PAPR reduction and Pre-distortion techniques against Non-linear Distortion of Satellite WiBro

Robin Shrestha,Myung Hwan Seo,Gyeong Wan Go,Byung Seub Lee 통신위성ㆍ우주산업연구회 2008 한국위성정보통신학회논문지 Vol.3 No.2

A major drawback of Orthogonal Frequency Division Multiplexing (OFDM) system is high peak-to-average power ratio (PAPR) of the transmitted signal which introduces inevitable non-linear distortion in the transmission due to the amplifier non-linear property. This causes both in-band distortion and out of band spectrum re-growth. A polynomial based pre-distortion is estimated using the non-linear and inverse non-linear polynomial achieved through the Least Square Error (LSE) method. A new technique of PAPR reduction called ‘Phase Realignment’ (PR) is proposed which has a optimal effect in improving the BER performance as well as considerable reduction in the PAPR. In this paper we used the PR method along with the ‘Peak Clipping’ (PC) method is used before the pre-distortion to remove the high peak present in the non constant amplitude of the OFDM signal responsible to drive the amplifier in near saturation region for better performance of the system.

Enhanced Phase Realignment Techniques for the PAPR Reduction in OFDM Systems

Shrestha, Robin,Kim, Jae Moung,Seo, Jong-Soo IEEE 2016 IEEE Transactions on Vehicular Technology VT Vol.65 No.9

<P>This paper presents a simple and effective solution for the peak-to-average power ratio (PAPR) problem in orthogonal frequency-division multiplexing (OFDM) systems. Since the OFDM signal has a high PAPR, it generates inevitable nonlinear distortions at the output of a high-power amplifier. A new technique of PAPR reduction called phase realignment (PR) and its modified version called modified PR (MPR) are proposed, which require neither side information nor do they impose any modification of the receiver. Furthermore, four possible variations of the MPR constellation structure are proposed and evaluated in terms of PAPR reduction and bit-error-rate (BER) performance. The proposed PR and MPR provide considerable reductions in PAPR. MPR<SUB>1</SUB> gives a comparatively high PAPR reduction, followed by MPR<SUB>3</SUB>, PR, MPR<SUB>2</SUB>, and MPR<SUB>4</SUB> in descending order. The PAPR reductions of MPR<SUB>3</SUB> and MPR<SUB>2</SUB> tend to converge to that of the PR as the modulation order increases, whereas MPR<SUB>4</SUB> gives a low PAPR reduction. These techniques are designed to allow a certain level of BER degradation, which is defined by a transmit error vector magnitude (EVM) limited to a specified threshold, to reduce the PAPR and the average power (equally important in terms of power efficiency). The proposed techniques produce the reduced out-of-band radiation compared with the original OFDM signal and have moderate computational complexity. The EVM threshold is imposed to design optimum amplitude and phase margins for the PR and MPRs. Furthermore, we present the analytic upper bound BER expressions of the proposed techniques.</P>

Compensation Techniques for TWTA non-linear intermodulation of Satellite WiBro

Robin Shrestha,Byung-Seub Lee 통신위성ㆍ우주산업연구회 2008 한국위성정보통신학회논문지 Vol.3 No.1

OFDM (직교 주파수 분할 다중화) 신호의 높은 PAPR은 시스템의 송신단에서 전력증폭기의 비선형적 특성으로 인해 비선형 왜곡이 불가피하게 발생한다. 이 현상은 대역 내 왜곡과 대역 외 방사를 초래한다. 본 논문에서는 다항식 (polynomial) 모델에 기반한 사전왜곡(pre-distortion) 기법으로 이러한 문제를 보상하는 기법을 제안한다. 비선형 및 역비선형 다항식 모델 추정은 LSE(Least Square Error) 알고리즘으로 수행한다. 또한 시스템의 성능 향상을 위해 피크제거와 클리핑 결합기법을 이용해 OFDM 신호가 전력증폭기의 포화 영역 근처에서 동작함으로써 발생하는 왜곡된 신호의 진폭을 제거한다. The high peak to average power ratio (PAPR) of OFDM (Orthogonal Frequency Division Multiplexing) system introduces inevitable non-linear distortion in the transmission due to the amplifier non-linear property. This causes both in-band distortion and out of band spectrum re-growth. In this paper we tried to compensate the problem by using polynomial based redistortion. Estimation of both the non-linear and inverse non-linear polynomial is achieved using the Least Square Error (LSE) method. Using these parameters closed form pre-distorter can be easily created. We also used the ‘partial peak cancellation and clipping’ method to remove the high peak present in the non constant amplitude of the OFDM signal responsible to drive the amplifier in near saturation region for better performance of the system

A Leakage-Based Solution for Interference Alignment in MIMO Interference Channel Networks

( Robin Shrestha ),( Insan Bae ),( Jae Moung Kim ) 한국인터넷정보학회 2014 KSII Transactions on Internet and Information Syst Vol.8 No.2

Most recent research on iterative solutions for interference alignment (IA) presents solutions assuming channel reciprocity based on the suppression of interference from undesired sources by using an appropriate decoding matrix also known as a receiver combining matrix for multiple input multiple output (MIMO) interference channel networks and reciprocal networks. In this paper, we present an alternative solution for IA by designing precoding and decoding matrices based on the concept of signal leakage (the measure of signal power that leaks to unintended users) on each transmit side. We propose an iterative algorithm for an IA solution based on maximization of the signal-to-leakage-and-noise ratio (SLNR) of the transmitted signal from each transmitter. In order to make an algorithm removing the requirement of channel reciprocity, we deploy maximization of the signal-to-interference-and-noise ratio (SINR) in the design of the decoding matrices. We show through simulation that minimizing the leakage in each transmission can help achieve enhanced performance in terms of aggregate sum capacity in the system.

