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.

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.

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

Ho-Van, Khuong,Sofotasios, Paschalis C.,Freear, Steven The Korea Institute of Information and Commucation 2014 Journal of communications and networks Vol.16 No.1

This work investigates two important performance metrics of underlay cooperative cognitive radio (CR) networks: Interference cumulative distribution function of licensed users and outage probability of unlicensed users. These metrics are thoroughly analyzed in realistic operating conditions such as imperfect fading channel information and strict transmit power constraint, which satisfies interference power constraint and maximum transmit power constraint, over Nakagami-m fading channels. Novel closed-form expressions are derived and subsequently validated extensively through comparisons with respective results from computer simulations. The proposed expressions are rather long but straightforward to handle both analytically and numerically since they are expressed in terms of well known built-in functions. In addition, the offered results provide the following technical insights: i) Channel information imperfection degrades considerably the performance of both unlicensed network in terms of OP and licensed network in terms of interference levels; ii) underlay cooperative CR networks experience the outage saturation phenomenon; iii) the probability that the interference power constraint is satisfied is relatively low and depends significantly on the corresponding fading severity conditions as well as the channel estimation quality; iv) there exists a critical performance trade-off between unlicensed and licensed networks.

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.

BER Performance of Cooperative Transmission for the Uplink of TDD-CDMA Systems

Ho Van Khuong,공현윤 한국전자통신연구원 2006 ETRI Journal Vol.28 No.1

In time division duplex (TDD) code division multiple access (CDMA) systems, chip-synchronous transmission in the uplink is obtainable, thus leading to free multiple access interference in flat Rayleigh fading channels plus additive white Gaussian noise. This motivates us to develop a novel cooperative transmission strategy that allows single-antenna devices to benefit from spatial diversity using orthogonal signature sequences. The proposed cooperation is applicable to many digital modulation methods and achieves the fullest diversity level, low implementation complexity, and a full data rate. Closed-form bit-error-rate expressions were also derived and compared to simulation results in order to evaluate its validity. A variety of numerical results demonstrated the cooperation’s superiority over single transmission under the same transmit power constraint.

무선 중계 네트워크의 협력 통신 방법에 대한 LLR 적용 연구

Ho Van Khuong,공형윤,이동운 한국정보처리학회 2006 정보처리학회논문지 C : 정보통신,정보보안 Vol.13 No.6

Decode-and-forward cooperative communications protocol (DFP) allows single-antenna users in wireless medium to obtain the powerful benefits of multi-antenna systems without physical antenna arrays. For this protocol, so far the relays have used SNR to evaluate the reliability of the received signal before deciding whether to forward the decoded data so as to prevent their unsuccessful detection. However, SNR only characterizes the long-term statistic of Gaussian noise and thus leading to inaccurate assessment. Therefore, we propose using log-likelihood ratio (LLR) which accounts for the instantaneous noise in the received signal as an alternative to SNR. A variety of simulation results reveal the significant superiority of the LLR-based DFP to the SNR-based DFP regardless of threshold level and relay position under the flat Rayleigh fading channel plus AWGN (Additive White Gaussian Noise). 복호 후 전달 협력 통신 프로토콜(DFP: Decode-and-Forward Cooperative Communication Protocol)은 하나의 안테나를 쓰는 사용자에게 물리적인 안테나의 증가 없이 다중 안테나 시스템의 강력한 장점을 얻을 수 있는 통신 방법이다. 지금까지는 신호 대 잡음 비율(SNR: Signal to Noise Ratio)이나 경로이득 크기의 루트를 취한 값들을 이용하여 중계 노드에서 복호한 데이터를 전달할지 안 할지를 결정하여 중계 노드에서 부정확한 검파가 되지 않도록 방지하였다. 본 논문에서는 기존의 DFP에서 사용되던 SNR을 대체할 수 있는 LLR(Log-Likelihood Ratio)를 이용하는 방법을 제안하였다. 여러 가지 많은 모의 실험을 통해 레일리 페이딩 환경과 AWGN환경에서 기준값이나 중계 노드의 위치에 상관없이 LLR기반의 DFP가 SNR기반의 DFP보다 아주 우수한 성능을 보이는 것을 확인하였다.

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.

레일레이 페이딩 채널의 무선 네트워크에서 ¾ STBC를 사용한 협력신호 구조에 관한 연구

Ho Van Khuong,공형윤,최정호 한국정보처리학회 2006 정보처리학회논문지 C : 정보통신,정보보안 Vol.13 No.7

Cooperative communications (CC) have received a great deal of attention recently as an efficient way to obtain the spatial diversity without physical arrays. Thus, space-time block codes (STBC) which are well-known for use in co-located multi-antenna systems can be still utilized for single-antenna users in a distributed fashion. In this paper, we propose a cooperative signaling structure using the ¾-rate STBC and derive closed-form BER expression which takes the effect of network geometry and transmit power constraint into account. A variety of simulated and numerical results demonstrated the cooperation considerably outperforms the direct transmission when partners are located in appropriate positions. 최근 협력통신은 안테나의 물리적인 배치 없이 공간 다중화를 얻을 수 있는 효과적인 방법으로 많은 주목을 받고 있다. 따라서 협력적인 위치를 가진 다중 안테나 시스템에서 잘 알려진 시공간 블록 코드(STBC, Space Time Block Code)는 분산환경 하의 단일 안테나 사용자들에게도 이용될 수 있다. 본 논문에서는 3/4 시공간블록 코드를 사용한 협력적인 신호 구조를 제안하고, 수학적으로 증명한 전송 전력 제한과 네트워크 기하학을 이용하여 이론상의 BER (Bit Error Ratio) 표현을 유도한다. 다양한 환경에서의 시뮬레이션과 잘 증명된 수학적인 결과는 파트너가 적절한 위치에 있을때, 직접 전송보다 매우 성능이 우수한 협력 통신을 증명하였다.

Performance Analysis of a Novel Cooperative Communications Signaling Structure in CDMA network

Ho Van Khuong,Hyung-Yun Kong,Doohee Nam 대한전자공학회 2005 ITC-CSCC :International Technical Conference on Ci Vol.2005 No.4

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.