Energy Savings in OFDM Systems through Cooperative Relaying

Khuong Ho Van,공형윤 한국전자통신연구원 2007 ETRI Journal Vol.29 No.1

Energy savings in orthogonal frequency division multiplexing (OFDM) systems is an active research area. In order to achieve a solution, we propose a new cooperative relaying scheme operated on a per subcarrier basis. This scheme improves the bit error rate (BER) performance of the conventional signal-to-noise ratio (SNR)-based selection relaying scheme by substituting SNR with symbol error probability (SEP) to evaluate the received signal quality at the relay more reliably. Since the cooperative relaying provides spatial diversity gain for each subcarrier, thus statistically enhancing the reliability of subcarriers at the destination, the total number of lost subcarriers due to deep fading is reduced. In other words, cooperative relaying can alleviate error symbols in a codeword so that the error correction capability of forward error correction codes can be fully exploited to improve the BER performance (or save transmission energy at a target BER). Monte-Carlo simulations validate the proposed approach.

Bit Error Rate of Underlay Decode-and-Forward Cognitive Networks with Best Relay Selection

Khuong Ho-Van,Paschalis C. Sofotasios,George C. Alexandropoulos,Steven Freear 한국통신학회 2015 Journal of communications and networks Vol.17 No.2

This paper provides an analytic performance evaluation of the bit error rate (BER) of underlay decode-and-forward cognitive networks with best relay selection over Rayleigh multipath fading channels. A generalized BER expression valid for arbitrary operational parameters is firstly presented in the form of a single integral, which is then employed for determining the diversity order and coding gain for different best relay selection scenarios. Furthermore, a novel and highly accurate closed-form approximate BER expression is derived for the specific case where relays are located relatively close to each other. The presented results are rather convenient to handle both analytically and numerically, while they are shown to be in good agreement with results from respective computer simulations. In addition, it is shown that as in the case of conventional relaying networks, the behaviour of underlay relaying cognitive networks with best relay selection depends significantly on the number of involved relays.

Underlay Cooperative Cognitive Networks with Imperfect Nakagami-m Fading Channel Information and Strict Transmit Power Constraint: Interference Statistics and Outage Probability Analysis

Khuong Ho Van,Paschalis C. Sofotasios,Steven Freear 한국통신학회 2014 Journal of communications and networks Vol.16 No.1

This work investigates two important performance metricsof underlay cooperative cognitive radio (CR) networks: Interferencecumulative distribution function of licensed users andoutage probability of unlicensed users. These metrics are thoroughlyanalyzed in realistic operating conditions such as imperfectfading channel information and strict transmit power constraint,which satisfies interference power constraint and maximum transmitpower constraint, over Nakagami−m fading channels. Novelclosed-form expressions are derived and subsequently validated extensivelythrough comparisons with respective results from computersimulations. The proposed expressions are rather long butstraightforward to handle both analytically and numerically sincethey are expressed in terms of well known built-in functions. Inaddition, the offered results provide the following technical insights:i) Channel information imperfection degrades considerablythe performance of both unlicensed network in terms of OP and licensednetwork in terms of interference levels; ii) underlay cooperativeCR networks experience the outage saturation phenomenon;iii) the probability that the interference power constraint is satis-fied is relatively low and depends significantly on the correspondingfading severity conditions as well as the channel estimation quality;iv) there exists a critical performance trade-off between unlicensedand licensed networks.

General expression for pdf of a sum of independent exponential random variables

Khuong, Ho Van,Kong, Hyung-Yun IEEE 2006 IEEE communications letters Vol.10 No.3

In order to evaluate exactly the performance of some diversity schemes, the probability density function (pdf) of a sum of independent exponential random variables (r.v.'s) must be known. This paper proposes a simple method to find it by using characteristic function., The resultant pdf is successfully applied to formulate the closed-form BER expression of 2 Tx-J Rx transmit diversity as well as the outage probability of repetition coding.

Error Probability Evaluation of a Novel Cooperative Communications Signaling Strategy in CDMA Systems

Khuong Ho-Van,Kong Hyung-Yun The Korea Institute of Information and Commucation 2006 Journal of communications and networks Vol.8 No.3

The powerful benefits of multi-antenna systems can be obtained by cooperative communications among users in multiple access environments without the need for physical arrays. This paper studies a novel cooperative signaling strategy that achieves high performance and low implementation complexity for synchronous code division multiple access (CDMA) wireless mobile networks. The validity of the proposed strategy under slow flat Rayleigh fading channel plus additive white Gaussian noise (AWGN) is verified through closed-form error probability expressions and MonteCarlo simulations. A variety of analytical results reveal that the new cooperative strategy significantly outperforms direct transmission subject to the same spectral efficiency and transmit power constraint.

