# Opportunities and Challenges for Error Control Schemes for Wireless Sensor Networks: A Review
## 3 Standard, Network and Channel Models
### Channel Models  
WSN has various types of applications and also has different types of nodes depending on application. The distance between two nodes depends on the application. The distance between nodes creates different types of communication links depending on distance, deployment scenarios, LOS and NLOS scenarios. In the practical WSN scenario, there exist various types of communication links between nodes. Therefore, various types of channel models are considered in the previous research works to present these communication links. These channel models are roughly categorised into two: theoretical model and empirical model. AWGN is the simplest and basic channel model used in telecommunication, but this channel model is not suitable for WSN because this does not consider frequency selectivity, fading, interference, signal dispersion, etc. It is a suitable model for noise affecting wireless communication systems and deep space communication links. However, it is not a good model for terrestrial links in harsh industrial environments, due to multipath, terrain blocking and interference. Furthermore, [49] shows that impulse noise in industrial environments cannot be modeled only with AWGN. AWGN can be used in a wireless channel if there is LOS. However, the assumption of AWGN channel is not realistic in NLOS as there is multipath fading. Fading in wireless communication conveys the variation of the attenuation of a signal with various parameters including time, geographical position, and frequency. Communication channels that  
Figure 8: Multi-hop WSN model. Figure 7: A single hop WSN model. Figure 9: Modified multi-hop WSN model. experience fading are called fading channels, and the fading may be caused by multipath propagation, weather and some obstacles in the wave propagation path. The reflectors in the transmission path means that multiple signals can arrive at the receiver, but each copy of signal can experience different attenuation, delay and phase shift. Fading can be categorised as slow fading vs. fast fading or selective fading or block fading. Some of the popular fading channels are: Rayleigh fading channel, Rician fading channel, block fading channel, etc. As the nodes in most of the WSN are static, multipath fading has particular importance in WSN [50]. Other important phenomenon is shadowing, which can cause deep fade if the sensor is under the shadow of a tall building or similar structure. Rayleigh fading channel [51] assumes that the magnitude of a received signal will vary randomly according to a Rayleigh distribution. The transmitted signal may take multiple paths to reach a destination due to reflections, scattering, and diffraction. Hence, a signal can be described as a Rayleigh fading channel when there are a large number of scatters, distractions or reflections, and there is no LOS path between the receiver and transmitter. If there is a dominant LOS component, then Rician fading is a good model. In Block fading channels, the fading varies slowly in time. This model was introduced in [52] to model correlated slowly varying fading, and it is particularly useful in wireless communications' circumstances containing slow time-frequency hopping or multi-carrier modulation using Orthogonal Frequency Division Multiplexing (OFDM). Design criteria for codes in block fading channels are different from AWGN and a fully-interleaved fading channel. The main difference is that information is not stable in Block fading channels; reliability function is zero for any finite SNR [53]. Free Space Optics (FSO) is transmission of modulated infrared beams to implement optical wireless communication. FSO can operate over several kilometers when there is a clear LOS; however, it is highly susceptible to various impairments such as: atmospheric fluctuations, scattering, pointing errors and building sway [54]. In FSO communication systems, fading due to atmospheric turbulence can be modeled with gamma-gamma distribution. Error control coding can be applied to mitigate turbulence-induced fading in FSO [55].