206 broadleaf trees caused a slightly lower signal loss of 4.8 dB. The measured attenuation (Table 1) represented a significant differ- ence in signal strength (tree in LOS versus a clear LOS) for the 7 APs evaluated both in the summer (P = 0.009), and in winter (all trees: P < 0.001; defoliated broadleaf trees only: P = 0.009). Notably, in none of the 28 measurements did the tree-caused attenuation reduce the (average) received signal strength below -75 dBm (the minimum for a reliable Wi-Fi connection). The re- ceived signal approached -75 dBm in two cases: (1) with nine 10 m tall Liquidambar styraciflua in LOS, causing a 7.6 dB at- tenuation at the 100 m (328 ft) AP-to-computer distance (Figure 6); and (2) with seven large conifers in LOS, causing an 18.7 dB attenuation at 90 m (295 ft) AP-to-computer distance (Table 1). Presence of trees in LOS had little effect on RF noise. The slight reduction in measured noise (0.03 to 0.2 dB) with trees in LOS was not statistically significant in either season, and the dif- ferences were so small as to be below the minimum-detectable val- ues in the outdoor environment (±0.5 dB; Dobkin, pers. comm.). GLM analysis (Table 2) indicated that the three tree charac- teristics useful in explaining attenuation were consistent for SNR (r2 = 0.65) and signal strength (r2 = 0.55): tree size (class), and tree leaf type (broadleaf or conifer), with tree canopy depth also contributing to the model but with a much smaller coefficient. The model-building procedure indicated that none of the mea- sured variables significantly explained variation in RF noise. When canopy depth, a factor indicated by GLM as important in signal and SNR variation, was plotted against signal attenuation (Figure 7), a linear relationship was observed, albeit with only modest fit [attenuation = 3.1 + 0.11 * canopy depth (m); r2 = 0.23]. Laćan and McBride: City Trees and Municipal Wi-Fi Networks DISCUSSION This is the first study to examine the effect of multiple tree char- acteristics on wireless signal transmission in the context of a cur- rently-operational municipal Wi-Fi network. The urban trees con- sidered in this study measurably attenuated the microwave signal, but did not reduce the signal strength below the minimum required for the wireless link (-75 dBm). That is, the trees did not interfere with the practical operation of the municipal WLAN, thus meet- ing the engineering objective of the network planners (see below). The calculated signal attenuation was rather low, ranging from almost none in the cases of the newly planted P. × acerifolia, to low (<10 dBm) in case of medium-sized and even some large trees (L. styraciflua, S. sempervierns, respectively). The greatest attenuation was observed with a dense and deep [75 m (246 ft) canopy depth] “screen” of tall conifers (mostly S. sempervirens), which is unlikely to be experienced very often in a typical ur- ban landscape. The second-greatest attenuation was observed in the case of a single large tulip poplar (L. tulipifera), having a dense canopy and leaves with dimensions almost equivalent to the wavelength of the Wi-Fi signal: 12 cm (4.7 in). It also is in- teresting to note the drooping habit of the L. tulipifera leaves and compare the similarly-sized but horizontally-oriented L. styraci- flua leaves, which appeared to produce much smaller attenuation. Trees had little effect on RF noise, which is not surprising because RF noise is caused by presence of other RF sources (e.g., cellular telephone towers, microwave ovens, two-way ra- dios; Dobkin 2005). The slight decrease in RF noise with trees in LOS was expected (as the other RF sources are also attenuated by trees; Dobkin, pers. comm.), but proved to be insignificant. Table 1: Tree characteristics and signal parameters (in dB) measured in a municipal Wi-Fi network in Mountain View, California. # of Location Bernardo Street Bernardo Street Cuesta Park Villa Street Glenborough Street Cuesta Park Cuesta Park Tree sp. Platanus × acerifolia Prunus cerasifera Liriodendron tulipifera Pistacia chinensis Liquidambar styraciflua Pinus radiata, Sequoia sempervirens Pinus 120 x 120 70 x 20 (240 x 170) 100 x 120 100 x 1.5, 18 x 40 (150 x 20) 140 x 1.5 pinea Average attenuation: 1 4 3 9 4 1 7 3 1 2 3 41 22 52 21 5 75 35 10 25 30 10 54 23 100 60 30 90 45 20 60 (all trees: n = 7): (defoliated broadleaf: n = 5): zNegative values indicate a stronger signal (or lower noise) with tree(s) in LOS. ySignificant differences (P < 0.05) in signal level between clear & tree condition. xDifferences of < 0.5 dB cannot be measured reliably in the outdoor environment. ©2009 International Society of Arboriculture 13.1 11.8 -- 3.6 7.6 2.2 2.6 18.7 5.1 2.1 4 5.3y 11 2.7 -- 4.2 3.9 5.1 2.3 14 5.6 2.2 9.4 5.6y 4.8y 0 0.4 -- 0.9 0 -0.4 0.1 -0.1 -1.2 -0.5 0.3 -0.03x -1 -1.7 -- 0.8 -0.7 -0.1 0.1 1.1 -0.3 1.2 -0.1 -0.04 -0.20 Leaf size (mm x mm) 100 x 100 to 160 x 180 40 x 80 trees in LOS 11 6 3 4 Canopy depth (m) 24.5 14 6.5 23 Dist. to AP (m) 150 90 30 90 ∆ Signalz summer -0.3 -1 1.9 7.4 , Clear - Tree in leaf ∆ Signal, winter Clear - Tree defoliated 3.5 3.2 3.2 8.7 ∆ Noise, summer Clear - Tree in leaf 0.2 0.3 -0.1 -0.3 ∆ Noise, winter Clear - Tree defoliated 0 0.1 0.7 -0.7
July 2009
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
