properties of error vectors are analyzed to show that all lattice points in an infinite lattice almost surely have four nearest neighbors after random channel transformation. Using this information and the minimum distance obtained by the modified sphere decoding algorithm, we formulate the analytical performance of vector symbol error rate(VSER) over the given channel. To verify the results, we simulate ML performance over the various random channels which are classified into three categories: unitary channel, dense channel, and sparse channel. From the simulation results, it is verified that the derived analytical result gives a good approximation for the performance of ML detector over all random MIMO channels

A New Approximation of the Receive Minimum Distance and Its Application to MIMO Systems

A new method to approximate the receive minimum distance is presented. In the proposed approximation, the geometric mean of the singular values of the channel matrix is used instead of the conventional minimum singular value. Numerical experiments show that the proposed approximation has less mean squared error than the minimum singular value bound and outperforms the minimum singular value bound in terms of bit error rate when they are applied to the antenna subgroup selection system.

Transmit Antenna Selection Scheme for Iterative Receivers in MIMO Systems

In this letter, we consider a transmit antenna selection scheme that provides selection diversity gain by exploiting a limited feedback information, and we propose a selection criterion appropriate for iterative receivers. Since the conventional selection criterion does not consider the convergence behavior of iterative receivers, a two-step antenna selection criterion is proposed, taking into account performance improvement as a function of iteration. Based on simulation results, the proposed scheme demonstrates superior performance compared to the conventional scheme as the number of iteration increases.

An efficient code timing acquisition for MIMO-CDMA system

Two code timing acquisition algorithms for a MIMO-CDMA system are proposed. While in conventional methods pilots are transmitted continuously, in the proposed algorithms, pilots are cyclically transmitted over each transmit antenna, so that it is very efficient with respect to the code scarcity problem. At the receiver, code timing is acquired by giving estimated delays the weighted average, which is in proportion to the peak of the output obtained from the matched filter of each channel. Simulation results show that the proposed methods give better code timing acquisition performance than the conventional method.

An eigen-based MIMO multiuser scheduler robust to spatial channel correlation

An eigen-based MIMO multiuser scheduler which is robust to spatial channel correlation is proposed. The proposed scheduler uses the principal right singular vector for reduced feedback information and exploits the eigen-mode transmission using a precoder matrix, whose columns are the principal right singular vectors of selected users. Since the proposed scheduler uses only each principal eigen channel per user, it is not affected by spatial channel correlation even if all spatial channels are equal, which means only one dominant eigen channel. Moreover, since the distribution of singular value is concentrated on the principal singular value under the spatially correlated environments, more channel gain can be obtained. By computer simulations, it is shown that the proposed scheduler approaches closer to the Satos bound and has better fairness performance under the moderate spatial channel correlation than the conventional schedulers using partial feedback.

Performance analysis of code selection algorithm based on quasi-orthogonal space-time block code

A quasi-orthogonal space–time block code (QO-STBC) has a rate one but a loss of diversity gain, compared with an orthogonal space–time block code such as Alamouti code. In this study, a code selection algorithm between QO-STBCs is proposed for a diversity gain improvement of a system based on a QO-STBC with four transmit antennas. The proposed algorithm improves diversity gain without a rate loss and provides better performance than the conventional closed-loop algorithm based on QO-STBC. Advantageously, the proposed algorithm can be applied to simple linear receivers, such as the zero-forcing receiver and minimum mean square error receiver, instead of the maximum likelihood receiver, which has a high computational complexity problem. The probability density function of two signal-to-interference and noise ratios (SINRs) is derived and by the derived pdf, an approximated bit error rate (BER) performance of the proposed system is also derived. The BER performance of the proposed system outperforms previous algorithms at least by about 0.5 dB SNR gain at BER 10−4.

An Efficient Multi-Mode Precoding for MIMO Systems with Full CSI

For the closed loop multiple-input-multiple-output (MIMO) systems, Fishers adaptive bit loading algorithm gives the best error performance by jointly optimizing the transmit powers, rates, and number of streams. However, its good performance comes at the cost of high and variable computational complexity for the joint optimization. In this letter, we propose an efficient multi-mode precoding algorithm using a simplified mode table. Numerical results show that the proposed algorithm provides almost the same performance as Fischers with much less computational complexity.