Reduced Complexity Scheduling Method with MIMO Interference Alignment for Mutually Interfering Broadcast Channels

Haewook Park(박해욱),Seok-Hwan Park(박석환),Hakjea Sung(성학제),Inkyu Lee(이인규) 한국통신학회 2012 韓國通信學會論文誌 Vol.37 No.8

MMSE-Based Filter Designs for Cognitive Multiuser Two-Way Relay Networks

Haewook Park,Changick Song,Hoon Lee,Inkyu Lee IEEE 2015 IEEE Transactions on Vehicular Technology VT Vol.64 No.4

<P>This paper considers cognitive multiuser two-way relay networks (MU-TRNs) where multiple secondary users (SUs) concurrently exchange their data with other SUs with an aid of a two-way relay in licensed primary networks. In this paper, we particularly focus on linear relay filter designs that effectively minimize the mean square error of SUs while providing a certain level of quality of service for the primary network. To this end, we first propose an iterative algorithm that identifies a solution based on an alternating optimization technique. Then, a closed-form relay filter design is presented, which employs a high signal-to-noise ratio approximation to reduce the computational complexity. Through the simulation results, we confirm that the proposed closed-form relay filter design outperforms conventional approaches and provides the performance close to the proposed iterative algorithm with much reduced complexity.</P>

Achieving reliable coincidence resolving time measurement of PET detectors using multichannel waveform digitizer based on DRS4 chip

Park, Haewook,Lee, Seungeun,Ko, Guen Bae,Lee, Jae Sung Institute of Physics in association with the Ameri 2018 Physics in medicine & biology Vol.63 No.24

<P>Coincidence resolving time (CRT) is one of the most important physical-performance measures for positron emission tomography (PET), as reconstruction with accurate time-of-flight information enhances the lesion detectability in patient studies. Accordingly, various PET detector designs and high-performance front-end readout circuits have been actively investigated to improve timing performance. The resulting PET detectors are often evaluated using multichannel waveform digitizers for versatile data analysis of the output signals. However, we have found that inappropriate data acquisition (DAQ) using a multichannel waveform digitizer based on the domino-ring-sampler 4 (DRS4) chip can lead to a considerable error when determining CRT. To address this issue, we performed CRT measurements using a pair of Hamamatsu R9800 photomultiplier tube based PET detectors. Then, considering intra- and inter-chip sampling, we employed four different combinations of input channels into the CAEN DT5742B waveform digitizer and obtained 2D CRT maps according to the leading-edge discriminator threshold for assessing each DAQ scheme. The intra-chip CRT measurement exhibited unusual streak patterns in the 2D CRT map and yielded the artificially-low CRT information in PET detector pairs, whereas the inter-chip CRT measurement provided the reliable estimation of timing resolution. Further, we could prevent the high-frequency signal crosstalk among input channels within the DRS4 chip using the inter-chip CRT measurement. We expect that our findings will also be useful for achieving the reliable CRT measurements when using other single-chip-based multichannel waveform digitizers.</P>

Weighted Sum MSE Minimization under Per-BS Power Constraint for Network MIMO Systems

Haewook Park,Seok-Hwan Park,Han-Bae Kong,Inkyu Lee IEEE 2012 IEEE COMMUNICATIONS LETTERS Vol.16 No.3

<P>We study joint processing (JP) for network MIMO systems where base stations exchange the user's message and channel state information under per-BS power constraint. In this letter, we propose a weighted sum mean square error (WS-MSE) minimization algorithm for the JP systems by considering the channel gain as the weight factor in the MSE metric. To efficiently solve the formulated WS-MSE problem, an alternating optimization method which iteratively finds a local optimal solution is employed in our algorithm. The simulation results confirm that the proposed algorithm provides the sum rate performance close to the near-optimal gradient ascent approach and outperforms conventional schemes. In addition, we also propose a modified WS-MSE design which is robust to channel mismatch caused by channel estimation and feedback errors.</P>

Hybrid charge division multiplexing method for silicon photomultiplier based PET detectors

Park, Haewook,Ko, Guen Bae,Lee, Jae Sung Institute of Physics in association with the Ameri 2017 Physics in medicine & biology Vol. No.

<P>Silicon photomultiplier (SiPM) is widely utilized in various positron emission tomography (PET) detectors and systems. However, the individual recording of SiPM output signals is still challenging owing to the high granularity of the SiPM; thus, charge division multiplexing is commonly used in PET detectors. Resistive charge division method is well established for reducing the number of output channels in conventional multi-channel photosensors, but it degrades the timing performance of SiPM-based PET detectors by yielding a large resistor–capacitor (RC) constant. Capacitive charge division method, on the other hand, yields a small RC constant and provides a faster timing response than the resistive method, but it suffers from an output signal undershoot. Therefore, in this study, we propose a hybrid charge division method which can be implemented by cascading the parallel combination of a resistor and a capacitor throughout the multiplexing network. In order to compare the performance of the proposed method with the conventional methods, a 16-channel Hamamatsu SiPM (S11064-050P) was coupled with a 4  ×  4 LGSO crystal block (3  ×  3  ×  20 mm<SUP>3</SUP>) and a 9  ×  9 LYSO crystal block (1.2  ×  1.2  ×  10 mm<SUP>3</SUP>). In addition, we tested a time-over-threshold (TOT) readout using the digitized position signals to further demonstrate the feasibility of the time-based readout of multiplexed signals based on the proposed method. The results indicated that the proposed method exhibited good energy and timing performance, thus inheriting only the advantages of conventional resistive and capacitive methods. Moreover, the proposed method showed excellent pulse shape uniformity that does not depend on the position of the interacted crystal. Accordingly, we can conclude that the hybrid charge division method is useful for effectively reducing the number of output channels of the SiPM array.</P>

