Propagation measurements of wireless channels performed in the tunnel environments at 6 GHz are presented in this paper. Propagation characteristics are simulated and analyzed based on the method of shooting and bouncing ray tracing/image (SBR/IM). A good agreement is achieved between the measured results and simulated results, so the correctness of SBR/IM method has been validated. The measured results and simulated results are analyzed in terms of path loss models, received power, root mean square (RMS) delay spread, Ricean Kfactor, and angle of arrival (AOA). The omnidirectional path loss models are characterized based on closein (CI) freespace reference distance model and the alphabetagamma (ABG) model. Path loss exponents (PLEs) are 1.50–1.74 in lineofsight (LOS) scenarios and 2.18–2.20 in nonlineofsight (NLOS) scenarios. Results show that CI model with the reference distance of 1 m provides more accuracy and stability in tunnel scenarios. The RMS delay spread values vary between 2.77 ns and 18.76 ns. Specially, the Poisson distribution best fits the measured data of RMS delay spreads for LOS scenarios and the Gaussian distribution best fits the measured data of RMS delay spreads for NLOS scenarios. Moreover, the normal distribution provides good fits to the Ricean Kfactor. The analysis of the abovementioned results from channel measurements and simulations may be utilized for the design of wireless communications of future 5G radio systems at 6 GHz.
The next generation (5G) of wireless communications will use systems operating from 500 MHz to 300 GHz [
Many extensive EHF wireless channel measurement campaigns have been investigated for different scenarios in multiple outdoor and indoor environments, yielding empirically based path loss models and delay dispersion properties. Some research projects, such as METIS [
Although a large number of channelmeasured results can provide reliable channel model, the expense of highprecision measurement equipment is very high and the number of observation points is limited in measurement campaigns, which are both great challenges for channel sounding. As an another approach, the shooting and bouncing raytracing/image (SBR/IM) method [
Despite many measurement campaigns were conducted up to now, there are still omitted environments and frequency bands, which require to be intensively investigated. Hence, extensive actual channel measurements and simulations should be performed for various scenarios and significant frequency bands. By analyzing the measured results and simulated results, a standardized channel model should be proposed. The contribution of this paper is fivefold. First of all, channel measurements and simulations in the tunnel environments are performed and analyzed at 6 GHz, which have not been detailed in previous work. Therefore, propagation characteristics are analyzed at 6 GHz in the tunnel environment. Second, the omnidirectional path loss models are characterized based on closein (CI) freespace reference distance model and the alphabetagamma (ABG) model. In addition, the comprehensive parameter table of path loss models including measured results and simulated results for all scenarios is given. Third, the statistical analysis of the RMS delay spread for tunnel scenarios is described using measurement datasets. The fourth part of the contribution is to investigate the different distribution models based on the cumulative distribution functions (CDF) in terms of the parameters of received power, Ricean Kfactor, and angle of arrival (AOA). Finally, some important angle of arrival is extracted based on the subtractive clustering algorithm.
The rest of the paper is organized as follows. The measurement setup and environments are described in Section
Measurements were conducted in the frequency domain in which the channel impulse responses (CIRs) were measured and recorded. Figure
Overview of the measurement setup.
Channel sounding system parameters used at 6 GHz.
Parameters  Value 

