Transmission improvement due to pre-emphasis of multi-band OFDM-UWB signals in LR-PONs using directly modulated lasers

摘要

The transmission improvement obtained through the usage of pre-emphasis (PE) of single-band and multi-band orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) signals using directly modulated lasers (DML) along long-reach passive optical networks is experimentally evaluated. Additionally, OFDM-UWB sub-bands with central frequencies above the cut-off frequency of the DML are experimentally considered in order to evaluate extending the transmission capacity through the usage of PE. In case of single-band transmission, PE improves the bit error ratio (BER) of several OFDM-UWB sub-bands by more than two orders of magnitude in back-to-back conditions as well as after transmission along 100 km of standard single-mode fiber transmission (SSMF). Such improvement is limited by the reduced intensity modulation response variation of the DML within the passband of each OFDM-UWB sub-band, as fixed modulation index is considered in all tested cases. Considering multi-band transmission along 100 km of SSMF, PE allows doubling the number of transmitted sub-bands before a BER exceeding 10~(?3) is achieved for any of the sub-bands and an optical signal-to-noise ratio of 25dB@0.1 nm. Such improvement is obtained as PE changes the effective modulation depth of each transmitted OFDM-UWB sub-band, so that equalized signal power after detection and almost identical BER in all OFDM-UWB sub-bands occur.