This paper presents results from a wide band singleinput–singleoutput (SISO) and 16
To ease the growing population pressure on urban traffic, major cities in China have accelerated subway construction. It is expected that by 2020 more than 45 cities in China will have subways, and all new subways under construction must have a large rush hour capacity. Increasing train running speed and reducing the interval between adjacent trains enable a passenger capacity increase, but at the same time this introduces some security risks such as those due to train malfunction or other disasters (e.g., fires). Thus, these trains require various levels of monitoring to ensure safe and efficient operation. For example, video information should be obtained in real time at various locations.
A wide band and MIMO wireless communication system can satisfy the increasing data transmission requirements for both train control systems and passenger communications. Before any MIMO system is deployed underground, the MIMO channel should be accurately characterized. Much work in the literature has discussed MIMO link performance by measurement and various theoretical methods.
Extensive work has been done on propagation and MIMO channel capacity based on measurements in subway tunnels. Using electromagnetic field theory, for example, [
The authors of [
The organization of the paper is as follows. In Section
Our measurement campaign was conducted in the Tunnel Laboratory in Zhongtian Technology Group Company, which is located in Nantong City, Jiangsu Province. The laboratory was built to test the performance of the company’s leaky cables. The tunnel is made of reinforced concrete and consists of two parts of different crosssectional shape, rectangular and circular; see Figure
Tunnel laboratory in Zhongtian Technology Group.
The measurement system is shown in Figure
Measurement parameters.
Center frequency  1.4725 GHz 
Signal bandwidth  91 MHz 
Antenna type  Biconical 
Antenna pattern  Omnidirectional (azimuth) 
Antenna deployment  SISO 
Probe signal  ZC sequence, length 2047 
Transmit power  10 dBm 
Measurement system.
ZadoffChu (ZC) sequences are often used as synchronization signals due to their good orthogonality and low peaktoaverage power ratio properties. The complex value of each sequence is given by [
The channel impulse response (CIR)
As noted, the length of the ZC sequence used here is 2047. Hence the maximum delay that can be measured is (1/91 MHz) × 2047 =
To measure the path loss (PL) in the tunnel, the Tx was moved from one end to the other in 1 m intervals. The PL is given by
The path loss results obtained from the SISO measurement and those from the free space model and ray tracing simulations, all versus linear distance, are shown in Figure
The path loss can be divided into two sections: before and after a distance of approximately 45 m. For the short range region, the measured path loss is up to 70 dB. In this short range section, the measured path loss is similar to, but slightly less than, the path loss of free space. After reaching 70 dB at 45 m, the measured path loss fluctuates around a value of approximately 60 dB. The drop at 45 m hence appears to come from the reduction of the cross section at the middle of the tunnel (transition from rectangular to circular), where the waveguide effect evidently increases; this waveguide effect is also expected to be stronger at longer link distances. The steady trend after 45 m is attributable to low order reflected multipath components (MPCs) adding to the LOS component; these MPCs are strongly affected by waveguide effect. The ray tracing results exhibit the same basic trend as the measurements but show more fluctuations in both sections. We point out that these path loss results did not average out small scale fading effects; hence the results are not used for path loss modeling but rather to observe overall trends in this twosection tunnel.
The RMSDS is given by [
Figure
SISO path loss measurement.
Path loss versus distance for measurement, ray tracing, and free space results.
RMSDS versus distance for measurement and ray tracing results.
RMSDS with rear door reflection
RMSDS without rear door reflection
As Figure
Measurement location illustration for virtual MIMO measurements.
Figure
A PDP example for the 25 m link distance.
The measured PDPs for link distances from 15 m to 85 m appear in Figures
PDP at 15 m.
PDP at 25 m.
PDP at 35 m.
PDP at 65 m.
PDP at 75 m.
PDP at 85 m.
In Figures
The PDPs show that the reflections from the rear door are strong at each position and can be separated from the LOS component. It also can be seen that in tunnel I the duration of reflected multipath from the rear door, as part C in Figure
In general, in small scale MIMO systems, antenna correlation and the MIMO capacity are of primary concern. But in medium or large scale MIMO systems, the received signal strength is also of interest, because when the number of antennas increases, distinct fading across the array may appear in some scenarios. In addition, in confined spaces like indoors and in tunnels, the capacity of any MIMO system is always influenced by specific characteristics of the antenna deployment. Thus novel metrics may be of use to help us understand how the signal propagates; ideally these metrics would be straightforward to compute and intuitive. An
Table
Number of multipath components using fixed threshold and SAGE.
Position  15 m  25 m  35 m  65 m  75 m  85 m 

