Asymmetric Sum and Difference Beam Pattern Synthesis With a Common Weight Vector

Kwak, Semin,Chun, Joohwan,Park, Dongmin,Ko, Young Kwan,Cho, Byung Lae IEEE 2016 IEEE antennas and wireless propagation letters Vol.15 No.-

<P>A radar with a large number of antennas requires efficient design in its feeding network or transmit/receive modules (TRMs). We propose a method for simplifying the antenna system of a monopulse radar by attaching a single common weight to each antenna, both for sum and difference beams. A major novelty of our work lies in that the proposed technique can synthesize arbitrarily asymmetric monopulse beams with a common weight vector, using computationally efficient linear programming, not resorting to slow global optimization techniques. Therefore, common-weight monopulse beams can be produced in real time to reject ground clutter or null jamming signals.</P>

Lattice Reduction Aided MMSE Decision Feedback Equalizers

Jaehyun Park,Joohwan Chun,Luk, F T IEEE 2011 IEEE transactions on signal processing Vol.59 No.1

<P>The LLL algorithm developed by Lenstra, Lenstra, and Lovász has been widely studied to improve the conditioning of linear systems of equations that arise in multi-input multi-output (MIMO) antenna systems. In the first part of this correspondence, we show how a certain class of matrices can be transformed so that the LLL algorithm can be carried out efficiently, without compromising its performance. Then, the proposed transformation is applied to the matrix with block lower triangular Toeplitz matrices that arises in the decision feedback equalizer (DFE) to improve the bit error rate (BER) performance with a slight computational overhead.</P>

Improved Lattice Reduction-Aided MIMO Successive Interference Cancellation Under Imperfect Channel Estimation

Jaehyun Park,Joohwan Chun IEEE 2012 IEEE transactions on signal processing Vol.60 No.6

<P>Lattice reduction (LR) and successive interference cancellation (SIC) are two well-known techniques that can be used to improve detection performance over linear detectors for multiple-input multiple-output (MIMO) systems. However, the LR technique and the SIC technique usually need perfect knowledge on the channel at the receiver, and the use of these techniques with an erroneous channel matrix even worsens the detection performance compared to linear detectors. In this correspondence, we shall show how to modify these techniques to make them robust under imperfect channel estimation. Information needed for the proposed algorithm is moderate; the variance of channel estimation error is the only requirement. Furthermore, our algorithm is not sensitive to the error in the variance of channel estimation error.</P>

Degrees of Freedom for a MIMO Gaussian <tex> $K$</tex> -Way Relay Channel: Successive Network Code Encoding and Decoding

Namyoon Lee,Joohwan Chun IEEE 2014 IEEE transactions on information theory Vol.60 No.3

<P>This paper studies a network information flow problem for a multiple-input multiple-output (MIMO) Gaussian wireless network with K users and a single intermediate relay having M antennas. In this network, each user sends a multicast message to all other users while receiving K-1 independent messages from the other users via an intermediate relay. This network information flow is termed a MIMO Gaussian K-way relay channel. For this channel, it is shown that the optimal sum degrees of freedom (sum-DoF) is KM/K-1, assuming that all nodes have global channel knowledge and operate in full-duplex. A converse argument is derived by cut-set bounds. The achievability is shown by a repetition coding scheme with random beamforming in encoding and a zero-forcing method combined with self-interference cancelation in decoding. Furthermore, under the premise that all nodes have local channel state information at the receiver only and operate in half-duplex mode, it is shown that a total K/2 DoF is achievable when M=K-1. The key to showing this result is a novel encoding and decoding scheme, which creates a set of network code messages with a chain structure during the multiple access phase and performs successive interference cancelation using side-information for the broadcast phase. One major implication of the derived results is that efficient exploitation of the transmit message as side-information leads to an increase in the sum-DoF gain in a multiway relay channel with multicast messages.</P>

Numerically extrapolated discrete layer-peeling algorithm for synthesis of nonuniform fiber Bragg gratings.

Choi, Youngchol,Chun, Joohwan,Bae, Jinho Optical Society of America 2011 Optics express Vol.19 No.9

<P>The discrete layer-peeling algorithm (DLPA) requires to discretize the continuous medium into discrete reflectors to synthesize nonuniform fiber Bragg gratings (FBG), and the discretization step of this discrete model should be sufficiently small for synthesis with high accuracy. However, the discretization step cannot be made arbitrarily small to decrease the discretization error, because the number of multiplications needed with the DLPA is proportional to the inverse square of the layer thickness. We propose a numerically extrapolated time domain DLPA (ETDLPA) to resolve this tradeoff between the numerical accuracy and the computational complexity. The accuracy of the proposed ETDLPA is higher than the conventional time domain DLPA (TDLPA) by an order of magnitude or more, with little computational overhead. To be specific, the computational efficiency of the ETDLPA is achieved through numerical extrapolation, and each addition of the extrapolation depth improves the order of accuracy by one. Therefore, the ETDLPA provides us with computationally more efficient and accurate methodology for the nonuniform FBG synthesis than the TDLPA.</P>

