Conclusion

Why Leaky Cables?

Until now we have only evaluated the leaky coaxial cable. However, there are still two other signal transmission technologies which can be chosen for the project. The wireless sensor network (WSN) for mobile underground coverage is possible by using either electromagnetic (EM) waves system or magnetic induction (MI) system. There are pros and cons for each technology.

The traditional terrestrial wireless technology uses EM waves, mainly microwave. But EM wave can no longer be propagated in air in an underground environment. WSN has three main problems in the underground situation, namely high path loss, dynamic channel condition and large antenna size. Firstly, the EM waves will be heavily attenuated by soil, rock and water. Because space is limited in underground tube stations, the power available for the equipment will be limited as well, hence causing transmission range of EM waves to be less than 4 metres. Moreover, the path loss varies dynamically, depending on the soil properties such as water level and soil density, which will also be dramatically different depending on time and location. This channel condition means that there will be a difficulty designing a network with reliable connectivity and high power efficiency. In order to reduce the path loss, the frequency of EM waves can be lowered. But this means larger antennas are needed for signal transmission, which doesn't satisfy the small sensor size requirement of a limited space .

Instead of the troublesome EM waves system, MI system could be a better form of wireless underground sensor network (WUSN) because MI communication system can resolve the three major problems faced by the EM waves system. Basically the transmission and the reception occur by using a coil of wire.

Unlike EM waves transmitter, we don't have to consider the radiation power of MI transmitter. The ratio of transmitting and receiving power is nearly one, if the resistance of the coil is small. This feature ensures almost no transmission power lost to the surroundings . This technology is not affected by the dynamic channel condition because magnetic permeability of different components of soil is relatively constant so the channel condition remains the same for most of the time.

The system operates with small-sized coil so there is no need for big antenna, thus saving space. For MI system, if the transmission distance increases by d, then the magnetic field strength decreases by a factor of (d3 . It thus makes MI and EM wave systems hard to be applied in the real world due to their short transmission ranges, both less than 10 metres : "the MI system provides larger transmission range (around 10 m) than that of the EM wave system (around 4 m)" . Currently there is no technical solution to improve their transmission range and thus making the leaky coaxial cable the most practical solution!

Final Words?

Over the past 20 years, 80% of the world population own mobile phones. This demonstrates how reliant people have become on mobile phones and there is no doubt that you can get phone coverage almost everywhere you go except… the underground.

Mobile phones lack network reception in tunnels and underground stations because radio waves cannot travel through walls. Therefore, we suggested leaky cables to be used as a solution to this problem.

But, as explained above, there are many approaches available in addressing this problem. Such approaches have already been implemented in some countries. Therefore, the delay in solution implementation in this country could be due to a number of factors; underdeveloped technology is certainly not one of them.

Security can be one such factor. Considering the geological conditions of the channel, as the oldest railway in the world, the network could be too old and too fragile to support any installation of new devices. Besides, Londoners are heavily dependent on tube; therefore any engineering service will disrupt the daily service. Hence, any upgrade to the system should be thoroughly planned to avoid any further disruptions. Apart from that, using phone underground may cause signal interference among the equipment, just like using mobile phone in flight will interfere with the operation of some avionics instrument installed on aircraft.

However, with technology advancement, we are positive that these problems could be overcome as long as there is adequate funding for further research to improve the suggested solution and substantial support given by both the authorities and the general public.

With that, science and technology can hopefully succeed in achieving its ultimate purpose of making our lives a lot easier!

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