Scattered Chatter

Tall buildings are often the bane of cell phone users when making calls in big cities, but such scattering structures may someday enhance communication. Researchers at the Laboratoire Ondes et Acoustique in Paris recently demonstrated the effect with ultrasonic antennas in a water-filled tank. When the space between a 23-element transmitter array and an array of 5 receivers was devoid of scattering structures, the error rate in transmission of a set of 5 messages sent simultaneously to the receivers was about 28%. By placing a forest of randomly arranged steel rods between the transmitter and receivers, the researchers found that they could transmit the same messages without any error at all.

The effect is due to the fact that, without scatterers, there is a substantial amount of cross-talk between the receivers - that is, each antenna detects some of the data intended for its neighbors, and has no way to distinguish between the message it is supposed to receive and the messages it should ignore. By adding scatterers, the researchers ensured that signals intended for different antennas were distinct. The situation can be understood by imagining a single antenna sending a signal to multiple receivers. Without scatterers, each receiver detects very nearly the same thing. Adding scatterers distorts the signal, and each antenna detects something slightly different. In fact, by reversing the experiment and sending signals from each of the receivers back to the transmitter (an arrangement known as a time-reversal antenna, Update 190) it is possible to ensure that with the scatterers, a transmitter array can send multiple unique signals that are only detectable by the intended receivers.

In essence, scatterers make it possible to establish multiple communication channels, and the more scatterers between the transmitters and receivers, the more channels that are available. For the time being, the communication technique is limited to ultrasonic communication - the electronics necessary for exploiting scatterers with wide-band time-reversal antennas at cell phone frequencies simply don't yet exist. But when they are developed, the buildings that currently hamper wireless communication will become a cell phone user's boon. (A. Derode et al., Physical Review Letters, 10 January 2003)