Ozgur Ergul

This paper inspects a reconfigurable intelligent surface (RIS)-aided non-orthogonal multiple access (NOMA) wireless network where users convey to the base station (BS) through the support of RIS. The study investigates the impact of channel estimation error (CEE) and hardware impairment (HWI) at BS and end users in this RIS-enabled NOMA scheme. Additionally, phase errors in the RIS and the influence of imperfect successive interference cancellation (SIC) on NOMA users are incorporated into the analysis. Exact closed-form analytical expressions for the outage probability (OP) and ergodic rate (ER) of the NOMA users are derived to evaluate system performance. Furthermore, the asymptotic OP was also examined. The theoretical findings are validated through simulations, which demonstrate that 1) HWI and CEE significantly affect the OP of the NOMA near user (NU) more than far user (FU); 2) CEE impacts the OP of FU more than HWI; 2) the effect of phase noise at the RIS on OP is substantially less significant than that of transceiver HWI and imperfect SIC; 4) the number of RIS reflecting elements plays a crucial role in enhancing the system performance of the RIS-assisted NOMA system; 5) varying power allocation coefficient between users can improve user fairness.

Wireless sensor nodes equipped with a cognitive radio, i.e., cognitive radio sensor networks (CRSN), can access the spectrum in an opportunistic manner and co-exist with licensed users to mitigate the crowded spectrum problem and provide ubiquitous remote event monitoring and tracking for cyber-physical systems. While opportunistic spectrum access (OSA) increases spectrum utilization, it also aggravates the challenges related to reliability and energy-efficiency for event reporting in CRSN. So far, there has been very limited research on intermittent connectivity and spectrum heterogeneity caused by spectrum sensing and spectrum handoff. In this paper, a novel transport layer protocol for CRSN, spectrum-aware energy-adaptive reliable transport (SERT) protocol is presented to enable energy-adaptive collaborative event sensing in spectrum-scarce cyber-physical systems. To the best of our knowledge, this is the first attempt to specifically devise a reliable event transport scheme for CRSN. SERT adapts to spectrum opportunities to provide spectrum-aware event detection reliability in CRSN while maximizing network lifetime. Extensive simulation experiments reveal that SERT reduces fluctuations in reliability caused by spectrum mobility and reduces energy consumption due to spectrum opportunity losses.

In light of recent natural catastrophes and terrorist activities, it has become evident that new architectural approaches are needed for next generation public protection and disaster relief networks. These architectures should be adaptable to the conditions at the event site and resilient enough to operate under adverse conditions in an emergency. Furthermore, they should enable rapid gathering of crucial event data and its delivery to the responder units at the site as well as the command and control center that is off-site. In this article, we first examine the state of the art in areas related to communication in PPDR systems, and discuss the open research issues for each topic. Then we propose a novel architecture that meets the aforementioned requirements and relies on a novel device called an ICG. An ICG enables flexible use of the spectrum and facilitates data gathering from all lower-tier devices and relays this data to relevant units through the higher-tier public or commercial backhaul networks. Finally, we provide some results that justify the need for these devices in emergency scenarios.

Summary: In the near future, billions of wireless devices are expected to be operational. To enable the required machine to machine communications, two major problems must be addressed. How to obtain the required spectrum efficiency, and how to deliver the required power to these devices. The most promising answers to these questions are cognitive radio and energy harvesting, respectively. Energy harvesting enables deployment of sensors and devices without having to worry about their battery lifetime. Cognitive radio increases the utilization of spectrum by accessing unused spectrum dynamically. Energy harvesting from electromagnetic waves is suitable for these low power, low cost devices used in machine to machine communications because only minimal additional hardware is required for such energy harvesting. With this idea as the starting point, we first present an analysis on how much throughput can be obtained from a cognitive, electromagnetic energy harvesting wireless network. Then, we show when and how cooperation among network nodes may increase performance. We believe that our results will provide insight for the development of future cooperative cognitive energy harvesting networks. Copyright © 2015 John Wiley & Sons, Ltd.

As the demand for wireless bandwidth rapidly increases, alternative methods to radio frequency-based communication are investigated to overcome the limited bandwidth problem. Visible light communication (VLC) using light emitting diodes (LEDs) is one of these alternatives. LEDs are estimated to replace the incandescent bulbs within the decade. Since, LEDs can be intensity modulated faster than the human eye can detect, illumination and communication can both be provided by the same lighting system. Indoors communication constitutes 70% of the overall traffic, and VLC is a promising technology to complement Wi-Fi and cellular wireless systems. However, proper handover mechanism should be developed for VLC to be a complete indoors solution. In this paper, we present two soft handover methods for VLC. Simulation results indicate our solutions provide higher data rate for both the overall system and individual users in the handover region.

The number of applications that use industrial, scientific and medical (ISM) radio bands increase every day, creating interference problem for the wireless sensor networks (WSN) that generally operate on these bands. Cognitive radio sensor network (CRSN) has been proposed as a promising solution to this problem. However, since sensor nodes are energy-constrained devices energy efficient spectrum sensing methods are needed for CRSN. To address this need, we propose a novel cooperative coarse sensing scheme for CRSN (CC4C). CC4C is based on sequential sensing, thus, it is simple and fast. Simulation results show that CC4C incurs less sensing delay and provides significant energy conservation compared to energy detection based coarse sensing schemes, and single stage sensing schemes where no coarse sensing is performed.

Discontinuous reception (DRX) is supported in 3GPP Long Term Evolution (LTE) to reduce power consumption of user equipments (UEs). Power conservation achieved via DRX can be further increased with a packet scheduler that takes DRX states into consideration. Thus, in addition to quality of service (QoS) and fairness factors, which have been the main focus so far in scheduler design, energy efficiency must also be considered in scheduling. In this paper, we introduce a DRX and QoS-aware uplink packet scheduling algorithm (DQEPS) for LTE networks. One of the main reasons of poor DRX utilization is the continuous uplink packet traffic generated by applications working in the background. Accordingly, we first layout the cumulative distribution functions (CDF) of inter-packet arrival durations constructed by inspecting uplink packet transmission for various applications. Then, we form metrics for each bearer using these CDFs along with the DRX states, QoS parameters, channel conditions, and the buffer status of the bearers. Using these metrics, we develop a scheduling algorithm for the uplink, which aims to maximize power conservation of DRX mechanism by scheduling packets in a way that tries to minimize ON duration, while meeting the QoS requirements. Performance evaluations indicate that DQEPS reduces power consumption significantly compared to the previously proposed methods for LTE.

Cognitive radio sensor network (CRSN) is an emerging sensor networking paradigm that aims to incorporate opportunistic spectrum access capability to the wireless sensor networks. Since sensor nodes are energy-constrained devices, design of efficient spectrum sensing schemes is imperative for the implementation of CRSNs. In order to address this need, a cooperative spectrum sensing scheme (CSS), specifically designed for CRSNs, is presented in this paper. CSS aims to minimize power consumption and delay during spectrum sensing, while meeting the performance requirements in terms of accuracy with minimal complexity. Simulation results indicate that significant power savings can be achieved with the proposed solution.