Challenges in Realizing a Chronic High-Resolution Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 144

(converted from image information by signal processing unit) to the implant.
Besides the image information, forward data telemetry also needs to relay config-
uration information to the implant. Reverse data telemetry transfers information
to the external unit from the implant. The information includes implant status
such as temperature and pH, received power level, impedance measurement
data, etc.
Forward Data Telemetry
To achieve maximum flexibility of stimulation, forward data rate needs to be as
high as possible. For high-density (1000 pixels) retinal implant, the estimated
forward data rate is about 1 to 2Mbps, as mentioned earlier. There are several
ways to transmit information to the implant. The simplest method of data trans-
mission to and from the implant is a percutaneous wire connecting the two units.
The disadvantages of using a tethering wire as discussed in the power telemetry
section apply here as well. Optical telemetry is another option for transmitting
data, where a modulated beam of infrared light is used to encode information.
This information would then be decoded at the receiving end using optoelectronic
detectors and further processed using electronics. However, optical transmission
efficiency decreases rapidly with skin thickness and it is more sensitive to
relative dislocation between the transmission element and the receiving element
compared to magnetic coupling with coils [24] as well as the ambient light
sources. The challenges associated with inductive method of data telemetry will
be discussed below.
To transmit data into implant, one way is to utilize existing power trans-
mission link by modulating the data onto the power carrier. Another way is
to use an RF link with frequency different from the power carrier. In the first
case, the power carrier is modulated with the data, thus employing the power
carrier as the data carrier. This type of telemetry system is termed 'single band
telemetry' referring to the fact that the same carrier frequency is used for trans-
mitting both power and data through the same physical means. The method has
the obvious advantage of not needing a dedicated coil pair for data telemetry.
However, it faces the challenge of high data rate required by high-density stimu-
lation. Normally the power carrier frequency is less than 10MHz to penetrate
the human body without significant absorption and achieve high efficiency of
power transmission. Achieving high data rates (1 to 2Mbps) at a relatively low
carrier frequency is a great engineering challenge. Another approach to high data
rate and high efficiency power transmission, termed 'dual-band telemetry', has
been proposed in Ref. [25]. Shown in Figure 7.9, the power and data are trans-
mitted through separate pairs of coils using two different carrier frequencies,
forming a hybrid dual-frequency link which allows optimization of efficiency
power telemetry and data rate of the forward telemetry through allocation of
different frequencies. The power carrier frequency f L is lower than the data
carrier frequency f H .