Research On Spatial Dimming Technique In Multi-user MIMO Visible Light Communication Systems

The current surge in the number of emerging applications and wireless terminals have led to a rapid increase in the demand for wireless data traffic,as a result,the scarcity of ratio frequency(RF)spectrum has become increasingly prominent.Along with the popularization of new lighting technologies such as light-emitting diode(LED),LED-based visible light communication(VLC)technology provides a new method to meet the requirement of indoor green lighting and high-speed communication.Considering the increasing equipment of indoor users,the current point-to-point VLC link has many difficulties to meet the requirements of indoor multi-user high-speed communication in the future.Due to the downlink broadcast nature of visible light signals and the convenience of easy controlled of LED light source,multi-user multiple input multiple output(MIMO)VLC system can be served as a new method to alleviate the wireless traffic demand of indoor users.Lighting is the primary function of LEDs.The study of dimming technology for multi-user MIMO VLC system is of great significance for realizing the high-speed data communication for multi-users compatible with illumination,which can promote the practical application of multi-user MIMO VLC system.Traditional dimming methods in VLC is operated in different signal dimensions(amplitude,duty cycle)to regulate optical power.In multi-user MIMO VLC system,there exist some problems such as the multi-user interference(MUI),high user channel correlation,and inconsistent user channel state.Due to the limitation of signal dimensions that can be exploited for optical power adjustment,traditional dimming methods may sacrifice great communication performance.In fact,the channel state information(CSI)among the multiple LEDs at the transmitter side of multi-user MIMO VLC system is also an important resource that can be utilized.Based on the traditional dimming schemes,the dissertation studies the spatial dimming scheme in multi-user MIMO VLC system by make full user of the spatial domain resources introduced by CSI information,which can effectively realize the compatibility of efficient illumination and multi-user high-speed communication.Through the spatial information processing,the CSI of the multi-user MIMO VLC system can be fully explored,and the new theory and method of spatial dimension dimming are studied,which effectively avoids the communication performance degradation caused by the traditional dimming schemes due to the adjustment of the amplitude or the duty cycle of forward current.The goal of improving user communication performance at different dimming level can be achieved.The specific research contents are as follows:First,in order to solve the compatibility problem of MUI elimination and dimming,the spatial dimming based on user interference mitigation is studied.Zero forcing precoding based spatial dimming(SD-ZF)and minimum mean-squared error precoding based spatial dimming(SD-MMSE)schemes are proposed for the multi-user multiple input single output(MU-MISO)system.SD-ZF and SD-MMSE schemes directly change the number of working LEDs to adjust the optical power,which can effectively suppress the clipping distortion of the optical signal.Meanwhile,transmit antenna selection is performed to improve the the performance ZF and MMSE precoding.Theoretical analysis and simulation results reveal that,compared with ZF precoding based analogue dimming(AD-ZF)and MMSE precoding based analog dimming(AD-MMSE)schemes,SD-ZF and SD-MMSE schemes are.effectively reduce the clipping noise of each user and improve the system communication performance.For example,to achieve the target BER threshold of 10-3 at the 70%dimming level,SD-ZF can realize a signal-to-noise ratio(SNR)gain of about 4 dB compared with AD-ZF.Secondly,to solve the problem that the user channel correlation limits the dimming accuracy,the spatial dimming based on user channel decorrelation is studied.Joint analogue spatial dimming(JASD)scheme is proposed in the multi-user multiple input multiple output(MU-MIMO)VLC system.The number of working LEDs and DC bias of forward current are jointly adjusted by JASD,which can effectively reduce the number of LEDs at the transmitter side with high dimming accuracy,and the user channel correlation can be improved accordingly.Moreover,the maximized minimum singular value based TAS(TAS-MMSV)algorithm is applied in JASD with the utilization of CSI for the optimal working LED subset,which could improve the channel gain for each user.Compared with the analogue dimming(AD)scheme,JASD can effectively reduce the clipping noise at lower dimming levels and improve the user communication performance.For example,to achieve the target BER threshold of 10"3 at 80%dimming level,JASD can achieve a SNR gain of about 13 dB against AD scheme.Finally,to overcome the limitation of the inconsistent user channel state on the communication capacity,the spatial dimming based on user channel state adaptation is studied.A joint power analogue spatial dimming(JPASD)scheme is proposed in the MU-MIMO VLC systeIm through the adaptive power allocation based on the user channel state.Firstly,the electrical signal power of joint analogue space dimming in different dimming level compared with analog dimming is analyzed.And the joint analogue space dimming scheme is shown to realize higher electrical power compared with analogue dimming,which can pave the way for the association of power domain to improve user communication performance.Then,the adaptation the difference of user channel state under different dimming levels,and CSI is utilized to perform user power allocation at the transmitter side.In order to improve the communication capacity of users in different dimming levels,a power allocatior algorithm with user channel state adaptation is proposed.Theoretical an simulation results show that JPASD can effectively reduce th communication capacity loss caused by channel state inconsisten compared to the uniform power allocation scheme.Besides,compar with the joint power analogue dimming(JPAD)scheme,JPASD c achieve higher electrical signal power under different dimming lev therefore the system.communication capacity can be significantly improved.For example,to achieve a target system capacity threshold of 40bit/s/Hz at 80%dimming level,JPASD could achieve SNR gains of 3dB,5dB,and 7dB,respectively,compared with JPAD,JASD,and AD scheme.