# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 3 Performance of Single Link
### Asymmetry of the Link  
The complicated forest environments usually lead to high link asymmetry, that is the qualities of a link on two directions are conspicuously different. Figure 6 shows the difference of the PRRs on two directions of the experimental link, in which all pairs of two-way PRRs are plotted considering different transmit powers and link distances. Clear link asymmetry can be seen in Figure 6. Previous empirical evaluations in the open field show that the link of higher quality or the link of lower quality has better symmetry. However, such a result cannot well hold in our experiment, especially in the Bajia forest: e.g., the PRR of link \(B\)-\(A\) varies too much even though the PRR of link \(A\)-\(B\) almost keeps at 100%. In the Campus forest, the links with PRR close to 100% are rendered with better symmetry than their counterparts in the Bajia forest. A later section will continue to discuss the time-varying property of link asymmetry.  
We use the absolute difference of the two-way PRRs of a link, denoted by \(D_{prr}\), to quantitatively evaluate the link asymmetry in forests with different experimental settings. For a given transmit power, in detail, we calculate the average and the variance of all of the \(D_{prr}\) measurements under different link distances. In total, the average and the variance of the \(D_{prr}\) profile are the distribution and the fluctuation of link asymmetry, respectively. Figure 7 shows the PRR asymmetry. In the Bajia forest, the link asymmetry increases as the transmit power level is tuned from 31 down to 19. Noticeably, the average and the variance of \(D_{prr}\) both decline when the transmit power ranges from 15 to 11. Prior studies [23; 24] find that the low-power wireless link tends to be relatively symmetric when the link quality is very low or very high and is significantly asymmetric when the link quality (the receiver resides in the gray region) is intermediate. However, the result of Figure 7a does not effectively support  
Figure 5: PRR distributions vs. transmit powers of the link B-Aof the distance of 28 m under the Bajia forest.  
their findings, because in our experiment, the PRR of long-distance links with lower transmit powers is often close to zero simultaneously in two directions; the \(D_{prr}\) values are subsequently weakened.  
In the Campus forest, the \(D_{prr}\) measurements differ much over transmit power levels. Under the three higher power levels, 31, 27 and 23, the links almost keep symmetric: the PRRs on two directions are both close to 100%, which is also demonstrated in Figure 4. The link asymmetry in the Campus forest starts experiencing a rising trend when the transmit power is lower than 19. By comparing the results of two forests, we find that the in-forest shrubs and short trees (in the Campus forest) will unnecessarily abate the symmetry of low-power wireless links. Additionally, we cannot conclude a significant grey region for the links in the Campus forest experiment.  
Besides the PRR, we also use the two-way RSSI and LQI absolute differences of a link, denoted by \(D_{rssi}\) and \(D_{lqi}\), respectively, to evaluate the link asymmetry, and the according results are plotted in Figures 8 and 9. When the transmit power decreases, in Figure 8, the \(D_{rssi}\) in the Bajia forest stably keeps going down, while the \(D_{rssi}\) in the Campus forest totally goes down with a little fluctuation. Figure 9 shows the LQI asymmetry for the two forests. It is interesting to note that the average and the variance of \(D_{lqi}\) have variations similar to the \(D_{prr}\) of Figure 7, especially in the Campus forest environment. To some extent, we can practically resort to the LQI asymmetry, rather than the RSSI asymmetry, to predict the link asymmetry for resource-efficiency purposes.  
Figure 6: Illustration of link asymmetry under two forest environments. (**a**) Bajia; (**b**) Campus.  
Figure 7: The asymmetry (\(D_{prr}\)) of PRR under two different forest environments. (**a**) Bajia; (**b**) Campus.