Review on Tunnel Communication Technology

Abstract

Tunnels account for an increasing proportion of highways. Due to the semi-closed structure of tunnels, signal communication is difficult in tunnels. This review analyzes the signal data transmission requirements of intelligent network management systems, such as lighting systems, wind protection systems, fire protection systems, and vehicle and pedestrian positioning systems in tunnels. The selection of signal coverage and transmission methods are also discussed. The advantages and disadvantages of various networking methods are analyzed. This paper summarizes the wireless signal transmission, wired signal transmission and signal transmission modes of different tunnel types.

1. Introduction

With the rapid development of the economy and increasing transportation, tunnels have become an important part of the transportation network. According to statistics, as of 2020, the number of highway tunnels in China has exceeded 20,000, reaching 21,316, and the total length has exceeded 20 million meters, reaching 21.999 million meters. At present, China has the largest number of tunnels and the largest construction scale in the world. However, the shape of tunnels is generally pipe-shaped, and the space is relatively confined. An outdoor macro base station signal cannot cover a tunnel. Thus, the signal for communication in the tunnel is weak [1,2]. A growing number of experts and scholars have measured and modeled signal transmission in tunnels [3] and proposed methods for mobile data signal detection in tunnels and methods to improve the performance of wireless communication in tunnels [4,5,6,7]. The network signals in tunnels were tested by the Communications Authority of Guizhou Province [8]. According to the tests, tunnels are the main sections prone to communication network problems. It is urgent to propose a communication signal coverage solution for tunnels.

The situation in tunnels is complicated, there are many obstacles and vehicles often pass by, so the information transmission is not affected. Some scholars consider designing a reliable device deployment topology to ensure the reliability of data transmission in highway tunnels. A high-gain antenna suitable for a tunnel environment is studied, and a new method using a binary phased array antenna system to improve the quality of signal transmission in a tunnel is proposed, which satisfies the urgent need to realize high-speed and high-quality wireless communication in tunnel environments [9]. Since road tunnels are often underground or under mountains, it is unavoidable that there is often accumulated water in the tunnels. Data transmission nodes should be securely encapsulated, and many scholars have studied encapsulation methods to ensure that nodes can work accurately in the tunnel for a long time [10,11,12,13]. In order to satisfy all road tunnels using sensor terminals to monitor the tunnels, when there are a large number of sensors transmitting data through a wireless network, if the power consumption is too large, the terminal consumes power rapidly, and it takes a lot of manpower and material resources to replace the battery of the sensor terminal. Low-power terminals can save a lot of complicated work, so many scholars conduct further research on terminal low-power technology [14,15,16].

According to different data transmission requirements, such as lighting systems, wind protection systems, fire protection systems and the positioning systems of vehicles and pedestrians in the tunnel. We reasonably select the signal coverage and transmission mode and analyze the advantages and disadvantages of various networking modes. This paper summarizes the methods of signal coverage and signal transmission in the tunnel by wireless communication and wired communication. Therefore, the paper is divided into four parts, as shown in Figure 1, the first part is about the research background and significance of highway tunnel communication technology, the second part is about the wireless communication signal source, the tunnel coverage antenna feeder system, the transmission method of wireless communication, and the wireless signal networking method to introduce the use of wireless communication technology in highway tunnels, and analyze its advantages and disadvantages. The third part summarizes the commonly used wired signal transmission methods in the tunnel environment and summarizes their advantages and limitations. The fourth part is a typical tunnel signal transmission method. According to the survey, there is no comprehensive research on signal transmission in highway tunnels. This paper provides a reference for future research on signal transmission in tunnels.

2. Wireless Transmission Technology

At present, wireless communication networks have basically achieved full coverage in highways. However, among the wireless signal coverage in tunnels, the wireless communication network still has the problem of difficult coverage in some parts of tunnels with long lengths and many curves. Different tunnel shapes also have a great impact on wireless communication signals in tunnels [17,18], such as rectangular tunnels and circular tunnels [19,20]. To address the difficulty of wireless signal coverage in tunnels, it is important to establish a wireless sensor network [21] for wireless signal transmission and to calculate the communication cost of the whole network [22,23]. This paper summarizes three aspects of wireless signal coverage in tunnels, including wireless communication signal sources, tunnel coverage antenna feeder systems, and transmission methods of wireless communication. The purpose of signal coverage in tunnels is to increase the transmission distance and improve communication quality.

2.1. Wireless Communication Signal Sources

According to the coverage status near the expressway tunnel, tunnel length, station construction conditions, base station distribution, traffic distribution and other factors, a suitable signal source should be selected. At present, the main signal sources of the tunnel include cellular base stations, microcellular base stations, wireless fiber optic repeaters, wired fiber optic repeaters and relay equipment.

2.1.1. Cellular Base Stations

Cellular base stations are not normally installed inside road tunnels. The wireless signals from cellular base stations outside tunnels have many blind spots, so repeaters are often used to amplify the signals from cellular base stations into the tunnel signal blind spots. Cellular base stations are often used in short straight tunnels to transmit baseband signals. The advantages of using cellular base stations are that they provide more channel resources, easier expansion and better coverage of a single base station; the disadvantages are the need to use cables to bring in signal coverage tunnels from the server room where the Base Transceiver Station equipment is located, which increases feeder losses, in addition to the need for larger server rooms and other ancillary equipment and high cost of total investment.

Wang et al. [24] measured and simulated the signal distribution in two curved highway tunnels for the control signal of a cellular base station. The effects of tunnel curvature, length of curved section and frequency on the received power along the tunnel are evaluated and discussed. The installation of base station antennas in straight or curved tunnels is determined to have good coverage in the tunnel.

2.1.2. Microcellular Base Stations

Microcellular base station equipment is often used as a signal source in the tunnel to eliminate the blind spots of wireless signal coverage in highway tunnels when there is a complex wireless network environment around the tunnel, or the coverage signal of the macro station around the tunnel cannot meet the requirements [25]. Microcellular base stations have the advantage of requiring less space for equipment, less supporting equipment and lower total investment costs. However, transmission resources are required, and the equipment needs to be replaced during capacity expansion. Among them, China Mobile Communications Group Design Institute [26] chose the distributed micro-base station AAU3240 product to complete the 4G wireless network coverage in the tunnel of Tongmei Expressway in Jilin Province, China. Wu et al. [27] extract real-time information (such as speed, position and moving direction) of each road user by using LIDAR through wireless communication technologies, such as cellular communication and share it with traffic facilities and other road users.

