Evaluation of the Quality of the Wi-Fi Channel of the Rolling Stock

Network

Lyubov Mikhailovna Zhuravleva, Vladislav Vitalievich Levshunov, Denis Alexandrovich Ryzhkov,

Anton Anatolyevich Antonov and Mikhail Alekseevich Nilov

Russian University of Transport (RUT), Russia, Moscow

Keywords: Monitoring, Wi-Fi channel, error probability, signal fading.

Abstract: The paper presents the results of research on the quality of the Wi-Fi channel used for technical monitoring

of the condition of rolling stock and railway transport infrastructure. For this purpose, a method has been

developed for calculating the probability of an error in the Wi-Fi channel between stationary base stations and

mobile in local network of the train, taking into account the features of modulation formats and experimental

data on communication quality. The presented methodology is based on the analysis of the effect of slow and

fast fading of the signal at the receiver input during the movement of the train, which lead to deep channel

speed dips. The reliability of the channel quality estimates obtained using the presented methodology is

confirmed by the results of calculating the error probability based on the processing of experimental data.

1 INTRODUCTION

Currently, information transmission systems based on

Wi-Fi technology are widely used in railway transport

for the implementation of technical monitoring of

rolling stock and infrastructure facilities. The main

advantage of Wi-Fi technology is the possibility of

two-way information exchange and constant

monitoring of the infrastructure without deterioration

of the electromagnetic environment. With the help of

a network of base stations (BS) deployed along the

railway track, technical monitoring data can be sent

to car depots, marshalling yards, centralized traffic

control and safety center (Popov, 2020).

To organize monitoring using Wi-Fi technology,

it is necessary to assess the quality of the channel

between stationary and train base stations. The

solution of this problem is connected with the

development of a methodology for calculating the

probability of error Р



., taking into account the

peculiarities of the propagation of decimeter range

signals and the modulation formats used.

Thus, as a result of interference of reflected waves

at the receiver input and changes in the level of the

desired signal during the movement of the train, slow

fading (s/f) and fast fading (f/f) of the envelope of the

carrier signal occur. This leads to dips in the signal-

to-noise ratio (s/n) and a deterioration in the quality

of communication. To combat these phenomena,

stationary BS (SBS) and train BS (TBS) are

periodically switched – handover and correction of

the modulation format: code positionality а(from 4 to

1024) and the type of modulation (BPSK, QPSK,

QAM), the encoding rate k/s (from 1/2 to 5/6), the

protective interval (cyclic prefix from 400ps to

800ps), the number of spatial streams from 1 to 4 (

IEEE 802.11ax, 2022; Denisov, 2019)

2 MATERIALS AND METHODS

As shown by experimental data obtained from the

input of the TBS (MaximaTelecom, 2021; Antonov,

2022; Zhuravleva, 2022), the consequence of the

occurrence of s/f and f/f are adaptive changes in the

modulation format, which lead to sharp dips in

channel speed (c/s) and relatively long fluctuations in

c/s ranging from 700Mb/s to 60Mb/s and below (fig.

1). The consequences of handover, fluctuations and

sharp failures of the c/s directly affect the efficiency

and quality indicators of Wi-Fi technology, and above

all, increase the probability of errorР



So, the fading of the signal at the input of the

demodulator (DM) of the receiver under the action of

s/f and f/f should be evaluated using parameters in the

form of the ratio of signal power and noise (s/n): a)

for slow fading through

𝛼

/



(relative to the sensitivity

Zhuravleva, L., Levshunov, V., Ryzhkov, D., Antonov, A. and Nilov, M.

Evaluation of the Quality of the Wi-Fi Channel of the Rolling Stock Network.

DOI: 10.5220/0011577000003527

In Proceedings of the 1st International Scientiﬁc and Practical Conference on Transport: Logistics, Construction, Maintenance, Management (TLC2M 2022), pages 38-41

ISBN: 978-989-758-606-4

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

threshold of the receiver); b) for fast fading through

𝛼

/



(relative to random fluctuations of the carrier

envelope) (Ratynsky, 1998).

A decrease in the signal level during the

movement of the train (𝛼

/



) and fluctuations in the

envelope (𝛼

/



) as a result of the reflected signals

lead to abnormal errors. The moments of error

occurrence are marked in Fig.1 by sharp dips or

fluctuations in c/s at the output of the channel

decoder, which reacts to the deterioration of signal

propagation conditions. The magnitude of the

probability of an abnormal errorР

ан

, meaning an error

in at least one character, depends on the values

𝛼

/



and 𝛼

/



(Gorelov, 2013). As a result of adaptive

correction of the modulation format, for example, a

decrease in positionality аwith a decrease in s/n, it is

possible to maintain the quality of communication



at a given level at the cost of a sharp decrease in

the information transfer rate.

