Ji-Hoon Choi

Interpolation based transmit beamforming for MIMO-OFDM with limited feedback

Transmit beamforming and receive combining are simple methods for exploiting spatial diversity in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. Optimal beamforming requires channel state information in the form of the beamforming vectors for each OFDM subcarrier. This paper proposes a limited feedback architecture that combines beamforming vector quantization and smart vector interpolation. In the proposed system, the receiver sends a fraction of information about the optimal beamforming vectors to the transmitter and the transmitter computes the beamforming vectors for all subcarriers through interpolation. A new spherical interpolator is developed that exploits parameters for phase rotation to satisfy the phase invariance and unit norm properties of the transmitted beamforming vectors. The beamforming vectors and phase parameters are quantized at the receiver and the quantized information is provided to the transmitter. The proposed quantization system provides only a moderate increase in complexity versus over comparable approaches. Numerical simulations show that the proposed scheme performs better than existing diversity techniques with the same feedback data rate.

Interpolation Based Unitary Precoding for Spatial Multiplexing MIMO-OFDM With Limited Feedback

Spatial multiplexing with linear precoding is a simple technique for achieving high spectral efficiency in multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Linear precoding requires channel state information for each OFDM subcarrier, which can be achieved using feedback. To reduce the amount of feedback, this paper proposes a limited feedback architecture that combines precoder quantization with a special matrix interpolator. In the proposed system, the receiver sends information about a fraction of the precoding matrices to the transmitter and the transmitter reconstructs the precoding matrices for all the subcarriers. A new interpolator is proposed inspired by spherical interpolation that respects the orthogonal columns of the precoding matrices and the performance invariance to right multiplication by a unitary matrix. The interpolator is parameterized by a set of unitary matrices; a construction of a suitable set is briefly described. Simulations illustrate the performance of limited feedback precoding with coding, estimation or prediction error, and time variation for bit error rate (BER), mutual information, and mean squared error (MSE)

Adaptive MIMO decision feedback equalization for receivers with time-varying channels

In an attempt to reduce the computational complexity of vertical Bell Labs layered space time (V-BLAST) processing with time-varying channels, an efficient adaptive receiver is developed based on the generalized decision feedback equalizer (GDFE) architecture. The proposed receiver updates the filter weight vectors and detection order using a recursive least squares (RLS)-based time- and order-update algorithm. The convergence of the algorithm is examined by analysis and simulation, and it is shown that the proposed adaptive technique is considerably simpler to implement than a V-BLAST processor with channel tracking, yet the performances are almost comparable.

Interpolation based unitary precoding for spatial multiplexing MIMO-OFDM with limited feedback

Spatial multiplexing with linear precoding is a simple technique to take advantage of the high spectral efficiency provided by multiple-input multiple-output (MIMO) systems, and it can be simply implemented over frequency selective channels by using orthogonal frequency division multiplexing (OFDM). This requires channel state information (CSI) at the transmitter in the form of the precoding matrices for all OFDM subcarriers, which can be achieved using feedback. To reduce the feedback requirements, this paper proposes a new precoding method incorporating quantization of a fraction of precoding matrices followed by a special interpolation at the transmitter. Since the precoding matrices are unitary, a new interpolator is proposed on the space of unitary matrices for which the outputs are also unitary. The parameters of the interpolator are optimized based on a mean square error (MSE) or mutual information criterion. Simulations show that performance near that of complete CSI can be achieved with a reasonable amount of feedback.

TRANSIENT THERMOELASTIC ANALYSIS OF DISK BRAKES IN FRICTIONAL CONTACT

The transient analysis of the thermoelastic contact problem of automotive disk brakes with frictional heat generation is performed using the finite element method. To analyze the thermoelastic phenomenon occurring in disk brakes, the coupled heat conduction and elastic equations are solved with contact problems. In the present work, the fully implicit transient scheme for the thermoelastic analysis is used to improve the accuracy of computations at every time step. The numerical simulation for the thermoelastic behavior of disk brakes is obtained in drag brake condition. The computational results are presented for the distributions of pressure and temperature on each friction surface between the contacting bodies. Also, the thermoelastic instability (TEI) phenomenon (the unstable growth of contact pressure and temperature) is investigated in the present study. The effects of the rotating speed of the disk on thermoelastic behaviors, such as the temperature distribution and contact ratio of the friction surfaces, are investigated.