Fixed threshold  9  12  12  12  9  9 
SAGE  12  13  12  12  10  10 
So the MIMO amplitude matrix
For purposes of comparison, we use the normalized MIMO amplitude matrix, which is written as
The normalized MIMO amplitude matrices are displayed from 15 m to 85 m in Figure
Measured amplitude matrices at (a) 15 m, (b) 25 m, (c) 35 m, (d) 65 m, (e) 75 m, and (f) 85 m.
Simulated (ray tracing) amplitude matrices at (a) 15 m, (b) 25 m, (c) 35 m, (d) 65 m, (e) 75 m, and (f) 85 m.
Illustrations of the two propagation modes at 25 m and 35 m based upon the amplitude matrices.
25 m
35 m
Ray tracing results for the amplitude matrices are shown in Figure
To help us judge the simulation accuracy, we have computed two metrics to compare the agreement between amplitude matrices of simulations and measurements: the rootmeansquare error (RMSE) (
The results listed in Table
Estimation error metric values versus link distance (m).
Position  

15  25  35  65  75  85  
Metric  

0.3626  0.3974  0.3932  0.2933  0.3059  0.3040 

0.8377  0.8027  0.8018  0.8533  0.8553  0.8536 
Estimation error metrics after averaging.
Metric  Average over  

Tx  Rx  Tx & Rx  

0.3084  0.2792  0.2004 

0.9387  0.9333  0.9725 
Measurement (a)–(c) and simulated (d)–(f) amplitude average results over 4 antennas on Tx and Rx and both on Tx and Rx antennas at 25 m.
Tx avg
Rx avg
Tx and Rx avg
Tx avg
Rx avg
Tx and Rx avg
The azimuth angle characteristics of the channel can be extracted from our linear array measurements [
AOA and AOD at each position.
Position  AOA  AOD 

15 m 


25 m 


35 m 


65 m 


75 m 


85 m 


RMSAS of AOA and AOD versus link distance.
The capacity of a wireless communication link depends on the properties of the complex channel matrix
Curves in Figure
Normalized singular values at each value of link distance.
The maximum capacity of a memoryless
Ergodic capacity versus link distance.
Link distance (m)  15  25  35  65  75  85 
Capacity (bits/s/Hz)  34.2  37.29  35.85  28.65  23.1  34.63 
Rayleigh channel from 8 × 8 to 16 × 16 MIMO capacity.
Antenna configuration  8 × 8  9 × 9  10 × 10  11 × 11  12 × 12  13 × 13  14 × 14  15 × 15  16 × 16 

Capacity (bits/s/Hz)  21.77  24.52  27.21  29.96  32.73  35.44  38.20  40.81  43.58 
In this article, results of a wide band SISO and virtual MIMO measurement in a tunnel laboratory at a frequency of 1.4725 GHz have been presented. In the SISO case, fairly good agreement was found between the measurement and ray tracing results for path loss and RMSDS characteristics. The path loss presents strong waveguide effects for distances larger than approximately 45 m. The RMSDS is near 20 ns without the rear door reflection and such remote reflectors can obviously increase the RMSDS in tunnel scenarios.
In the MIMO case, the measurement was conducted at six link distances. The simulated PDPs show fairly good agreement with the measurements; a slight vertical shift of one antenna does not affect the PDP significantly. Amplitude matrices were introduced, and these show strong regularity characteristics at all of the six positions; specifically a strong diagonal transmission characteristic was found. This observation held more strongly when local averaging was applied to the amplitude matrices, and this may be of use in increasing the MIMO system performance. The simulation accuracy of the amplitude matrices based on ray tracing was analyzed as well, and the result also showed acceptable agreement between measurements and simulations. Angular characteristics and channel matrix singular values were also illustrated, and these were in accordance. Finally, although there are rich reflections in the tunnel, the MIMO capacity shows some resemblance to key hole effects according to the narrow angular characteristics and channel matrix singular values.
The authors declare that there are no conflicts of interest regarding the publication of this article.
The research was supported in part by the NSFC project under Grant no. 61132003, the Open Research Fund of National Mobile Communications Research Laboratory, Southeast University, under Grant no. 2014D05, and Beijing Natural Science Foundation project under Grants no. 4152043 and no. 4174102.