Research On Adaptive Coding And Bit-loading Algorithm In MB-OFDM Ultra-wideband Over Fiber System

With the rapid development of mobile application,the demand of bandwidth continues to grow exponentially.Multiband orthogonal frequency division multiplexing ultra-wideband(MB-OFDM UWB)over fiber system,owing to its wide spectrum,low energy consumption,broad coverage and other advantages,has been attracting great interest of wordwide researchers.However,MB-OFDM UWB signal can be subject power penalty and nonlinear effect in optical fiber transmission system.The spectrum efficiency and transmission quality can be further enhanced in MB-OFDM UWBoF system.In intensity-modulation and direct-detection(IM/DD)MB-OFDM UWB over fiber system,the bit-loading combined with adaptive coding algorithm and relevant performance of the digital signal processing(DSP)technology is proposed and experimentally investigated in this paper.The main works are as follows:1.An adptive LDPC code rate algorithm is proposed and investigated in intensity-modulation direct-detection(IM/DD)MB-OFDM UWB over fiber system.By utilizing the ALCR algorithm,according to the results of anticipating channel state of subcarriers,allocating the LDPC code rate for UWB signal with the optimal iteration.Therefore,the coding gain of system will be optimized.The experimental results show that,at hard-decision forward error correction(HD-FEC)limitation,compared to the same expanse of encoding,the signal-to-noise(SNR)is improved by 2.6 dB for128 QAM MB-OFDM UWB system after 100 km SSMF transmission using ALCR algorithm.Moreover,DFT-spread combined with ALCR algorithm is demonstrated inMB-OFDM UWBoF system.At the HD-FEC limitation.compared with code rate of75% MB-OFDM UWB signal without DFT-spread scheme,the 32 QAM MB-OFDM UWB signal by using DFT-spread combined with ALCR algorithm obtains power gainof 2.56 dB over 100 km SSMF transmission.In addition,a Mach-Zehnder modulation(MZM)with negative chirp parameter is utilized in proposed system.Compared to the BTB case,at the HD-FEC limitation,the average negative power penalty of 0.9 dB in128/64/32 QAM MB-OFDM UWB signal are measured over 50 km SSMF transmission.2.A new linear-approximation based bit-loading combined ALCR algorithm(BL&ALCR)is proposed and simulated in software-reconfigureable MB-OFDM UWBoF system.According to the condition analysis of channel state by estimating the SNR on each subcarrier with optimal iteration,the BL&ALCR is used to allocate theoptimal LDPC code rate and modulation format for MB-OFDM UWB signal.Thesimulation results present that,at the HD-FEC limitation,the value of Q factor is improved by 3.93 dB and the system transmission bit rate can be improved by 1.5 Gbps over 100 km SSMF transmission,compared to the fixed modulation with uncoded scheme at the same spectrum efficiency(SE).The highest transmission bit rates of MB-OFDM UWBoF system over different SSMF by using BL&ALCR algorithm are also investigated.