Adaptive Group Loading and Weighted Loading for MIMO OFDM Systems

( Robin Shrestha ),( Jae Moung Kim ) 한국인터넷정보학회 2011 KSII Transactions on Internet and Information Syst Vol.5 No.11

Adaptive Bit Loading (ABL) in Multiple-Input Multiple-Output Orthogonal Frequency-Division Multiplexing (MIMO-OFDM) is often used to achieve the desired Bit Error Rate (BER) performance in wireless systems. In this paper, we discuss some of the bit loading algorithms, compare them in terms of the BER performance, and present an effective and concise Adaptive Grouped Loading (AGL) algorithm. Furthermore, we propose a “weight factor” for loading algorithm to converge rapidly to the final solution for various data rate with variable Signal to Noise Ratio (SNR) gaps. In particular, we consider the bit loading in near optimal Singular Value Decomposition (SVD) based MIMO-OFDM system. While using SVD based system, the system requires perfect Channel State Information (CSI) of channel transfer function at the transmitter. This scenario of SVD based system is taken as an ideal case for the comparison of loading algorithms and to show the actual enhancement achievable by our AGL algorithm. Irrespective of the CSI requirement imposed by the mode of the system itself, ABL demands high level of feedback. Grouped Loading (GL) would reduce the feedback requirement depending upon the group size. However, this also leads to considerable degradation in BER performance. In our AGL algorithm, groups are formed with a number of consecutive sub-channels belonging to the same transmit antenna, with individual gains satisfying predefined criteria. Simulation results show that the proposed “weight factor” leads a loading algorithm to rapid convergence for various data rates with variable SNR gap values and AGL requires much lesser CSI compared to GL for the same BER performance.

A Parallel Combinatory OFDM System with Weighted Phase Subcarriers

( Hui Zheng ),( Robin Shrestha ),( Jaeho Hwang ),( Jaemong Kim ) 한국인터넷정보학회 2012 KSII Transactions on Internet and Information Syst Vol.6 No.1

Orthogonal Frequency Division Multiplexing (OFDM) is usually regarded as a spectral efficient multicarrier modulation technique, yet it suffers from a high peak-to-average power ratio (PAPR) problem. Among all the existing PAPR reduction techniques in OFDM systems, side information based PAPR reduction techniques such as partial transmit sequence (PTS) and selective mapping (SLM) schemes, have attracted the most attention. However, the transmission of side information results in somewhat spectral loss and this does not significantly improve the bit error rate (BER) performance. Parallel combinatory (PC) OFDM yields higher spectral efficiency (SE) and better BER performance on Gaussian channels, while is a little but not obvious PAPR improvement over the ordinary OFDM system. This investigation aimed to design a `perfect` OFDM system. We introduce the side information to rotate the subcarrier phases of our novel PC-OFDM system structure, and call this new system the SIPC(Side information based Parallel Combinatory)-OFDM system. The proposed system achieves better PAPR and SE performance. In addition, considering the tradeoff of system parameters, the proposed system also has the properties of a higher BER.

LETM1 is required for mitochondrial homeostasis and cellular viability

Li, Yuwen,Tran, Quangdon,Shrestha, Robin,Piao, Longzhen,Park, Sungjin,Park, Jisoo,Park, Jongsun D.A. Spandidos 2019 MOLECULAR MEDICINE REPORTS Vol.19 No.5

<P>Leucine zipper/EF-hand-containing transmembrane protein 1 (LETM1) has been identified as the gene responsible for Wolf-Hirschhorn syndrome (WHS), which is characterized by intellectual disability, epilepsy, growth delay and craniofacial dysgenesis. LETM1 is a mitochondrial inner membrane protein that encodes a homolog of the yeast protein Mdm38, which is involved in mitochondrial morphology. In the present review, the importance of LETM1 in WHS and its role within the mitochondrion was explored. LETM1 governs the mitochondrion ion channel and is involved in mitochondrial respiration. Recent studies have reported that LETM1 acts as a mitochondrial Ca<SUP>2+</SUP>/H<SUP>+</SUP> antiporter. LETM1 has also been identified as a K<SUP>+</SUP>/H<SUP>+</SUP> exchanger, and serves a role in Mg<SUP>2+</SUP> homeostasis. The function of LETM1 in mitochondria regulation is regulated by its binding partners, carboxyl-terminal modulator protein and mitochondrial ribosomal protein L36. Therefore, we describe the remarkable role of LETM1 in mitochondrial network physiology and its function in mitochondrion-mediated cell death. In the context of these findings, we suggest that the participation of LETM1 in tumorigenesis through the alteration of cancer metabolism should be investigated. This review provides a comprehensive description of LETM1 function, which is required for mitochondrial homeostasis and cellular viability.</P>

