INVESTIGATION OF WI-FI INDOOR SIGNALS UNDER LOS AND NLOS CONDITIONS

8/12/2019 INVESTIGATION OF WI-FI INDOOR SIGNALS UNDER LOS AND NLOS CONDITIONS

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Investigation of WI-Fi indoor signals under LOS and NLOS

conditions

S. Japertas, E. Orzekauskas

Department of Telecommunications, Kaunas University of Technology,Studentu str. 50, LT-51368 Kaunas, Lithuania

e-mail:[email protected],[email protected]. Slanys

Telecommunications Department, JSC Lietuvos dujos,Aguonu str. 24, LT-03212 Vilnius, Lithuania

e-mail:[email protected]

ABSTRACT

In this work the propagation of radio wavesat 2.4 GHz in NLOS conditions has beenstudied. The study was carried out using two

transmitters operating on different standards:802.11g (D-Link) and 802.11n (TrendNet).

The experiments were carried out underdifferent scenarios in order to investigate theeffect of the walls on signal propagation.

Experimental results were processed usingstatistical methods and were compared with

"log-distance" and free space models and

with the network simulation programme"Aerohive online planner" results. A newsignal prediction model, which allows

predicting signal propagation depending on

the number of walls, was created. In thiswork 802.11g and 802.11n standards werealso compared. The results can be used tofurther investigate radio wave propagationin indoor NLOS conditions and in the

development of wave propagation models.

KEYWORDS

WLAN, 802.11 signal indoor propagation,LOS, NLOS.

1 INTRODUCTION

Data transfer is a key component of the

Information Society. Its impact on dailylife increases constantly.

Wireless Local Area Networks (WLAN)technologies and the application thereof

in the indoor environment have gainedespecially high acceleration. Signals aretransmitted under the line-of-sight (LOS)

and/or non LOS (NLOS) conditions in

such environments. During the WLAN

design, it is necessary to pre-evaluateLOS and NLOS radio-wave propagation

in the indoor environment. But inpractice systems generally work under

NLOS conditions. The main problem isthat it is very difficult to predict indoor

radio wave propagation in the absence of

direct visibility between the transmitterand the receiver. Although some number

of radio waves propagation prediction

methods [1-4] have been proposedrecently, it is still difficult to predict how

the radio frequency waves act. The

existing simulation programmes aremainly intended to simulate signalpropagation under LOS conditions.The goal of this research paper was theexperimental investigation of the 802.11g and n standards (further 802.11g/n)radio waves NLOS propagation in themulti partitions indoors. Experimentalresults were compared with the log-

International Journal of Digital Information and Wireless Communications (IJDIWC) 2(1): 26-32The Society of Digital Information and Wireless Communications, 2012(ISSN 2225-658X)

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distance path losses and free space lossesmodels as well as with the results of thesimulation programme "Aerohive onlineWi-Fi planner". Based on the results ofexperiments, a new model for theevaluation of influence of multipartitions on the propagation of suchsignals was proposed.

2 RADIO WAVE NLOS INDOORS

PROPAGATION

2.1 Experiments and Models

There is a sufficient number ofexperiments carried out in various

NLOS indoor environments. In most

researches, the experimental results areapproximated by some models. It is

quite difficult to compare the results of

different authors because the transferred

signal is affected by a variety ofpropagation mechanisms (absorption,

scattering, reflection, and diffraction)

under the NLOS conditions.However, insome cases it is difficult to explain the

experimental results according the

existing effects and describe them

according the well-known models.On the other hand, there still is an

insufficient number of studies,

particularly in investigating the 802.11nstandard, which would allow a full

assessment of radio wave propagation

indoors.In order to predict the NLOS signal

propagation, the following models [1-4,

6] are usually applied: Open Space,

Traditional, Multi-Partitioning, Log-

Distance Path Loss Model, Log-NormalShadowing, Two-Ray Model, Free

Space Losses, and other.