3 RESULTS AND DISCUSSIONS.

METHODS FOR ASSESSING

THE QUALITY OF THE WI-FI

CHANNEL

The method of calculating the error probability



of channel quality is based on the analysis of

experimental data on the operation of the train's Wi-

Fi channel and the assessment of anomalous

distortions of the desired signal from the action of s/f

and f/f. Abnormal symbol errors occur due to

fluctuations in the signal envelope at the output of the

demodulator (DM), which lead to time shifts (𝛥𝑡) of

the pulse position within its interval (Fig.2) (Fomin,

1975).

Figure.1. Experimental characteristics of the functioning of the channel between the SBS and the TBS of the Wi-Fi

network.

100

200

300

400

500

600

700

00414.795559

00419.415604

00424.035562

00428.655557

00433.275616

00437.895779

00442.515564

00447.135781

00451.755563

00456.375573

00460.995559

00465.615567

00470.235561

00474.855561

00479.475582

00484.095591

00488.715547

00493.335558

00497.955555

00502.575562

00507.195568

00511.815564

00516.435564

00521.055551

00526.097496

00530.717506

00535.337503

00539.957524

00544.998498

00549.618487

00554.238480

00558.858486

00563.478476

00568.098482

00572.718480

00577.338474

Mbps

dBm

Time

TxMCS (Complex parameter of the modulation characteristic and coding scheme)

RSSI (The indicator of the desired signal)

HO_CurrM (Handover of mobile BS)

TxRate (Channel speed of WI-Fi connection)

HO_CurrAP (Handover of stationary BS)

Evaluation of the Quality of the Wi-Fi Channel of the Rolling Stock Network

Figure 2. The mechanism of formation of time shifts of the

symbol due to envelope fading: and, accordingly, the

envelope of the pulse and noise from f/f; and the moments

of operation at the level (points A and B) accordingly, in

the absence of noise and with noise.

If the value 𝛥𝑡 exceeds the protective interval

(cyclic prefix 𝜏

/

), an error of false symbol

discrimination will occur. With the threshold DM

algorithm (as the simplest in technical

implementation), the first emission of the signal

envelope beyond the boundaries of the interval is

triggered. Thus, an abnormal error will occur if two

conditions are met: 1) the magnitude 𝛥𝑡  𝜏

/

𝛥𝑡 ≻

𝜏

/

; 2) the amplitude of the symbol А



𝑉



. The

fulfillment of the first condition is estimated by

probability Р𝛥𝑡  𝜏

/

 ; the second – by the

probability of false triggeringР

л/ср

. To calculate the

probability Р𝛥𝑡  𝜏

/

 , knowledge of the

elementary probability law (EPL) of envelope

fluctuations due to f/f is required. Noise or distortion

of the envelope from f/f is the result of interference

(the superposition of many reflected rays of a desired

signal having different initial phase and amplitude).

Therefore, the EPL law of random fluctuations of the

envelope corresponds to the normal law (Wentzel,

1983). The magnitude of the variance of these

fluctuations 𝜎





can be determined on the basis of

experimental data using the 3- sigma rule. So, taking

into account the sensitivity of the TBS

receiver р

/

−95 𝑑𝐵𝑚, the average value of s/n

𝛼

/



= 65 dBm and the magnitude of the fluctuations

of the parameter 𝛼

б/з



(5𝑑𝐵𝑚), the noise dispersion

is equal to: 𝜎





≅10



Вт.

Time shift 𝛥𝑡 (of the pulse) of the symbol is a

random variable, which is also distributed according

to the normal law, since it is a consequence of the

impact of noise from f/f. The variance 𝜎





of the time

shift is determined through the variance 𝜎





and the

steepness of the pulse 𝐷



at the threshold level 𝑉



follows (Gorelov,2013; Fomin, 1975):

𝜎

𝛥𝑡



𝜎

ℎ

𝐷

𝑠

. (1)

For angles (between threshold and envelope) at a

level 𝑉



≈





close to the maximum steepness of the

pulse 890, the variance of the time shift 𝛥𝑡will be the

value 𝜎





≅0,3⋅10



The probability Р𝛥𝑡  𝜏

/

 for 𝜏

/



400 𝑝𝑠 (IEEE 802.11ax, 2022; Denisov, 2019) is

calculated from the expression:

