Optical data transmission system and method
An optical data transmission system and method employs positive only data modulation, with an offset applied to the positive modulated values before generating modulated drive current signals for a light emitting component. The offset is such that the minimum drive current falls in a range where the electron-to-photon efficiency of the light emitting component is substantially constant. The drive current thus falls in a more linear part of its current vs. intensity characteristic. This reduces distortion and hence enables increased data rate.
1. An optical data transmission system, comprising:
an input for receiving a data signal;
a light emitting component;
a modulator for modulating the data signal to provide a modulated data signal having only positive modulated values;
an offset unit for adding a positive DC offset to said only positive modulated values; and
a converter for generating current signals based on the offset and modulated values, thereby providing a modulated drive current signal for driving the light emitting component,
wherein the data signal is a bi-polar data signal and the modulated data signal comprises a non-negative OFDM signal having a probability density function of the modulated output which has a peak at zero and wherein as a result of the positive DC offset added to the modulated values, the minimum drive current falls in a range where the electron-to-photon efficiency of the light emitting component is substantially constant.
2. The system as claimed in claim 1 , wherein the light emitting component comprises an LED and the minimum LED drive current provided by the modulation is between 0.05 and 0.2 times the maximum LED drive current provided by the modulation.
3. The system as claimed in claim 1 , wherein the modulated drive current signal comprises a LED drive current.
4. The system as claimed in claim 1 , wherein the data signal is converted by a non-linear operation into only positive modulated values.
5. The system as claimed in claim 1 , wherein the light emitting component comprises an LED, and wherein the linearity of the LED light output intensity with respect to the LED drive current is greater between the minimum LED drive current and a maximum LED drive current than between zero and said maximum LED drive current.
6. The system as claimed in claim 1 , wherein the system comprises a digital to analog converter, wherein the modulator and the offset unit are provided in the digital domain before the digital to analog converter.
7. The system as claimed in claim 1 , wherein the system comprises a digital to analog converter, wherein the modulator is provided in the digital domain before the analog to digital converter and the offset unit is provided in the analog domain after the digital to analog converter.
8. The system as claimed in claim 1 , further comprising a controller for selecting the positive DC offset in dependence on the characteristics of the light emitting component to be driven by the drive currents.
9. An optical transceiver system, comprising:
the optical data transmission system as claimed in claim 1 ; and
an optical data reception system comprising
a photo diode arranged to receive light transmitted by the optical data transmission system and convert it to the electrical domain, and
an non-negative OFDM signal demodulator arranged to demodulate the data signal from the photo diode output.
10. An optical data transmission method, comprising:
modulating a data signal to provide a modulated data signal having only positive modulated values;
adding a positive DC offset to said only positive modulated values;
generating light emitting component drive currents based on the offset and modulated values,
driving a light emitting component using the drive currents,
wherein the data signal is a bi-polar data signal and
the modulated data signal comprises a non-negative OFDM signal having a probability density function of the modulated output which has a peak at zero and
wherein as a result of the positive DC offset added to the modulated values, the minimum drive current falls in a range where the electron-to-photon efficiency of the light emitting component is constant.
11. The method as claimed in claim 10 , wherein the light emitting component comprises an LED and the linearity of the LED light output intensity with respect to the LED drive current is greater between the minimum LED drive current and a maximum LED drive current than between zero and the maximum LED drive current.
12. The method as claimed in claim 10 , wherein modulating the data signal comprising applying a non-negative OFDM modulation to the bipolar data signal.
13. A non-transitory computer readable medium having a computer program stored therein, which is adapted, when said program is run on a computer, to implement the processing step of the method of claim 10 .