- Research
- Open Access
Base-band involved integrative modeling for studying the transmission characteristics of wireless link in railway environment
- Dawei Li^{1, 2}Email author,
- Junhong Wang^{1, 2},
- Meie Chen^{1, 2},
- Zhan Zhang^{1, 2} and
- Zheng Li^{1, 2}
https://doi.org/10.1186/s13638-015-0316-3
© Li et al.; licensee Springer. 2015
- Received: 6 June 2014
- Accepted: 2 March 2015
- Published: 20 March 2015
Abstract
Base-band involved integrative modeling method (BIMM) is proposed for studying the transmission characteristics and bit error rate of the wireless communication, in which the transmitting and receiving antennas, wave propagation environment, and the modulation and demodulation modules are modeled and simulated integratively. Comparing with the conventional wave propagation method used in prediction of field coverage, BIMM is capable of taking the interaction between antenna and environment into consideration, and can give the time-domain waveforms of signals in different places of the wireless link, including those in the base-band modules. Therefore, the distortion reason of signal and in what place it happens can be found, and the bit error rate of the wireless communication can be analyzed. The BIMM in this paper is implemented by finite-difference time-domain (FDTD) method and is applied to the wireless link of railway communication. The effect of the electric spark generated by the power supplying network of the express train on the transmitting property and bit error rate is analyzed.
Keywords
- Integrative modeling method
- Wireless communication link
- FDTD
- Base-band signal
- Railway environment
1 Introduction
With the rapid development and wide application of wireless communication, the prediction of the wave propagation and field coverage of the wireless link becomes important in characterizing the radio channel of the wireless communication, especially for the case of complicated environment, such as the railway environment. Usually, measurement is taken to get the field coverage of wireless communication systems, but measurement is expensive and time consuming and cannot give realistic results in some cases. For example, the results measured by an antenna in free space (normally used in measuring field strength) cannot reflect the realistic case of an express train system, in which the antenna is installed on top of the locomotive and is influenced by the locomotive body. Besides the measurement, numerical methods are also used in prediction of field coverage. There are basically two kinds of numerical methods used, one is the high-frequency method and the other is the full-wave method. The typical high-frequency method used is the ray-tracing method [1-4], which can give results with acceptable accuracy for simple and regular environments [3,4]. However, ray-tracing method is not suitable for analyzing the field coverage of the wireless link in complicated environments, so full-wave method is used in such environments. The typical full-wave method used is the finite-difference time-domain (FDTD) method [5,6], which has been used in studying the wave propagation in the micro-cell of mobile communication [7], between different floors of buildings [8], and in indoor environment [9]. It has also been combined with ray-tracing method to study the effect of wall on the indoor field coverage [10-12]. But these works did not consider the interaction between the transmitting/receiving antennas and environments and can only give the field distribution (field parameters). In our previous work, integrative modeling method (IMM) is proposed for characterizing the wireless link in complicated environments [13,14]. In IMM, the wireless link is decomposed into three parts, namely the transmitting antenna with its neighboring environment, the wave propagation environment between transmitting and receiving antennas, and the receiving antenna with its neighboring environment. These three parts can be integratively modeled and simulated by full-wave method only (small problem) or by hybrid methods involving full-wave methods and high-frequency methods (large problem). The difference between the IMM and other available methods is that IMM can not only give the field distribution in environment but also give the voltages/currents of input and output signals of the antennas (circuit parameters). With IMM, the effect of the environment on the output signal and the reason of signal distortion can be analyzed. In [13,14], FDTD is used to model and simulate the transmitting/receiving antennas together with their neighboring environments, and ray-tracing method is used to calculate the wave propagation in the rest environment.
However, IMM can only deal with the RF link of the wireless system and can give the reason of distortions of RF signals. But how these distortions affect the base-band signal? And how the environment influences the bit error rate of the wireless system? These are the motivations of this paper. In this paper, the modeling and analyzing of RF link (IMM) and base-band modules are combined together, and we call it the base-band involved integrative modeling method (BIMM). By this method, the time-domain waveform of signal at different places of the wireless link can be obtained, including those intermediate signal waveforms in base-band modules. So the reason of signal distortion and in what place it happens can be found, and the bit rate error of the complete wireless link can be studied. To our knowledge, there are no similar available works that can be used to analyze the relationship between distortion of base-band signal and environment.
In this paper, the wireless link of the railway communication is studied. Global system for mobile communications railway (GSM-R)-modulating signal, which is extensively used in express train, is adopted as the excitation signal. The detail description of the problem and the exciting signal involving the base-band information are given in Section 2. In Section 3, several examples are given to show the efficiency of the BIMM. Conclusions are drawn in Section 4.
2 Problem description and method
2.1 Problem description
Digital base-band signal modulates the carrier, and the modulated RF signal transmits to the space through the transmission antenna. When the wireless link environment is small enough, the transmitting and receiving antennas and the environment can be directly involved into the FDTD meshes by setting corresponding parameters at different mesh grids. The FDTD iteration will produce full-wave effects including the reflection, diffraction, and refraction between the antennas and environment. These effects will impact on the time-domain waveforms of the output signal of the receiving antenna. When the wireless link environment is too large to deal with by available computer resource, FDTD method could combine with ray-trace method to solve long-range propagation problem as mentioned in [13]. In this paper, we only consider the small-scale wireless link environment, so only FDTD method is used. The RF signal output from the receiving antenna is restituted through the demodulation module. By comparing the difference between input and output base-band signals, the transmission property of the whole wireless link and the error of bit rate can be studied.
