Jemin Lee

Outage Probability of Cognitive Relay Networks with Interference Constraints

This paper evaluates the outage probability of cognitive relay networks with cooperation between secondary users based on the underlay approach, while adhering to the interference constraint on the primary user, i.e., the limited amount of interference which the primary user can tolerate. A relay selection criterion, suitable for cognitive relay networks, is provided, and using it, we derive the outage probability. It is shown that the outage probability of cognitive relay networks is higher than that of conventional relay networks due to the interference constraint, and we quantify the increase. In addition, the outage probability is affected by the distance ratio of the interference link (between the secondary transmitter and the primary receiver) to the relaying link (between the secondary transmitter and the secondary receiver). We also prove that cognitive relay networks achieve the same full selection diversity order as conventional relay networks, and that the decrease in outage probability achieved by increasing the selection diversity (the number of relays) is not less than that in conventional relay networks.

Reliability Improvement Using Receive Mode Selection in the Device-to-Device Uplink Period Underlaying Cellular Networks

A new interference management scheme is proposed to improve the reliability of a device-to-device (D2D) communication in the uplink (UL) period without reducing the power of cellular user equipment (UE). To improve the reliability of the D2D receiver, two conventional receive techniques and one proposed method are introduced. One of the conventional methods is demodulating the desired signal first (MODE1), while the other is demodulating an interference first (MODE2), and the proposed method is exploiting a retransmission of the interference from the base station (BS) (MODE3). We derive their outage probabilities in closed forms and explain the mechanism of receive mode selection which selects the mode guaranteeing the minimum outage probability among three modes. Numerical results show that by applying the receive mode selection, the D2D receiver achieves a remarkable enhancement of outage probability in the middle interference regime from the usage of MODE3 compared to the conventional ways of using only MODE1 or MODE2.

Sensing Performance of Energy Detector With Correlated Multiple Antennas

This paper describes the derivation of detection and false-alarm probabilities for energy detectors in cognitive radio networks when a sensing node of the secondary system has correlated multiple antennas. The sensing performance degradation due to the antenna correlation is then investigated based on the performance analysis. The conclusions of the analysis are verified by numerical simulation results.

Capacity enhancement of secondary links through spatial diversity in spectrum sharing

Previous investigations on capacity of secondary users in spectrum-sharing environments have determined the capacity of a secondary link based on the interference power threshold set at the primary receiver. In contrast to these previous works, we show that the capacity of a secondary link is determined based on a geographical relationship expressed as the ratio of the distance between the primary receiver and secondary transmitter to the distance between the secondary transmitter and receiver. Proceeding from that and in an effort, to enhance the capacity of the secondary user, which is limited by this distance-ratio, we adopt a secondary transmitter with M antennas. Furthermore, we analyze the capacity achieved using a simple antenna selection process.

Capacity of Secondary Users Exploiting Multispectrum and Multiuser Diversity in Spectrum-Sharing Environments

In spectrum-sharing environments, secondary users are permitted to share the primary users spectrum only if limited interference to the primary user can be guaranteed. Hence, the capacity of the secondary link is limited by interference constraints given by the primary user. This fact motivated us to investigate selection diversity as a way of enhancing secondary link capacity. Selection diversity in conventional licensed wireless communication systems can be achieved only by selecting the user with the strongest channel gain. On the other hand, spectrum-sharing environments allow the secondary user to select not only the best secondary receiver with the strongest channel gain multiuser diversity (MUD) but the best primary spectrum with the weakest interference link gain multispectrum diversity (MSD) as well, thus doubling the opportunities for the secondary user. In this paper, we analyze the capacity gain of a secondary user exploiting MSD and MUD in spectrum-sharing environments in the form of a closed-form expression in a Rayleigh-fading channel. We then separately extract the MSD and MUD capacity gains, which results in an asymptotic capacity expression. Our results show that MSD and MUD play different roles in capacity enhancement in the spectrum-sharing environment, with capacity enhancement for secondary receivers as a whole and transmit capacity enhancement for the secondary transmitter.

