• The Journal of Korean Institute of Communications and Information Sciences
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• v.39B no.11
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• pp.803-809
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• 2014

In this paper, we investigate physical layer security in a cooperative cognitive radio networks (CRN) with a relay selection in the presence of a primary user and an eavesdropper. To protect the CRN from wiretapping by the eavesdropper, we propose employing an opportunistic relay selection scheme and multiple antennas at the destination that work based on the availability of channel state information at the receivers. Under these configurations, we derive an exact closed-form expression for the secrecy outage probability of the CRN, and also derive an asymptotic probability. Numerical results will be presented to verify the analysis.

• Journal of Communications and Networks
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• v.10 no.2
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• pp.175-185
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• 2008

In this paper, we investigate the effect of cooperative and selection relaying schemes on multiuser diversity in downlink cellular systems with fixed relay stations (RSs). Each mobile station (MS) is either directly connected to a base station (BS) and/or connected to a relay station. We first derive closed-form solutions or upper-bound of the ergodic and outage capacities of four different downlink data relaying schemes: A direct scheme, a relay scheme, a selection scheme, and a cooperative scheme. The selection scheme selects the best access link between the BS and an MS. For all schemes, the capacity of the BS-RS link is assumed to be always larger than that of RS-MS link. Half-duplex channel use and repetition based relaying schemes are assumed for relaying operations. We also analyze the system capacity in a multiuser diversity environment in which a maximum signal-to-noise ratio (SNR) scheduler is used at a base station. The result shows that the selection scheme outperforms the other three schemes in terms of link ergodic capacity, link outage capacity, and system ergodic capacity. Furthermore, our results show that cooperative and selection diversity techniques limit the performance gain that could have been achieved by the multiuser diversity technique.

• Journal of Communications and Networks
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• v.15 no.5
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• pp.469-475
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• 2013

In this paper, we propose an efficient interference management technique which controls the number of resource blocks (or subcarriers) shared with other cells based on statistical interference levels among cells. The proposed technique tries to maximize average throughput of a femto cell user under a constraint on non-real time control of a femto cell network while guaranteeing a target throughput value of a macro cell user. In our proposed scheme, femto cells opportunistically use resource blocks allocated to other cells if the required average user throughput is not attained with the primarily allocated resource blocks. The proposed method is similar to the underlay approach in cognitive radio systems, but resource block sharing among cells is statistically controlled. For the statistical control, a femto cell sever constructs a table storing average mutual interference among cells and periodically updates the table. This statistical approach fully satisfies the constraint of non-real time control for femto cell networks. Our simulation results show that the proposed scheme achieves higher average femto user throughput than conventional frequency reuse schemes for time varying number of users.

• Journal of Communications and Networks
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• v.17 no.4
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• pp.394-405
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• 2015

This paper considers a wideband cognitive radio network which can simultaneously sense multiple narrowband channels and thus aggregate the detected available channels for transmission and proposes a novel cognitive radio system that exhibits improved sensing throughput and can save power consumption of secondary user (SU) compared to the conventional cognitive radio system studied so far. More specifically, under the proposed cognitive radio system, we study the problem of designing the optimal sensing time and power allocation strategy, in order to maximize the ergodic throughput of the proposed cognitive radio system under two different schemes, namely the wideband sensing-based spectrum sharing scheme and the wideband opportunistic spectrum access scheme. In our analysis, besides the average interference power constraint at primary user, the average transmit power constraint of SU is also considered for the two schemes and then a subgradient algorithm is developed to obtain the optimal sensing time and the corresponding power allocation strategy. Finally, numerical simulations are presented to verify the performance of the two proposed schemes.

• Journal of Communications and Networks
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• v.17 no.3
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• pp.267-274
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• 2015

Due to the difficulty of coordination in the cellular uplink, it is a practical challenge how to achieve the optimal throughput scaling with distributed scheduling. In this paper, we propose a distributed and opportunistic user scheduling (DOUS) that achieves the optimal throughput scaling in a single-input multiple-output interfering multiple-access channel, i.e., a multi-cell uplink network, with M antennas at each base station (BS) and N users in a cell. In a distributed fashion, each BS adopts M random receive beamforming vectors and then selects M users such that both sufficiently large desired signal power and sufficiently small generating interference are guaranteed. As a main result, it is proved that full multiuser diversity gain can be achieved in each cell when a sufficiently large number of users exist. Numerical evaluation confirms that in a practical setting of the multi-cell network, the proposed DOUS outperforms the existing distributed user scheduling algorithms in terms of sum-rate.

