说明：  The inherent robustness of OFDM against multipath effects makes it an excellent choice for situations where multiple transmitters are used simultaneously (to avoid shadowing effects) and a path difference to the receiver exists. One of the major disadvantages of an OFDM based system is the characteristic high crest factor of the time domain signal. In traditional RF based systems, this usually necessitates a transmitter with a high dynamic range and hence results in reduced power efficiency. In the proposed optical LED system, this disadvantage is turned to an advantage - the time domain OFDM signal is used to modulate (IM-DD) the optical source.