# Two Hop Adaptive Vector Based Quality Forwarding for Void Hole Avoidance in Underwater WSNs
## 1 Introduction  
In recent years, underwater wireless sensor networks (UWSNs) have gained much attention due to their growing application horizons like ocean exploration, disaster prevention, oil and gas extraction, technical surveillance for defense, etc. In underwater medium, acoustic waves are used as a means of communication instead of radio waves that absorb in water within the very short distance from the transmitter. Optical waves require alignment of the transmitter and receiver for link establishment. Therefore, acoustic communication is preferred for underwater wireless communication. However, the underwater acoustic communication faces unique challenges like limited available bandwidth, high end-to-end delay, severely impaired channel, etc. [1]. The network architecture of UWSNs comprises randomly-deployed sensor nodes equipped with acoustic links to communicate with each other and with the sink; whereas the sink is equipped with both acoustic and radio modcommunication and the land communication, respectively. Sensor nodes report data to the sink, which are further offloaded to the monitoring center offshore. The propagation speed of the acoustic signal is five-times less than the radio frequency, which results in higher end-to-end delays. Unstable network structure due to water currents in UWSNs is another challenge for traditional routing mechanisms to work in the underwater environment. Other factors like the low data rate are an outcome of limited available bandwidth of the acoustic channel. Similarly, severe channel impairments cause a high bit error rate. Both of these factors together affect the communication efficiency of UWSNs. The aforementioned reasons and constraints restrict the performance of conventional terrestrial schemes in the underwater environment. Moreover, the dynamic nature of UWSNs, where sensor nodes are mobile due to water currents may create void holes. Void hole creation may also occur due to energy fatigue of a sensor node. In these scenarios, if sensor nodes send data towards void regions, it may result in a packet drop that directly affects the packet delivery and energy consumption [2]. Therefore, throughput maximization has been extensively addressed in research because a high packet drop rate leads to high energy consumption. Hence, we need to apply throughput-focused routing techniques for energy conservation and void hole avoidance for reliable data delivery. The proposed schemes, two hop adaptive hop by hop vector-based forwarding (2 hop-AHH-VBF) and quality forwarding adaptive hop by hop vector-based forwarding (QF-AHH-VBF), aim to maximize network throughput and minimize delay. Firstly, the 2 hop-AHH-VBF void hole avoidance scheme checks the void hole status of forwarding nodes before sending data packets. Secondly, the forwarding node selection is optimized to ensure reliable data transmission with minimized end-to-end delay particularly in local sparse regions. The other scheme, QF-AHH-VBF, formulates a composite priority function (\(F_{p}\)) after exchanging of the control packets in the data packet-forwarding stage. To carry out optimal forwarder selection at each hop throughout the routing path, every node calculates \(F_{p}\). After relative comparison, the node with a high \(F_{p}\) value is selected as the qualified forwarder. Afterwards, objective functions with linear constraints for either to maximize or minimize the performance parameters are defined under the linear programming-based problem formulation. As a final step, simulations are conducted to validate our schemes, which perform better than counter vector-based schemes in terms of successful transmissions and energy tax.