Adaptive modulation for MIMO systems with V-BLAST detection

Adaptive modulation for multiple-input multiple-output (MIMO) systems with zero forcing (ZF) V-BLAST is considered. Motivated by the observation that the detection ordering of the original V-BLAST algorithm, called the forward ordering, is not suitable for adaptive modulation, a new detection ordering for V-BLAST-based adaptive modulation is proposed. This ordering, called the reverse ordering, selects the symbol corresponding to the minimum gain at each iteration, in contrast to the forward ordering which always gives priority to the maximum gain. The advantage of the reverse ordering over the forward ordering is demonstrated through analysis and simulation. It is shown that the reverse ordering can provide about 2 dB gain as compared with the forward ordering and almost comparable to the optimal ordering which is found through an exhaustive search. The V-BLAST based adaptive modulation with reverse ordering performs somewhat worse than the singular value decomposition (SVD) based method [G.G Raleigh et al., March 1998], but the former is shown to be considerably simpler to implement than the latter.

Adaptive MIMO decision feedback equalization for receivers in time-varying channels

In an attempt to reduce the computational complexity of the vertical Bell Labs layered space time (V-BLAST) processing for time-varying channels, an efficient adaptive receiver is developed based on the generalized decision feedback equalizer (GDFE) architecture. This receiver updates the filter weight vectors using a recursive least squares (RLS)-based time- and order-update algorithm. In time-varying channels, the proposed adaptive technique can be considerably simpler to implement than the V-BLAST processor with the channel tracking, yet its performance is almost comparable to that of the latter.

A bimodal approach for GPS and IMU integration for land vehicle applications

In this paper, we present a novel idea to integrate low cost inertial measurement unit (IMU) and differential global positioning system (DGPS) for land vehicle applications. Since the usage of high performance inertial navigation systems (INS) is restricted by their high price and the government regulation, low cost IMU is generally used for most land vehicle applications. However, low cost IMU produces large positioning and attitude errors in very short time due to the poor quality of inertial sensor assembly. More specifically, when GPS information is available, a good navigation solution can be obtained by incorporating DGPS data. However, a reliable result cannot be provided for land vehicle navigation when GPS signal is not available. To conquer the limitation, we present a bimodal approach for integrating IMU and DGPS, taking advantage of positioning and orientation data calculated from CCD images based on photogrammetry and stereo-vision techniques. The positioning and orientation data from the photogrammetric approach are fed back into the Kalman filter to reduce and compensate IMU errors and improve the performance. Experimental results are presented to show the robustness of the proposed method that can provide accurate position and attitude information for extended period for nonaided GPS information.

Modified leaky LMS algorithm for channel estimation in DS-CDMA systems

A simple adaptive least mean square (LMS) type algorithm for channel estimation is developed based on certain modifications to finite-impulse response (FIR) Wiener filtering. The proposed algorithm is nearly blind since it does not require any training sequence or channel statistics, and it can be implemented using only noise variance knowledge. A condition guaranteeing the convergence of the algorithm and theoretical mean square error (MSE) values are also derived. Computer simulation results demonstrate that the proposed algorithm can yield a smaller MSE than existing techniques, and that its performance is close to that of optimal Wiener filtering.

TRANSIENT ANALYSIS OF THERMOELASTIC CONTACT BEHAVIORS IN COMPOSITE MULTIDISK BRAKES

The transient analyses of the thermoelastic contact problem are performed for carbon–carbon composite multidisk brakes subject to mechanical and frictionally excited thermal loads. The finite element method, based on the coupled theory in which displacement and temperature fields are mutually affected, is applied for the numerical simulations. In the present study, to improve the accuracy of computations and stabilize the algorithm, the implicit transient scheme is used for the thermoelastic analysis. The law of action and reaction, Signorinis law of contact, Coulombs law of friction, and Archards law of wear are applied to be valid locally at each point for friction surfaces. The computation results are obtained for the antiskid brake conditions and presented for the transient evolution of contact pressure, temperature on each friction surface between the bodies, and thermoelastic deformations. It is also found that the high thermal stresses due to considerable temperature difference during the landing process occur in multidisk brakes.