Shadow Fading Analysis for High-Speed Railway Station Channels Pengyu Liu1, Bo Ai1, David Matolak2, Ruoyu Sun2, and Yan Li1 1 State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, P. R. China, 100044, [email protected], [email protected] 2 Department of Electrical Engineering, University of South Carolina, USA, 29208, [email protected], [email protected] Abstract High-speed railways are a key component for modern transportation, as they can provide safe and convenient service for passengers and goods. In order to ensure efficient and well-organized operation of high-speed railways, GSM for railways (GSM-R) has been used to connect high-speed trains with control system centers. In order to optimize radio performance, one must quantitatively characterize the radio propagation channel for high-speed railways. In this paper, we report on measurement campaigns along the Zhengzhou-Xi’an and Beijing-Shanghai high-speed railway lines at Gongyinan, Sanmenxia, Xingyangnan and Suzhou Dong high-speed railway stations. We present shadow fading analysis for the measurements made at these stations. 1. Introduction High-speed railway (HSR) is an economical, flexible and safe transportation method, very competitive in comparison to air and road transportation. Railways require moderate area, can effectively use energy, and are influenced little by severe weather. Therefore, railways have increasingly become attractive to governments, industries, and academia for coordinated development. Reliable communication links are vital to maximizing railway efficiency, and the wireless propagation channel’s effects must be quantified to attain this communication reliability. Propagation channel modeling is a significant research area for HSR in the design of GSM for railway (GSM-R) wireless communication systems. In recent years, high-speed railway channel characterization has been studied by numerous researchers. There are various propagation environments along HSR lines, such as plains, viaducts, cuttings, tunnels, crossing bridges and mountain areas. Each of these scenarios is likely to impose unique characteristics on the HSR propagation channel. Thus, it is indispensable to find accurate channel models for each scenario. Researchers have proposed channel characterization models for viaducts [1]-[2], cuttings [3]-[4], tunnels [5]-[9], and crossing-bridges [10]. These channels have been characterized in terms of the propagation path loss exponent, shadowing losses, fade depths and fading distributions, and often the level crossing rate and Ricean K-factor. The railway station is yet another unique propagation area. The railway station is a place for loading and unloading goods or passengers. There are typically several parallel rail tracks, multiple platforms, and awnings in the railway station. Awnings are built above the platform to shelter the station from weather. For radio propagation, this awning structure can block some radio signal energy between base stations and the high-speed trains, reducing received signal strength. To date, the high-speed railway station as a special semi-open area propagation environment has been covered by only a very limited amount of work. Reference [11] proposed two empirical models for the extra propagation loss in the high-speed railway stations at 930 MHz. The authors associated the extra loss with four parameters: the distance between transmitter (Tx) and train station, type of train station, rail track and propagation zone. However, they did not address the effect of shadowing for the high-speed railway station. The authors of reference [12] conducted measurements at Beijing-Tianjin HSR station at 2.1 GHz with 3.84 MHz bandwidth. They presented broadband channel results including 978-1-4673-5225-3/14/$31.00 ©2014 IEEE maximum root-mean square delay spread (RMS-DS) and probability density functions (PDF) of path numbers, but likewise did not consider shadowing. To fill this gap, in this paper, we perform an analysis of shadowing for the high-speed railway station. This includes a statistical distribution of the shadowing loss, and shadowing spatial correlation. Section 2 describes the four high-speed railway stations where we conducted our measurement campaigns, and summarizes the channel measurement equipment. In Section 3 we analyze our measured shadowing loss. Section 4 concludes this paper. 