A QAM/QPSK Modulation Scheme With IM/DD In Optical Wireless Communication

Atmospheric laser communication combines the advantages of traditional wireless communication such as optical fiber communication and microwave communication systems.It has the advantages of low cost,convenient installation,secure communication and no need for frequency license.It has gained extensive attention and become a new type of broadband wireless access communication.However,the capacity of optical communication systems is limited with the emergence of new technologies such as multi-beam transmission technology(i.e.multiple input multiple output,MIMO)applications.Multiple input multiple output(MIMO)technology can increase the channel capacity of existing systems,but in conventional optical MIMO systems,intensity modulation schemes such as OOK PPM,MPPM and so on are commonly used.Although these modulation methods have the advantages of simple realization,small size and light weight however,their modulation efficiency is low.In order to effectively solve the problem,high-order modulation schemes(such as quadrature amplitude modulation,QAM;quadrature phase shift keying,QPSK)are applied.However,the QAM / PSK modulation scheme uses phase to represent information,this results in complex and negative signals in these modulation schemes.The complex and negative signal cannot be expressed by the intensity of light.Therefore,this paper studies the transmission scheme suitable for intensity modulation / direct detection(IM / DD)mode for QAM / PSK modulation.The main research work is as follows:1.In view that the complex and negative signals in the QAM / PSK modulation cannot be transmitted directly in the IM / DD link,using the inverse fast Fourier transform(IFFT)method,a DC component QAM/PSK is proposed.The results show that the proposed scheme can not only improve the efficiency of data transmission,but can also effectively resist the scintillation effect caused by atmospheric turbulence and improve the system performance with the increase of antenna number.2.DC component QAM / PSK modulation scheme can effectively solve the problem that QAM / PSK modulation cannot be directly used for IM / DD modulation,however,its optical power utilization is low.Therefore,this paper proposes the signal mapping method so that the IFFT output of the real signal has Hermitian symmetry to remove the negative signal and proposes a much more suitable scheme for IM / DD modulation;asymmetrically clipped QAM/PSK modulation scheme.The results show that the BER performance of the scheme is better than that of the DC component QAM / PSK modulation scheme when the total power of the system is constant.