2.1.3. Wireless Fiber Optic Repeaters

The maximum transmission distance of a fiber optic repeater is 15 km [28], so for long and narrow tunnels in general, a fiber optic repeater can be used for coverage as long as the transmission distance is not exceeded. Wireless fiber optic repeaters are divided into wireless co-channel repeaters and wireless frequency shifting repeaters, both of which are used in conjunction with directional antennas. The advantages of wireless co-channel repeaters are flexible installation, low investment and effective channel utilization in the cell where the signal source is located; the disadvantages are that they cannot be processed independently, which are prone to self-excitation and need to consider antenna isolation; the advantages of wireless frequency shifting repeaters are that the signal is pure and does not cause self-excitation problem, a purer signal, less feeder cable being required for installation, no self-excitation problems and more flexibility in network design. The disadvantage is that additional frequency resources for transmission are required, and the transmission antennas are required to be visible without blocking.

2.1.4. Wired Fiber Optic Repeaters

The main difference between wired and wireless fiber optic repeaters is the difference in transmission methods, with wireless fiber optic repeaters using space to transmit signals and wired fiber optic repeaters using fiber optics to transmit signals. Wired fiber optic repeaters are widely used where there is a good GSM (Global System for Mobile Communications) signal and the coverage area has sufficient network capacity. The signal source selection is carried out mainly on the basis of the length of the tunnel, the conditions of the base station, the distribution of traffic and the distribution of base stations. Pure signal sources are available from wired fiber optic repeaters, which can extend the signal to longer distances, and the signal source can be coupled from the base station or from other repeaters; the parameter settings of the source base station and the base stations around the coverage target need to be considered when using them. Consider neighborhood switching relationships, same neighborhood frequency interference and other issues. Currently, there are three main ways to solve the neighborhood switching problem.

(1)

At the intersection of each tunnel and the ground, outdoor directional antennas are used to radiate the signal inside the tunnel in the direction of the tunnel to the outside, when the underground tunnel extends to the ground, the signal field strength inside the tunnel and the signal field strength outside the tunnel maintain a smooth transition state.

(2)

When the signal environment outside the tunnel is relatively good, the RF repeater is used to amplify the signal from outside the tunnel and then introduce it into the tunnel, so that the signal field strength inside the tunnel and outside the tunnel can be maintained in a smooth transition state in the underground tunnel near the tunnel entrance.

(3)

Using the characteristics of the leaky coaxial cable to feed the RF signal, the leaky cable will be extended out of the tunnel opening and continue to lay along the ground for a certain distance, so that when the underground tunnel is extended to the ground, the signal field strength inside and outside the tunnel maintains a smooth transition state.

Experiments have shown that both Option 1 and Option 3 have a certain impact on the wireless network outside the tunnel [29], and Option 3 is more difficult to construct; while Option 2 can control the switching area well inside the tunnel.

2.1.5. Relay Equipment

Relay equipment generally consists of both GRRU digital repeaters and distributed base stations [30].

GSM Digital Remote RF Units (GRRU) digital repeater is a wireless network solution that directly couples the base station signal and transmits it to the remote end for coverage using digital transmission; it consists of a digital access control unit (DAU) and a digital RF far-reaching unit (DRU).

Distributed base stations generally consist of a BBU (Base Band Unit) and an RRU (Remote Radio Unit). The RRU is connected to the BBU through optical fiber, and the fiber connection interface follows the CPRI standard. The RRU supports a maximum of six consecutive sites in a common cell, effectively reducing the number of switching times and improving network performance; the distance of frequency multiplexing increases, improving the utilization of frequency resources; it also has the function of time delay adjustment, making network planning simple.

2.2. Tunnel Overlay Antenna System

Once the corresponding tunnel coverage source is identified, the appropriate antenna system is selected to cover the tunnel according to the actual terrain conditions, environmental characteristics, and tunnel features, and different antenna configurations are tested for different polarizations and orientations [31]. At present, coaxial passively distributed antenna systems, fiber optic active distributed and leaky cables are the three commonly used deployment methods in the tunnel.

2.2.1. Tunnel Coverage Antenna Selection

Directional antennas and narrow beam antennas are generally chosen to be used for coverage because of the obvious directionality of the tunnel coverage [32]. Considering the installation of the antenna and the reflection of the signal by the tunnel wall, a dual-polarized antenna is generally chosen. For highway tunnels with a length of not more than 2 km, a low gain (10~12 dBi) antenna can be chosen. For longer tunnels, a high gain (22 dBi) narrow beam antenna can be used for coverage. When the vehicle travels through the highway tunnel, the remaining space in the tunnel is still large, so a larger size of antenna can be chosen in order to obtain a higher gain and larger coverage. Flat antennas, Yagi antennas, indoor wall antennas, parabolic antennas, bidirectional antennas and leaky cables are often used for signal coverage in tunnels [33]. With the rapid development of digital signals, some new antennas with higher performance have been applied to tunnel communications. For example, China Unicom, together with several partners, has proposed and validated the “new 4T4R wall-mounted antenna” solution for highway tunnel coverage, which has successfully achieved uniform coverage of wireless signals in tunnel scenarios and improved the coverage range of wireless signals in tunnel scenarios [34].

Depending on the characteristics of the different tunnels, different antenna coverage systems are chosen [35]. The simplest way to cover a tunnel is to choose a single directional antenna, which is not only very inexpensive but also very suitable for short distances tunnels of up to 2 km in length, where the traffic density is relatively low. For tunnels with a length of more than 2 km or even longer, a directional antenna can be appropriately added to the tunnel, and the signal can be evenly distributed in a cascade manner, so as to ensure the coverage quality of the signal in the tunnel. For tunnels with a length of more than 5 km or even longer [36], leaky cables or distributed antennas are used to solve the coverage problem in the tunnel. For tunnels in mountainous areas, it is common to use microwave directional antennas at the base stations at either end of the tunnel port to transmit the RF signal.

