• 대한임베디드공학회논문지
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• 제15권4호
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• pp.189-196
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• 2020

In cognitive radio networks, secondary transmitters should cease its transmission immediately on detecting of primary transmission in the spectrum they are accessing. Then they should exploit another idle spectrums and handoff to the newly found idle spectrums, which is called spectrum rendezvous. With regards to spectrum rendezvous, most of related work presume the existence of dedicated common control channel used by secondary users for exchanging the information of idle spectrums. However, this presumption is not feasible in real world cognitive radio scenario. Therefore we address a blind spectrum rendezvous scheme with no need of separate control channel. Furthermore we consider maintaining one or more extra spectrums (channels) to expedite the spectrum rendezvous. Our scheme lets secondary users maintain extra spectrums by exchanging the spectrum information periodically during normal communications. The one of the extra spectrums are regarded as a candidate spectrum that the users can handoff to on detecting the primary transmission. We evaluate that our blind scheme can help to reduce the rendezvous delay in a real world cognitive radio environments with USRPs.

• Journal of Computing Science and Engineering
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• 제7권2호
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• pp.81-88
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• 2013

In cognitive radio or dynamic spectrum access networks, a rendezvous represents meeting two or more users on a common channel, and negotiating to establish data communication. The rendezvous problem is one of the most challenging tasks in cognitive radio ad hoc networks. Generally, this problem is simplified by using two well-known mechanisms: the first uses a predefined common control channel, while the second employs a channel hopping procedure. Yet, these two mechanisms form a life cycle, when they simplify the rendezvous problem in cognitive radio networks. The main purpose of this paper is to point out how and why this cycle forms.

• 융합보안논문지
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• 제21권1호
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• pp.209-219
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• 2021

허가 된 무선 채널 스펙트럼의 혼잡 문제를 해결하는 방법으로 인지무선네트워크(CRN, Cognitive Radio Networks)가 많은 주목을 받고 있다. 보조 사용자 (SU) 간의 랑데뷰를 보장하기 위해 여러 알고리즘들이 존재하며 점점 더 효율적인 알고리즘들이 개발되어 다양한 시나리오에서 빠른 랑데뷰가 가능해지고 있다. 동시에 개선되고 있는 랑데뷰 알고리즘을 공격하기 위해 여러 재밍 알고리즘들이 개발되고 있다. CRN의 목표는 강력한 알고리즘으로 이러한 재밍 공격을 최소화 하여 랑데뷰를 보장하는 것입니다. 이 논문에서는 다양한 재밍 기술들을 분류하고 잘 알려진 여러 랑데뷰 알고리즘들의 재밍공격 상황하에서의 성능을 분석하였다.

• 한국통신학회논문지
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• 제32권10B호
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• pp.621-629
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• 2007

CR 네트워크에서의 무선 채널은 일반적으로 우선 사용자에 의해 점유되어 사용하지 못하는 채널과 점유되지 않은 사용 가능한 채널로 분류된다. 하지만 이러한 채널 분류는 CR 노드와 우선 사용자의 시간적 특성만 고려하고, 공간적 특성을 고려하지 않아 유휴 주파수 자원 확보에 비효율적이다. 이에 본 논문에서는 중앙 집중형 CR 네트워크 환경에서 기존방식의 단점을 극복하여 유휴 주파수 자원을 보다 효율적으로 확보할 수 있는 새로운 채널 분류 방법을 제안하며, 제안하는 채널 분류 방법의 효율성을 수학적으로 분석한다. 이와 더불어 정의한 채널분류를 이용하여 주파수 자원이 매우 동적으로 변화하는 환경에서도 상하향 제어 채널 확보를 용이하게 확보할 수 있는 랑데부 알고리즘을 제안한다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제12권6호
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• pp.2513-2533
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• 2018

In cognitive radio technology, the overall efficiency of communications systems can be improved without allocating additional bands by allowing a secondary system to utilize the licensed band when the primary system, which has the right to use the band, does not use it. In this paper, we propose a fast and reliable common channel initialization protocol without any exchange of initialization messages between the cluster head and the member nodes in cognitive ad-hoc networks. In the proposed method, the cluster and member nodes perform channel-based spectrum sensing. After sensing, the cluster head transmits a system activation signal through its available channels with a predetermined angle difference pattern. To detect the cluster head's transmission channels and to join the cluster, each member node implements fast Fourier transform (FFT) and computes autocorrelation for the angle difference sequence of the received signal patterns. This is compared to the predetermined reference angle difference pattern. The join-request and channel-decision procedures are presented in this paper. Performance evaluation of the proposed method is presented in the simulation results.

