# Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests
## 3 Performance of Single Link
### Effect of Distance and In-Forest Surroundings  
Figures 2 and 3 show the distributions of RSSI measured by the receiving node (from two directions) with four transmit power levels (tx), 31, 27, 19 and 15. We can see that for the two forest environments, the RSSI values of both directions roughly decline as the link distance increases. When the link distance is beyond 48 m in the Bajia forest, or 36 m in the Campus forest, the obtained RSSI is reduced almost to \(-90\) dBm, the sensitivity threshold of the CC2420 transceiver. In addition, with each of the four transmit powers, the Campus and the Bajia forests both experience obvious decreased RSSI when the link distance is not too long; for instance, when tx = 31 and the link distance increases from 4 m to 20 m, the RSSI decreases from \(-66\) dBm to \(-80\) dBm in the Bajia forest and from \(-62\) dBm to \(-78\) dBm in the Campus forest, respectively. However, as the link distance is longer than 20 m with tx = 31 or 16 m with tx = 27, the RSSI of the Campus forest descends gently until no radio carrier can be detected, different from the case in the Bajia forest. Clearly, in forest environments, the RSSI value is not linearly correlated with the communication distance. Compared with the RSSI of the Campus forest, the RSSI of the Bajia forest fluctuates more significantly in the descending trend for each transmit power. This observation implies that the reflection and scattering caused by relatively dense vegetation and sparse shrubs (in the Campus forest) do not impact the low-power radio link as much as we had thought; the vegetation or the short shrubs will possibly improve the radio propagation to some degree. Moreover, such an observation is also partially proven in Figure 4, illustrating the PRR dynamics under two forest environments. Note here that there are a few missing points in Figures 2 and 3 because no probes are successively received in the Bajia experiment where the link distance and the transmit power are set to be 28 m and 15, respectively; however, this does not imply that this experimental configuration in forests always cannot produce valid links. Nevertheless, we believe that such a point loss represents the real status of low-power wireless links in forests. Figure 3: Link-quality indicator (LQI) 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. Figure 2: RSSI 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. The LQI's distributions are shown in Figure 3. For the CC2420 transceiver, an LQI value close to 110 indicates a highest-quality packet, and in contrast, the link of around 50 LQI measurement is typically the lowest-quality link [22]. It can be seen from Figure 3 that for each forest environment, the LQI shows a downtrend as the transmit power is reduced from 31 to 15. The LQI of the Bajia forest is pretty much unstable with sharper transitions than that of the Campus forest. For example, with higher transmit powers, 31 and 27, the LQI of the Bajia forest ranges from 70 to 108, while the LQI in the Campus forest from 100 to 108. This observation is almost inconsistentwith the aforementioned RSSI distributions in Figure 2: a small quantity of vegetation in the Campus forest does not necessarily impact the low-power radio link too much. Compared to the RSSI distributions, the LQI shows a relatively moderate descending tendency when the link distance is not too long. In the Bajia forest with tx = 31, for instance, the RSSI shows an exact declining tendency as the link distance is not beyond 24 m, while the corresponding LQI does not change significantly.