Multiplexing of Optical Biotelemetry with Spread Spectrum Technique

Recently the problems of electromagnetic interference in the hospital are serious. An optical biotelemetry technique using indirect light transmission was considered to be one of the solutions. However with this technique, the discrimination of the different transmitters has been difficult. To solve this problem, we have applied the spread spectrum (SS) technique to the optical biotelemetry. An experimental system was developed which consists of two transmitters and one receiver. In a transmitter, original signal is multiplied with a pseudo-noise code in time domain so as to have extremely wide frequency spectrum. And the resultant signal is transmitted out in light. In the receiver, the received signal is multiplied with the same pseudo-noise code as the one in the transmitter. Using different types of the pseudo-noise code for each transmitter, many channels can be multiplexed in common space. This technique is less susceptible than other techniques to the interference due to extrinsic noises, multi-path propagation and signal of other channels. Using this system, some fundamental characteristics of this technique have been investigated. Internal noises of the system and threshold values for satisfactory transmission were measured. Finally, simultaneous transmission of ECG's from two transmitters were performed. The quality of transmitted signal was generally good having no cross-talk between two channels. With this technique the usefulness of optical biotelemetry in clinical use will be expanded.