# MIMO in tunnel: impact of polarization and array orientation on the channel characteristics

Frédéric Challita, Pierre Laly, M. Lienard, Davy Gaillot, Pierre Degauque, Jose-Maria Molina-Garcia-Pardo, Wout Joseph

► To cite this version:

Frédéric Challita, Pierre Laly, M. Lienard, Davy Gaillot, Pierre Degauque, et al. MIMO in tunnel: impact of polarization and array orientation on the channel characteristics. 6th IEEE Radio and Antenna Days of the Indian Ocean, RADIO 2018, Oct 2018, Mauritius, Mauritius. pp.100-101, 10.23919/RADIO.2018.8572376. hal-03346247

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# MIMO in tunnel: impact of polarization and array orientation on the channel characteristics

Frédéric Challita, Pierre Laly, M. Lienard, Davy Gaillot, Pierre Degauque, Jose-Maria Molina-Garcia-Pardo, Wout Joseph

► To cite this version:

Frédéric Challita, Pierre Laly, M. Lienard, Davy Gaillot, Pierre Degauque, et al. MIMO in tunnel: impact of polarization and array orientation on the channel characteristics. 6th IEEE Radio and Antenna Days of the Indian Ocean, RADIO 2018, Oct 2018, Mauritius, Mauritius. pp.100-101, 10.23919/RADIO.2018.8572376. hal-03346247

HAL Id: hal-03346247

https://hal.science/hal-03346247v1

Submitted on 17 Jun 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# MIMO in Tunnel: Impact of Polarization and Array Orientation on the Channel Characteristics

F. Challita, P. Laly, M. Lienard, D. P. Gaillot and
P. Degauque

Lille University/IEMN/TELICE
Villeneuve d'Ascq, France
Martine.Lienard@univ-lille1.fr

J. M. Molina-Garcia-Pardo

Dpto. Tecno. de la Información y las Comunicaciones
Universidad Politecnica de Cartagena
Cartagena, Spain
josemaria.molina@upct.es

W. Joseph

Ghent University/iMinds
Gent, Belgium
wout.joseph@ugent.be

***Abstract***—Polarimetric characteristics of the propagation channel in a straight tunnel and in a frequency band around 1.3 GHz, are deduced from measurements using a real-time MIMO channel sounder. Emphasis is on the cross-polarization discrimination factor and, for a MIMO configuration, on the correlation between transmitting and receiving array elements, depending on the array orientation and on the polarization of the transmitting elements.

## I. INTRODUCTION

Wireless communications in confined areas, as in road and railway tunnels have already been widely studied. Path loss and delay spread have first been determined for a single-input single-output (SISO) link but the new generation of communication systems is based on multiple-input multiple-output (MIMO) systems. For frequencies on the order or greater than 1 GHz, it was shown that the channel capacity can be improved by using MIMO instead of SISO links. However, this improvement is strongly related to MIMO channel characteristics as, for example, the correlation between the transmitting (Tx) / receiving (Rx) array elements which are usually co-polarized [1]. To decrease this correlation while keeping non prohibitive element spacing, a possible solution could be to combine MIMO and polarization diversity. Determination of the statistical polarimetric channel properties and the wideband channel characteristics, are of prime importance.

Such MIMO measurements are often made by using a vector network analyzer or an equivalent system, a cable or a fiber optics thus connecting Tx and Rx as in [2]. The main drawbacks are firstly that the number of measurement points along the tunnel is quite limited and, secondly, that the tunnel must be closed to traffic. In this paper measurements have thus been carried out with a real time channel sounder, the receiving part being onboard a vehicle. Different orientations of the antenna arrays together with various polarizations of the array elements have been tested to show their influence on the mean path loss, on the cross polar (X-polar) discrimination factor

(XPD) and on the correlation between Tx or Rx elements. Section II briefly recalls the main features of the sounder that we have designed, while Section III describes the geometry of the tunnel and the different configurations of the Tx/Rx antenna arrays. Section IV presents some results on the influence of polarization on channel characteristics.

