present a study of electromagnetic attenuation over different environments of Mexico City, for an empirical modification of Free Space Attenuation (FSA). Our work uses results of in-site measurements in different places in the city.
hemos hecho el experimento para diferentes estándares de comunicaciones, presentamos en este trabajo los resultados para WiMAX en 3.5 GHz. de atenuación en zonas urbanas, WIMAX.
Wireless communication systems are evolving rapidly, EM attenuation in urban areas is now a most for the system design.
models as
Hata [1],
Cost 231 (a modification of that of Hata), Erceg [3], Shittu [4] and others, are used all around the world to predict propagation losses, for each site should be tested to prove at least, the deviation between prediction and measurement. We have done measurements all over Mexico City in different frequencies, comparing them with prediction of most popular models, for different communications standards as WiMAX (Worldwide Interoperability Microwave Access), which is a metropolitan area service used with one or more base stations at different frequencies: 2.3 GHz, 2.5 GHz, 3.3 GHz, 3.5 GHz and 5.8 GHz [5,6].
Our analysis considers comparison of two models: COST 231 and Erceg, for similar environment as the scenario 2 (shown below); for COST we use the big city model, applying all communications system parameters to equation (1)
= 46.3 + 33.9 ∗ log − 13.82 ∗ log − ( ) +(44.9 − 6.55 ∗ ) ∗ + … (1)
f = 3.5 GHz.
AT = 30 m (Transmission height antenna) d = 100 – 1200 m C = 3
(correction factor) Receiver height antenna correction: (ℎ ) = 3.2 ∗ [ EIRP=19.5 dB
(11.75 ∗ )]2 − 4.97, with AR = 1.5 m
PFSA. = Free space loss d0 = Reference distance (100 m).
For Erceg Model we use the A zone, meaning hills and a medium density of trees. Then we use equation 2: = . + 10 ∗ log ( ) + ;
≥ 0 …(2) 0 = ( − ∗ ℎ + ) + ∗ ; 10
≥ ℎ ≥ 80 = Using Erceg parameters, we define:
= + ∗ = 4.6
= 0.0075 = 12.06 = 10.6 = 2.3
= 0.57
Results of our comparison between measurements made in Mexico City and actual models, show important differences, as seen in figure 1.
Figure 1 shows that Mexico City environment is different to those where, the models are defined. This paper, analyze our own scenarios and adjust the Free Space Model (FSM) trying to find the best relationship between measurements and
model modifications. Considering those differences of figure 1, we Copyright © 2015 for the individual papers by the papers’ authors. Copying permitted for private and academic purposes. This volume is published and propose to use a modification of FSA equation, comparing it with regression curves of measurement.
The idea is to measure field attenuation in an area with defined characteristics, make modification of FSA equation and after that, take measurement in other different place in the city, with similar characteristics and compare the new measurement regression curves with former modification of FSA. For the experiment we choose to different places, 30 Km apart, one in the north and the other in the south of Mexico City. Experiment starts with choosing similar scenarios in both sites, we find 4 areas with similar characteristics, those from 1 to 4. The fifth scenario was in downtown, which is a unique place in the city and do not have a similar site to be compared with, but we presented because it was part of the measurements.
We propose to use Free Space Attenuation, modifying the equation exponent. As is known, FSA supposes a free obstacle region between transmitter and receiver; it is given by [3]: = ( 4 ) 2
(3) where d: distance between transmitter and receiver f: operating frequency c: speed of light Equation (3) is often expressed in dB as: 4 = 20 log + 20 log + 20 log
(4) The new proposed equation is: = ( 4 valid only for the used frequency, for a different one we have to repeat the procedure. On the other hand we want to emphasize that we are comparing only attenuation due the environment, this means that we take a measurement power at some distance of base station and relate it to the measurement at other distance.
We are still working in the procedure, and we are now measuring in similar areas in other places in the city, to compare the original results.
II. MEASUREMENT METHODOLOGY
Measurements were performed using base stations located in
University (UNAM) with base station in the
Building at the south of the city, in Instituto Politécnico Nacional (IPN) with station located in the Dirección de Cómputo y Comunicaciones (DCyC) at the city’s north and the Instituto de Ciencia y Tecnología (ICYT) in downtown. Three scenarios were chosen due their similarities but also by their differences.
