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Talking underground

Published by
World Coal,

Dr Wisam Farjow, PBE, discusses the choice of backbone architecture for underground mine communications systems.

The issue of safety in coal mines has gained high global visibility in recent years, following a series of dramatic incidents that led to human casualties, major structural damages and lengthy disruptions of coal mines’ operation systems, with hard-felt impact on regional economies. Effective mitigation of accidents and their far-reaching consequences is paramount. This requires an holistic approach to safety, where it becomes as much an integral part of incident management processes as of day-to-day operations. Widely adopted safety legislation means that comprehensive communication for emergency services is an essential requirement within all types of mines. In many instances, multiple wireless networks are required to operate alongside one another.

In general, the objective of a mine communications system is to provide links between all parts of the mine and the surface. A typical communications network consists of an equipment head-end, a backbone network, wireless channels and wireless network devices. The overall communications network connects the equipment head-end to applications used by personnel or devices in the mine. The approachand design philosophy of the overall system are dependent on many factors, including:

  • Permanent system vs construction phase solutions.
  • Type of mines and mine layout.
  • Application requirements.
  • Redundancy requirements.
  • Data throughputs and frequency bands.

This article will consider the choice of backbone architecture of a mine communications system. Three common backbone architectures are radiating cable, fibre-optic and hybrid.

Radiating cable direct feed architecture

A radiating cable network direct feed architecture uses a radiating cable network (RCN) to distribute services to confined areas of the mine. The RCN feeds, usually via coaxial cable, into head-end equipment, which in turn may connect to outdoor donor antennae. Head-end equipment typically consists of repeaters, radio frequency (RF) components, data head-ends and user interfaces, including software.

Radiating cable has historically been used to provide communications services throughout mines, as it is a reliable technology well suited to mine expansions. An RCN must be carefully planned for each underground environment, as signal losses are incurred via cable loss and in-line components, such as power dividers. Radiating cable exploits the leakage property of RF signals along cables to provide wireless coverage in the vicinity of the cable. Radiating cable is specifically designed to uniformly radiate signals in specific frequency bands along its length. It also acts as an antenna and as a cable to distribute signals and sometimes power throughout the mine.

Fibre-optic cable architecture

An alternative backbone network to radiating cable is a fibre-optic cable backbone. Unlike its counterpart, fibre-optic cable does not leak signals into the surrounding vicinity. This quality is advantageous in areas where RF coverage is not required. This type of cable has much lower losses than radiating cable losses. It can also be integrated with antennae to create a distributed antenna system (DAS).

A DAS may be implemented using a variety of equipment (Figure 1). In this arrangement, an on-surface donor antenna interfaces to a bi-directional amplifier (BDA) or repeater at the head-end, which is similar to the radiating cable architecture. The architectures differ in the way RFs are converted to optical frequencies at the fibre interface, taking the downstream signal path as an example. The fibre-optic cable distributes optical frequency signals to amplifiers, which in turn interface to indoor transmitting and receiving antennae. The upstream signal path follows the reverse process. Antennae are positioned throughout confined environments based on coverage requirements and signal coverage simulations. Signal coverage is not as uniform as in radiating cable architectures, but it can be optimised to ensure system requirements are met.


Figure 1. Fibre-optic cable backbone architecture.


Hybrid architecture

In large mine networks, hybrid architectures, which consist of radiating cable and fibre-optic cable, may be used to preserve signal integrity. Losses in expansive RCNs can be significant, so, in order to balance the losses, amplifiers are used. While amplifiers boost weak signals throughout the network, they also raise the noise floor of the system. Large networks, with many amplifiers, may have a high noise floor. A system’s noise floor affects its dynamic range, which can have consequences on voice and data signal transmissions. To alleviate this problem, hybrid networks may be used to minimise system loss and preserve signal integrity. Furthermore, hybrid networks are suitable solutions for networks requiring large amounts of data transmission that are sensitive to noise levels.

In a hybrid architecture, radiating cable is placed throughout the mine to provide continuous wireless coverage, while fibre-optic cable is used to feed amplified signals into the RCN at specific points via fibre-fed amplifiers. Fibre-fed amplifiers do not increase noise floor levels in the same manner as in-line radiating cable amplifiers. Moreover, high signal integrity is preserved throughout the RCN, thus minimising error rates.

In Figure 2, the fibre interface connects to several fibre-optic cable branches that run to fibre-fed amplifiers placed along the radiating cable network. The fibre-fed amplifiers inject amplified radio frequency signals into the RCN or receive signals from it. As signals are distributed to the RCN from various points, this architecture has built-in redundancy for cable breaks.


Figure 2. Sample hybrid architecture.



Backbone architectures for particular systems must be designed considering many factors. By carefully evaluating these factors, engineers may choose to implement radiating cable-based, fibre-based or hybrid networks. While the backbone network ties a communications network together, wireless devices – and their functionalities within mines – define the communications system. The wireless devices operate in a mine propagation environment, which must be modelled carefully to predict device performance.

Note: This is an extract of an article that first appeared in the July 2014 issue of World Coal.

Written by Dr Wisam Farjow. Edited by .

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