According to an experimental investigation of tunnels conducted by a TV station in Shanghai province, China [37], using a single antenna mode for coverage in tunnels with low traffic density and up to 2 km in length is the most economical way of coverage. With the arrival of the 5G era, in order to achieve all-around 5G signal coverage in the tunnel [38]. It is also a common to use an achromatic antenna and micro base station to cover the tunnel network in the tunnel or at the tunnel entrance. The commonly used achromatic antenna is a log periodic antenna. The coverage method shall be reasonably selected according to the tunnel length and other elements. In linear short-distance tunnels, the installation of achromatic antennas at both ends of the tunnel entrance is generally chosen to achieve internal network coverage. To ensure that terminals can switch smoothly inside and outside of the tunnel, the other side of the antenna can be installed at the tunnel entrance to extend coverage towards the outside of the tunnel, at which time both sides of the tunnel entrance should be set as the same cell. In the network coverage of long distance it is not only needed to install a special antenna at the tunnel entrance but also install it inside the tunnel; a long distance tunnel will pass through the cell generally, so it can make full use of the cell for network coverage, and set up a switching area in the tunnel during the construction.

2.2.2. Coaxial Passive Distributed Antenna System

Coaxial passive distributed antenna systems consist of a sender antenna that couples the radio system to the cellular base station and redirects the signal to the target transmission antenna using a coaxial transmission line [39]. The distributed antenna system mainly uses a single antenna for tunnel signal coverage, mostly in short tunnels, where the directional antenna needs to be extended from the tunnel entrance to the interior of the tunnel. Coaxial passive distributed antenna systems have the advantages of being easy to install, inexpensive and do not generate large cable feed tube fading.

2.2.3. Fiber Optic Active Distributed Antenna System

The fiber optic active distributed antenna system can be used in more complex tunnel environments. It can replace coaxial passive distributed antenna systems to some extent. It can use thinner fiber optic cable, can reduce electromagnetic interference, and the design is flexible in complex networks. However, its disadvantage is the high cost.

In urban tunnels, a distributed antenna system with an antenna spacing of 400 m is sufficient to meet the reception threshold [40]. Distributed antenna systems can receive more multipath components because multiple antennas transmit the same signal. Aitjilal R et al. [40] applied the MIMO system to a tunnel communication system. MIMO takes advantage of the multipath propagation in a rich scattering environment and uses multiple transmitting and receiving antennas to increase capacity.

2.2.4. Leakage Cables

The structure of existing highway tunnels in China is based on straight-through tunnels and a large number of leaky coaxial cables are used for communication. This method has a simple structure and an even distribution of signal field strength and is suitable for wireless signal propagation in all types of tunnels. The signals of several different wireless systems can share the same leaky cable through a combined circuit, which makes the installation of multiple antenna systems less complex. By installing a leaky cable in the tunnel wall, uniform coverage of the wireless signal in the tunnel is achieved. The advantages and disadvantages of the leaky cable approach compared to the traditional antenna approach are shown in

Table 1. The advantages and disadvantages of the leaky cable and traditional antenna.

	Traditional Antenna	Leaky Cables
Distance covered	Short	Long
Coverage density	Uneven	Uniformity
Applicable conditions	Low traffic density	No restrictions
Construction conditions	Small restrictions	Leakage cable to be laid
Construction cycle	Short	Long
Construction costs	Low	High

.

Given the good nature of leaky cables, scholars agree that leaky coaxial cable communication systems are an effective means of solving the problem of tunnels facing traffic congestion, dramatic increases in traffic volumes and density, high-speed safe travel, as well as an emergencies. Due to the narrow shape of the tunnel, signal transmission sometimes undergoes fading, for signal propagation in the tunnel there are interference problems [41]; for the base station and fiber optic repeater, the length of the leaky cable can be properly adjusted, and the transmission signal is truncated and isolated to reduce interference as much as possible. In the tunnel, two frequency groups are used for multiplexing the whole line with the base station coverage method; the tunnel leakage cable field strength overlaps area through the adjacent base station’s RF signal for coverage and switching will automatically occur in the area of the system. At least two base stations are used in tunnel coverage to solve signal fading and inter-code crosstalk from the same frequency band by using signal interleaving.

The technology for installing leaky cables in tunnels has matured and is now being implemented and used in major tunnels. In 1988, Philips Radio Communication Systems of Cambridge University and EB Communications [42] jointly designed a leaky cable-based communication system that would install two communication networks, one for maintenance and the other for relaying emergency information which can provide various alarm information about highway traffic conditions and road emergencies ahead. In 1991, the leakage cable communication system [43] of the Dapu Road River crossing tunnel passed the laying and testing, and the communication effect was good, which was highly praised by relevant authorities.

In 1999, the first highway tunnel communication in Slovakia used leaked cable technology to provide voice information from the main control room and emergency control room [44]. In 2015, the Heda high-speed wireless signal coverage [45] in the extra-long or narrow space inside the tunnel walls to install leaking cables. A tunnel in the Taizhou section of the Yongtaiwen Expressway in China with a length of 2.4 km [46], previously had 2G network coverage by installing panel antennas in the tunnel, but the signal effect in the tunnel was poor, with frequent call drops and even off-grid at some locations. The existing tunnel was modified by means of a leaky cable, which was used to cover both sides of the tunnel from the middle section of the air pass and the two human passes. After operational testing, both the upstream and downstream links of the tunnel meet the coverage requirements. With the advent of the 5G era, the tunnel leakage cable mainly includes a 1-5/8” leakage cable, full-band 1-1/4” leakage cable and low-loss 1-1/4” leakage cable, of which the 1-1/4” type supports 5G high-frequency signal transmission and different leakage cables can support different scenarios of tunnel 5G coverage [47].

2.3. Wireless Signal Networking Methods

With the development of wireless communication technology, there are various ways of networking wireless communication at this stage. As light, temperature and humidity in tunnels need to be detected and able to be viewed remotely, the use of wireless sensor networks for communication to view information in tunnels is common.

2.3.1. ZigBee

ZigBee is a near-range, highly reliable, low-complexity, low-power, low-data-rate, low-cost wireless network technology, it is mainly for near-range wireless connectivity. In the entire network range, each ZigBee network data transmission module can communicate with each other, and each ZigBee network node itself can be used as a monitoring object; its connected sensors directly transmit for data collection and monitoring, and also automatically relaying data from other network nodes [48,49,50]. In addition, each ZigBee network node can be wirelessly connected to multiple isolated sub-nodes within its own signal coverage area that do not undertake the task of relaying network information, extending the wireless signal coverage. Currently, the Zigbee transmission method has been widely used in cable tunnels, where it is used in conjunction with 4G for data transmission [51]. This method is only applicable to short tunnels, and the relay nodes are accessed too much to guarantee the reliability of data transmission, and the system has instability. For the long mileage of highway tunnel construction, complex working environment, many operating units, shortage of management personnel, etc., ZigBee has been widely used for personnel positioning [52,53,54,55].

