A State-of-the-Art Survey on Multidimensional Scaling-Based Localization Techniques <s> E. Underwater Exploration <s> Underwater wireless technologies demand to transmit at higher data rate for ocean exploration. Currently, large coverage is achieved by acoustic sensor networks with low data rate, high cost, high latency, high power consumption, and negative impact on marine mammals. Meanwhile, optical communication for underwater networks has the advantage of the higher data rate albeit for limited communication distances. Moreover, energy consumption is another major problem for underwater sensor networks, due to limited battery power and difficulty in replacing or recharging the battery of a sensor node. The ultimate solution to this problem is to add energy harvesting capability to the acoustic-optical sensor nodes. Localization of underwater sensor networks is of utmost importance because the data collected from underwater sensor nodes is useful only if the location of the nodes is known. Therefore, a novel localization technique for energy harvesting hybrid acoustic-optical underwater wireless sensor networks (AO-UWSNs) is proposed. AO-UWSN employs optical communication for higher data rate at a short transmission distance and employs acoustic communication for low data rate and long transmission distance. A hybrid received signal strength (RSS) based localization technique is proposed to localize the nodes in AO-UWSNs. The proposed technique combines the noisy RSS based measurements from acoustic communication and optical communication and estimates the final locations of acoustic-optical sensor nodes. A weighted multiple observations paradigm is proposed for hybrid estimated distances to suppress the noisy observations and give more importance to the accurate observations. Furthermore, the closed form solution for Cramer-Rao lower bound (CRLB) is derived for localization accuracy of the proposed technique. <s> BIB001 </s> A State-of-the-Art Survey on Multidimensional Scaling-Based Localization Techniques <s> E. Underwater Exploration <s> Location is one of the basic information required for underwater optical wireless sensor networks (UOWSNs) for three main purposes: 1) Relating the sensing measurements with precise sensor positions, 2) Enabling efficient routing techniques by exploiting the sensor locations, and 3) Sustaining link connectivity and performance via pointing and alignment mechanisms between the nodes. Even though various two-dimensional UOWSNs localization methods have been proposed in the past, the directivity of optical wireless communications and three dimensional (3D) deployment of sensors require to develop 3D underwater localization methods. Therefore, we propose a robust 3D localization method for partially connected UOWSNs with a limited number of anchors. The proposed method achieves an accurate 3D localization of all the nodes in the network by using a novel low-rank matrix approximation and outliers removal method. The performance of the proposed method is compared with the well known iterative majorization approach. The numerical results indicate that the proposed method outperforms the iterative majorization method substantially. <s> BIB002 </s> A State-of-the-Art Survey on Multidimensional Scaling-Based Localization Techniques <s> E. Underwater Exploration <s> Underwater optical wireless links have limited range and intermittent connectivity due to the hostile aquatic channel impairments and misalignment between the optical transceivers. Therefore, multi-hop communication can expand the communication range, enhance network connectivity, and provide a more precise network localization scheme. In this regard, this paper investigates the connectivity of underwater optical wireless sensor networks (UOWSNs) and its impacts on the network localization performance. First, we model UOWSNs as randomly scaled sector graphs where the connection between sensors is established by point-to-point directed links. Thereafter, the probability of network connectivity is analytically derived as a function of network density, communication range, and optical transmitters’ divergence angle. Second, the network localization problem is formulated as an unconstrained optimization problem and solved using the conjugate gradient technique. Numerical results show that different network parameters such as the number of nodes, divergence angle, and transmission range significantly influence the probability of a connected network. Furthermore, the performance of the proposed localization technique is compared to well-known network localization schemes and the results show that the localization accuracy of the proposed technique outperforms the literature in terms of network connectivity, ranging error, and number of anchors. <s> BIB003 </s> A State-of-the-Art Survey on Multidimensional Scaling-Based Localization Techniques <s> E. Underwater Exploration <s> This paper proposes a received signal strength (RSS)-based localization framework for energy harvesting underwater optical wireless sensor networks (EH-UOWSNs), where the optical noise sources and channel impairments of seawater pose significant challenges on range estimation. In UOWSNs, energy limitation is another major problem due to the limited battery power and difficulty to replace or recharge the battery of an underwater sensor node. In the proposed framework, sensor nodes with insufficient battery harvest ambient energy and start communicating once they have sufficient storage of energy. Network localization is carried out by measuring the RSSs of active nodes, which are modeled based on the underwater optical communication channel characteristics. Thereafter, block kernel matrices are computed for the RSS-based range measurements. Unlike the traditional shortest-path approach, the proposed technique reduces the estimation error of the shortest path for each block kernel matrix. Once the complete block kernel matrices are available, a closed form localization technique is developed to find the location of every optical sensor node in the network. An analytical expression for the Cramer–Rao lower bound is also derived as a benchmark to evaluate the localization performance of the developed technique. The extensive simulations show that the proposed framework outperforms the well-known network localization techniques. <s> BIB004
Robust and accurate localization techniques for underwater sensor networks (USNs) is a necessary but challenging task due to the harsh aquatic environment. MDS is used in various works for the localization of underwater sensor nodes. For example, an MDS based localization scheme was proposed in BIB003 where localization of multi-hop UOWSNs was formulated as an unconstrained optimization problem and solved using the conjugate gradient technique. Similarly, MDS-based localization technique was used in BIB002 ,  for three-dimensional underwater optical wireless networks which take into account the outliers in ranging and optimize the anchor's location. Moreover, centralized MDS-based technique was recently used in BIB001 , BIB004 which also considers energy harvesting in the underwater environment to improve the connectivity and localization of the network. All of these MDS-based localization techniques developed of USNs are centralized and thus have high complexity. Therefore, semicentralized and distributed MDS-based techniques need to be developed for the USNs.