# A Survey on Physical Layer Techniques and Challenges in Underwater Communication Systems
## 3 Modeling of UWA Channel Problems and Challenges
### Delay Spread  
When sound propagates from the transmitter to the receiver, refractions are observed in the path of the sound waves. In addition to the direct path it follows, auxiliary paths result from surface reflections, sea floor reflections, and irregularities in the speed of sound.  
The channel delay spread is defined as the difference in time between the first and last received path. The fastest rate of data transmission without ISI is determined by the delay spread. The length of the delay spread is frequently stated in terms of transmitted symbol intervals, and the transmitted symbol rate in terms of the transmitted symbol rate. In UWC routes, delay spreads of tens or hundreds of symbols are uncommon. On the other hand, the ISI symbols in an RF channel are usually about three. The paths are different from the transmitter to the receiver causing the delay spread in the UWCs channels. The spread of channel delay in shallow water is induced due to the bottom and top surface of sea reflections. Some reflections result from objects present in the column of water. Since they exist due to macro features in the surroundings, such reflections are described as macro-multipath. These attributes are typically presumed to be approximately time-invariant upon a timescale that is considerably bigger in comparison with the data rate [170].  
The acoustic rays rather than two-dimensional lines are preferably modeled as three-dimensional tubes. When the tubes come across an obstacle, the reflections commonly are not pointed reflections but rather are reflections from an area, for instance, an uneven sand patch or rough surface of the sea. This results in each path of the ray exhibiting a time spread, sometimes of the order of a few milliseconds. The micro-multipath is perceived as a multipath due to the small-scale properties, such as random oscillations in the water and surface roughness. The micro-multipath is non-specular and only a few parts of the small-scale random variations can be statistically modeled [170]. The micro-multipath concept in acoustics literature is well-known as ray tubes [150].  
Figure 17: The range-dependent SNR for UWA links as a function of frequency [150].  
In deep-water atmosphere, completely refracted paths are detected in the sound speed profile by a local minimum in addition to the bottom and surface recoils. The sound \"favors\" parts with slow sound speeds and eventually turns towards them. The ocean channel of deep sound, i.e., the channel for the ranging and fixing of sound has the slowest sound speed (SOFAR). The bending of rays occurs due to Snell's law employed on channels exhibiting a constantly varying sound speed. In comparison to time and scales of equalizer adaption relevant to acoustic transmission, the channel of deep water may have a considerable time spread but is generally sparse and progressively time-varying. Shadow zones are defined as sections of slight acoustic penetration owing to the geometry of the system and the profile of sound speed. These are a result of obstacles (e.g., mounts in the sea) or more notably due to the physics of waveguide propagation. Shadow zones can occur in both the deep and shallow channels of water [150; 160]. A little signal processing can be employed to adjust the shadow zones therefore their compensation is achieved using mission design and system placement.