Comparison chart of fitted values from the free space model and actual measured values at antenna heights of A 0. The two-ray model was used to estimate the wireless channel path loss in rice fields in three developmental stages and four antenna heights. Figure 5 shows the performance of the two-ray model in estimating path loss in rice field wireless channels.

Under the antenna heights of 0. At the tillering stage when antenna height was increased from 0.

## Journal of Optical Technology

At the jointing stage when antenna height increased from 0. At the grain filling stage when antenna height increased from 0. This shows that, when the two-ray model was used, the node antenna height had a greater effect on model results. As the antenna height changed, the distance between the node antenna and crop canopy also changed. This changed the reflection and transmission distance of the signal through the crop canopy and affected the precision of the estimation using the two-ray model.

Comparison chart of fitted values from the two-ray model and actual measured values at antenna heights of A 0. When the antenna height was 0. When the antenna height was 1. When the antenna height was 2. In the two-ray model, the progress of the developmental stage had a greater effect on the model results. The main reason for this is that the two-ray model considers the combined effects of direct transmission and reflection on channels. As rice plants grow, the reflection caused by changes in plant height and canopy coverage had greater effects on channels. At the tillering stage, plant height and coverage were relatively low and wireless signals are mainly affected by ground reflection.

## Wireless Channel Propagation Characteristics and Modeling Research in Rice Field Sensor Networks

At the jointing stage, wireless signals are also affected by reflection caused by plants in addition to ground reflection. During the grain filling stage, plant growth is more luxuriant and plant reflection is stronger compared to earlier growth stages and reflection effects are more significant.

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Therefore, the applicability of the model will improve. Compared to the free space model, the two-ray model considers the effects of direct waves, reflected waves caused by crops, and node height on signal transmission in addition to the foundation of wireless signal frequency and propagation distance. When antenna heights were 1.

This shows that the two-ray model has utility for rice field environments at antenna heights greater than 1. The least squares method was used for regression analysis of measured path loss based on Equation 6. Table 1 shows the regression parameters of the one-slope log-distance model for measured path loss at different developmental stages. In Table 1 , the path loss factor n increases with decreasing antenna height, indicating that the transmission environment worsens with decreasing antenna height.

At the same antenna height, n differs in different developmental stages but all stages showed similar variation trends that were increases with the advance of developmental stages. With developmental stage progression, rice plant height and canopy coverage increase, the canopy layer becomes more luxuriant and the superposition of leaf layers increases.

The reflection, refraction, and diffraction of wireless channels will intensify with changes in plant height, reducing the wireless transmission environment of rice fields and increasing path loss factors. A one-slope log-distance model based on Table 1 data was used to estimate the wireless channel path loss in rice fields in three developmental stages and at four antenna heights.

Figure 6 shows the performance of the one-slope log-distance model in estimating path loss in wireless channels in rice fields. These were significantly less than the free space model and the two-ray model. This indicates that the one-slope log-distance model has better performance than the free space model and the two-ray model and has better applicability in modeling of wireless channels in rice fields. However, when the antenna heights were 0.

This shows that the model has a degree of error when used for estimating path loss in rice fields. Comparison chart of fitted values from the one-slope log-distance model and actual measured values at antenna heights of A 0. Least squares regression analysis of measured loss was performed according to Equations 7 and 8. Table 2 shows the regression parameters for the modified two-slope log-distance model in different developmental stages. The results showed that, under the four antenna heights 0. The modified two-slope log-distance model based on model regression parameters Table 2 was used to estimate the wireless channel path loss in rice fields at three developmental stages and four antenna heights.

Figure 7 shows the performance of the two-slope log-distance model in estimating the path loss of wireless channels in rice fields. These values were significantly reduced compared with the free space model, two-ray model, and one-slope log-distance model. With the exception of the 0. This shows that the application of the modified two-slope log-distance model in rice fields had better applicability in estimating path loss. The modified two-slope log-distance model had better estimation precision than the one-slope log-distance model and appears more suitable for wireless channel propagation path loss analysis and modeling in rice fields.

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Comparison chart of fitted values from the modified two-slope log-distance model and actual measured values at antenna heights of A 0. The application of WSN in agriculture should consider the interference caused by plant height, leaves, and canopy coverage on channel transmission and effects on data transmission rate and channel loss [ 25 ]. These data show that an increase in the developmental stage had a great effect on model estimation results.