However, due to the weak penetration capability of ZigBee [56], the situation of communication difficulties occurs in narrow and obstacle-ridden tunnels. To avoid communication difficulties, The large capacity of ZigBee networks can be used to deploy ZigBee devices to special obstacle locations. For the actual situation of road tunnel electromechanical systems, Wang et al. [57] designed the framework of the ZigBee positioning system from an overall perspective; using the ZigBee module with CC2431/CC2430 chips as the core, the hardware platform of the system was built to achieve monitoring and positioning of specific objects in the tunnel, which focuses on the hazardous material transport vehicles, maintenance and construction vehicles and personnel running in the tunnel. The system focuses on the positioning of objects, such as dangerous goods vehicles, maintenance and construction vehicles and personnel.

ZigBee networking technology is used for the 3 m precise location system for intelligent tunnel personnel Entian, in Yunnan Province, in China. Southeast University [58] developed a wireless health detection system for tunnels, which consists of wireless sensors, repeaters, base stations and user terminals, and uses ZigBee technology to achieve wireless transmission of monitoring data. The relative position of the repeater and the wireless sensor has a significant impact on the transmission distance, and the transmission distance of the wireless sensor in the tunnel is not less than 100 m; the wireless sensor has a strong self-grouping capability and can achieve mutual relaying.

2.3.2. Bluetooth

Bluetooth uses frequency hopping technology to split the transmitted data into packets, which are transmitted separately over 79 designated Bluetooth channels. Each channel has a bandwidth of 1 MHz. Bluetooth 4.0 uses a 2 MHz pitch and can accommodate up to 40 channels. A Bluetooth master device can communicate with no more than seven devices in a piconet, and the roles can be switched between devices via protocols, and slave devices can switch to master devices [59,60,61].

Bluetooth can be used to locate people inside the tunnel during the construction phase. Tian [59] designed a professional network monitoring network by integrating Bluetooth communication technology with wired telephony and the internet, building a network between professional equipment with Bluetooth capabilities and achieving wireless communication and external Ethernet access to realize local centralized control and remote control of the intelligent monitoring of the road tunnel construction and production safety network platform.

2.3.3. Wi-Fi

Wi-Fi connections can be used for networking within the effective range of the wireless router’s airwave coverage. The data security of Wi-Fi is relatively poor [62,63,64]. Ordinary road tunnels have difficulty in providing adequate power facilities with limited funding. If a tunnel needs to be monitored for its condition during construction, Wi-Fi can be used for wireless communication because of the large number of staff at the time of construction, and the ease of meeting the requirements for deploying Wi-Fi equipment. In the case of normal tunnel health monitoring, the use of this method requires careful judgment due to the high cost and power consumption.

The research on multi-terminal tunnel monitoring systems and emergency plans was carried out in the Pingchang Line Tunnel of Shanxi Provincial in China [65] by means of Wi-Fi network communication technology monitoring and combined with the actual situation, which can achieve real-time and accurate traffic flow and traffic operation monitoring around the clock, discovering various abnormal situations and taking emergency measures anytime and anywhere in time. Based on Wi-Fi wireless network, Yang [66] designed a highway tunnel lighting control system. The lighting nodes in the tunnel are connected in a tree-like manner according to the length of the highway tunnel so that each lighting node can be controlled by the wireless network. Various information from the vehicle is sent to the wireless controller in the form of communication via the Wi-Fi wireless sensor, and the information sensed is processed; this received information is sent to the nodes of the road tunnel lighting control via the wireless transmitter to make the lighting appropriate.

Cheng et al. [67] used wireless sensor-based positioning and tracking, self-organizing Wi-Fi and two-way communication in a tunnel to prevent accidents during changing and dynamic lifts. The network consists of small sensor nodes with communication, computational capabilities and an ‘intelligent’ system, it can independently perform the assigned tasks according to the environment and requirements. When one sensor is damaged or the signal is blocked, another sensor can transmit the signal to a computer server.

2.3.4. NB-IoT

Built on cellular networks, NB-IoT consumes only about 180 KHz of bandwidth and can be deployed directly on GSM networks, UMTS (Universal Mobile Telecommunications System) networks, or LTE networks. NB-IoT is an emerging technology in the IoT space that supports cellular data connectivity for low-power devices over wide area networks, also known as LPWAN (Low Power Wide Area Networks). It supports efficient connectivity for devices with long standby times and high network connectivity requirements. It also provides very comprehensive coverage of cellular data connections within tunnels [68,69,70,71].

NB-IoT cellular networks are also widely used in tunnels. Xiamen Telecom [72] installed NB-IoT smart detectors in tunnels, which were automatically alerted once the water level exceeded the threshold and prompted flooding warnings in the electronic signage bar at the tunnel entrance to ensure traffic safety. Ji et al. [73] introduced IoT (Internet of Things) technology into the tunnel lighting control system and used NB-IoT control technology to design an autonomous control system for tunnel lighting and ventilation according to the design requirements of tunnel lighting and ventilation intelligent control, which saves energy and saves costs for the tunnel operation under the premise of protecting the safety of drivers. Shi et al. [74] designed a system for monitoring the status of NB-IoT and sensors in response to the frequent occurrence of accidents in large machinery tunnel excavations. In remote monitoring, NB-IoT technology is used for communication, and the collected status data are transferred to a cloud server and stored, effectively improving the efficiency of real-time status monitoring of road heading machines by users and manufacturers through the client.

2.3.5. LoRa

LoRa is an exclusive private technology monopoly by the USA company SEMTECH, and the terminal and gateway chip IP patents are controlled by SMETCH. The most important feature is that it propagates farther than other wireless methods under the same power consumption conditions, achieving the unification of low power consumption and long range, and it extends the distance three to five times more than traditional wireless RF communication at the same power consumption [75]. Low power consumption and high penetration are the more important indicators when judging the goodness of communication technologies in road tunnels. Among them, LoRa wireless communication technology has very low power consumption and greater penetration power. Moreover, LoRa enables completely wireless transmission. However, new signal towers or base stations are required, and the initial investment in human and financial resources is large. If communication devices need to be deployed intensively, the use of LoRa will result in spectrum interference. Its disadvantage is that the human and financial investment in the early stage is large. If the intensive deployment of communication equipment is required, spectrum interference may occur when using LoRa.