## II. MAIN FEATURES OF THE CHANNEL SOUNDER

The MIMO (8,16) sounder has a center frequency of 1.35 GHz with a transmission bandwidth of 80 MHz. It is based on an interleaved frequency division multiplexing technique. Since the size $N_{tx}$ of the Tx array can be chosen, the 6560 subcarriers can be allocated either to only one antenna or distributed among all Tx antennas. As an example for $N_{tx}=8$, the frequency spacing between subcarriers transmitted by each antenna is 97.66 kHz. In the time domain, the maximum duration of the channel impulse response is 10.24 µs with a time resolution of 12.5 ns.

A switch with 50 dB isolation is used to change the polarization of each dual-polarized Tx antenna to avoid doubling the RF front end. Therefore, 2 OFDM symbols are successively sent, the first one for one polarization and the other one for the orthogonal polarization. On the contrary, at Rx, the signals measured at each of the 2 outputs of dual-polarized patch antennas are simultaneously stored. With symbol duration of about 82 µs, the acquisition time of a full polarimetric channel matrix (deduced from the reception of 2 successive symbols) is about 300 µs.

## III. GEOMETRICAL CONFIGURATION

Measurements have been carried out in a 2-way road tunnel but one way was closed to traffic. The configuration of the tunnel and the implementation of the Tx and Rx arrays are shown in Fig. 1. The tunnel, 600 m long, has a maximum width of 9.7 m and a maximum height of 4.6 m. Each antenna of the 8-element uniform arrays (numbered 1 to 8 in Fig. 1) is a dual-polarized patch antenna, the port isolation and cross
# MIMO in Tunnel: Impact of Polarization and Array Orientation on the Channel Characteristics

F. Challita, P. Laly, M. Lienard, D. P. Gaillot and
P. Degauque

Lille University/IEMN/TELICE
Villeneuve d'Ascq, France
Martine.Lienard@univ-lille1.fr

J. M. Molina-Garcia-Pardo

Dpto. Tecno. de la Información y las Comunicaciones
Universidad Politecnica de Cartagena
Cartagena, Spain
josemaria.molina@upct.es

W. Joseph

Ghent University/iMinds
Gent, Belgium
wout.joseph@ugent.be

***Abstract***—Polarimetric characteristics of the propagation channel in a straight tunnel and in a frequency band around 1.3 GHz, are deduced from measurements using a real-time MIMO channel sounder. Emphasis is on the cross-polarization discrimination factor and, for a MIMO configuration, on the correlation between transmitting and receiving array elements, depending on the array orientation and on the polarization of the transmitting elements.

## I. INTRODUCTION

Wireless communications in confined areas, as in road and railway tunnels have already been widely studied. Path loss and delay spread have first been determined for a single-input single-output (SISO) link but the new generation of communication systems is based on multiple-input multiple-output (MIMO) systems. For frequencies on the order or greater than 1 GHz, it was shown that the channel capacity can be improved by using MIMO instead of SISO links. However, this improvement is strongly related to MIMO channel characteristics as, for example, the correlation between the transmitting (Tx) / receiving (Rx) array elements which are usually co-polarized [1]. To decrease this correlation while keeping non prohibitive element spacing, a possible solution could be to combine MIMO and polarization diversity. Determination of the statistical polarimetric channel properties and the wideband channel characteristics, are of prime importance.

Such MIMO measurements are often made by using a vector network analyzer or an equivalent system, a cable or a fiber optics thus connecting Tx and Rx as in [2]. The main drawbacks are firstly that the number of measurement points along the tunnel is quite limited and, secondly, that the tunnel must be closed to traffic. In this paper measurements have thus been carried out with a real time channel sounder, the receiving part being onboard a vehicle. Different orientations of the antenna arrays together with various polarizations of the array elements have been tested to show their influence on the mean path loss, on the cross polar (X-polar) discrimination factor

(XPD) and on the correlation between Tx or Rx elements. Section II briefly recalls the main features of the sounder that we have designed, while Section III describes the geometry of the tunnel and the different configurations of the Tx/Rx antenna arrays. Section IV presents some results on the influence of polarization on channel characteristics.

## II. MAIN FEATURES OF THE CHANNEL SOUNDER

The MIMO (8,16) sounder has a center frequency of 1.35 GHz with a transmission bandwidth of 80 MHz. It is based on an interleaved frequency division multiplexing technique. Since the size $N_{tx}$ of the Tx array can be chosen, the 6560 subcarriers can be allocated either to only one antenna or distributed among all Tx antennas. As an example for $N_{tx}=8$, the frequency spacing between subcarriers transmitted by each antenna is 97.66 kHz. In the time domain, the maximum duration of the channel impulse response is 10.24 µs with a time resolution of 12.5 ns.

