Optic Wireless Communications (OWC), also called Free Space Optics (FSO) represent an attractive technological solution in terms of throughput to obtain broadband Internet access on board trains. Indeed, FSO technologies offer large unregulated bandwidth allowing throughput up to Gbps, in addition to immunity to electromagnetic interferences and low Bit Error Rates (BER). Moreover, optical signals cannot penetrate walls and optical “print” being easily defined, transmission can be completely secured.
Studies in Japan
The Railway Technical Research Institute in Japan tested this technique, in collaboration with the Keio University [86-90]. Throughputs up to 700 Mbps are obtained at TCP level, for a speed of 130 km/h.
First works investigated a ground-to-train communication system using FSO technology . Some BER experimental results using test train are given, showing that the proposed system is a promising candidate for train communication from the view point of BER characteristics. Works are then pursued by improving the system [87, 90]. Three different methods are tested:
- • The leaky optical fiber method: this method requires installation of optical fiber along the track. It uses laser beam that flee through the fiber to establish a communication. The method allows to obtain continuous communication with the train;
- • The “fan-shaped laser beam” method: this method uses a laser beam diffused with a concave lens. The lens radiates the laser beam in one horizontal direction. At reception, the laser beam is caught by the condenser lens. This is one of the characteristics of this method: laser transmitter can communicate with a wide area receiver;
- • The “laser beam tracking” method: the transmitter consists of a laser transmission device and a mobile mirror. It transmits laser beams towards the receiver. This one is identified using an infrared beacon light. With the mobile mirror, the transmitter can follow the receiver and establish a continuous communication.
Preliminary tests were performed in order to compare the different methods. It follows that laser beam tracking method is the most efficient. It allows obtaining throughputs up to 400 Mbps (against 100 Mbps for the two other methods). Moreover, transmission distance is more important and dynamic mirror makes the solution much more flexible.
Authors detail the communication system by laser beam tracking adapted to the railway constraints. The communication device is embedded on board train (the mobile station) and its ground counterparts (base stations) send a laser signal in order to establish a bidirectional communication. Each of them transmits a light beacon signal, standing for an identifying signal, with a different wavelength from laser signal. To apply the laser beam tracking method to railway environment, it requires the deployment of many base stations in order to cover the entire railway line. Therefore, the system requires a handover mechanism between the different base stations. Problems to solve are then as following:
- 1. Connection has to be maintained whatever the speed of the train and the possible vibrations, for high speed. The mobile mirror has to operate in a very dynamical way;
- 2. Handover has to be performed rapidly and dynamically, even at high speed, connection being completely interrupted during the handover.
A developed prototype was able to record theoretical throughputs up to 1 Gbps. It consists of a mirror able to move in all 3D directions, which allows reducing size and weight of the device. Details of the development of the tracking mechanism and the optical equipment (lens, diodes and mobile mirror) are described in . The minimization of the size and the weight of the lens is studied, in addition to the study of the beacon laser power and the types of lens to be used at reception (telephoto lens preferred to wide spectrum lens).
Handover mechanism between different base stations is also described. An optimized handover is implemented by improving standard protocol . In this paper, measurement results in an emulated environment where a handover occurs every 5 s showed a packet loss rate of 2 % during the handover. The network is then divided in subnetworks because of its large size. Two types of handover have to be considered: the handover performed inside a same subnetwork, which is realized at layer 2 level, the link layer, and handover performed between two different subnetworks, realized at layer 3 level, the network layer. The system is based on the mobility protocol IPv6. Enhancements are performed from IPv6 at different steps of the handover, in order to minimize its duration.
Experiments are then realized in order to fix the “ideal” transmission distance depending on the number of base stations deployed along the track, which allow keeping a continuous communication during the entire trip. It follows that a distance between 300 and 400 m seems to be optimal.
Authors are interested in the influence of atmospheric conditions on the quality of the communication. The study is quite succinct and without numerical data. The given conclusion is that the quality of the communication depends on the visibility.
First tests of the entire system are set up. Initially, tests in static are performed. A first communication between two devices allows obtaining throughput at TCP level up to 923 Mbps. The transmission distance was tested until 360 m maintaining a communication. A glass was placed between two devices in order to simulate the train window. The transmission distance is reduced to 200 m, but without loss of throughput. A last test is implemented: a communication between a fixed base station and a mobile station put in a car moving at 100 km/h is realized; a maximal throughput of 656 Mbps is obtained.
After these preliminary tests, the system was tested on a train. Three bases stations are positioned along the track and connected to a control center. They are separated by 100 m from each other. At a speed of 130 km/h, throughputs between 500 and 700 Mbps are achieved. An important packet loss rate of about 20-30 % is observed, which represents a subject to improve. The handover time also remains high (about 0.4 s), which is due to the train vibrations causing instability of the infrared link. There is no significant observation regarding the influence of atmospheric conditions. A special effort is still to provide a protection of devices against condensation.
Finally, works conclude with HST trials on Shinkansen trains. The speed is about 240-270 km/h. A single base station could be installed. The handover mechanism could then not be tested. However, the communication between the mobile station on board train and the fixed base station was tested. The two stations could catch the beacon light for 0.7 s. A communication at PHY layer could be realized during 6 ms. However, no packets transmission could have been tested.
In , authors propose a collaboration of the Infrared Communication Device (IR-CD), presented above, and the LCX system deployed in the Shinkansen, presented earlier. The proposed system installs the IR at the upstream of the LCX system to keep the modification of the existing LCX system as small as possible. The LCX system and the IR system are not used at the same time; switch is performed when IR system is not available. The proposed system was implemented in Linux in order to evaluate the handover processing time (which is important in the case of the LCX system). It appears that handover time is short enough for passengers on Shinkansen (around 200 ms, near LCX handover time).