With the development of IoT technology, multi-sensor monitoring in tunnels has become common, and data transmission from sensors using LoRa has been a common networking method among tunnels. The data of the sensor are transmitted to nodes which transmit the data to gateways or base stations, and the gateways or base stations transmit the data to servers or clouds [76]. Wang et al. [77] built a communication LAN for the wireless LoRa of tunnel monitoring sensors, developed a communication protocol between monitoring central nodes and terminal nodes, and designed a low-power supply strategy for monitoring terminal nodes to meet the needs of long-term data collection under battery power supply of monitoring terminals. Lin et al. [78] used LoRa wireless communication to achieve 0–10 V intelligent dimming power switch control and automatic detection and positioning of luminaire faults, while the system established a smart tunnel lighting monitoring and operation service platform to achieve tunnel information fusion and intelligent control. Compared to conventional tunnel lighting, the energy saving rate is over 60%. The signal transmission method uses LoRa wireless transmission and has excellent features, such as low power consumption, long distance and a self-organizing network. While the output is abnormal, the server will issue a command to trigger an alarm to achieve safe construction.

2.3.6. RFID

RFID (Radio frequency identification technology) is a kind of non-contact automatic identification technology. Its basic principle is the use of radio frequency signal and space coupling (inductive or electromagnetic coupling) or the transmission characteristics of radar reflection, to achieve the automatic identification of the identified object [79,80,81].

Given its good nature, it is commonly used for wireless signal coverage of long highway tunnels and positioning in tunnels [82]. The Donghuangmiao Tunnel of the Hang-Xin-Jing Expressway in China [83], uses RFID technology to transmit and cover signals in long highway tunnels and uses its users for intelligent management of long highway tunnels. Yan Wenxin et al. [84] applied RFID technology to a new type of tunnel construction personnel location. By setting up base stations outside the tunnels, laying cables inside the tunnels and allowing construction personnel to wear identification cards, which enables functions in turn, such as real-time dynamic display of tunnel construction personnel. In the Huangqu South Expressway Jiangduwu Tunnel in Zhejiang Province in China [85] RFID-based automatic detection systems for highway tunnel traffic events are used and readers are deployed every 300 m. When the vehicle carrying the electronic tag reaches the effective reading range of the reader, the tag is activated, at which point the activated tag sends the vehicle information stored in the card to the reader. It can better carry out traffic event detection based on a single vehicle. Guo et al. [86] established the tunnel electromechanical equipment operation monitoring system based on RFID technology. The system requires RFID tags to be integrated into each tunnel equipment, collecting the inherent information of each piece of equipment and the address information of the equipment; the information collected by the RFID tags is transmitted to the routing device via radio frequency, realizing a user-oriented static and dynamic monitoring information service.

2.3.7. GPRS

GPRS, known in English as a General packet radio service, is a transitional industry from 2G to 3G, providing fast and instant connectivity that can significantly improve the efficiency of some transactions (e.g., credit card reconciliation, remote monitoring, etc.). With GPRS technology, users can be continuously connected. GPRS services are available wherever GSM coverage is available, allowing users to connect to the Internet when other services, such as 3G or HSDPA are not available [87,88,89]. It is suitable for intermittent, sudden data transfers or frequent, continuous mass data transfers, especially for vehicle mobile terminals [90].

Amedeo Manuello and the AE research department of the Politecnico di Torino, in collaboration with the Italian company Lunitek SRL [91], have developed an AE mission system for structural and seismic monitoring based on AE data acquisition and wireless transmission technology. Eight PZT sensor arrays are connected to this multi-channel system (each with a dedicated memory of 64 Mb), which automatically stores and processes the important parameters of each detected signal waveform. The processed data are sent periodically by the system to a remote server via GPRS, thus allowing continuous, simultaneous monitoring of individual structural elements or entire structures that may be located in different locations and the real-time investigation of the damage evolution of two precast concrete arch elements in a highway tunnel. Rong et al. [92] designed a distributed temperature monitoring system for tunnels in high-altitude cold regions, which consists of a control center, multiple on-site monitoring data receiving stations, multiple data acquisition terminals and corresponding sensor probes, using long-term GSM \GPRS wireless data transmission in the tunnel. The distributed temperature monitoring can fully meet the actual needs of tunnel temperature monitoring.

2.3.8. 3G/4G/5G

3G/4G/5G is the abbreviation for but third, fourth and fifth generation mobile communication technologies, respectively, which refers to cellular mobile communication technology supporting high-speed data transmission. It has the advantages of fast transmission rates, wide transmission range and relatively pure content, with the disadvantages of having high costs in terms of base station deployment and small coverage distances [92,93,94,95]. It is widely used in all kinds of tunnels.

A communications Construction Company in Shaanxi Province, China [96] designed an exclusive trunked intercom system using mobile communications 3G network in the Qinling Terminal Mountain highway tunnel to meet the daily operation and management and emergency rescue communications of the growing highway tunnel.

Chao et al. [97] applied a 4G mobile communication network for the remote transmission of sensor data to achieve cloud-based storage and real-time viewing of tunnel environmental monitoring data. China Mobile Communications Group Design Institute selected the distributed micro base station AAU3240 product to complete the 4G wireless network coverage in the tunnel of Tongmei Expressway in Jilin Province, China.

Recently, China Unicom, together with ZTE, Henxin Technology and Kingxin Communications, completed the first pilot deployment and trial validation of 5G in a road tunnel at the Yangling Highway Tunnel in Yixing, China [98]. It successfully achieved uniform wireless signal coverage and enhanced coverage in the tunnel scenario, improving coverage and capacity capabilities.

A one-way highway tunnel with a double cavern [99], a total length of 1665 m, using an 8T8R 50WNR RRU source and spotlight antenna scheme was studied; antennas and equipment are fixed in the gap on the two sides of the tunnel next to the sandwich wall, 8T8R equipment are under the two hanging four-port 5G spotlight antenna, antennas are installed back to back, the antenna spacing laid in the tunnel was between 300 and 500 m, after experiments and tests, this solution can achieve low-cost 5G coverage in road tunnels.

