Research On Novel Modulation Formats In Short-reach Optical Communication Systems

The rapid development of high-speed services,such as cloud computing,Internet protocol television(IPTV)and high-definition video creates a higher demand for both bandwidth and bit-rate for short-reach optical access network.However,coherent detection utilized in long-haul optical communication systems is too expensive for short-reach optical communication system and solutions that use intensity modulation and direction detection(IM-DD)are seen as more practical.Meanwhile,to meet the increasing end-user' s demands for broadband services,modulation formats applied in IM-DD system should be more spectrally efficient.In this paper,novel modulation formats with high spectral efficiency(SE)for short-reach optical fiber system and visible light communication system are experimentally investigated.The main works are as follows:1.An adaptive pulse amplitude modulation(APAM)scheme is proposed and experimentally demonstrated in the IM-DD optical communications system using directly-modulated laser(DML).In the proposed scheme,the channel is divided into two sub-channels,and different PAM mapping can be chosen for different sub-channel according to the fading conditions to optimize the bit error rate(BER)performance and SE.Using intra-symbol frequency-domain averaging(ISFA)to improve the accuracy of channel estimation,frequency domain equalization is applied for the proposed modulation format with high SE.The 20-km standard single mode fiber(SSMF)transmission of 20 and 24 Gbps 8/4-APAM signal with the SE up to 5 bit/s/Hz is experimentally demonstrated.The experiment results show that the bit error rate(BER)of the 8/4-APAM signal can be achieved less than 2.4×10-2,which is the threshold of the soft-decision forward-error-correction(SD-FEC).2.An APAM transmission system with low-density parity-check(LDPC)pre-coding scheme is proposed and experimentally demonstrated in the IM-DD optical fiber communications system.The experimental results show that,after transmission over 20-km SSMF,at a BER of 3.8×10-3,the receiver sensitivities have been improved 2.76 and 4.17dB for 87.5%and 75%code rate,respectively,compared to the uncoded case.3.Based on a cost-effective DML and blue laser diode(LD),a hybrid fiber-visible laser light communication(Fiber-VLLC)system using orthogonal pulse amplitude modulation and discrete multitone modulation(O-PAM-DMT)is proposed.Fiber-VLLC system integrates the advantages of fiber-to-the-x(FTTx)technology and VLLC.Meanwhile the peak to average power ratio(PAPR)of O-PAM-DMT is effectively reduced compared to the conventional DMT.Through adjusting the ratio of PAM and DMT signal,three types of O-PAM-DMT signals are investigated.The experimental results show that,after transmission over 20-km SSMF and 5-m free-space VLLC,at a BER of 1×10-3 the receiver sensitivity for 4.36Gbps O-PAM-DMT signals can be improved by 0.4,1.4,and 2,7 dB,respectively,compared with a conventional DMT signal.