Cross-relation-based frequency-domain blind channel estimation with multiple antennas in orthogonal frequency division multiplexing systems

Jaehyun Park,Joohwan Chun,Byung Jang Jeong IET 2013 IET communications Vol.7 No.16

<P>Frequency domain blind channel estimation methods for orthogonal frequency division multiplexing (OFDM) systems with multiple antennas are proposed, in which the cross relation between the channel gains and a single snapshot of the received signal on each subcarrier is utilised. Necessary and sufficient conditions for the identifiability of a single-input multiple-output OFDM channel estimator are derived and, furthermore, using a first-order perturbation analysis, the approximation of mean-squared-error (MSE) is analytically obtained, which shows that the proposed estimator is approximately unbiased at high signal-to-noise ratio. The proposed estimator exhibits lower MSEs than those of both the existing subspace-based and deterministic blind channel estimators and is less sensitive to the estimation error of the number of multipaths. Finally, the extension of the proposed channel estimation to multiple-input-multiple-output OFDM is presented.</P>

Efficient Lattice-Reduction-Aided Successive Interference Cancellation for Clustered Multiuser MIMO System

Jaehyun Park,Joohwan Chun IEEE 2012 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY Vol.61 No.8

<P>The lattice-reduction (LR)-aided successive interference cancellation (SIC) is a well-known detection technique achieving near-maximum-likelihood (ML) performance in the multiuser system. However, the LR algorithm and the computation of sequential nulling vectors still incur considerable computational complexity overhead for a large number of mobile stations (MSs). To overcome it, we propose a decoupled LR-aided SIC for clustered MSs, in which the entire system is linearly decoupled into multiple lower dimensional subsystems before the LR-aided SIC is applied. In addition, efficient MS grouping algorithms are proposed to optimize the detection performance of the decoupled LR-aided SIC. The proposed LR-aided SIC with linear preprocessing exhibits a full diversity order, compared with the conventional LR-aided SIC, with considerable computational savings. Finally, to further improve the system performance, a nested LR-aided SIC with preprocessing is also developed.</P>

DTV receivers using an adaptive switched beamformer with an online-calibration algorithm

Jaehyun Park,Joohwan Chun IEEE 2010 IEEE transactions on consumer electronics Vol.56 No.1

<P>We propose DTV receivers using an adaptive switched beamformer with an online calibration algorithm in the presence of unknown gain and phase errors of an antenna array. The proposed calibration algorithm is based on the subspace method which does not require a prior knowledge of the angle of arrivals (AOAs) of the multipath signals. The calibration algorithm provides us with the estimates of the AOAs of multipaths as well as calibration information of the antenna gain and phase. By utilizing the AOA estimates, an adaptive switched beamformer which has lower hardware complexity than the conventional switched beamformer is developed. To demonstrate our proposed scheme, we develop a complete DTV beamforming receiver, equipped with the proposed calibrator, synchronizer, and equalizer, which is compatible with a wide variety of commercially available VSB demodulator chips.</P>

Pilot-based non-adaptive equalizers using an array antenna for DTV receivers

Jaehyun Park,Joohwan Chun IEEE 2009 IEEE transactions on consumer electronics Vol.55 No.1

<P>In this paper, a pilot-based single-input multiple-output (SIMO) minimum mean square error (MMSE) decision feedback equalizer (DFE) using an array antenna is proposed to overcome the long transient period problem of adaptive equalizers for a severely distorted digital television (DTV) channel. Furthermore, for the sake of reduced complexity, a low complexity spatio-temporal equalizer is developed, wherein one spatial equalizer and multiple single input single-output (SISO) MMSE-DFEs are cascaded. To further improve the spatio-temporal equalizer, we also propose an orthogonalization technique of the beamforming weights. Two implementation issues, the channel estimation and computational complexities of the proposed algorithms, are also investigated. The simulation results indicate that the spatio-temporal equalizer has a slightly degraded performance with a much lower complexity than the SIMO MMSE-DFE.</P>

The Schur Algorithm Applied to the One-Dimensional Continuous Inverse Scattering Problem

Youngchol Choi,Joohwan Chun,Taejoon Kim,Jinho Bae IEEE 2013 IEEE transactions on signal processing Vol.61 No.13

<P>The one-dimensional continuous inverse scattering problem can be solved by the Schur algorithm in the discrete-time domain using sampled scattering data. The sampling rate of the scattering data should be increased to reduce the discretization error, but the complexity of the Schur algorithm is proportional to the square of the sampling rate. To improve this tradeoff between the complexity and the accuracy, we propose a Schur algorithm with the Richardson extrapolation (SARE). The asymptotic expansion of the Schur algorithm, necessary for the Richardson extrapolation, is derived in powers of the discretization step, which shows that the accuracy order (with respect to the discretization step) of the Schur algorithm is 1. The accuracy order of the SARE with the <I>N</I>-step Richardson extrapolation is increased to <I>N</I>+1 with comparable complexity to the Schur algorithm. Therefore, the discretization error of the Schur algorithm can be decreased in a computationally efficient manner by the SARE.</P>