There are two main solutions for using mobile signal repeaters to cover tunnels :  Fiber optic repeaters : these repeaters use fiber optic cables to transimit the signal from the cell phone tower to the tunnel .fiber optical cables are less susceptitle to signal attenuation than copper cables,making them ideal for use in tunnels .  DAS(distributed antenna system ):this is a more complex solution that involves installing a series of antennas and amplifiers throughout the tunnel . DAS systems can provide better coverage than fiber optic repeaters .but they are also more expensive to install and maintain