Cognitive Radio (CR) is a new paradigm which offers a viable solution to deal with the spectrum shortage problem and enhance the spectrum utilization. In Cognitive Radio Ad Hoc Networks (CRAHNs), data routing is one of the most challenging tasks due to frequent topology changes and intermittent connectivity caused by the activity of Primary Users (PUs). This paper proposes a joint path and spectrum diversity based routing protocol with an optimized path selection for CRAHNs, referred to the Enhanced Dual Diversity Cognitive Ad-hoc Routing Protocol (E-D2CARP). The Expected Path Delay (EPD) routing metric used in the protocol for path decision is also introduced. The protocol utilizes the joint path and spectrum diversity and circumvents the PU regions during path establishment phase in order to make the transmission path less vulnerable to the impact of PU activities and provide efficient route recovery in presence of path failures resulting from PU activities. The performance evaluations are conducted through simulations using the NS-2 simulator. Simulation results obviously demonstrate that the E-D2CARP can achieve better performance in terms of average throughput, packet loss, average end-to-end delay, and average jitter as compared to the recently proposed D2CARP protocol in identical scenarios.
Underwater Optical Wireless Communication (UOWC) is identified as a promising technology because it offers higher bandwidth than acoustics and radio frequency techniques. This paper investigates the performance of light wave propagation for UOWC through experimental approach. An experimental set-up is developed consists of a transmitter, receiver and a glass chamber to emulate the water channel. Three types of water including clear, sea and cloudy are used to investigate their interaction with the light emitted by light emitting diode (LED) and laser diode. The geometrical loss (GL) analysis shows the white LED suffered a severe GL (GL<<1) as the transmission link increases due to the wide viewing angle while green and yellow LED obtained an equal GL due to the same size of viewing angle. However, red laser does not experience any GL. Therefore, the received power by red laser is 35% higher than by green LED. The analysis deduces that the estimated attenuation coefficient c had an increase of 15% and 55% for green LED and red laser respectively when the UOWC medium changed from clear water to sea water. This study contributes to identify the potential and limitations of different parameter design in order to optimize UOWC performance.
Idle Sense (IS) is a simple yet efficient access method to provide optimized throughput and fairness in wireless local area networks (WLAN). Each station dynamically controls its contention window (CW) size by monitoring the mean number of idle slots between transmission attempts. In general this quantity is subject to estimation error that is related to the number of transmissions, M , over which it is measured. Previous works in IS assumed all stations use similar CW sizes to contend the channel; an assumption that is not always valid in reality. Therefore, this paper studies the behaviour of the scheme when stations have different CW sizes. An analysis shows that bias caused by the additive increase multiplicative decrease (AIMD) algorithm used to control the CW sizes, combined with varying lengths of M can cause a fairness problem. Two classes of stations develop, with the first class gaining most of the channel bandwidth while the second class stays idle.
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