The most prevalent model, which iscommonly used to compare the

experimental results, is the free space

losses (FSL) model:

FSL= 92,45 + 20 logf+20lgd, (1)

where:

f - the frequency in GHz,

d - the distance between the transmitter

and receiver in km.The log-distance model (2) [1, 5] or itsmodifications are amongst the most

widely used ones:

0

0 log10)()(d

dndPLdPL , (2)

where:

PL(d)the path loss in the distance d,PL(d0)the path loss in the distance d0,

d0 the close-in reference distance in

meters,nthe path loss exponent.

The multi-partitioning model tries to

evaluate the signal propagation in

buildings with multiple partitions, walls,and other obstacles [2]. The model's

mathematical expression is:

MAffdd

GGPP trtr

)log20

6.32(

00

, (3)

and

,1

cbN

aN

NAA (4)

where:

A an additional signal attenuation dueto the building partitions, walls, or other

barriers;

Nthe number of the partitions;A1 the average attenuation of the

partition;

constants a, b, and c are determined

during the experiment.

As it is seen, this model is not very easyto use in the practicable simulations.

2.2 Simulation tools

Currently, there are a number ofsimulations tools that allow indoor

signal propagation under LOS and

International Journal of Digital Information and Wireless Communications (IJDIWC) 2(1): 26-32The Society of Digital Information and Wireless Communications, 2012(ISSN 2225-658X)

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NLOS conditions to be predicted. In this

work In order to simulate signal

propagation in indoor conditions the"Aerohive online Wi-Fi planner" tool

was chosen. This programme has a web-

based interface and helps the indoorwireless network to be designed andallows the RF environment, where the

wireless network will be installed to be

simulated; it also allows setting theaccess point (AP) parameters

(frequency, power, standard, etc.). There

is the possibility to design your indoor

map, to put the AP into the desiredlocation and obtain a simulated

coverage.

3 MEASUREMENT SCENARIOS

The experiments were carried out on the

fourth floor of the building of theTelecommunications and Electronics

Department of the Kaunas University of

Technology (Fig. 1). Measurementswere made by increasing the distance

between the transmitter and receiver step

by step in small portions. It is the way

for detecting effects that could cause thiscomplex geometry of the corridor. The

height of the corridor and rooms is 3.00

m. The total length of the corridor is 155m. The measurements were carried out

up to 90 m. The length of the rooms

varied. The signal reflection andscattering effects from the surface of the

restrictive corridors have a large impact

on the measurements. As it is known,

these two effects strongly depend on thedielectric properties and signal

frequency [10]. Reflective surfaces in

the corridor and rooms include glass and

wooden doors and plaster walls. In thisresearch the dielectric properties of these

materials were not specifically

measured. Two scenarios (Scenarios 1for the LOS conditions and Scenarios 2

for the NLOS conditions) were applied

when a wireless router was fixed at a

height of 1.47 m and a receiver wasmoved along the central axis of rooms or

the corridor:

The following two wireless routers wereused as the original signal sources: D-Link DIR300 version 2.01, which

supports the IEEE 802.11g standard, and

Trendnet TEW410 APB, which supportsthe IEEE 802.11n standard. The main

specifications are: Module technique

16QAM/ 64QAM/BPSK/QPSK with

OFDM, frequency is 2462 MHz,transmit power is 16 dBm for DIR300;

Module technique is OFDM, frequency

is 2437 MHz, transmit power is 15.5dBm for TEW410APB.

Measurements were carried out by

means of the spectrum analyzer Anritsu

Cell Master MT8212A. The spectrumanalyzer was lifted to a height of 1.47

meters and was moving along the axial

line of the corridor or the hall.10 measurements were made in each

point, and the average value of these

results was calculated. The statistical

treatment of these results for the LOSconditions (Scenario 1) showed that the

root mean square deviation was

approximately 2.8 dBm for the 802.11gstandard and about 3.3 dBm for the

802.11 n standard (depending on the

distance from the transmitter). Thecorrelation coefficient was about 0.88

and 0.83 for the 802.11 g and 802.11 n

standards respectively. The root mean

square for the NLOS conditions(Scenario 2) was 2.1 dBm for the 802.11

standard and 2.4 dBm for the 802.11 n

standard. The correlation coefficient was

0.945, which shows a strong relationbetween the results. As can be seen, the

deviation for the 802.11g standard is less

than for the 802.11n standard. These

International Journal of Digital Information and Wireless Communications (IJDIWC) 2(1): 26-32

The Society of Digital Information and Wireless Communications, 2012(ISSN 2225-658X)

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statistical results illustrate the accuracy

of the experiment measurements.