I shows characteristics of communications system, transmitter antenna height is almost the same for all scenarios.
TABLE I COMMUNICATION SYSTEM FEATURES
analyzer, becoming in computer txt files containing latitude, longitude, distance and input power. Database is used to construct linear regression curves, to be compared with adjustments of FSA.
III. IDENTIFICATION OF SCENARIOS
We identified 5 scenarios accordingly of existing features of different areas in Mexico City, which are described in following paragraphs: A. Scenario 1: low buildings with low tree density
This scenario considers an area with a low tree density, and low high buildings, as shown in Figure 3. B. Scenario 2: low buildings with medium tree density
The scenario considers an area with medium tree density and a low height of the buildings as the one shown in figure 4. C. Scenario 3: area with a high tree density
D. Scenario 4: tall buildings with medium tree density
The scenario considers an area with medium tree density and high height buildings.
E. Scenario 5: colonial city Scenario 5 is a unique environment of Mexico City (and many Latin American cities), it is located at historic downtown; it has some very unique construction features such as: large width walls, tall buildings, and narrow streets.
Figures 8 and 9 show UNAM (south of the city) with yellow mark, showing position of base station at Humanidades II building and the one at IPN (north of the city) with mark in DCyC building (Dirección de Cómputo y Comunicaciones). Measurements were taken in both zones for scenarios 1 through 4, each one bounded for different colors. The blue frame define scenario 1, while scenario 2 is red, green is the scenario 3 and scenario 4 orange.
Each scenario was visually distinguished from both base stations photos. As an example, area for scenario 1 is mostly filled by buildings distinguished in photographs from tree density. from the photo the differences with other scenarios, is possible to distinguish them from position, we created tables with each point measurements and then constructed with those taken in IPN to validate X and Y parameters. is made changing the exponent value (X) for the slope and (Y) for losses magnitude, in equation 5 considering a difference no higher than 3 dB: = ( 4 meaning scenario 4 needs further analysis. measurement no higher than 1.5 dB. As seen the exponent value do not change (X=2);
magnitude of attennuation factor is selected as Y = -28 for scenario 2.
In the same way we compare curves for scenario 3, as shown in Figure 13; again the slope of regression curves are similar, with a difference of power no greater than 5 dB. After adjustment, we select a curve for FSA between both regression curves; lost equation is expressed as:
4 3 = − (
) −
Following same procedure, we compare results for scenario 4. Regression curves are shown in Figure 14. As seen slopes of both curves are different, although no more than 1.5 dB between Although two power slopes are different, a good FSA adjustment was found as: 4 = − (
) + 4 4
As scenario 5 is unique, is not possible to compare with any other curve. Figure 15 shows FSA adjustment of regression curve. As can be seen from Figure 15, there is a sharp slope fall, similar to that of scenario 3, meaning a zone of high attenuation, due the tall buildings with very dense walls and narrow streets.
5 = − ( ) −
As seen in Table II, scenarios 1 and 4 have similar slopes, with an exponent of 3. Furthermore the loss adjustment for FSM is increased with 29 and 21 dB respectively.
For scenarios 2 and 3, the slope has the same exponent of 2, and loss requires an adjustment of -28 and –31 dB respectively, only a 3 dB difference.
After identification of some common scenarios in Mexico City as: low buildings-low tree density; low buildings-medium tree density; high tree density zone; tall buildings-medium tree density; Colonial City, we compare measurements over city streets with FSA, to find a relationship between them.
Considering similarities for two base stations, we validate scenarios selection, at least for 4 of them, leaving scenario 5 as unique.
Comparing measurements for each scenario with adjusts of exponent and amplitude of FSA losses, we conclude that model can predict path loss for WiMAX or similar communication standard. We think that 5 dB differences is a good margin, to predict propagation of mobile communication systems over an environment as Mexico City. Probably we have to define an accepted margin, but accordingly with our experience a 10 dB could be a good number.