Achievable Transmission Capacity of Secondary System in Cognitive Radio Networks

This paper evaluates the achievable transmission capacity of the secondary system in cognitive radio networks, defined by the spatial density of successful transmissions while guaranteeing the target outage probabilities of the secondary and the primary systems. By using stochastic geometry, the effects of the spatial densities and the transmission powers on the achievable transmission capacity is presented. Subsequently, the optimal spatial density of the secondary system and the optimal transmission power ratio of the primary system to the secondary system are derived. Furthermore, the maximum achievable transmission capacity is defined using the derived optimal values. From the theoretical results, it is shown that the optimal transmission power ratio is affected by not the density of the primary system, but the system parameters including the target outage probability. In addition, the achievable transmission capacity of the secondary system decreases as the spatial density of the primary system increases at the decreasing rate determined by the system parameters of the primary system.

QoS-guaranteed Transmission Scheme Selection for OFDMA Multi-hop Cellular Networks

In this paper, a problem for efficient use of subcarriers in downlink OFDMA multi-hop cellular networks is studied based on quality of service (QoS) guarantee of each mobile station (MS). We consider three possible transmission schemes in multi- hop networks: single-hop, multi-hop, and multi-hop with spatial reuse of subcarriers. We verify that the required number of sub-carriers to guarantee the target BER and the target data rate is changed according to the transmission scheme and QoS requirements as well as the channel condition. Based on this result, we propose a new transmission scheme selection algorithm to select the best performance scheme for the efficient utilization of the limited subcarriers. Through the numerical results, the performance of this algorithm is demonstrated by the number of supported MSs in the system and the blocking probability.

QoS-guaranteed transmission mode selection for efficient resource utilization in multi-hop cellular networks

This paper considers the problem of efficient usage of subcarriers in downlink orthogonal frequency-division multiple access (OFDMA) multi-hop cellular networks. Multi-hop transmission can either save or waste subcarriers depending on quality of service (QoS) requirements, interference from other cells, and the location of a given mobile station (MS). Preventing unnecessary usage of multi-hop transmission requires the use of a transmission mode selection (TMS) scheme, and we propose two types of TMS in this paper. The first of these is distance-based TMS (TMS-D), which determines the transmission mode based on the MSiquests location, and the second is subcarrier-based TMS (TMS-S), which selects whichever transmission mode uses fewer subcarriers. Numerical results on blocking probability demonstrate that TMS improves overall subcarrier usage efficiency, meaning that more MSs can be supported with low blocking probability within a cell. Furthermore, the performance of TMSS, even given its higher complexity, is shown to be superior to that of TMS-D.

Throughput Analysis and Optimization of Sensing-Based Cognitive Radio Systems With Markovian Traffic

This paper analyzes the secondary user throughput of a sensing-based cognitive radio system with Markovian traffic. Under the Markovian traffic assumption, imperfect packet capture, which occurs upon the random arrival of primary user packets, considerably affects the secondary user throughput. Considering both the primary user packet capture and energy detection of the captured packet, we derive the secondary user throughput. Additionally, using the tradeoff between the sensing accuracy and the secondary user throughput, we solve a joint optimization dealing with both the sensing duration and the sensing period to maximize the secondary user throughput with an interference constraint for the primary user.

Sensing threshold control for fair coexistence of heterogeneous systems in open spectrum

A critical problem in open-spectrum communications is fairness with respect to the coexistence of heterogeneous systems with different resource units and traffic models. In addition, the sensing performances of different systems can also lead to unfair resource utilization between systems. To address this problem, we derive a continuous-time Markov chain model to show the effect of sensing performance on system coexistence. The analysis derived from this model is then used as the basis for a sensing threshold control (STC) scheme to achieve fairness. The proposed STC determines the sensing threshold for each system as a way of balancing resource utilization among systems, while guaranteeing target detection probability. Numerical results on the amount of resource utilization by each system demonstrate that the proposed STC achieves a full degree of fairness.