The Yangtze River Tunnel in Wuhan Province, China [100] uses a new four-way new 5/4″ leaky cable for 5G construction in the tunnel. For tunnels without 4G leaks, a new 5/4″ single cable is built for 4G and 5G sharing in the coverage scenario, and at least 1*5/4″ single cable and 1*13/8″ single cable are built for 4G and 5G sharing in the capacity scenario. A single cable is used for 4G and 5G splitting. Moreover, log-periodic antennas are installed at the entrance and exit of the ramp for outward coverage to ensure normal signal switching.

When new road tunnels are built [101], the use of panel antenna coverage is recommended, taking into account construction costs and implementation difficulties. The antenna spacing can be set through the link budget according to the different systems accessed by the operator, the degree of tunnel curvature, etc. It is recommended to be between 200 m and 300 m to ensure good coverage of each system. To ensure a 4G/5G MIMO effect, the spacing between two leaky cables should be no less than 30 cm.

2.3.9. UWB Ultra-Wideband Technology

UWB technology has high accuracy, high communication rates and strong multi-path capabilities, and is widely used indoors and in tunnels [102]. UWB technology uses extremely narrow pulse signals to achieve carrier-free wireless communication, with signals having a bandwidth in the order of gigahertz [103,104,105]. Its advantages are low power consumption, high interference immunity, high penetration power and centimeter-level accuracy; its disadvantage is high cost.

Outdoor UWB base stations are installed at 100 m intervals along the tunnel to configure a UWB-based intelligent traffic system. The intelligent traffic system works well in the tunnel, where the vehicle positioning system not only provides the intelligent control algorithm with the ID and location of the vehicle tag but also detects the speed of the moving vehicle. In the construction of the Karakorum Highway tunnel [106], the UWB positioning base station was integrated with the function of a wireless AP. A positioning base station is installed every 100 m in the tunnel to achieve full wireless signal coverage in the tunnel to meet the data transmission requirements of the positioning system. A wired connection is used between the base station and the server at the tunnel entrance to ensure real-time and accurate data transmission. In the Fenghuang section of the Zhangjihuai Railway tunnel in China [107], UWB positioning tags are attached to the workers’ helmets, and the workers’ positions are detected in real-time through each slave base station. The master base station collects the data from the slave base stations and sends it to the core gateway through the digital transmission gateway, and then transmits it to the cloud or the monitoring server of the project department through the core gateway. In the event of an emergency, an alarm is raised through the loudspeaker and the alarm message is sent to the worker’s mobile phone.

2.3.10. Wireless Bridges

Wireless bridging can amplify the signal of the main router, so as to achieve the effect of covering a larger area. It has the advantages of being erected, increasing links at any time, and convenient installation and expansion. The disadvantage is that two bridges communicating cannot be sheltered from each other, and therefore, more wireless bridges need to be deployed in tunnels with more curves, which increases the cost [108,109,110].

Jeon et al. [111] erected a 5.8G wireless bridge backbone transmission unit in a tunnel and used alternating non-interference channels in the actual deployment, which solved the signal attenuation problem caused by the bend, and the accumulation of wireless signal interference, and achieved the purpose of stable wireless network transmission. The stability of wireless communication in the tunnel is improved. Jiangxi Feishang Technology Co., Ltd. [112] adopted the wireless bridge transmission method of communication in the Jiangxi An‘ding Expressway in Jiangxi Province, China, which can achieve the function of wireless remote data transmission and acquisition in highway tunnels. The Bayu tunnel [113] is 13.073 km long, with a total of 12.242 km of predicted rockburst sections in the main cavern, accounting for 94% of its length. The remote bridge solution was used to build a network that could keep track of the server and microseismic monitoring system operation in the tunnel in real-time. Wang et al. [114] used a decentralized arrangement of wireless bridges to solve the problem of many monitoring projects and complex mountainous terrain. The wireless bridges were connected via switches to transmit data. When the distance is too large, the HD camera and the wireless bridge are connected through twisted-pair cables. The wireless bridge can be used to build a front-end transmission network, and the data are received from the receiving end of the wireless bridge and the data are transmitted from the construction site and the transmitting end of the prefabricated site. For tender sections with complex links, relay points need to be added. The data are finally uploaded to the public network via the project’s independent broadband. The mapping is completed by binding the IP address of the project’s independent broadband to the server, which processes the uploaded data and make it available to users.

2.3.11. Satellites

The communication distance is up to 13,000 km in the satellite beam coverage area; it is not subject to any complex geographical conditions between the two points of communication; it is not affected by any natural disasters or man-made events between the two points of communication; the communication quality is high and the system is highly reliable. However, it has a large transmission delay: high latitude areas are difficult to achieve which is a disadvantage of this communication. Some scholars have also applied this communication method in highway tunnels.

The Mengshan section of the Jinxiu Tunnel in the Guangxi Zhuang Autonomous Region in China [115], used VSAT satellites to set up a three-way communication networking platform emergency broadcasting system to provide communication in the event of blockages, traffic accidents and fires in the tunnel. Cui et al. [116] used two satellite navigation simulators to simulate real satellites transmitting satellite navigation signals at each end of the tunnel. At the same time, a coaxial leakage cable was laid inside the tunnel to connect the transmitting ends of the two satellite navigation simulators, and the signals were radiated to the outside world using the slotted holes of the coaxial leakage cable, which greatly reduced the impact of the near and far effect problem and achieved one-dimensional positioning in straight tunnels.

3. Wired Transmission Technology

When data transmission is by wire, there are disadvantages, such as difficult wiring, which can limit the range of sensor placement, limited wire life and high energy consumption. As shown in

Table 2. Wired transmission technology.

Wired Transmission Method	Communication Methods	Communication Distance	Transmission Method	Transmission Rate
RS485	Half-duplex communication	Up to eight trunks can be added, which means that theoretically a maximum transmission distance of 10.8 km can be achieved with RS485	Asynchronous transfer	10 Mbps
PLC (Power Carrier)	Full duplex communication	Unstable	Synchronous transmission	Varies considerably depending on the programme

.

3.1. RS485

RS485 is one of the mainstream serial communication interfaces and does not require a signal to be detected relative to a reference point; the system simply detects the potential difference between the two lines. It is extremely resistant to common mode interference and supports multipoint data communication. The network topology generally adopts the bus type structure with terminal matching, which means that each node is connected in series by a bus. It does not support ring or star networks, and it supports 32 nodes at most [117,118,119].

