I don't understand why in much article and book about VLC IM-DD it's written that one can use an OFDM + QAM modulation.
The trick should be the hermitian simmetry imposed on the transmitter but, if i impose that the signal becomes real.
If the signal is real how can i transmit a QAM? How can i receive the phase?
see this paper:
Fundamentals of Coherent Optical Fiber Communications
Kazuro Kikuchi, Fellow, IEEE, Fellow, OSA
(Tutorial Review) 2016
http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7174950
Khalid, this article speak about coherent receiver and not IM-DD, i want to know (if i understand correctly) if the IM-DD can use a QAM modulation.
M-ary Quadrature Amplitude Modulation Subcarriers Multiple Intensity Modulation / Direct Detection (M-ary QAM-SCM-IM/ DD)
From The above article:
The IM·DD scheme has a significant advantage in that the receiver sensitivity is dependent neither on the carrier phase nor on the state of polarization (SOP) of the incoming signal, which randomly fluctuate in real systems.
On the other hand, a receiver, in which the signal is interfered with a local oscillator (LO) so as to extract the phase information of the signal, is called a coherent receiver; heterodyne and homodyne receivers.
In the case of coherent receivers, we can restore full information on optical carriers, namely, in- phase and quadrature (IQ) components (or amplitude and phase) of the complex amplitude of the optical electric field and the SOP of the signal. In order to gain such a significant advantage, coherent receivers are highly sensitive to random variations in the phase and SOP of the incoming signal.
To deal with this problem, the configuration of coherent systems becomes much more complicated than that of IM·DD systems.
The IM·DD scheme has a significant advantage in that the receiver sensitivity is dependent neither on the carrier phase nor on the state of polarization (SOP) of the incoming signal, which randomly fluctuate in real systems.
Here say that the carriar phase randomly fluctuate in real system.
But in that:
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6415964
with the impose of hermitian simmetry seem to be possible to use IM-DD with QAM OFDM without local oscillator
Dear Stefano Truzzi,
I hope the following article clears your question.
http://ieeexplore.ieee.org/document/4785281/?tp=&arnumber=4785281
Since VLC only works as an IM-DD system only, therefore QAM may not be directly implemented on a VLC signal (i.e. visible light). However, a QAM-OFDM signal can be employed to modulate visible light after applying necessary amendments such as dc-biasing or clipping the negative portion of the signal (asymmetrical clipping) altogether. This results in a real-valued and uni-polar signal that is required to modulate visible light. Hence, the receiver does not need to detect the phase of the received signal.
Please see:
http://www.homepages.ed.ac.uk/hxh/Li-Fi_PAPERS/14_optical_attocells.pdf
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