A switch with 50 dB isolation is used to change the polarization of each dual-polarized Tx antenna to avoid doubling the RF front end. Therefore, 2 OFDM symbols are successively sent, the first one for one polarization and the other one for the orthogonal polarization. On the contrary, at Rx, the signals measured at each of the 2 outputs of dual-polarized patch antennas are simultaneously stored. With symbol duration of about 82 µs, the acquisition time of a full polarimetric channel matrix (deduced from the reception of 2 successive symbols) is about 300 µs.

## III. GEOMETRICAL CONFIGURATION

Measurements have been carried out in a 2-way road tunnel but one way was closed to traffic. The configuration of the tunnel and the implementation of the Tx and Rx arrays are shown in Fig. 1. The tunnel, 600 m long, has a maximum width of 9.7 m and a maximum height of 4.6 m. Each antenna of the 8-element uniform arrays (numbered 1 to 8 in Fig. 1) is a dual-polarized patch antenna, the port isolation and cross
polarization being larger than 30 dB and 25 dB across the
whole frequency band, respectively.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Cross section of the tunnel and positions of the Tx and Rx arrays.</figcaption></figure>

The spacing between the centers of each successive Tx/Rx element is 17 cm, i.e. 0.76 wavelength (λ) at 1.35 GHz. The orientation of each path in the cross section of the tunnel can be changed and, in the experiments, the dual polarizations of each patch are either vertical (V) / horizontal (H) or +45°/-45°. Successive orientations of the Tx array were also considered: H, V or 45° as shown in Fig. 1. In any case, the Rx array is horizontal and put at the rear of a small van, at a height of about 2 m above ground

IV. EXPERIMENTAL RESULTS

In a first step, some results on the polarization effect on
path loss are presented, including statistical values of the
discrimination factor. Then the correlation between array
elements are studied as a function of their polarization.

A. Field Variation and Cross Polar Discrimination Factor

As an example, curves in Fig. 2 show the variation of the channel gain G between antennas Tx1 and Rx1, for co-polar (VV) or cross-polar (VH) polarizations, the first and second letters referring to the polarization of Tx and of Rx, respectively. G has been averaged over the whole transmitting bandwidth to improve the clarity of the Figure by avoiding strong fluctuations of the curves when deep fading occur in this frequency selective channel. Fig. 2 shows that the average additional attenuation for X-polar does not depend on the distance *d* between Tx and Rx.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Channel gain for co- and cross-polarization</figcaption></figure>

The cumulative distribution function (cdf) of G related to
the previous configuration is plotted in Fig. 3. We see that
curves related to VV and VH are parallel and that XPD is
about 14 dB. The same approach has been made for the
various antennas and polarizations mentioned in Section III in
order to deduce statistical results.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Cdf of the gain for co- and X-polarizations</figcaption></figure>

B. *Correlation between Array Elements*

For co-polarized Tx or Rx elements, the correlation
increases with the distance Tx-Rx. For example, for an element
spacing of 34 cm (1.5 λ), the correlation varies from 0. 7 to 0.9
when d increases from 50 m to 300 m. To decrease correlation
while minimizing the spacing between array elements, a
solution could be using Rx and/or Tx polarization diversity, the
elements of the linear array being H and V successively
polarized. However, X-polar reception leads to a strong
decrease of the Rx power as shown previously. To cope with
this problem, a possible solution could be to use Tx antennas
having a polarization of 45° and to receive the H component on
an Rx element and the V component on an adjacent element.

For the same spacing of Rx elements as previously, this
leads to an average correlation of 0.6 at 300 m instead of 0.9
when using successive co-polar elements. A statistical
approach has been made by considering all Tx and Rx
elements. Lastly, the increase in MIMO ergodic capacity by
introducing polarization diversity was also studied.

ACKNOWLEDGEMENT

This work has been done in the frame of the ELSAT2020
project co-financed by the European Union with the European
Regional Development Fund, the French state and the Hauts de
France Region Council.