Feedback Bit Allocation Schemes for Multi-User Distributed Antenna Systems

Eunsung Park,Heejin Kim,Haewook Park,Inkyu Lee IEEE 2013 IEEE COMMUNICATIONS LETTERS Vol.17 No.1

<P>In this paper, we propose a feedback bit allocation algorithm for multi-user downlink distributed antenna systems with limited feedback. We consider a composite fading channel with small scale fadings and path loss, and assume the case where each user is served by only one distributed antenna (DA) port while each DA port can support any number of users. In order to efficiently determine bit allocation, we propose an iterative algorithm which minimizes an upper bound of a mean rate loss. Compared to conventional bit allocation methods, the proposed algorithm can be applied to more general system configurations. Simulation results show that our proposed algorithm offers a performance gain of 20% over an equal bit allocation scheme.</P>

간섭 Broadcast 채널에서 다중 사용자 Diversity 를 이용한 간섭 정렬 기법

박해욱(Haewook Park),박석환(Seok-Hwan Park),이인규(Inkyu Lee) 한국통신학회 2009 한국통신학회 학술대회논문집 Vol.2009 No.11

Distributed Beamforming Techniques for Weighted Sum-Rate Maximization in MISO Interference Channels

Seok-Hwan Park,Haewook Park,Inkyu Lee IEEE 2010 IEEE Communications Letters Vol.14 No.12

<P>This letter proposes a beamforming technique for weighted sum-rate (WSR) maximization in multiple-input single-output (MISO) interference channels. In order to solve the WSR maximization problem in a distributed manner, we obtain a decoupled problem by applying high signal-to-interference-plus-noise-ratio (SINR) approximation. When there are more than two users, further approximation is employed to fully decouple the problem. Then, the decoupled problems are solved by using a zero-gradient (ZG) based algorithm which converges to a local optimal point with only a few iterations. Unlike the conventional distributed schemes where additional information should be exchanged at each iteration, each transmitter of the proposed scheme utilizes only local channel information to compute its beamformer.</P>

New Beamforming Techniques Based on Virtual SINR Maximization for Coordinated Multi-Cell Transmission

Seok-Hwan Park,Haewook Park,Hanbae Kong,Inkyu Lee IEEE 2012 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS Vol.11 No.3

<P>In this paper, we propose new beamforming techniques based on virtual signal-to-interference-plus-noise ratio (VSINR) for weighted sum-rate (WSR) maximization in coordinated multi-cell transmission. In earlier works based on the VSINR maximization, the parameters which control the interference power and the noise variance were set to fixed values regardless of channel realizations and the signal-to-noise ratio level. In order to obtain an improved WSR performance, we propose a method which adaptively adjusts the parameters after establishing a connection between the WSR and VSINR. Our proposed method can be applied to the cases of various coordination levels among base stations. To address practical implementation issues, a decentralized implementation of the beamforming techniques is also proposed based on local channel state information. Numerical results confirm that the proposed centralized schemes provide near-optimal WSR performance and the proposed decentralized methods show a negligible performance loss compared to the centralized algorithms with reduced system complexity.</P>

Regularized Transceiver Designs for Multi-User MIMO Interference Channels

Seok-Hwan Park,Haewook Park,Hakjea Sung,Inkyu Lee IEEE 2012 IEEE TRANSACTIONS ON COMMUNICATIONS Vol.60 No.9

<P>For multi-user interference channels (IC), an altruistic approach based on the zero-forcing (ZF) criterion shows the near-optimal performance at high signal-to-noise ratio (SNR), whereas its performance at low SNR becomes poor compared to a simple egoistic algorithm (selfish beamforming). Thus, balancing between the egoism and the altruism has been an important issue to achieve good sum-rate performance at overall SNR regime. In this paper, we propose a new approach for enhancing the performance by regularizing the ZF based transceivers. To this end, we start with investigating efficient ZF transceivers for 2-user and 3-user ICs. First, coordinated spatial multiplexing (CSM) is proposed for 2-user IC. For the 3-user case, it is shown that the enhanced interference alignment (E-IA) introduced in our previous work is the optimal ZF transceivers in terms of the sum-rate performance. Next, to improve the performance of the CSM and E-IA schemes at low SNR, we propose a non-iterative regularization method under the high SNR approximation. The distributed implementation of the proposed regularization method is also presented where each node is able to compute its own precoding or decoding matrix using local channel state information. From simulations, it is observed that the proposed regularized design outperforms the conventional schemes in overall SNR regime. Also, we confirm that our distributed approach provides a substantial performance gain over the conventional distributed scheme with reduced computational complexity.</P>