Experimental Study of Depolarization and Antenna Correlation in Tunnels in the 1.3 GHz band Frederic Challita IEMN/TELICE University of Lille Villeneuve d’Ascq, France Frederic.Challita@univ-lille.fr Davy P. Gaillot IEMN/TELICE University of Lille Villeneuve d’Ascq, France Davy.Gaillot@univ-lille.fr Pierre Laly IEMN/TELICE University of Lille Villeneuve d’Ascq, France Pierre.Laly@univ-lille.fr Pierre Degauque IEMN/TELICE University of Lille Villeneuve d’Ascq, France Pierre.Degauque@univ-lille.fr Martine Lienard IEMN/TELICE University of Lille Villeneuve d’Ascq, France Martine.Lienard@univ-lille.fr Wout Joseph IMEC-WAVES Ghent University Ghent, Belgium Wout.Joseph@Ugent.be Abstract— Measurements have been carried out in a low- traffic road tunnel to investigate the influence of the polarization of the transmitting and receiving antennas on the channel characteristics. A real-time channel sounder working in a frequency band around 1.3 GHz has been used, the elements of the transmitting and receiving arrays being dual- polarized patch antennas. Special emphasis is made on cross- polarization discrimination factor and on the spatial correlation between array elements which has a great influence on the performances of transmit/receive diversity schemes. Various polarizations both at the transmitter and the receiver have been tested to minimize this spatial correlation while keeping the size of the array as small as possible. Keywords—propagation, tunnel, polarization, spatial correlation I. INTRODUCTION Wireless communications in road tunnels, as Vehicle-to- Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications, are essential to develop safe and efficient autonomous driving. A large number of papers have already been published on the channel characteristics in tunnel environment, deduced either from theoretical models or from measurements. An analytical approach usually assumes that the tunnel has a canonical shape, rectangular or circular, in order to get closed-form expressions of the field distribution inside the tunnel. Such an approach, for example based on the modal theory, can be useful for interpreting the results of the simulation [1], [2]. In [3], a simple testing system was used for measuring radio frequency power attenuation with distance in a railway tunnel. The theoretical predictions based on ray tracing and modal methods are compared to field measurements. To take the real structure of the tunnel into account, a fine discretization of the walls and distributed obstacles as cables, lamps, vents is needed but leads to important computational time. Furthermore the presence of vehicles moving in the tunnel is not taken into account. During the last decade, many experiments have thus been carried out, mainly in the GHz band and for a V2I configuration. If narrow band and wide band channel characteristics are sufficient for optimizing a Single Input Single Output (SISO) link, measurements of the H transfer channel matrix are needed for Multiple Input Multiple Output (MIMO) communication or for optimizing transmit/receive diversity schemes. In [4], ray-tracing was utilized to obtain the MIMO channel matrices at 1.8 GHz, while in [5] channel characteristics including correlation between antennas and eigenvalue distribution of the H matrix for MIMO systems are deduced from measurements in the 3 GHz and 5 GHz band. Nearly all these papers assume that the transmitting antenna (Tx) and the receiving antenna (Rx) have the same polarization, usually horizontal or vertical. Furthermore, if the tunnel is closed to traffic, a vector network analyzer (VNA) can be used, Tx and Rx being connected to the VNA owing to fiber optics [5]. In this case, the longitudinal step between 2 successive measurement points is between 5 and 10 m while virtual arrays were obtained by also moving the antennas in the transverse plane of the tunnel. To overcome the previous constraints, results presented in this paper are deduced from measurements made in a low- traffic road tunnel by using a real-time channel sounder working at a center frequency of 1.35 GHz. Since both path loss and correlation between array elements play a leading role not only in MIMO systems but also on the performances of Single-Input Multiple-Output (SIMO) or Multiple-Input Single-Output (MISO) transmission techniques, the influence of the Tx and Rx polarizations on the amplitude of the received signal and on the spatial correlation is studied [6]. To keep the size of the arrays as small as possible while keeping an acceptable value of the spatial correlation, polarization diversity schemes will also be investigated. In Section II, the channel sounder characteristics are briefly recalled, more details being given in [7], while Section III describes the configuration of the experiments. Channel gain for various polarizations and cross-polar discrimination factor (XPD) are studied in Section IV. The last Section deals with spatial correlation between array elements by introducing polarization diversity in order to keep arrays of small size. II. PRINCIPLE AND PERFORMANCES OF THE CHANNEL SOUNDER The sounder is based on a space-frequency division multiplexing and an OFDM transmission technique. The numbers Ntx and Nrx of Tx and Rx ports are equal to 8 and 16, respectively. The 6560 subcarriers of the OFDM scheme are distributed on a 80 MHz band, the spacing between each subcarrier being thus 12.2 kHz. Since the size of the Tx array can be chosen between 1 and 8, the subcarriers can be allocated to only one antenna or distributed among all Tx elements. Fig. 1 shows the subcarrier allocation between 8 transmitting antennas. In this case, the spacing between the 1024 subcarriers sent to each antenna is 97.6 kHz.