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This paper presents channel propagation characteristics of different multiple-input multiple-output (MIMO) systems using ray tracing approach in a confined area at 1.8 GHz according to the LTE-M standards. Leaky coaxial cables were exploited at different transmitted locations to visualize the fluctuated radiated field under different polarization combinations. In order to encounter this vision, the reflected and line-of-sight paths are under consideration for both vertically and horizontally polarized waves emitting from the leaky coaxial cables (LCXs). Emphasis is given to understand the effect of LCX configuration on the channel correlation coefficient and capacity (C) in the confined area. The exploration of experimental results reveals that the MIMO channel using LCXs has significant performance, specifically in the case of horizontal polarization. Furthermore, it is inferred that for the longer distance between transmitter and receiver, the correlation coefficients have higher magnitude.

Long-Term Evolution for metro technology is well-known as the stable communication approach for advanced urban rail transit system due to the few drawbacks of the wireless local area networks (WLANs) in the communication-based train control (CBTC) systems. The important findings for the LTE-M are improved spectral efficiency and high data rates communication. LTE-M utilizes LCX for reliable and long-term communication in underground tunnels, subways, malls, etc. [

Due to supplementary degree of freedom provided by the spatial multiplexing, multiple-input multiple-output (MIMO) technology is recognized as a promising technique to achieve higher spectral efficiency. In [

The influence of signal correlation on the

Since there exists an enormous interest for the MIMO system deployment for the LTE-M system in the tunnel environment, we propose a radiated model for the different MIMO channels at 1.8 GHz. For measurement campaign, LCXs with the vertical and horizontal polarizations are considered, which are laid parallel to each other at some finite distance from the receiver. For evaluating the feasibility of MIMO channel, radiated field from LCXs is examined for the scattered (NLOS) and the line-of-sight (LOS) paths. Moreover, the correlation coefficients and channel capacities are investigated for different MIMO systems. We found that the MIMO system using LCX deployment in a confined area has favorable performance in terms of higher capacity and lower correlation. The paper is organized as follows. Section

The procedure of computing the scattered radiated field of slots on the conductor sheet is very complex. When one side of the leaky coaxial cable is associated to the source, the transversal radio waves propagating within the cable diffuse from the continuous slots. The transverse proportion has insignificant impact compared to the wavelength, and the radiations from the respective slot are equivalent to the individual magnetic dipole in a free space. In this paper, we considered the horizontally and vertically polarized LCXs. The length of LCX is regarded as

The slot structure of LCX (a). The radiated field of LCX (b).

The LCXs are located along

The entire radiated field is acquired by accumulating the effects of all individual slots of the whole LCX

The time-domain pseudonoise (PN) sequence correlation technique was assumed in the measurements. Keysight E8267D vector signal generator (VSG) constitutes the transmitter (Tx), and it generates a BPSK signal modulated by PN sequence of 511 chips. The chirp rate was considered as 40.8 MHz which was the same as the transmitted signal bandwidth. The signal is fed to one side of the LCX, and a 50-ohm load was attached to the other port. For the receiver side, a dipole antenna was associated with the R&S FSG spectrum analyzer. The measurement setup configuration is shown in Figure

The measurement setup sketch.

The inner view of the Nantong tunnel (a, b). Tunnel construction in the wireless InSite software (c).

Measuring parameters.

Unit | Parameter |
---|---|

Carrier frequency | 1.8 GHz |

Transmitted power | 20 dBm |

Bandwidth | 40.8 MHz |

Tx-antenna | ZTT-LCX of 50 m length |

Rx-antenna | UHA9125D dipole antenna |

Antenna gain | 2.15 dBi |

Height of LCX at location 1 | 2.7 m |

Height of the Rx antenna | 1.6 m |

LCX spacing | Loc1 and Loc3, 0.8 m |

LCX spacing | Loc1 and Loc5, 1.2 m |

LCX spacing | Loc3 and Loc5, 0.4 m |

Slot period of LCX | 0.6 m |

Sampling rate | 81.6 MHz |

Resistance | 50 ohms |

Measurement time | 50 ms |

We divided the total rectangular tunnel into three regions, and each region constitutes a rectangular grid of measurement points (

The virtual antenna array view of the Nantong tunnel.

The first LCX was vertically polarized (periodic slots were directed obliquely), and the other type of LCX was horizontally polarized (periodic slots were directed perpendicular to the LCX axis). As compared to the conventional antennas, LCXs can provide more predictable and homogeneous signal strength for indoor environment. The relationship between different virtual receiving antenna arrays and the analytical received power by considering LOS path only for the horizontally polarized

The received power for horizontally polarized LCX MIMO system in region 1.