Р𝛥𝑡  𝜏

/

















𝑒𝑝𝑥−











,

where 𝑥𝛥𝑡. (2)

Using the tabular integral 𝑉𝑥(complement to the

probability integral) (Gorelov, 2013), we obtain the

following value forР𝛥𝑡  𝜏



:

Р𝛥𝑡  𝜏

ц/пр

Р𝑥







√





ехр−











𝑑𝑧

where

𝑥













,⋅



,⋅



2,31

;

Р𝛥𝑡 ≻𝜏

pref

 9,176 ⋅ 10



(3)

The probability of false triggering Р

𝑓/𝑡𝑞𝑟

, taking

into account the Rayleigh elementary probability law

of the envelope at the output of the DM (Gorelov,

2013; Fomin, 1975), is determined by the formula:

/

ехр−𝑉





/2𝜎



, (4)

where 𝜎

is the variance of the signal envelope

amplitude.

Based on experimental data of the signal power

level spread from s/f (Fig.1) within -45 dBm and -25

dBm from an average value of -35 dBm, the value

𝑠

 3,162⋅ 10

−4

and 𝜎

 7,115⋅ 10

−5

can be

estimated.

Hence, the probability Р

𝑓/𝑡𝑔𝑟

ехр−𝑉



2𝜎

  ехр−2,1 ≅ 0,122.

The probability of an abnormal error Р

𝑎𝑏𝑛

is:



Р𝛥𝑡𝜏

/

⋅Р

/

≅ 0,011 (5)

To calculate the probability of error Р



when

transmitting a frame with the number of elementary

characters кof the order of a thousand, it is necessary

ан

to divide by 𝐾, namely:

𝑒𝑟𝑟





1,11⋅10

−5

(6)

The final value of the Wi-Fi channel quality can be

obtained if we take into account the action of the

channel decoder, which, due to the block-

convolutional code, reduces the probability of error

by at least two orders of magnitude (Gorelov, 2013).

It is possible to confirm the correctness of the

method of analytical calculation of the error

probability proposed above using the results obtained

after statistical processing of the data obtained as a

result of the experiment (MaximaTelecom, 2021;

Antonov, 2022).

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CONSTRUCTION, MAINTENANCE, MANAGEMENT

Analysis of the nature of fluctuations in the channel

speed and the frequency of sharp dips of c/s below

50Mb/s showed that during 1200s, when the train was

moving at a speed of no more than 50 km/h, 22 dips

of c/s occurred with an average duration of 5ms. Each

dip of the c/s is a reaction of the channel decoder to a

decrease in s/n due to slow and fast fading in order to

"soften" the effect of envelope distortions and reduce

the magnitude of the error in character recognition.

This interpretation of deep dips of c/s allowed us to

calculate the value of the probability of dips of

c/s Р

с/𝑠

, namely: Р

𝑐/𝑠

≈9,17⋅10

−5

(IEEE

802.11ax, 2022; Denisov, 2019). Hence, the

magnitude of the error probability calculated

analytically and the estimate obtained statistically

have the sa me order. This indicates the correctness

of the task, the validity of the developed methodology

and the reliability of the results obtained.

4 CONCLUSIONS

1. The proposed method of calculating the quality

of the Wi-Fi channel using the probability of

error allows us to evaluate the capabilities of

wireless technology for technical monitoring in

railway transport.

2. The results of assessing the quality of the Wi-

Fi channel, obtained on the basis of

experimental data and using the developed

methodology, indicate the possibility of using

it to calculate the probability of error.

3. The acceptable values of the error probability

should be evaluated taking into account the

features of monitoring railway transport

facilities, for example, a crossing carried out

using intelligent video surveillance systems

(IVSS).

4. Based on the requirements for the values of the

probabilities of false and correct detection of

dangerous objects at crossings, it is possible to

obtain acceptable values of the probability of

error in the Wi-Fi channel used to transmit

video surveillance information to the train

IVSS recognition device.

REFERENCES

Popov, P., 2020. Development of unmanned technologies in

railway transport, https://habr.com/ru/post/502202/.

IEEE 802.11ax, 2022.

https://ru.wikipedia.org/wiki/IEEE_802.11ax

Denisov, D., 2019. Overview of Wi-Fi technology,

https://nag.ru/material/35534.

MaximaTelecom, 2021. https://maximatelecom.ru/.

Antonov, A., A., Zhuravleva, L., M., Levshunov, V., V.,

Ryzhkov, D., A., 2022. Prospects for the use of wireless

technologies in railway transport. In Automation,

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Evaluation of the Quality of the Wi-Fi Channel of the Rolling Stock Network