Li et al.

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They concluded that the intensification of channel loss was caused by the continuous increase in plant height and stems and leaves becoming more luxuriant as the crop grew and developed. This caused changes in the signal propagation environment when wireless sensor networks are used in farmland and propagation loss can significantly increase. Therefore, the effects of crop growth on network performance should be considered when establishing wireless sensor network nodes in rice fields.

When the free space model was used for path loss modeling of 2. The free space model only considers the effects of wireless signal frequency and propagation distance on channel propagation loss and does not consider the effects of plant height, planting density, stems and leaves in the wireless propagation environment on propagation loss in rice fields. Therefore, the free space model cannot be directly used for analysis of path loss in farmland wireless channels.

This conclusion is consistent with the results of Zhang and Zhang [ 14 ]. When the two-ray model was used for path loss modeling, the RE and RMSE between the estimated values and measured values at the three developmental stages gradually decreased with plant growth progression. This shows that rice growth caused significant reflection of propagation channels and changing node antenna height can decrease the estimation precision of the model. Mou et al. Crop cover causes irregular reflection, absorption, and scattering.

We found that the lower the node antenna height, the greater the RE and RMSE difference between estimated values and measured values. This is because the two-ray model only considers the effects of height, reflection, and distance on channel loss and neglects the effects of scattering and diffraction.

Therefore, the two-ray model cannot be directly used for wireless channel path loss modeling in rice fields. Ma et al. This further increases signal attenuation at propagation paths, causing the channel transmission environment to gradually worsen. Du et al. When we used the one-slope log-distance model for path loss modeling, RE and RMSE were significantly reduced compared with the free space model and the two-ray model. This is primarily because the one-slope log-distance model comprehensively considers the effects of plant growth changes in rice fields on channel propagation, and path loss factors are obtained from fitting of measured values to construct a channel model.

This has better applicability for wireless channel modeling in rice fields and is consistent with previous studies [ 30 , 31 , 32 ]. This shows that some error in path loss remains when this model is used in rice fields.

## Modeling the Wireless Propagation Channel | Wiley Online Books

When the least squares method was used for regression analysis of measured loss, we found that the path loss factors during the three developmental stages before the breakpoint were all smaller than those after the breakpoint. The Fresnel zone radius of channels increases as the propagation distance increases. After a certain propagation distance is reached the breakpoint , crop height and canopy in the field will cause the area of the Fresnel zone to increase to a threshold value that reduces communication quality. This caused the attenuation speed of the wireless signals to increase and was manifested as an increase in path loss factors after the breakpoint.

Path loss factors after the breakpoint were greater than path loss factors before the breakpoint. This result is consistent with that obtained by Zhang Haihui [ 14 ] in the analysis of the path loss modeling method of a 2. By setting of breakpoints in the model, the modified two-slope log-distance model increased the precision of the one-slope log-distance model and improved the error produced by the one-slope log-distance model for modeling wireless channels in rice fields.

Compared with the one-slope log model, the modified two-slope log-distance model had better applicability for channel propagation in rice fields. Existing models with consideration of the effects of the aforementioned factors can be used to further improve model precision. This can provide a basis for the engineering, application and networking and node layout for wireless sensor networks in rice fields.

In the future, large-scale networking experiments should be conducted to test the performance of the double fold line logarithmic distance model in rice fields. Testing the performance of the wireless sensor network, as an important step before applying the network in precision irrigation, will be our next work. We conducted channel loss tests in rice fields by comparing wireless channel propagation variation trends at four antenna heights and three rice developmental stages.

The degree of RSSI attenuation showed monotone decreases with changes in antenna height, but the transmission range showed monotone increases with antenna height. The effects of different node height on wireless channel propagation characteristics in rice fields were significant. The transmission environment of wireless channels in rice field sensor networks deteriorated with developmental stage increase.

The optimal height for node antennas was 2. The free space model is unsuitable for the estimation of path loss in wireless channels in rice fields. The two-ray model can be used in rice fields when antenna height exceeds 1. The modified two-slope log-distance model increased the performance of the one-slope log-distance model. This model can provide a basis for modeling of sensor network channels and construction of wireless sensor networks in rice fields.

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