To understand the LCX-based MIMO channel performance, we considered two properties such as channel correlation coefficients and channel capacity.

The appropriate condition for the MIMO channel efficiency is the existence of orthogonal subchannels. The degree of independence of the subchannels can be tested through the correlation analysis. The computation technique for the correlation of the MIMO channel gains from two independent transmitters with the same receiving antenna is discussed comprehensively in [

The

The spatial correlation coefficients’ relation with the virtual antenna array for

The correlation coefficients comparison for

In practice, the MIMO channel capacity (C) does not depend only on channel multipath but also on the overall received power and the average signal-to-noise ratio (SNR) through the channel. The following two important aspects are generally not independent and approximately some compromise needs to be practiced.

Assuming equal power is transmitted throughout all transmitters and no channel state information (CSI) is available at the transmitter but developed perfectly at the Rx, then the capacity of the

As a result, the capacity of the MIMO channel can be achieved by considering the effects of all measurement points in each local region.

Wireless system engineering (WiSE) is a software-based system that calculates both outdoor and indoor radio channel characteristics using ray tracing method. Wireless InSite provides few antenna patterns, for example, half-wave dipole, Lambert’s law, automatic, and isotropic [

The ECDF comparison for

For further investigation of MIMO systems, we considered the Massif Central tunnel in south central France to examine the correlation coefficients for different MIMO systems [

Each of the regions is 100 m long. LCXs were positioned at 0.2 m away from the sidewall. Region 1 covers area from 0 m to 100 m, region 2 contains area from 350 m to 450 m, and region 3 consists of area from 700 m to 800 m. Initially, we assumed the 2 m distance between two consecutive receivers. For each region, we measured the MIMO channel characteristics at 51 different receiving locations which were at least 4.1 m away from the LCXs and they were 1.6 m high from the ground level as shown in Figure

The different measuring locations of the Massif Central tunnel in south central France.

The correlation coefficients for vertically and horizontally polarized LCX-based

The mean value of correlation coefficients for the 2 m and 5 m separation distances.

Separation distance | Polarization type | Region 1 | Region 2 | Region 3 |
---|---|---|---|---|

2 m | Horizontal | 0.22 | 0.32 | 0.35 |

Vertical | 0.38 | 0.35 | 0.30 | |

5 m | Horizontal | 0.23 | 0.34 | 0.30 |

Vertical | 0.36 | 0.36 | 0.23 |

The empirical distribution function (ECDF) of the capacity for the Massif Central tunnel was further examined for the fixed SNR of 10 dB and the 1.8 GHz frequency by utilizing Eq. (

The ECDF comparison of capacity for

Afterward, we estimated the capacity gain for the

The mean value of capacity for the lambda (

Polarization | Massive tunnel, |
Massive tunnel, | ||||
---|---|---|---|---|---|---|

Region 1 | Region 2 | Region 3 | Region 1 | Region 2 | Region 3 | |

Horizontal | 6.0501 | 7.6854 | 6.4409 | 5.6440 | 6.2031 | 5.8050 |

Vertical | 5.6305 | 5.8452 | 5.7363 | 5.5716 | 5.5629 | 5.4603 |

The mean value of capacity for the half-lambda (

Polarization | Massive tunnel, |
Massive tunnel, | ||||
---|---|---|---|---|---|---|

Region 1 | Region 2 | Region 3 | Region 1 | Region 2 | Region 3 | |

Horizontal | 6.6213 | 6.7603 | 6.2468 | 5.4893 | 5.8439 | 5.7145 |

Vertical | 5.5483 | 5.8283 | 5.5026 | 5.4658 | 5.4584 | 5.4062 |

By using Figures

This paper studies the LCX-based MIMO systems’ measurement campaign in a tunnel environment based on radiated theory and ray tracing method at 1.8 GHz. In order to realize the

The measurement data used to support the findings of this study have not been made available because of the protection of intellectual property rights. Moreover, the data are shared with our partner company and will be used in our future work. Also, the measurement work costs a lot of manpower and equipment.

The authors declare that they have no conflicts of interest.

This work was supported by the National Natural Science Foundation of China under Grants 61571282 and 61871261 and a fund from the Shanghai Cooperative Innovation Center for Maglev and Rail Transit.