Carrier frequency  6 GHz 
RF bandwidth  100 MHz 
Excitation sequence  Multicarrier signal 
Subcarrier number  2560 
Effective subcarrier number  2048 
TX/RX antenna  Omnidirectional/omnidirectional 
Transmission power  10 dBm 
TX/RX antenna gains  20 dBi/20 dBi 
TX/RX antenna height  1.8 m/1.6 m 
TXRX synchronization  Supported 
Two tunnel propagation measurement campaigns were conducted in the tunnel chamber of Beijing Jiaotong University (BJTU) in Beijing, China, in two different scenarios as shown in Figures
Measurement environment. (a) Photo of tunnel scenario 1, (b) photo of tunnel scenario 2, (c) sketched plan of tunnel scenario 1, and (d) sketched plan of tunnel scenario 2.
The shooting and bouncing raytracing/image (SBR/IM) method is developed to deal with the radio wave propagation for complex environment. It can track all the triangular ray tubes bouncing with high accuracy and computational efficiency. If the RX is within a ray tube, the ray tube will have contribution to the received field at RX and the equivalent source (image) can be determined. So SBR/IM method is an effective method which can be used to predict the propagation characteristics at 6 GHz.
The raytracing simulation performs the method of SBR/IM using the software tool, Wireless InSite [
The path loss is a significant parameter which can be applicable to describe the largescale effects of the propagation channel [
Two wellknown models are used to develop omnidirectional path loss models in this paper. First of all, the equation for the CI model is given by
The ABG model is another famous model which can be used to discuss the frequency dependence of path loss. It can be defined as [
Different path loss models have been deduced based on extensive wideband measurements at 6 GHz in terms of LOS and NLOS scenarios. Figures
Path loss variation with TXRX separation distance using CI and ABG models. (a) LOS1 path in tunnel scenario1, (b) LOS2 path in tunnel scenario 1, (c) LOS3 path in tunnel scenario 2, and (d) NLOS path in tunnel scenario 1.
Parameters in the ABG, CI, and CIopt path loss models in tunnel scenarios in terms of LOS and NLOS paths at 6 GHz. Dis. Ran. denotes distance range. Number of data points denotes the number of data points.
Sce.  Env.  Number of data points  Dis. Ran. (m)  ABG  CI  CIopt  









 
Tunnel 1  LOS1  Mea.  20  21  1.74  33.32  2  2.59  1.72  1.56  0.75  1.75  1.39 
Sim.  191  21  1.69  31.54  2  2.36  1.66  3.00  0.98  1.61  2.10  
LOS2  Mea.  12  13  2.12  31.64  2  2.35  1.69  0.53  1.23  1.45  0.38  
Sim.  111  13  2.02  31.11  2  8.27  1.50  2.80  0.97  1.55  0.51  
NLOS  Mea.  32  33  2.58  32.19  2  7.68  2.18  4.32  3.95  2.41  1.38  
Sim.  308  33  2.57  29.42  2  7.41  2.20  7.50  2.86  2.11  2.74  


Tunnel 2  LOS3  Mea.  17  18  2.33  30.99  2  1.06  1.58  1.56  1.51  1.45  1.39 
Sim.  80  18  2.32  31.40  2  6.52  1.74  3.00  1.03  1.71  2.10 
From Figure
From these analyses, we can conclude that CI model with the reference distance of 1 m provides more accuracy and stability in LOS and NLOS tunnel scenarios at 6 GHz. These simulated results are in agreement with previous works [
In wireless communication channels, the signal is transmitted and then undergoes direct reflection, transmission, scattering, and diffraction. Hence, the signal arriving at the receiver is the superposition of the various multipath components [
Cumulative distribution function (CDF) of the received power at 6 GHz in LOS and NLOS tunnel scenarios. (a) LOS1 and NLOS paths and (b) LOS2 and LOS3 paths.
Received power values for all scenarios.
Scenarios  Fitted parameters  



Max (dB)  Min (dB)  Median (dB)  
Tunnel 1  LOS1  Mea.  −53.76  6.45  −38.55  −62.31  −53.68 
Sim.  −49.53  5.90  −33.80  −64.56  −49.51  
LOS2  Mea.  −45.36  6.24  −33.37  −60.62  −45.28  
Sim.  −45.89  5.03  −34.32  −57.71  −45.82  
NLOS  Mea.  −61.4  11.49  −38.55  −81.62  −60.44  
Sim.  −56.25  10.88  −33.80  −86.13  −54.10  


Tunnel 2  LOS3  Mea.  −48.12  4.15  −37.78  −52.71  −48.99 
Sim.  −50.38  5.26  −38.28  −56.49  −52.65 
The mean excess delay and rootmeansquare (RMS) delay are two important parameters used to characterize the temporal dispersive properties of multipath channels. The mean excess delay
The RMS delay spread for LOS2, LOS3, and NLOS paths is shown in Figure
RMS delay spread variation with TXRX separation distance in LOS and NLOS tunnel scenarios. (a) LOS2 and LOS3 paths and (b) NLOS path.
CDF for RMS delay spreads at 6 GHz in LOS and NLOS tunnel scenarios. (a) LOS1 path, (b) LOS2 path, (c) LOS3 path, and (d) NLOS path.
RMS delay spread values for all scenarios.
Scenarios  Fitted parameters  