• Journal of Communications and Networks
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• v.17 no.2
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• pp.172-183
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• 2015

To provide an efficient link maintenance approach, we propose a cross layer medium access control (LM-MAC) protocol for multichannel cognitive radio ad hoc networks. Link establishment and reliable transmission are two key problems for a perfect link maintenance mechanism. Since the cognitive user (CU) pairs have to reestablish their links each frame, in the proposed MAC protocol, three different access modes are designed to guarantee transmission efficiency in continuous frames. To enhance the transmission reliability, each CU will create a father spectrum list (FSL) after joining in the network. FSL is divided into three groups of sub-channels with different functions to compensate the packet loss caused by the primary users' appearance and the deep fading. Meanwhile, since the transmitter and the receiver will share the same FSL, periodical cooperative sensing is adopted to further optimize the former problem. Finally, compared with the existing opportunistic multichannel (OMC)-MAC protocol, the proposed LM-MAC protocol achieves better system performance in terms of saturation throughput, continuity and access delay.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.11
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• pp.2218-2229
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• 2015

Recently Wi-Fi Alliance defined Wi-Fi direct, which can communicate through a direct connection between the mobile device anytime, anywhere. In Wi-Fi direct, all devices are categorized by group of owner (GO) and client. Since portability is emphasized in Wi-Fi direct devices, it is essential to control the energy consumption of a device very efficiently. In order to avoid unnecessary power consumed by GO, Wi-Fi direct standard defines two power management schemes: Opportunistic power saving scheme and Notice Of Absence (NOA) scheme. But, these two schemes do not consider the traffic pattern, so we cannot expect high energy efficiency. In this paper, we suggest an algorithm to enhance the energy efficiency of Wi-Fi direct power saving, considering the characteristics of multimedia video traffic. Proposed algorithm utilizes the statistical distribution for the size of video frames and adjusts the length of awake interval dynamically. Also, considering the inter-dependency among video frames, the proposed algorithm assigns priorities to video frames and ensures that a video frame with high priority is transmitted with higher probability than other frames with low priority. Simulation results shows that the proposed method outperforms the traditional NOA in terms with average delay and energy efficiency.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.11A
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• pp.1013-1020
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• 2008

Cognitive radio(CR) techniques have been considered as a strong solution of frequency scarcity due to the limitation of frequency resource, In this paper, an orthogonal beamforming method is proposed that enables cognitive radio systems to coexist with a primary users' system in the same spectrum and region without causing interference to primary systems. The orthogonal multiple beams for multi-users of CR systems are obtained through the generation of orthogonal subspaces from primary users' channel state information. To increase the sum-rate of the CR systems, the proposed scheme adopts opportunistic radio resource allocation techniques. Simulation results provides that the proposed scheme achieves 2.72bps/Hz, the sum-rate capacity at 20 cognitive users in an area with two antennas at the cognitive radio basestation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.8A
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• pp.712-722
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• 2011

Wireless body area network(WBAN) provide communication service in the vicinity of the body. Since WBANs utilize both MICS frequency band for implant medical applications and ISM frequency band for medical and consumer electronics(CE) applications. Therefore, MAC protocols in WBAN should be designed considering flexibility between medical and CE applications. In this paper, we identify the requirements of WBAN MAC protocols and propose a WBAN MAC protocol which satisfies the requirements. In other to provide transmission flexibility for various applications. we present the dynamic CFP allocation and opportunity period. Extensive simulation result show that the proposed protocol achieves improved throughput and latency in WEAN environment cimpared with IEEE 802.15.4.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.7B
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• pp.536-549
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• 2012

An ad hoc network consists of nodes with limited energy. Therefore, the data transmission can fail abruptly due to lack of energy of transmitting node. A previous work PAMP proposed to build multiple low-energy paths to support stable packet delivery exploiting the nodes with low energy. It has energy-reservation scheme and multi-path selection scheme for stationary wireless ad hoc networks. In this paper, we propose an extended version of PAMP by incorporating network coding opportunity in path selection process. The simulation results show that our proposed scheme shows better packet delivery ratio and lower energy consumption compared with PAMP and a legacy energy-aware multipath routing protocol REAR.