In this paper, we consider cooperative horizontal and vertical underwater visible light communication (UWVLC) systems employing best relay selection. In the vertical UWVLC system, the source is placed at the sea surface and the destination is placed at a depth of 60 m. The link between the source and the destination is modeled as concatenated layers considering inhomogeneous environmental conditions. The underwater parameters change with depth, causing a variable refractive index, which leads to non-uniform optical turbulence. The horizontal communication link is analyzed for two different levels of turbulence at 10 m and 50 m depths. Turbulence is modeled using a log-normal distribution, whose statistical parameters depend on the temperature and salinity at different depths. We have also taken into account the path losses caused by absorption and scattering, and carried out a comprehensive performance comparison between the horizontal and the vertical UWVLC systems. Insights show that compared to horizontal links, vertical links need an extra relay in order to achieve the same level of matching performance. Moreover, it is shown that the horizontal system’s outage performance improves with depth.

<!-- *** Custom HTML *** --> Visible light communication (VLC) has the ability to provide a high data rate up to Mbps in underwater environments for real-time communication systems. In underwater VLC (UWVLC), two major impairments are the turbulence-induced fading due to variation of salt and temperature of seawater and incremental path loss with distance due to absorption and scattering. We consider these two impairments and derive the closed form expressions for average symbol error probability (ASEP), asymptotic relative diversity order, and ergodic capacity for UWVLC dual-hop cooperative communication system. We consider multiple receiver branches with selection combining to combat the effect of fading. The impact of temperature on the fading parameters and system performance is highlighted. We conduct a comparative analysis of ASEP for four-pulse amplitude modulation and four-square quadrature amplitude modulation schemes and draw useful insights. We prove the accuracy of the derived analytical expression using Monte Carlo simulations.

Visible light communication (VLC) has gained attention due to its ability to provide a high data rate in underwater communication. We consider an underwater VLC (UWVLC) system and vertical propagation through the column of water with L layers (channels) and M relays between the transmitter and the receiver. We consider the real-time adverse effects of depth-dependent weak turbulence and path loss. We model each of the L channels with a log-normal distribution, where its statistical parameters mean and variance are functions of the parameters representing the adverse effects of the turbulence. For this concatenated L layered UWVLC, using Gauss–Hermite integral technique, we derive closed-form expressions of outage probability and average symbol error probability taking square quadrature amplitude modulation and rectangular quadrature amplitude modulation modulations with different configurations and assuming imperfect channel state information available at the receiver for detection. We have observed that there is a drastic degradation in the performance as the depth (distance between the transmitter and the receiver) is increased. However, the severity can be alleviated by installing multiple relays between the transmitter and the receiver.

LEACH (Low-Energy Adaptive Clustering Hierarchy) is a distributed routing protocol that is used in wireless sensor networks to help reduce energy consumption. The protocol utilizes a hybrid routing scheme in which each node is assigned a random number between one and the maximum number of nodes in the network. The nodes then form clusters based on their numbers, and each node acts as a cluster head. The cluster heads then communicate messages to the other nodes in the cluster. This allows data to be sent and received with a minimum amount of energy. By utilizing this hybrid routing scheme, LEACH is able to improve energy efficiency by reducing the number of transmissions made in the network and also by decreasing the number of hops required to send a message.

Efficient resource allocation is one of the most challenging facet in small cell deployment. With continuous research, many algorithms have been developed in this regard. The proposed algorithm is for small cell downlink on a large scale with the context of orthogonal frequency division multiple access. The proposed algorithm has 4 stages, one of which also incorporates device-to-device (D2D) communication inside a small cell. The algorithm uses distributed resource allocation for downlink model based on user demand. A simulation environment is developed to study the quantitative effects of proposed algorithm. Poisson random distribution is used to model the user and access point locations. The interference analysis to determine maximum distance of D2D connection within a small cell is also provided. Performance comparisons between fixed resource allocation, partially distributed allocation and the proposed, proportional resource allocation algorithm with and without D2D are also shown. The simulations are carried out in MATLAB 2013.