2.2 RF signal generation and base-band signal recover
where B is the bandwidth of the LPF and T is the symbol period. BT is a typical parameter of GMSK, and different BTs present different correlations between adjacent symbols. Due to the limitation of computer resource, the rate of symbols in this paper is set to 5.4166 × 10^{7} bps.
where E _{ c } is the energy of single bit, f _{0} is the center frequency of carrier, and ϕ _{0} is the random phase of carrier. In this paper, the orthogonal frequency modulation is used and the center frequency is set to 900 MHz.
2.3 Simulation scheme
In this paper, a small scale wireless link environment is modeled using FDTD algorithm, which includes the transmitting and receiving antennas and the propagation environment. To further enhance the simulation speed, we adopted the parallel algorithm. Combining message passing interface (MPI) with FDTD to realize parallel computation has been proven to be an efficient way in improving the computing speed. The parallel algorithm of FDTD utilizes a one-cell overlap region to exchange the information between adjacent sub-domains, and only the tangential magnetic fields are exchanged at each time step [16].
After all the parameters are set, the base-band signal is modulated into RF signal which excites the transmission antenna and starts the FDTD computation. The receiving antenna output the voltage amplitude in each time step of FDTD iteration for post-processing. This output RF signal involves the environment effect, such as the multi-path effect and the interference between antennas and nearby materials, and the demodulated base-band signal also involves the environment effect. Using this base-band-involved integrative modeling method, the entire wireless communication link can be simulated accurately and the reason of bit error can be found.
3 Implementation and results
3.1 Modeling of antennas and environment
Parameters of materials used in the model
Model name | ε _{ r } | μ _{ r } | σ (S/m) |
---|---|---|---|
Locomotive body | 1.0 | 1.0 | 2.494 × 10^{7} |
Vehicle windows | 5.5 | 1.0 | 0.0 |
Power supply wire | 1.0 | 1.0 | 2.494 × 10^{7} |
Steel rails | 1.0 | 1.0 | 1.1 × 10^{6} |
Earth plane | 14.0 | 1.0 | 0.01 |
The center frequency of the antennas is set to 900 MHz, and the frequency band covers the signal bandwidth. The length and cross section of the transmitting half-wavelength dipole are 150 and 25 × 25 mm, respectively, and the feeding gap between two arms of the dipole is 10 mm. The length and cross section of the monopole mounted on the locomotive top is 70 and 25 × 25 mm, respectively, and the feeding gap between locomotive top plane and monopole is 10 mm. The dipole antenna is located 1 m above the locomotive and with a distance of 14.3 m (in the rear of the locomotive) to the monopole antenna. Non-uniform grid technique and parallel FDTD algorithm are utilized. The general grid size of FDTD is set to 30, 30, and 30 mm in the x, y, z directions, respectively, while the non-uniform grid size in the antenna and nearby region is set to 12.5, 12.5, and 10 mm, respectively. The locomotive is located along the x-direction. The surrounding environment is divided into 1,109 × 218 × 312 ΔS grids. The feeding gap of the locomotive antenna is located at the coordinates of (755, 105, 190 ΔS), and the feed gap of the transmitting dipole is located at (105, 172, 254 ΔS). Δt, the time step of FDTD, equals 16.667 ps, and the total computation time steps are set to 11,070.
3.2 Impact of environment on transmission signal
3.3 Impact of EMI on base-band digital signal
Parameters of the electromagnetic spark used in this paper
Parameters | Values |
---|---|
\( {U}_0^p \) | 2.962 × 10^{4} V |
\( {t}_0^p \) | 4.5 ns |
\( {f}_0^p \) | 500 MHz |
\( {\tau}_0^p \) | 4.0 ns |
It is also found from Figure 12b that the influence of the spark on sampling and judgment lasts more than 200 ns although the spark only lasts 9 ns. This is due to the ringing of the spark pulse on the antenna. In addition, comparing Figure 12c with Figure 8, we can see that the impact of high-intensity electromagnetic pulse on bit error seems not so great (because only two errors occur). However, if comparing base-band waveforms in Figure 12b and Figure 10a carefully, it is found that the judgment of the symbols is at high risk in the period from 200 to 400 ns, because the waveform is significantly distorted. In other words, the accuracy of the symbol judgment cannot be guaranteed between the ninth symbol and the 20th symbol.
4 Conclusions
Estimation of the transmission characteristics of wireless link is an important work especially for the wireless communication in complicated environment, such as the high-speed railway environment. But conventional method can only give the field coverage and cannot give the effect of environment on the transmission property of the wireless link. This paper proposed a base-band involved integrative modeling method which can not only take the interaction between antennas and environment into consideration but also give the effect of environment on the base-band signal and bit error rate of the wireless communication. Railway environments are taken as examples to show the implementation and efficiency of the method, and the results show that the proposed method is effective in finding the reason of waveform distortion of base-band signal and can give the explanation of the occurring of bit errors.
Declarations
Acknowledgments
This work was supported in part by NSFC under grant no. 61331002 and in part by the National Key Basic Research Project under grant no. 2013CB328903.
Authors’ Affiliations
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