4 MEASUREMENT RESULTS AND

ANALYSIS

4.1 Scenario 1

Figure 2 shows the measurement data

when wireless routers D-Link andTrendnet are treated as the access points

(AP).

The results are compared with the free-

space model (FSL) results. It is clearlyseen that for the Tx-Rx distances higher

than 15 m path losses are less than for

the FSL for both D-Link and Trendnet.Such signal's levels especially gain

compared with FSL, and this is observed

approximately from the distance of 40

m. This can be explained by waveguideeffects [5, 79] which are minimized

multi path effects. The fact that the

corridor functions as a waveguide isproved by the fact that according to

formula (1) the best approximation of

the results is achieved with n < 1,6 for

the D-Link and n < 1,3for Trendnet. Asit is seen, n for the 802.11n is less than

for the 802.11g. This means that the

waveguide effect for 802.11n is strongerthan for the 802.11g.

The strong two maxima are clearly

visible at about 37 and 60 m from Tx.The comparison of the measured results

with losses in a free space show that at

the distance Tx-Rx approximately 60 m

the difference is about 11 dBm for theD-Link, while, for the Trendnet case, the

difference is about 17 dBm. At 37 m

distance this difference is stronger for

the 802.11 n cases and is about 23 dBm.For the 802.11g this difference only is

about 8 dBm. This reaffirms the fact that

the waveguide effect is stronger for the

802.11n technology than for the

802.11g.

Figure 1. Rooms and Corridor Plan. The basis of

arrows shows the wireless router fixed position;

the direction of the arrow shows the receiver

moving direction.

-80

-70

-60

-50

-40

-30

-20

-10

0

0 10 20 30 40 50 60 70 80 90 100

Receivedsignallevel,dBm

Tx -Rx, m

802.11g FSL 802.11n FSL

802.11g LOS 802.11n LOS

Figure 2. Received signal level vs Tx and Rx

separation for 802.11g and 802.11n standards.



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takes into account the distance from the

transmitter, the number of walls, single

wall absorption, the power of thetransmitter, the transmitter and receiver

antenna gain, and the transmitted

frequency. Mathematically, this model isdescribed as follows:

),log(, SKmSLFSLGTP rtr (5)

where:

Prthe received signal level, dBm;

Tthe transmitter power, dBm;

G the transmitter and receiver antennagain, dBi;

FSLfree space losses, dB;

SL the separate walls absorptionlosses, dBm;

SK the number of walls, m, is a

coefficient, which depends on the 802.11

standard.According to these measurements, in the

802.11n case, m is 16.2 and in the

802.11g case, m is 18.1. Themathematical description of this new

model, as can be seen, to some extent, is

similar to the multi-partitioning model.

However, in our opinion, the new modelmay be more convenient for the user

because it requires less measurements,

for example, in order to evaluate theconstants a, b, and c.

5 CONCLUSIONS

1. Experimental results showed that

under LOS conditions the waveguide

effect, which can affect the appearance

of the other effects when signal travelsthrough the complex geometry corridor,

is seen very clearly.2. The "Aerohive online Wi-Fi planner"simulation tool's results do not fullycoincide with the experimental results.The simulation provides higher signallosses than practical. So this simulation

tool suggests more than enough for acertain coverage to be achieved.3. A new model (5) for predicting802.11g/n standard NLOS signalpropagation in buildings withhomogenous partitions is proposed inthis research paper.4. Experimental results may be used to

improve models for WLAN datatransmission prediction in the indoor

NLOS environment.

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INVESTIGATION OF WI-FI INDOOR SIGNALS UNDER LOS AND NLOS CONDITIONS

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