REFERENCES

[1] P. Laly, M. Lienard, P. Degauque, C. Sanchis-Borras, and J. M. Molina Garcia Pardo, “Propagation in road tunnels: Predicted performances of LTE precoding techniques,” IEEE RADIO Conf., Proc. pp. 1-1, 25 – 28 Sept. 2017

[2] R. Sun, D. W. Matolak, C. Tao, L. Liu, Z. Tan dna T. Zhou, “Investigation of MIMO channel characteristics in a two-way section tunnel at 1.4725 GHz,” Hindawi Int. J. of Antennas and Propag., Article ID 3693149, 12 pages, 2017
polarization being larger than 30 dB and 25 dB across the
whole frequency band, respectively.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Cross section of the tunnel and positions of the Tx and Rx arrays.</figcaption></figure>

The spacing between the centers of each successive Tx/Rx element is 17 cm, i.e. 0.76 wavelength (λ) at 1.35 GHz. The orientation of each path in the cross section of the tunnel can be changed and, in the experiments, the dual polarizations of each patch are either vertical (V) / horizontal (H) or +45°/-45°. Successive orientations of the Tx array were also considered: H, V or 45° as shown in Fig. 1. In any case, the Rx array is horizontal and put at the rear of a small van, at a height of about 2 m above ground

IV. EXPERIMENTAL RESULTS

In a first step, some results on the polarization effect on
path loss are presented, including statistical values of the
discrimination factor. Then the correlation between array
elements are studied as a function of their polarization.

A. Field Variation and Cross Polar Discrimination Factor

As an example, curves in Fig. 2 show the variation of the channel gain G between antennas Tx1 and Rx1, for co-polar (VV) or cross-polar (VH) polarizations, the first and second letters referring to the polarization of Tx and of Rx, respectively. G has been averaged over the whole transmitting bandwidth to improve the clarity of the Figure by avoiding strong fluctuations of the curves when deep fading occur in this frequency selective channel. Fig. 2 shows that the average additional attenuation for X-polar does not depend on the distance *d* between Tx and Rx.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Channel gain for co- and cross-polarization</figcaption></figure>

The cumulative distribution function (cdf) of G related to
the previous configuration is plotted in Fig. 3. We see that
curves related to VV and VH are parallel and that XPD is
about 14 dB. The same approach has been made for the
various antennas and polarizations mentioned in Section III in
order to deduce statistical results.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Cdf of the gain for co- and X-polarizations</figcaption></figure>

B. *Correlation between Array Elements*

For co-polarized Tx or Rx elements, the correlation
increases with the distance Tx-Rx. For example, for an element
spacing of 34 cm (1.5 λ), the correlation varies from 0.7 to 0.9
when *d* increases from 50 m to 300 m. To decrease correlation
while minimizing the spacing between array elements, a
solution could be using Rx and/or Tx polarization diversity, the
elements of the linear array being H and V successively
polarized. However, X-polar reception leads to a strong
decrease of the Rx power as shown previously. To cope with
this problem, a possible solution could be to use Tx antennas
having a polarization of 45° and to receive the H component on
an Rx element and the V component on an adjacent element.

For the same spacing of Rx elements as previously, this
leads to an average correlation of 0.6 at 300 m instead of 0.9
when using successive co-polar elements. A statistical
approach has been made by considering all Tx and Rx
elements. Lastly, the increase in MIMO ergodic capacity by
introducing polarization diversity was also studied.

ACKNOWLEDGEMENT

This work has been done in the frame of the ELSAT2020
project co-financed by the European Union with the European
Regional Development Fund, the French state and the Hauts de
France Region Council.

REFERENCES

[1] P. Laly, M. Lienard, P. Degauque, C. Sanchis-Borras, and J. M. Molina Garcia Pardo, “Propagation in road tunnels: Predicted performances of LTE precoding techniques,” IEEE RADIO Conf., Proc. pp. 1-1, 25 – 28 Sept. 2017

[2] R. Sun, D. W. Matolak, C. Tao, L. Liu, Z. Tan dna T. Zhou, “Investigation of MIMO channel characteristics in a two-way section tunnel at 1.4725 GHz,” Hindawi Int. J. of Antennas and Propag., Article ID 3693149, 12 pages, 2017