위성 WiBro의 HPA 비선형 왜곡특성 및 보상기술

이병섭,박평주,서명환,Robin Shrestha 한국통신학회 2009 정보와 통신 Vol.26 No.6

본고에서는 위성을 이용하여 WiBro(Wide Broadband)서비스를 제공하는데 있어서 WiBro 신호를 위성중계기를 통해 송수신할 경우, OFDM(Orthogonal Frequency Division Multiplexing) 신호가 갖고 있는 높은 PAPR(Peak-to-Average Power Ratio) 특성으로 인해 발생할 수 있는 전력증폭기의 비선형왜곡 현상에 대해서 기술하였다. 그리고 이를 효과적으로 극복할 수 있는 여러 방법들을 소개하고 모의실험을 통해 그 유효성을 확인하였다.

Tamsulosin versus tadalafil as a medical expulsive therapy for distal ureteral stones: A prospective randomized study

Hari Bahadur KC,Anil Shrestha,Ganesh Bhakta Acharya,Robin Bahadur Basnet,Arvind Kumar Shah,Parash Mani Shrestha 대한비뇨의학회 2016 Investigative and Clinical Urology Vol.57 No.5

Purpose: This study aimed to compare the safety and efficacy of tamsulosin and tadalafil as medical expulsive therapy for distal ureteral stones. Materials and Methods: This prospective randomized study was conducted at the Department of Urology of Bir Hospital over a period of 12 months in patients with distal ureteral stones sized 5 to 10 mm. Patients were randomly divided into 2 groups: group A received tamsulosin 0.4 mg and group B received tadalafil 10 mg at bedtime for 2 weeks. Stone expulsion rate, number of ureteric colic episodes and pain score, analgesic requirements, and adverse drug effects were noted in both groups. Statistical analyses were performed by using Student t-test and chi-square test. Results: Altogether 85 patients, 41 in group A and 44 in group B, were enrolled in the study. The patients' average age was 31.72±12.63 years, and the male-to-female ratio was 1.5:1. Demographic profiles, stone size, and baseline investigations were comparable between the 2 groups. The stone expulsion rate was significantly higher in the tadalafil group than in the tamsulosin group (84.1% vs. 61.0%, p=0.017). Although the occurrence of side effects was higher with tadalafil, this difference was not significant (p=0.099). There were no serious adverse effects. Conclusions: Tadalafil has a significantly higher stone expulsion rate than tamsulosin when used as a medical expulsive therapy for distal ureteral stones sized 5–10 mm. Both drugs are safe, effective, and well tolerated with minor side effects.

Proportional 자원할당을 위한 OFDMA 시스템에서 채널 용량을 증대시키기 위한 향상된 전력 할당 기법

와풋닛(Puthnith Var),로빈(Robin Shrestha),김재명(JaeMoung Kim) 한국통신학회 2013 韓國通信學會論文誌 Vol.38 No.7C

Orthogonal Frequency Division Multiple Access(OFDMA)는 4세대 무선 통신을 위해 고려되는 변조와 다중 접속 기술이다. 본 논문에서는, OFDMA 시스템에서 더 나은 proportional rate의 자원할당 기법을 위해 전체 전력, 비트 오류율, 유저 간의 rate proportionality에 대한 제한 조건을 만족시키면서, 유저의 전송률을 최대화 시키는 전략 할당 알고리즘에 대해 기술하였다. 그리고 subcarrier 할당의 비율과 유저의 정규화된 proportionality 조건에 기반을 둔 전력 할당 방식을 제안하였다. 유저에게 subcarrier 할당을 위한 방법으로는 greedy 알고리즘과 waterfilling 기술이 적용되었다. 제안된 알고리즘에 대한 평가는 시뮬레이션을 통해서 이루어졌으며, 시뮬레이션을 통해서 제안된 알고리즘이 유저 간의 rate proportionality를 유지하면서 더 높은 시스템 채널 용량과 적은 소요 시간을 보인다는 것을 확인할 수 있었다. The Orthogonal Frequency Division Multiple Access (OFDMA) is considered as a novel modulation and multiple access technique for 4th generation wireless systems. In this paper, we formulate a base station’s power allocation algorithm for each user to maximize the user’s sum rate, subject to constraints on total power, bit error rate, and rate proportionality among the users for a better proportional rate adaptive (RA) resource allocation method for OFDMA based system. We propose a novel power allocation method based on the proportion of subcarrier allocation and the user’s normalized proportionality constant. We adapt a greedy algorithm and waterfilling technique for allocating the subcarriers among the users. In an end-to-end simulation, we validate that the proposed technique has higher system capacity and lower CPU execution times, while maintaining the acceptable rate proportionality among users.