A generalized solution to the spectral efficiency loss in unipolar optical OFDM-based systems

A number of unipolar optical orthogonal-frequency-division-multiplexing (OFDM) schemes have been proposed as a solution to the high energy consumption in the widely adopted direct-current-biased optical OFDM (DCO-OFDM) modulation scheme. These schemes have a reduced spectral efficiency due to the restrictions imposed on their frame structure. The enhanced unipolar OFDM (eU-OFDM) modulation scheme was recently introduced to compensate for the reduced spectral efficiency in unipolar OFDM (U-OFDM). The concept exploits the frame structure of U-OFDM and allows for multiple U-OFDM information streams to be combined, thus increasing the overall spectral efficiency of the communication system. In this paper, the concept of the enhanced U-OFDM scheme is generalized for arbitrary combinations of U-OFDM data streams with various constellation sizes and various power allocations. A closed-form theoretical bound on the bit error rate (BER) performance of the GeneRalizEd EnhaNcEd unipolaR OFDM (GREENER-OFDM) is derived and verified by comparison with the results of Monte Carlo simulations. The proposed scheme has an improved power efficiency compared with a spectrally equivalent DCO-OFDM. The GREENER-OFDM allows the gap in spectral efficiency between DCO-OFDM and the inherently unipolar optical OFDM schemes to be completely closed.