A lighting system in the tunnel was designed using a fusion of wireless and wired technologies [120], where the wired part was RS485 and the wireless part was covered by GPRS signals. An RS485 bus intelligent dimming control system [121] using stepless dimming, combined with illuminance sensors installed at the tunnel entrance, inside the tunnel and tunnel traffic detection sensors communicated with the LED dimmable power module (single light controller pass and power module) via RS485 to achieve stepless dimming. Zhao et al. [122] proposed a multiplexed data collector for highway tunnel lighting systems, RS485 was used to transmit the collected data, where Magnetic beads are set to suppress high-frequency noise and peak interference on RS-485 signal lines. Lai et al. [123] used a dimming controller with a four-way RS485 bus that is connected to the basic luminaires in the area, which allows dynamic dimming control and detection of the luminaires in the tunnel. It was also applied to a tunnel in Yunnan Province, China. Chen et al. [124] carry out the intelligent management of motorway tunnels by a combination of wired and wireless transmission, where the wired transmission includes RS484 and CAN bus, and the wireless propagation is 2.4G wireless propagation technology. The communication mode between the devices is I2C.

3.2. PLC (Power Carrier)

Powerline carrier communication is a method that transmits information by means of high frequency carrier signals over high or low voltage power lines. In practice, when the power line is unloaded, point-to-point carrier signals can be transmitted up to several kilometers. However, when the load on the power line is heavy, only a few ten meters can be transmitted [125,126,127].

In the Wankai Expressway tunnel monitoring system setup [128], the tunnel monitoring center monitoring host communicates with each PLC and the tunnel monitoring host inside and outside the tunnel via a network. A C/S architecture is used by the fire alarm and monitoring system.

In the eight tunnels of the Linfen to Xiangning Expressway in China [129], system monitoring data is collected and stored by using PLC communication technology, where single-mode optical fiber is used as the communication medium for the system and Ethernet is used for the communication network. PLC [130,131] was used to achieve real-time communication between the various components of tunnel ventilation monitoring and environmental detection. The Partridge Hill Tunnel is the longest highland road tunnel in China [132], and all 13 tunnel control systems are using an optical fiber redundant ring network, with RS485/RS422 or RS232 communication ports which are installed on each PLC, to achieve communication in the tunnel.

4. Typical Tunnel Scenario Coverage Options

Depending on the length of the tunnel, tunnels are generally classified as short, medium, long, and very long tunnels. However, there are different coverage options for continuous tunnel groups in motorway tunnel coverage, this article looks at short, medium long, extra-long tunnels and continuous tunnel groups.

4.1. Short Tunnels

Short tunnels, defined as road tunnels less than or equal to 0.5 km, are generally covered by cellular base stations with directional antennas, mainly in the form of cellular base stations that are pulled away and micro base stations that are built at the tunnel entrance [133].

(1)

Directional cellular base station coverage: By pulling far away from the macro base station signal at the tunnel entrance, a directional wide beam antenna is used to provide directional coverage of the tunnel entrance.

(2)

Micro base station coverage: through the construction of micro-stations on the monitoring poles at the tunnel entrance, the tunnel will be covered in a directional manner, such as a two-way separated tunnel that can be covered by an antenna power division.

4.2. Medium-Length Tunnels

Medium-length tunnels are road tunnels more than 0.5 km and less than or equal to 3 km. The strategy of long tunnel coverage can be divided into two types: low power multiple antennas and high power few antennas. For low power multiple antennas, the source power is basically within 10 W, the transmission medium is a feeder, the antenna is a low gain ceiling or plate type, and the coverage radius is within 15 m. Every 300–400 m, an active amplifier is required [1]. The advantages of this method are less equipment, lower engineering and maintenance costs, and easy implementation; the disadvantage is that the signal is easily blocked, fades fast, and the level fluctuates greatly. Medium and long tunnels are generally covered by leaky cables [134], with RRU pulling away and multiple RRU common cells to reduce switching in the tunnel. At the same time, the tunnel coverage extends to form an overlapping coverage zone with outdoor signals to ensure smooth switching. Presently, for the construction of medium and long tunnels on highways in China, there are the following examples.

Li et al. [135] solved the problem of switching the power supply at both ends of medium and long distance tunnels by means of optical fiber communication. Through fiber optic point-to-point communication, data interaction between the two ends is realized, and intelligent operations, such as virtual hardware electrical interlocking and standby power self-injection are realized by equipment self-injection devices at both ends. A wireless access fiber optic repeater is used in a tunnel with a total length of 820 m [26], with two holes and two lanes in one direction, and a height of about 6 m, with Yagi antennas for coverage inside each of the two tunnels and parabolic antennas outside the tunnel, while the isolation degree is solved by using the tunnel roof shading. Through testing, this coverage method expands the coverage of the base station and has no impact on the parameter index of the base station. The tunnel in the entrance section of the Yongtaiwen Expressway in Taizhou, China is 2400 m long [136] and covers the whole area of the tunnel for the Unicom GSM900 and 3G networks. According to the user’s requirements and the coverage area, the tunnel was covered by a nearby macro cellular base station as the source, using leaky cables from the middle section of the tunnel and two entry points on each side, and using RRU or outdoor fiber optic repeaters as the application equipment. Within the 1000 m length of the Jiangduwu tunnel of the Huangqu South Expressway in China [137], readers are placed at 300 m intervals to collect traffic parameters of vehicles in the tunnel using RFID technology.

4.3. Extra Long Tunnels

Extra-long tunnels are road tunnels greater than 3 km in length. In recent years, China’s road extra-long tunnels have developed rapidly, with the number of road extra-long tunnels in China growing to 1175, and the length of road extra-long tunnels reaching 5,217,500 m. In view of the complex wireless transmission environment in long distance tunnels, traditional communication technology cannot meet the needs of high quality communication of passengers. There is a lot of research in China on long road tunnel communications. In the case of the confined long tunnel scenario, a combination of two ideas, using a distributed source approach is preferable. The source is an optical fiber towed repeater or RF pull-out base station with a power of around 20 w, the transmission medium is optical fiber and the antenna is a 13 dB directional antenna with a coverage radius of 500 to 1000 m. This can effectively avoid the disadvantage of a single source causing the signal to be easily blocked, while drawing on the layout of multiple antennas, multiple sources and antennas can be arranged in the tunnel to meet the coverage requirements.

