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
38
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
Copyright
c
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
Р
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
Р
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.
0
100
200
300
400
500
600
700
0
10
20
30
40
50
60
70
80
90
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
39
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 Р𝛥𝑡 𝜏
/
; the second – by the
probability of false triggeringР
л/ср
. To calculate the
probability Р𝛥𝑡 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
as
follows (Gorelov,2013; Fomin, 1975):
𝜎
𝛥𝑡
2
𝜎
ℎ
2
𝐷
𝑠
2
. (1)
For angles (between threshold and envelope) at a
level 𝑉
≈
А
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 Р𝛥𝑡 𝜏
/
for 𝜏
/
400 𝑝𝑠 (IEEE 802.11ax, 2022; Denisov, 2019) is
calculated from the expression:
Р𝛥𝑡 𝜏
/
х
𝑒𝑝𝑥−
,
where 𝑥𝛥𝑡. (2)
Using the tabular integral 𝑉𝑥(complement to the
probability integral) (Gorelov, 2013), we obtain the
following value forР𝛥𝑡 𝜏
:
Р𝛥𝑡 𝜏
ц/пр
Р𝑥
√
ехр−
х
𝑑𝑧
where
𝑥
,⋅
,⋅
2,31
;
Р𝛥𝑡 ≻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:
Р
/
ехр−𝑉
/2𝜎
А
, (4)
where 𝜎
А
2
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
А
𝑠
2
3,162⋅ 10
−4
and 𝜎
А
2
7,115⋅ 10
−5
can be
estimated.
Hence, the probability Р
𝑓/𝑡𝑔𝑟
ехр−𝑉
2
/
2𝜎
А
2
ехр−2,1 ≅ 0,122.
The probability of an abnormal error Р
𝑎𝑏𝑛
is:
Р
Р𝛥𝑡𝜏
/
⋅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
Р
ан
to divide by 𝐾, namely:
Р
𝑒𝑟𝑟
Р
к
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).
B
B
th
TLC2M 2022 - INTERNATIONAL SCIENTIFIC AND PRACTICAL CONFERENCE TLC2M TRANSPORT: LOGISTICS,
CONSTRUCTION, MAINTENANCE, MANAGEMENT
40
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.
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