Design Problem: The Shinkansen Bullet Train and Wireless Coverage

Shinkansen is the name given to the Japanese high-speed bullet trains that run from Hachinohe on the northern end of Honshu to Fukuoka on the island of Kyushu.   The network of train lines offer connections between numerous major Japanese cities at speeds of 260 km/h and upwards.  At these remarkable rates of travel, wireless communication via mobile phones becomes an increasingly challenging problem.

Mobile phones rarely have a direct link to their base stations and instead rely on the tendency for electromagnetic waves to diffract and reflect.  These reflections and diffractions are what is generally received by the mobile unit in urban environments and provide an excellent example of the multipath problem.  As single spherical wave propagated from a base station propagates through an environment, different parts of that wave will incur different phase changes as it diffracts and reflects about various objects.  A mobile user in the environment will not receive one wave but a multitude of waves that have bounced and bent their way to the user from the base station.  Due to the varying phase changes of each wave, they will constructively and destructively interfere with each other in space.  As a mobile phone user travels through this environment, their signal strength is going to vary dramatically from one point to another.  This rapid signal variation as a function of space is called small-scale fading.

Because small-scale fading is a function of space, at high velocities where one’s location is changing rapidly, the effects of small-scale fading will be dramatic.  This brings us to the problem at hand.  The Shinkansen bullet trains operate at extremely high speeds and thus are very susceptible to the effects of small-scale fading.  The large fluctuations in signal strength due to small-scale fading will cause any potential wireless link to be unreliable.  This presents a difficult problem for cellular providers who wish to serve customers in a high-speed Shinkansen train.  Any provider who does manage to secure a reliable wireless link will have a significant marketing advantage over its competitors.