• 한국통신학회논문지
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• 제37C권11호
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• pp.1112-1128
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• 2012

인지 무선 환경에서의 부 사용자들은 주 사용자가 일시적으로 사용하지 않는 채널을 기회적으로 이용하여 통신을 수행한다. 이러한 통신은 부 사용자 상호간에 접근이 가능한 공통 채널을 통해 이루어지는데, 부 사용자들의 통신을 위한 공통 채널은 주 사용자의 채널 사용 현황을 바탕으로 동적으로 변경될 수 있다. 최근 인지무선통신을 위한 연구가 많이 진행 되고 있으나, 공통 채널을 구성하기 위한 세부적인 절차에 대한 연구가 부족한 실정이다. 본 논문에서는 인지무선 네트워크 환경에서 군사용 무전기의 점대점 통신을 위한 신속하고 신뢰성 있는 동적 공통 채널 설정 및 복원 방법에 대한 방법과 절차를 제안하고, 공통 채널 설정과 복원 과정을 위한 패킷 교환 시간, 채널 요청 대기 시간, 랑데부 시간과 같은 시간 파라미터들을 고려하여, 각각의 세부적인 절차들을 기술하였다. 또한, 수치 해석을 통해서 채널 설정 및 복원 지연 시간과 처리량 등을 도출하여 제안된 방법의 성능을 평가하였다.

• 천문학회보
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• 제46권2호
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• pp.58.1-59
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• 2021

Exploration of asteroids' internal structure is essential for understanding their evolutional history. It also provides a fundamental information about the history of coalescence and collision of the solar system. Among several models of the internal structures, the rubble-pile model, confirmed by the near-Earth asteroid (25143) Itokawa by Hayabusa mission [1], is now widely regarded as the most common to asteroids with size ranging from 200 m to 10 km [2]. On the contrary, monolithic and core-mantle structures are also possible for small asteroids [3]. It is, however, still challenging to look through the interior of a target object using remote-sensing devices. In this presentation, we introduce our ongoing research conducted at Seoul National and propose an idea to infer the internal structure of Apophis using available instruments. Itokawa's research provides an important benchmark for Apophis exploration because both asteroids have similar size and composition [4][5]. We have conducted research on Itokawa's evolution in terms of collision and space weathering. Space weathering is the surface alteration process caused by solar wind implantation and micrometeorite bombardment [6]. Meanwhile, resurfacing via a collision acts as a counter-process of space weathering by exposing fresh materials under the matured layer and lower the overall degree of space weathering. Therefore, the balance of these two processes determine the space weathering degrees of the asteroid. We focus on the impact evidence on the boulder surface and found that space weathering progresses in only 100-10,000 years and modifies the surface optical properties (Jin & Ishiguro, KAS 2020 Fall Meeting). It is important to note that the timescale is significantly shorter than the Itokawa's age, suggesting that the asteroid can be totally processed by space weathering. Accordingly, our result triggers a further discussion about why Itokawa indicates a moderately fresh spectrum (Sq-type denotes less matured than S-type). For example, Itokawa's smooth terrains show a weaker degree of space weathering than other S-type asteroids [7]. We conjecture that the global seismic shaking caused by collisions with >1 mm-sized interplanetary dust particles induces granular convection, which hinders the progression of space weathering [8]. Note that the efficiency of seismic wave propagation is strongly dependent on the internal structure of the asteroid. Finally, we consider possible approaches to investigate Apophis's internal structure. The first idea is studying the space weathering age, as conducted for Itokawa. If Apophis indicates a younger age, the internal structure would have more voids [9]. In addition, the 2029 close encounter with Earth provides a rare natural opportunity to witness the contrast between before and after the event. If the asteroid exhibits a slight change in shape and space weathering degree, one can determine the physical structure of the internal materials (e.g., rubble-pile monolithic, thick or thin regolith layer, the cohesion of the materials). We will also consider a possible science using a seismometer.