## 2 Acoustic Channel  
Designing an efficient acoustic channel is a challenge because of its underwater environment characteristics as stated, like multipath propagation which causes a signal power drop and phase oscillation. The Doppler effect is also another problem that is observed when there is a relative displacement of the sending and receiving nodes. The sound propagation speed and underwater noise are also considered to be taken into account, since they are factors which affect the performance of the acoustic channel [6].  
Capacity analysis and studies have been done for the channel and its dependence on the depth and temperature by considering losses as a result of dispersion and environment noise. The absorption rate estimation for the acoustic communication path is a function of the distance (between a pair of nodes) and the communication frequency used. The acoustic channel bandwidth increases with the increase of depth value and temperature and decreases by increasing the distance between the two nodes. A comparison between two methods used to determine the absorption coefficient is shown in literature, the first one by [PERSON] [7], while the latter is by [PERSON] [8]. While [PERSON]'s model considers a fixed value for depth and temperature (it does not consider changes in depth or temperature) for the calculation of acoustic channel capacity, instead the [PERSON] model takes into account the changes of these. In [8], it has been shown that the channel capacity and throughput increase with the increasing of temperature value and depth, in the case of small communication distances. The authors also explain that the nodes positioned in ocean deep areas achieve high speed and throughput. According to the results obtained in [8], a higher channel capacity would be possible if the same transmission power was maintained but with an increase of depth value. This result can be used in the design of adaptive transmission schemes for underwater mobile networks, or for the various power management methods that reduce transmission power to achieve a constant target bit rate with increasing depth. As a consequence of the bandwidth dependency from the transmission distance, we will get a high throughput only with multi-hop communication used and where messages are routed to some intermediate nodes. This is a desired feature to have in case of live streaming when using a single hop (great distance). In the analysis done in [9], regarding the physical model of the acoustic channel by taking into account losses due to spread and environment conditions, [PERSON] determined the bandwidth dependency from distance. Underwater acoustic channels suffer from attenuation of the signal along the acoustic communication path which depends not only on the space between the nodes but also on the signal frequency used.  
A variation of the signal frequency is considered to calculate the losses due to signal absorption rate for the communication, which occurs due to the conversion of the acoustic power in high temperature. A shorter communication link provides more bandwidth compared to a longer link in an acoustic system. Therefore, its use brings an increase in the throughput of received information, and more efficient usage of the acoustic channel, but as well as consuming even less energy. A significant error rate per velocity of data transmission and high delays are problematic features for the acoustic channel. In [10] several different techniques are compared in order to avoid different problems mentioned like the package size adaptation or forward error correction. Taking into account all these challenges related to underwater environments, there are required different methodologies to achieve a better usage of the acoustic channel, and this is of great interest for numerous researcher group, some of those that are useful for this study, will be mentioned in the following section.