# A Biologically Inspired Self-Organizing Underwater Sensor Network
## 2 Related Work  
Underwater communication plays an essential role in underwater wireless sensor networks (UWSNs) [5]. To meet increasing exploitation demands of submarine resources, various underwater communication devices and methods have been proposed [6; 7]. The technologies for the transmission of wireless underwater communication can be classified into three categories, i.e., _radio frequency (RF) communication, optical communication_, and _acoustic communication_[8]. Each technology is reviewed in detail in the following sections. RF communication technology is unsuitable for underwater environments since the high conductivity of seawater severely affects the propagation of electromagnetic waves [9]. [PERSON] et al. [10] have found that RF communication can be used in shallow-water environments. However, RF communication technology still suffers from many limitations such as serious signal interference by environmental noise [11] and huge energy loss in the propagation process [10]. Studies have proved that the RF data rate varies from 1 to 10 Mbps [12], and the communication distances are only a few meters [13]. Compared to RF technology, optical communication technology exhibits a better data rate. [PERSON] and [PERSON] revealed that the data rate of optical communication reached up to 1 Gbps [14]. However, the reliable communication distance is only tens of meters, from about 2 to 25 m [15]. [PERSON] et al. found that the attenuation of optical signals of different frequencies underwater varies, and the blue-green optical signal has a smaller propagation attenuation [16; 17]. Moreover, the attenuation and interference of optical signals are also affected by different water environments. Therefore, optical communication is more disadvantageous to underwater communication than RF communication [18]. Acoustic communication possesses more advantages than the other two technologies for underwater environments [6], and its communication distance can reach several kilometers [19]. Due to the speed difference between sound and electromagnetic waves [20], there exist three main losses--spreading loss, absorption loss, and scattering loss--that affect the propagation distance of acoustic waves [21]. [PERSON] et al. show that the data rate of sound waves is only 0.6 to 50 Kbps [22]. To address the problem of low acoustic wave propagation rates and the increasing number of deployed devices, wireless sensor networks (WSNs) [5] have become an efficient tool in the underwater environment. WSNs, however, suffer from serious problems such as resource allocation, interference management, anti-blocking, and deafness [5]. [PERSON] et al. used MAC protocol, which focuses on improving the performance and efficiency of communication [23], in their study to solve such problems in WSNs [24]. [PERSON] et al. indicated that it is significant to analyze MAC protocols in the study of WSNs [25]. [PERSON] et al. found that in WSNs, MAC protocols operate at the top layer of the physical layer [26]. Meanwhile, MAC protocols always consider energy efficiency [27]. All these elements are crucial for the lifetime and efficient operation of WSNs. Nonetheless, the MAC protocols used in terrestrial wireless sensor networks (TWSNs) are unsuitable for UWSNs because these MAC protocols exhibit low bandwidth, low throughput, high energy consumption, and high propagation delay [28; 29; 30]. Time synchronization in UWSNs is another difficult factor because effective operation cannot be confirmed between sensor nodes and higher propagation delay [31], which leads to too many re-transmissions and collisions in UWSNs [32]. There exist many MAC protocols used for UWSNs, such as OFDMAC, UW-OFDMAC, ED-MAC, and DL-MAC [23]. However, they cannot meet all the requirements in different underwater environments.