Intra-Organizational Factors Affecting Business Performance: An Empirical Study in Vietnam

Khuong Ngoc MAI,Thao Thi Thanh NGUYEN,Phuong Ngoc Duy NGUYEN,Khoa Tien TRAN 한국유통과학회 2021 The Journal of Asian Finance, Economics and Busine Vol.8 No.10

In the era of industry 4.0 with the robust digital transformation, especially under the trigger of the Covid-19 pandemic, the process of transforming businesses to achieve the desired business performance depends much on the mindset transformation of each member of the organization, beginning with the thoughts of leadership and stakeholders. This study will evaluate the relationship between leadership’s strategic reasoning perspectives on employee engagement or commitment and the company’s reputation, thereby directly or indirectly affecting organizational performance. The study examines data from 382 companies out of 500 samples in typical industries in Vietnam using the exploratory factor analysis (EFA) and partial least squares structural equation modeling (PLS-SEM) techniques. The results show that holistic thinking is closely related to employee retention and corporate reputation, thereby increasing the business outcomes of the organization, whereas there was no evidence to support analytical thinking in this study. As a consequence, transforming the business to achieve the desired business performance is heavily reliant on changing the mindset of each member of the organization, beginning with the top leaders and influencers of the business. This will assist Vietnamese leaders in gaining a comprehensive understanding of corporate governance and controlling the relationships between organizational constructs.

Exact Outage Probability Analysis of Proactive Relay Selection in Cognitive Radio Networks with MRC Receivers

Khuong Ho-Van 한국통신학회 2016 Journal of communications and networks Vol.18 No.3

Proactive relay selection in cognitive radio networks hasrecently received considerable attention. However, its outage probabilityanalysis is limited to partially-identical fading distributions,uncorrelation among received signal-to-noise ratios (SNRs), and nodirect channel. This paper completes this literature deficiency bygeneralizing the existing analysis for non-identical fading distributions,correlation among received SNRs, and with direct channel. Numerous results demonstrate that relay selection with a directchannel achieves a higher diversity order and superior performancethan that without a direct channel at virtually no cost ofpower and bandwidth. Further, proactive relay selection suffersan error floor at either a large maximum transmit power or largemaximum interference power; however, the error floor level can besignificantly remedied with an increase in the number of relays.

Cognitive Relay Networks with Underlay Spectrum Sharing and Channel Estimation Error: Interference Probability and BER Analysis

Khuong Ho-Van 한국통신학회 2014 Journal of communications and networks Vol.16 No.3

This paper proposes accurate interference probability and bit error rate formulas for cognitive relay networks with underlay spectrum sharing and channel estimation error (CEE). Numerous results reveal that the CEE not only degrades the performance of secondary systems (SSs) but also increases interference power caused by SSs to primary systems (PSs), eventually unfavorable to both systems. A solution to further protect PSs from this effect through reducing the power of secondary transmitters is investigated and analyzed.

Effect of Mutual Interference and Channel Estimation Error on Outage Performance of Reactive Relay Selection in Unlicensed Systems

Khuong Ho-Van 한국통신학회 2015 Journal of communications and networks Vol.17 No.4

This study addresses the effects of channel estimation error and mutual interference between licensed and unlicensed systems on outage performance of reactive relay selection in unlicensed systems over independent non-identical (i.n.i) Rayleigh fading channels and under both the maximum transmit power constraint and primary outage constraint. Toward this end, power allocation for unlicensed users is first recommended to satisfy both constraints and account for channel estimation error and mutual interference. Then, we derive an exact closed-form outage probability representation for unlicensed systems to quickly evaluate this effect in key operation parameters. Various results corroborate the derived expressions and provide useful insights into system performance.

Novel Reward Function for Autonomous Drone Navigating in Indoor Environment

( Khuong G. T. Diep ),( Viet-tuan Le ),( Tae-seok Kim ),( Anh H. Vo ),( Yong-guk Kim ) 한국정보처리학회 2023 한국정보처리학회 학술대회논문집 Vol.30 No.2

Unmanned aerial vehicles are gaining in popularity with the development of science and technology, and are being used for a wide range of purposes, including surveillance, rescue, delivery of goods, and data collection. In particular, the ability to avoid obstacles during navigation without human oversight is one of the essential capabilities that a drone must possess. Many works currently have solved this problem by implementing deep reinforcement learning (DRL) model. The essential core of a DRL model is reward function. Therefore, this paper proposes a new reward function with appropriate action space and employs dueling double deep Q-Networks to train a drone to navigate in indoor environment without collision.