2. Channel Measurement 2.1 Measurement Environment We conducted our measurement campaign at 930 MHz along Zhenghzou-Xi’an HSR in September-November 2009 and along the Beijing-Shanghai HSR in January 2011 HSR in China. Four HSR railway station (Xingyang Nan, Gongyi Nan and Sanmenxia Nan along Zhenghzou-Xi’an HSR and Suzhou Dong along Beijing-Shanghai HSR) were chosen for measurement. We list the latitude and longitude of the base stations, and starting and ending positions of the rail tracks in Table I. TABLE I. GEOGRAPHIC COORDINATES OF THE TRACKs AND BASE STATIONs Base Station Starting Position Ending Position NL 34.744652 NL 34.744812 NL 34.747158, Xingyang Nan E 113.419096 E 113.414577 E 113.384470 NL 34.671383 NL 34.670185, NL 34.657603 Gongyi Nan E 112.910598 E 112.908292 E 112.881738 NL 34.749251 NL 34.749150, NL 34.745868, Sanmenxia Nan E 111.154641 E 111.154628 E 111.129420 NL 33.67786825 NL 33.674503 NL 33.645105 Suzhou Dong E 117.24574664 E 117.246860 E 117.250690 We also provide pictures of the four HSR stations in Fig. 1. The red solid ellipses mark the locations of the awnings in the HSR stations. They block a portion of the first Fresnel zone between a base station and the high-speed trains, hence radio propagation energy at the receiver is weakened by these metal awnings. 2.2 Measurement Equipment An operational GSM for railway (GSM-R) network was utilized in the measurements. The transmitter was a GSM-R base station (BS) with dual-polarized directional antennas, 17 dBi gain, 65◦ horizontal and 6◦ vertical beam widths mounted on 50-meter towers. The broadcast control channel (BCCH) signal with a carrier frequency of 930 MHz is fed to the base station antenna as the transmission signal. The BCCH signal is a continuous signal with bandwidth approximately 200 kHz. The output power of the transmitter along Zhengzhou-Xi’an HSR line is 40 dBm, whereas that along the Beijing-Shanghai HSR line is 46 dBm. The receiving antenna on the train was an omnidirectional antenna, with 4 dBi gain, 65◦ vertical beam width, located in the middle part of the train, on the top of the carriage at a height of 30 cm above the roof. A Willtek 8300 Griffin fast measurement Rx was utilized to collect and store measured data. The distance sensor was set on a wheel of the locomotive to record the wheel speed. The measurement locations were resolved with a global positioning system (GPS) Rx. The data display and operation distance computing are conducted using ourself-designed“Liu-Jie” GSM-R field strength measurement software on a portable computer. (a) Xingyang Nan (b) Gongyi Nan (c) Sanmenxia Nan (d) Suzhou Dong Fig. 1. Four HSR railway stations. 3. Measurement Results Propagation path loss is useful for link budgets in the design of wireless communication systems. Path loss is modeled versus the logarithm of distance, and can be expressed as follows: L(d)=A+10nlog10(d/d0)+X, (1) where A is the fit intercept at reference distance d0, d is transmitter-receiver link distance, n is the path loss exponent, and X is a zero mean Gaussian random variable in dB with standard deviation σx. Shadow fading expresses the large-scale variation about the overall fitted path loss. Hence, for a given distance d, shadow fading can be calculated from the measurement results by subtracting the expected (fitted) path loss from the measurement data. We present the cumulative distributions of the shadow fading for the four HSR stations. We applied the Gaussian (normal) distribution to fit the measurement data. The minimum value of shadow fading (negative of the maximum of shadowing loss) for the HSR stations are -9 dB, -4 dB, -7 dB and -4.2 dB, respectively. The maximum shadow fading values of the four stations are 5 dB, 6.2 dB, 17.7 dB and 4.63 dB, respectively. Sanmenxia Nan has a shadowing standard deviation of approximately 3.63 dB, whereas that of other three stations are near 2 dB. 1 1 Shadowing fading for Xingyang Nan Normal distribution, Sigma=2.1 dB 0.8 0.8 0.6 0.6 0.4 0.4 Cumulative probability Cumulative probability 0.2 0.2 Shadowing fading for Gongyi Nan Normal distribution, Sigma=2.04 dB 0 0 −8 −6 −4 −2 0 2 4 −4 −2 0 2 4 6 Shadowing Fading (dB) Shadowing fading (dB) (a) Xingyang Nan (b) Gongyi Nan 1 1 0.8 0.8 0.6 0.6 0.4 0.4 Cumulative probability Cumulative probability 0.2 0.2 Shadowing fading for Sanmenxia Nan Shadowing fading for Suzhou Dong Normal distribution, Sigma=3.63 dB Normal distribution, Sigma=2.24 dB 0 0 −5 0 5 10 15 −4 −2 0 2 4 Shadowing fading (dB) Shadowing fading (dB) (c) Sanmenxia Nan (d) Suzhou Dong Fig. 2. Fitting distribution of shadowing for the four HSR stations. We also perform a spatial correlation analysis of the shadow fading. We use (d) to determine the correlation of shadowing as a function of change in receiver distance written as ாሼൣ௑ሺௗሻିா൫௑ሺௗሻ൯൧ൣ௑ሺௗା௱ௗሻିா൫௑ሺௗା௱ௗሻ൯൧ሽ ρ(d) = (2) ඥ௩௔௥ሾ௑ሺௗሻሿ௩௔௥ሾ௑ሺௗା௱ௗሻሿ where X(d) denotes shadow fading—with small-scale multipath fading averaged out-- at distance d, E[·] and Var[·] denote the expected value and the variance, respectively. We define a threshold where correlation coefficient goes below 0.5 as uncorrelated, thus the stationarity distance is where correlation remains above 0.5.