# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 3 Performance of Single Link
### Effect of Distance and In-Forest Surroundings
However, when the transmit power is set to be lower (e.g., 19 and 15) and the link distance increases up to 36 m, the RSSI in the Bajia forest declines gently, while the corresponding LQI decreases sharply. The comparison results here demonstrate different variation patterns of RSSI and LQI. Though the RSSI and the LQI can roughly profile the link quality in some scenarios, an effective and commonly-used approach of evaluating link quality in practice is to calculate the runtime PRR of a radio link, which is the reception ratio of a set of probing messages that are successively sent or broadcast within a pre-configured period of time. Figure 4 plots the PRR measurements in the two forest environments. In the Campus forest environment with more vegetation and sparse bushes, the link PRR generally performs better, in both directions of the link, than the link PRR of the Bajia forest does. The interesting behavior of the PRR of the Bajia forest is its irregular variations, although in the experiments, two nodes are always line-of-sight, and the link is formed under the canopy without penetrating any bushes or leaves. For example, when tx = 31 in the Bajia forest, the link of a distance of 8 m produces a PRR of 42%, while the link of a distance of 32 m produces a PRR close to 100%. Such an irregular variation also occurs in the Campus forest experiment in which the link is often non-line-of-sight or the receiver is surrounded by laighshrubs, but they are infrequent, even in the scenarios with lower transmit powers, say tx = 19 or tx = 15. Comparing Figure 3 to Figure 4, we can  
Figure 4: Packet reception ratio (PRR) distributions vs. distances with different transmit powers under two different forests. (**a**) Bajia, tx = 31; (**b**) Bajia, tx = 27; (**c**) Bajia, tx = 19; (**d**) Bajia, tx = 15; (**e**) Campus, tx = 31; (**f**) Campus, tx = 27; (**g**) Campus, tx = 19; (**h**) Campus, tx = 15.  
see that the PRR has a variation pattern similar to the LQI, indicating that in forest environments, the link quality in terms of PRR can be probably predicted with the LQI. This will be further investigated in a later section. In the experiments, we find that for a given communication distance, higher transmit powers cannot always lead to higher link quality, not conforming to what has been traditionally deemed in theoretic studies. Figure 5 shows the PRR distributions of link \(B\)-\(A\) against transmit powers under the Bajia forest with the link distance of 28 m. As the transmit power decreases from 31 to 23, the corresponding PRR sharply drops from 0.23 to 0.07; however, the transmit power of 19 achieves a relatively higher PRR of 0.49. In our experiments, we fix the sending node and move the receiving node to different positions, yet keeping the same distance, in order to form distance-identical links with different receiver's directions. Such extra controlled experiments prove that the result of Figure 5 is not rare for in-forest low-power wireless links. The reason behind this might be that higher transmit powers result in a more complex multi-path effect that degrades the radio propagation in the forest environment.