A wireless front-end for bioelectric recordings offers two important advantages: optimal interference reduction and safety is achieved due to a perfect patient isolation [1] and freedom of movement of the patient is retained as much as possible. Although recent developments such as fiber optic links [2] - which also offers a perfect isolation and a relatively large freedom for the patient - are satisfactory in many situations, some specialized applications such as long term continuous monitoring of epilepsy patients will benefit from a true wireless system.
Radiofrequent transmission is the method most often applied for wireless transmission links. The main advantage of RF transmission is the favorable ratio between range and power consumption. The main problems in clinical applications are the increasing amount of HF interference sources and the difficulty to comply with the various local regulations in terms of transmission power and frequency bands.
The main disadvantages of infrared transmission are the low efficiency of optical transmitter and receiver components and the requirement of an uninterrupted light path between the transmitter and the receiver. However, infrared transmission can have important advantages in clinical applications. In contrast to RF transmission, infrared signals remain confined to the experimentation room, which can be an advantage in many clinical situations since interference with other recordings and experiments is prevented. In addition, usual light sources (incandescent and fluorescent lights) produce hardly any infrared interference signals with a modulation frequency higher than a few kiloHertz. As a result, a virtually empty transmission channel is available.

 

II. BIDIRECTIONAL INFRARED TRANSMISSION

A digital modulation format was developed based on the following design rules:

  1. Optimal use should be made of the possibility of transmission between the mobile and the stationary section in both directions.
  2. The emitted optical power of the mobile section should be regulated depending on the distance between the mobile and stationary sections.
  3. The direction of emission of the mobile section should be adapted depending on the orientation of the mobile section with respect to the stationary section.

These principles were used in a digital IR transmission system of which a simplified block diagram is given in Fig. 1. Data is encoded in high frequency bursts of variable length. A 10 MHz burst is emitted by a one of the lasers for every 'one' bit while the length of the burst is adjusted so as to keep the magnitude constant of the converted burst detected by the stationary section. This results in a minimum power consumption of the mobile section.
A continuous timing signal transmitted by LEDs from the stationary section to the mobile section has a frequency of either 62.5, 104.2 or 312.5 kHz. Toggling between these odd harmonics enables information to be transmitted to the mobile section. In the present prototype the sampling rate and the burst length are regulated. The large frequency difference between the back (max. 312.5 kHz) and forth (10 MHz bursts) going signals prevents interference problems. Synchronized detection (lock-in detection) can be performed on the high-frequency bursts received by the stationary section since the 10 MHz signal during the bursts tracks with the master clock in the stationary section. The prototype of the mobile section was equipped with three independent optical receiver/transmitter units, covering a solid angle of approx. 0.5 sr. All three receivers are permanently on. The position of the stationary section relative to the mobile section can be deduced from the differences in magnitude of the output signals from the three receivers. This makes it possible to emit in the appropriate direction by transmitting only with the LASER aimed in the optimal direction.

 

III. RESULTS

The infrared transmission link was applied in a prototype 16-channel EEG recording system. The EEG signals are amplified, time-multiplexed and digitized by a 12-bit A/D converter. Solid state laser diodes with an optical output power of 20 mW are used as optical transmitters in the mobile section. The output of each of the lasers covers a solid angle of approx. 0.2 sr. The mobile section is powered by a single 6 V sealed lead-acid battery with a capacity of 1.2 Ah. The maximum continuous operating time is approx. 10 hours at the maximum distance of 5 meters between the mobile section and the stationary section. In the stationary section, the received 10 MHz bursts are mixed with a 10 MHz clock signal and integrated. The output signal of the integrator is the recovered bit stream from the A/D converter. Control words are transmitted with the data-stream to check if the optical output power of the mobile section should be increased or decreased by altering the lengths of the bursts. The signal to the mobile section is transmitted by 6 infrared LEDs with a combined output power of 60 mW. With this prototype a 16 channel EEG signal could be transmitted as long as the receiver of the stationary section stayed within the direct view of one of the three lasers in the mobile section.

 

IV. DISCUSSION

The prototype showed that infrared transmission could form an alternative for RF transmission in biomedical telemetry systems. However, with three laser emitters in the mobile section only a solid angle of approx. 0.5 sr could be covered which did not give the patient sufficient freedom of movement. A clinical useable system should be equipped with a larger number of lasers and receivers in order to cover at least a half hemisphere (about ten of the current lasers would be needed). We intend to achieve this in a future prototype with a further integration of components.

 

REFERENCES

[1] A. C. MettingVanRijn, A. Peper and C. A. Grimbergen, "The isolation mode rejection ratio in bioelectric amplifiers," IEEE Trans. Biomed. Eng., vol. 38, pp. 1154-1157, 1991.

[2] A. C. MettingVanRijn, A. P. Kuiper, A. C. Linnenbank and C. A. Grimbergen, "Patient isolation in multichannel bioelectric recordings by digital transmission through a single optical fiber," IEEE Trans. Biomed. Eng., vol. 40, pp. 302-308, 1993.


Fig. 1 Block diagram of the infrared telemetry system. A synchronization signal is transmitted from the stationary to the mobile section while 10 MHz burst signals are transmitted from the mobile to the stationary section.