Among them, the 22.1 km long Tianshan Shengli Tunnel is the longest motorway tunnel in the world, and the longest extra-long tunnel with high cold, high altitude and high ground stress among motorways under construction in China. At present, 50 KM of optical fiber cables are laid in the 22.1 km long tunnel [138], and 4G/5G base stations are built to achieve seamless 5G coverage, utilizing China Telecom’s low latency and large bandwidth characteristics to ensure the accuracy of video and audio and sensor information collection within the project, providing a “5G + Smart Tunnel “ multi-scene application and a solid foundation. The Yinbingshan Tunnel in Guangzhou Province, China [139] deployed a total of 20 km of optical fiber cables and tuned and opened 68 devices, eventually allowing Telecom and Unicom 4G signals to cover every corner of the tunnel. In order to meet the urgent need for high speed and high quality wireless communication in the tunnel environment, Zhong et al. [9] studied high gain antennas for the tunnel environment and proposed a new method to improve the signal transmission quality by using a binary phased array antenna system. Simulation results show that: compared with a single antenna, the minimum level of the synthesized electric field of the signal transmitted by the phased-array antenna system in the axial propagation range of 3000 m tunnel is increased by at least 19.6 dB; the minimum level is increased by at least 12.4 dB, achieving a better diversity optimization effect, eliminating the depth fading caused by the multipath effect, and solving the communication problems in the tunnel environment. Communication problems exist in the tunnel environment. Wang et al. [140] proposed a distributed tunneling communication system based on wavelength division multiplexing passive optical networks with RoF (Radio over Fiber). The experimental platform was built to enable high-quality and smooth signal transmission in extra-long tunnels. In very long and curved tunnels, due to the linearity of the wireless signal propagation and the attenuation of the leakage power itself, Yang [141] proposed that a certain area of metal mesh can be laid at each bend of the tunnel as a wave reflector, and the metal mesh can be replaced by metal powder coated on the tunnel wall. When the field strength at the entrance is appropriate, normal communication can be carried out in the tunnel by means of the reflector.

The G50 Shanghai–Chongqing Expressway Yuquanxi Tunnel [142] is located at the junction of Gaojiayan Chegou Village and Hejiaping Town Baifan Village in Changyang County, Yichang City, in China, with a total length of 5228 m and a width of 6 m. For the police wireless communication system, the 350 MHz fire-fighting communication system and the 800 MHz public security digital trunking system are used to provide full coverage of the internal area of the tunnel. The base station is deployed inside the highway tunnel and radiates to both ends of the tunnel through a splitter connected to two leaking coaxial cables, forming a composite base station system. This system is easy to set up, but the disadvantages are high transmission loss, a large number of base stations and high construction cost.

The mobile communication system wireless coverage project in a tunnel in Xiamen, China [143] is the first wireless tunnel coverage technology pilot in China to adopt the BBU + RRU method for the wireless distribution system in the tunnel, where the RRU is cascaded through optical fibers, and its source is introduced from a base station in the south of the tunnel. The first system is placed at the southern end of the tunnel, then the second system is placed at the northern end of the tunnel via fiber optics; the signal is then fed from the system into the tunnel using a 5/4 feeder. After the corresponding coupling and power division, it is transmitted by the back reflection antenna to complete the overall coverage of the tunnel. Nine pairs of backfire antennas are adopted in the whole distribution system, and the output power of the antenna port is 10~25 DBM. To avoid switching in the tunnel, the two systems are defined as the same cell. For very long tunnels, this approach can still be used to reduce cell switching within the tunnel while maintaining capacity. By dividing the long tunnel into segments, each segment is covered by a number of systems that allow for segmental traffic splitting and minimizes inter-cell switching within the tunnel to ensure the quality of service.

4.4. Continuous Tunnel Complex

For short and continuous tunnel groups, tunnel communications are relatively easy; coverage is basically achieved by building a base station in the middle of the tunnel group or at the tunnel entrance and aligning it with the tunnel using a panel antenna.

For long tunnels or sections of tunnel complexes [144], it is advisable to provide an emergency telephone system, for every 100 m in the tunnel, and to adopt a combination of wired broadcasting and emergency telephone systems, the consoles are shared and fiber optic communication transmission is used.

The left and right line tunnels of the Pingxi Highway in Shenzhen, China are about 590 m long and the left and right line tunnels of the Diffu Mountain are about 1700 m long, forming a tunnel complex [145]. Profibus fieldbus technology is used in the tunnel, along with LAN communication, large screen display technology and other advanced technologies to form a distributed computer monitoring system; an S/C (server/client) structure is adopted in the central monitoring system of the monitoring center and is interconnected via a 100 M high-speed Ethernet LAN and TCP/IP network protocol. The emergency telephone and cable radio system are connected to the control room controller via the RS232 interface of the mainframe and transmits the current status of all telephone extensions and cable radio to the central control system.

4.5. Summary

In this part, through the literature review, the tunnels are divided into short tunnels, medium-long tunnels, extra-long tunnels and continuous tunnel groups according to their lengths. We summarize its optimal layout and signal transmission limitations as shown in

Table 3. Typical tunnel scenario coverage options.

Tunnel Form	Signal Coverage Method	Limitation
Short tunnels	Extend the macro base station or build a micro base station at the tunnel entrance	Incomplete signal coverage, occasionally causing disconnection
Medium-length tunnels	Leaky cable coverage, adopt RRU remote method	The signal is easily blocked, fading quickly, and the level fluctuates greatly
Extra long tunnels	Distributed source method	High price, large power loss
Continuous tunnel complex	Build a base station in the middle of the tunnel group or at the entrance of the tunnel, and align the tunnel with a panel antenna for coverage	Great coverage, but low accuracy

.

5. Conclusions

Communication problems inside tunnels are still very difficult to manage, this paper reviewed wired transmission technology and wireless data transmission in tunnels. In the early days of tunnel development, broadcast and wired telephones were used to report and record emergencies in tunnels. With the development of wireless transmission technology, more and more tunnels are entering the field of wireless transmission, and the transmission of signals in vehicle-road cooperation is even more dependent on wireless signal transmission. At present, data transmission in tunnels is mostly carried out by a combination of wired and wireless transmission, with wired transmission connecting the various sensors in the tunnel, such as temperature sensors, humidity sensors, lighting systems and monitoring systems, which are connected together by wired signals, and then using wireless signals to form a network for the remote transmission of data. With the development of 5G, more and more new antennas and more reasonable networking methods are being used in tunnel communications, greatly improving the quantity and quality of signal coverage in tunnels.