Free Space Photonics Signalling Technique

A new bandwidth efficient method to implement orthogonal frequency division multiplexing (OFDM) on intensity modulated direct detection (IM/DD) channels is presented and termed auto-correlated optical OFDM. It is shown that a necessary and sufficient condition for a band limited periodic signal to be positive for all time is that the frequency coefficients form an autocorrelation sequence. Instead of sending data directly on the subcarriers, the autocorrelation of the complex data sequence is performed before transmission to guarantee non-negativity. In zdomain, the average optical power is linked to the position of the zeros and used for the design of signal sets. In contrast to previous approaches, auto-correlated optical OFDM is able to use the entire bandwidth for data transmission and does not require reserved subcarriers. Using a sub-optimal design technique with 9 subcarriers and 8 bits per symbol, autocorrelated optical OFDM has a 0.5 dB gain over ACO-OFDM at a BER of 10-5 and a reduction in peak-to-average ratio of more than 30%.