208 The presence of leaves resulted in slightly (but not significant- ly) higher attenuation than was seen with defoliated broadleaf trees, which was another expected result, as signal attenua- tion is largely attributed to water in the leaves (Dobkin 2005). Observed signal attenuation was considerably lower than that reported in previous studies. This may reflect the advances in Wi-Fi hardware since the publication of previous works. It is also likely that these measurements were less precise than those by the RF engineering researchers (Dalley et al. 1999; Perras and Bouchard 2002) who used purpose-built experiments. For example, this experimental setup did not exclude the possibil- ity of RF phenomena such as Fresnel zones (that are formed around obstacles, including trees), multipath reflections, or re- fraction around obstacles (Dobkin 2005), any of which could have an enhancing effect on RF signal, and are usually elimi- nated in controlled experiments. Instead, this study demon- strates the effect of trees on the performance of a municipal WLAN as a typical user might experience it: some signal at- tenuation, but no substantial interference with the wireless link. It must be emphasized, however, that signal attenuation by trees must be taken into account during installation of the mu- nicipal Wi-Fi, as is general practice (Blais and Kruse, pers. comm.) and had been done during the installation of Google-Fi Laćan and McBride: City Trees and Municipal Wi-Fi Networks in Mountain View. This is because tree-caused attenuation ne- cessitates a larger number of APs (“greater AP density,” in RF engineering terminology) than would be needed without trees. In addition, the calculation of how many APs are needed should be based on the canopy depth of full grown trees, even in locations where only newly-planted trees are present. For exam- ple, this study observed a “link margin” (the difference between the received signal strength and the -75 dBm required minimum) between 13 and 10 dB for AP on Bernardo Street, which is popu- lated with immature Platanus trees. Once fully grown, total can- opy depth of these 11 trees could easily expand to as much as 70 m (230 ft), and the resulting attenuation would then approach 10 dB (compared to the near-zero attenuation observed at present) —very close to the link margin. Thus, a need for close collabora- tion between the urban forester and the Wi-Fi network planner during planning and installation of the municipal Wi-Fi network should be stressed. Such collaboration could be useful in the fu- ture as well. For example, signal attenuation from trees that grew to obscure an AP could be alleviated by selective pruning, such as crown raising or directional thinning, in tree species that tolerate it. Continued development of other wireless network technolo- gies, such as 806.11n and WiMAX (the 802.16 standard; Dob- kin 2005) is likely to present continued challenges for urban network managers and to open new issues for future studies of the interaction between urban trees and wireless networks. Some of these alternative wireless network systems operate with low power similar to Wi-Fi, but differ from Wi-Fi in sig- nal frequency and data handling (Dobkin 2005), and thus may differ in their susceptibility to signal attenuation by urban trees. Other directions for future work include evaluating the impor- tance of differing tree crown architecture, and leaf size and po- sition to signal attenuation. Depending on their location, such studies could include palms (e.g., Washingtonia spp., Roystonea regia), evergreen broadleaf species (e.g., Magnolia spp., and Ficus spp.), and species with densely branched fine canopies (e.g., Cercidium floridum). Trees with pendulous leaves (e.g., most of the Eucalyptus species), or finely compound leaves (e.g., some Accacia species, Jacaranda mimosifolia) should also be examined for their potential to attenuate Wi-Fi signal. City arborists remain keenly interested in improving the per- Figure 7. The signal-attenuation increase with canopy depth. Dot- ted lines are 95% confidence bands for the fitted line. Note the high outliers at 5 m (16 ft) canopy depth: this is the large L. tulip- ifera, shown in Figure 2 and Figure 3. formance of urban trees, and one means to achieve this goal is to minimize the conflict with “grey infrastructure”, an idea en- capsulated in the slogan “right tree—right place.” Although generally positive, this concept nevertheless has resulted in Table 2: Results of general linear model (GLM) analysis describing the factors that influence the attenuation of Wi-Fi signal, noise, and signal-to-noise ratio (SNR). Factors tested Intercept Metric ∆SNR ∆Signal ∆Noise -7.836 -1.308 – w 0.001 < P < 0.05 ©2009 International Society of Arboriculture – – – z “Full model”: includes all variables. y “Final model”: includes only the variables that were retained by the stepwise procedure. x P < 0.001 – – – 4.77x 4.40x – 0.123w 0.124w – -4.89x -4.37x – – – – – – – – – – Distance to AP (m) Tree count Tree size (S,M,L) (coefficients for the factors retained in final model) Canopy depth (m) Broadleaf or conifer In-leaf or defoliated Leaf area Leaf dim (cm2 ) ratio Model r2 Fullz 0.62 0.64 0.25 Finaly 0.58 0.55 0.00
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