In this paper, we investigate the so-called noise propagation effect in a mixed radio-frequency/ free-space optical (RF/FSO) amplifying-and-forwarding (AF) relaying system that is applied for data transmission in wireless sensor networks. The noise propagation could be essentially severe when battery-charged sensor nodes have very limited transmit power. We provide an exact expression on the cumulative distribution function (CDF) of end-to-end signal-to-noise power ratio (SNR) for a dual-hop mixed RF/FSO AF relaying system. We assume a tightly power-constrained amplifying gain at the relay, which has been usually ignored in existing performance studies for the mixed RF/FSO AF system. It however should be considered to properly evaluate the noise propagation effect especially if the relaying power is not infinite or the sensor has a poor budget in transmit power. We apply the derived exact CDF to evaluate the system performances such as outage probability, average bit-error rate, and ergodic capacity. Numerical investigation is used to justify that the proposed analysis is exactly matched with the simulation and shows that the performance gap caused by the inclusion of the noise propagation effect is significant (about 2–12%) especially when the SNR per hop is in the medium- or the low-SNR ranges (i.e., at 10–20 dB).

Opportunistic cooperation (OC) dynamically switches between a cooperative mode (CM, using both direct and relay links) and a non-cooperative mode (NM, using only the direct link). OC achieves significant power gain compared with using CM only but needs full channel information. In this paper, we propose threshold-based OC (TBOC) in an amplify-and-forward (AF) relaying system where only partial channel information, so-called staying link information (SLI), is available and the switching-decision is made by comparing the SLI with a threshold to be determined. We consider three variants of TBOC and provide optimal thresholds, respectively. It is proved that the outage probabilities of the proposed TBOCs with the optimal thresholds are exactly the same to that of OC with full channel information. Numerical results are also provided to verify the analysis and to give insights on the outage performance of the TBOC.

This paper presents a price-searching model in which a source node (Alice) seeks friendly jammers that prevent eavesdroppers (Eves) from snooping legitimate communications by generating interference or noise. Unlike existing models, the distributed jammers also have data to send to their respective destinations and are allowed to access Alice’s channel if it can transmit sufficient jamming power, which is referred to as collaborative jamming in this paper. For the power used to deliver its own signal, the jammer should pay Alice. The price of the jammers’ signal power is set by Alice and provides a tradeoff between the signal and the jamming power. This paper presents, in closed-form, an optimal price that maximizes Alice’s benefit and the corresponding optimal power allocation from a jammers’ perspective by assuming that the network-wide channel knowledge is shared by Alice and jammers. For a multiple-jammer scenario where Alice hardly has the channel knowledge, this paper provides a distributed and interactive price-searching procedure that geometrically converges to an optimal price and shows that Alice by a greedy selection policy achieves certain diversity gain, which increases log-linearly as the number of (potential) jammers grows. Various numerical examples are presented to illustrate the behavior of the proposed model.

In this paper, we model and investigate the random access (RA) performance of sensor nodes (SN) in a wireless sensor network (WSN). In the WSN, a central head sensor (HS) collects the information from distributed SNs, and jammers disturb the information transmission primarily by generating interference. In this paper, two jamming attacks are considered: power and code jamming. Power jammers (if they are friendly jammers) generate noises and, as a result, degrade the quality of the signal from SNs. Power jamming is equally harmful to all the SNs that are accessing HS and simply induces denial of service (DoS) without any need to hack HS or SNs. On the other hand, code jammers mimic legitimate SNs by sending fake signals and thus need to know certain system parameters that are used by the legitimate SNs. As a result of code jamming, HS falsely allocates radio resources to SNs. The code jamming hence increases the failure probability in sending the information messages, as well as misleads the usage of radio resources. In this paper, we present the probabilities of successful preamble transmission with power ramping according to the jammer types and provide the resulting throughput and delay of information transmission by SNs, respectively. The effect of two jamming attacks on the RA performances is compared with numerical investigation. The results show that, compared to RA without jammers, power and code jamming degrade the throughput by up to 30.3% and 40.5%, respectively, while the delay performance by up to 40.1% and 65.6%, respectively.

한국전자파학회논문지 (The Journal of Korean Institute of Electromagnetic Engineering and Science)

This paper investigates optimal switching between coordinated Access Points (APs). Since a user has difficulty in gathering the full instantaneous channel information from all the surrounding APs, per-link information between an AP and the user is assumed available at a time and is compared with a predetermined threshold to determine either switching or staying. Unlike the existing studies on switch-and-stay combining techniques, AP switching in this paper deals with nonhomogeneous bandwidth allocation between potential switching branches. Optimal per-link thresholds are provided by assuming Rayleigh fading channels between AP and the user. The expected capacity achieved by the proposed threshold-based switching obviously depends on the order of comparison. With numerical investigation, the comparison sequence determined from a decreasing order of per-link average capacity is shown the best one. Another sequence obtained using an upper bound of the per-link average capacity is also shown promising, which is greatly easy to compute.