Max (ns)  Min (ns)  Median (ns)  
Tunnel 1  LOS1  Mea.  10.01  2.75  14.61  5.34  10.08 
Sim.  8.42  1.51  10.31  3.66  8.51  
LOS2  Mea.  5.41  0.86  8.76  4.40  5.28  
Sim.  7.18  2.41  12.09  2.77  2.41  
NLOS  Mea.  10.12  3.74  18.76  3.03  10.46  
Sim.  8.30  1.59  10.46  3.66  8.43  


Tunnel 2  LOS3  Mea.  9.85  3.23  18.28  5.32  3.23 
Sim.  8.18  1.73  10.02  3.67  8.48 
Kfactor is a significant parameter in wireless communications because it is able to characterize the type of fading environments [
Ricean Kfactor (KF) values at different distances between transmitter and receiver for LOS paths are shown in Figure
Ricean Kfactor variation with TXRX separation distance in LOS tunnel scenarios.
CDF for Ricean Kfactor in LOS tunnel scenarios.
The angles
In terms of the angle of arrival, some conclusions are presented. The distribution of AOA in the elevation plane has been researched [
CDF for mean AOA in tunnel scenarios.
In addition, some important arrival angles are extracted based on subtractive clustering algorithm [
Mean AOA in tunnel scenarios. (a) LOS1 path, (b) LOS2 path, (c) LOS3 path, and (d) NLOS path.
AOA parameters for all scenarios.
Scenarios  Cluster parameters  

TXRX (m)  Received power (dBm)  AOA (degree)  
Tunnel 1  LOS1  1.50  −38.03  162 
5.80  −47.32  153  
13.70  −51.68  90  
18.50  −54.05  267  
LOS2  2.70  −38.97  292  
2.90  −43.87  40  
3.40  −41.89  139  
6.20  −46.73  109  
7.10  −45.40  251  
10.30  −53.49  41  
10.90  −48.34  304  
4.40  −45.74  345  
NLOS  7.70  −47.70  142  
15.30  −50.32  296  
16.60  −50.37  47  
22.10  −60.96  147  
23.70  −65.14  288  
30.40  −75.21  122  


Tunnel 2  LOS3  2.30  −37.04  174 
5.10  −43.85  155  
7.70  −50.36  142 
In this paper, extensive measurements and characterizations of wideband tunnel channel have been proposed. Channel characteristics such as path loss models, received power, RMS delay spread, Ricean Kfactor, and AOA are described and modeled. Based on extensive radio channel sounding campaigns and simulations, it is found that the raytracing predictions agree fairly well with the measured results. Comparison with path loss models illustrates the fact that the CI model with the reference distance of 1 m was shown to be the most suitable because of its accuracy and simplicity in tunnel scenarios. The CI path loss models indicated that the PLEs vary between 1.50 and 1.74 in LOS scenarios and between 2.18 and 2.20 in NLOS scenarios. The CDF of received power follows the normal distribution. The normal distribution provides good fits to the Ricean Kfactor for LOS scenarios. The Poisson distribution model best fits the measured data of RMS delay spreads for LOS scenarios, and the Gaussian distribution model best fits the measured data of RMS delay spreads for NLOS scenarios. Moreover, the normal distribution and the uniform distribution reasonably fit the AOA well for all tunnel scenarios. In addition, propagation characteristics with the effects of human movement and vehicular motion on wireless channels require further measurement and analysis in the complex environments, which is the next research direction.
The authors declare that they have no conflicts of interest.
This work has been supported by the National Natural Science Fund under Grant no. 61372045 and by the Postgraduate Research & Practice Innovation Program of Jiangsu Province in 2016 under Grant no. KYLX16_0650. The work of this paper is funded by Radio Wave Environment Characteristics and Modeling Technology Key Laboratory under Grant no. 201600013. Moreover, the authors would like to thank Dr. R. C. Sun, Q. C. Li, Y. Lei, and Y. H. Wu for their help with the measurements and associate Professor L. Liu from Institute of Broadband Wireless Mobile Communications at Beijing Jiaotong University, Beijing, for providing radio propagation measurement equipment used for preliminary measurements.