Link Investigation of IEEE 802.15.4 Wireless Sensor Networks in Forests

Abstract

Wireless sensor networks are expected to automatically monitor the ecological evolution and wildlife habits in forests. Low-power links (transceivers) are often adopted in wireless sensor network applications, in order to save the precious sensor energy and then achieve long-term, unattended monitoring. Recent research has presented some performance characteristics of such low-power wireless links under laboratory or outdoor scenarios with less obstacles, and they have found that low-power wireless links are unreliable and prone to be affected by the target environment. However, there is still less understanding about how well the low-power wireless link performs in real-world forests and to what extent the complex in-forest surrounding environments affect the link performances. In this paper, we empirically evaluate the low-power links of wireless sensors in three typical different forest environments. Our experiment investigates the performance of the link layer compatible with the IEEE 802.15.4 standard and analyzes the variation patterns of the packet reception ratio (PRR), the received signal strength indicator (RSSI) and the link quality indicator (LQI) under diverse experimental settings. Some observations of this study are inconsistent with or even contradict prior results that are achieved in open fields or relatively clean environments and thus, provide new insights both into effectively evaluating the low-power wireless links and into efficiently deploying wireless sensor network systems in forest environments.

Keywords: forest monitoring; link performance; low-power wireless link; wireless sensor networks.

Figure 1

Three typical forest environments. (a) Campus forest; (b) Bajia Rural Park forest; (c) Jiufeng Mountain forest.

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.

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 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.

Figure 5

PRR distributions vs. transmit powers of the link B-Aof the distance of 28 m under the Bajia forest.

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.

Figure 8

The asymmetry ($D_{rssi}$) of RSSI under two different forest environments. (a) Bajia; (b) Campus.

Figure 9

The asymmetry ($D_{lqi}$) of LQI under two different forest environments. (a) Bajia; (b) Campus.

Figure 10

The PRR versus RSSI under two different forest environments. (a) Bajia; (b) Campus.

Figure 11

The PRR versus LQI under two different forest environments. (a) Bajia; (b) Campus.

Figure 12

RSSI distance fitting in the Bajia forest environment. (a) tx = 31; (b) tx = 11.

Figure 13

Overview of a ten-node wireless sensor network deployed in the Jiufeng forest (only a part of the wireless links is shown here for clarity).

Figure 14

Temporal variation of the topology of the Jiufeng wireless sensor network (shown here are only links incident to nodes S2 and S12). (a) The 10th minute; (b) The 20th minute; (c) The 30th minute; (d) The 40th minute.

Figure 15

PRR performance of directional links. (a) Over each valid directional link; (b) Over all valid direction links against time.

Figure 16

Symmetry variation of links with different link quality. (a) Good link; (b) Middle link; (c) Poor link.

Figure 17

Conceptual illustration of the link correlation. (a) Demo of incident links; (b) Patterns of the PRR variations.

Figure 18

Demo of the link correlation measured in the Jiufeng experiment. (a) Correlation = 0.63; (b) Correlation = 0.28; (c) Correlation = 0; (d) Correlation = −0.4.

Figure 19

Correlation of all incident link pairs. (a) Correlation distribution; (b) CDF of the absolute value of correlation.

Figure 20

Correlation coefficient of any two links from each sending node.