# Wireless Channel Models for Over-the-Sea Communication: A Comparative Study
## 2 Key Concepts in Modeling Over-the-Sea Channels
### Important Definitions and Parameters  
**Multipath propagation:** A propagation that causes radio signals to propagate through two or more paths toward an antenna placed at the receiving end can be regarded as multipath propagation. Multipath can be caused by refraction, reflection, atmospheric ducts, etc. Figure 3 presents a very basic diagram of multipath wave propagation. **Line-of-sight (LoS) propagation:** Electromagnetic radiations tend to transmit/travel from a source to a destination along a direct path. This inherent characteristic of electromagnetic (EM) propagation, without any obstacle between the transmitting and receiving antenna, is known as LoS propagation. It is possible that the waves or rays may be refracted, diffracted, and reflected in their path. Figure 4 presents a scenario of LoS propagation. **Non-Line-of-Sight (nLoS) and beyond nLoS:** Non-LoS (nLoS) is the transmission of radio signals along a path that has a physical object in its way and is partially obstructed due to this. Signals are partially obstructed in the innermost Fresnel zone. The Fresnel zone is an ellipsoidal region between the transmitting and receiving antennas. The region is widely used to comprehend the strength of the waves transmitted by the sending and receiving antennas. In addition, this zone is useful for estimating whether any obstruction close to the line connecting the transmitter and receiver causes significant interference. Beyond nLoS (bLoS) is a special case of propagation, where the nLoS propagation confronts links blocked by a huge terrain, an earth bulge, etc., due to transmission over long distances. Figures 5 and 6 show the nLoS and bLoS propagations, respectively. **Evaporation duct boundary**  
Figure 4: An example of line-of-sight (LoS) propagation pathway. Figure 5: An example of a non-line-of-sight (nLoS) propagation pathway. Figure 3: An example of multipath wave propagation between transmitter (Tx) and receiver (Rx). **Slow and fast fading:** Slow fading is caused when the delay requirement of the application is relatively less than the coherence time of the channel. In contrast, fast fading occurs when the delay requirement of the application is greater than the coherence time. **Rayleigh and Rician fading:** Rayleigh fading is a statistical model that indicates the effect of the propagation environment on radio signals. According to this model, a signal passing through the transmission medium (communication channel) will fade in a random manner. In addition, this fading will occur in accordance with the Rayleigh distribution, which is the radial component of the summation of two uncorrelated Gaussian random variables. A radio propagation anomaly can occur due to partial cancellation of radio signals by themselves. The stochastic model of this phenomenon is regarded as Rician fading. **Frequency selective fading:** It is a radio propagation distortion that occurs due to partial cancellation of radio signals. If there is a signal propagating through two different paths, one of them will change. In this kind of fading, the bandwidth of the signal is larger than the coherence bandwidth. **Fade margin:** The additional margin in the average received signal power, which is required to achieve a certain level of availability for a given modulation and bit error rate performance in real-time channels, can be considered as a fading margin. Slow fading and fast fading that occur due to changes in the height of the evaporation duct have the safest value that can apply to both. This is considered as a complete fading margin. **Delay spread:** It is possible for multipath components to reach their destination at different times. The delay spread can be defined as the difference between the arrival time of the first and last multipath components. **Inter-symbol interference (ISI):** Suppose the symbol period is \(T_{s}\) and the delay spread is \(T_{m}\). If \(T_{s}<T_{m}\), then ISI occurs. It is a kind of signal distortion that causes one symbol to interfere with subsequent ones.