Research On Superposed Constellation For MIMO Visible-light Communication Systems

In the era of AI intelligence,massive data has put forward higher requirements for Communication network and high-speed access technology.Due to the unique advantages,Visible Light Communication（VLC）has become one of the most promising emerging wireless Communication methods in B5G/6G ultra-high speed Communication network.Visible light communication can not only meet the requirements of lighting,but also provide safe,green,anti-electromagnetic interference,rich spectrum resources of high-speed communication services.However,the limited modulation bandwidth of the emitting device LED and the fading channel of the high frequency of VLC seriously affect the system performance and transmission rate.Multiple input multiple output（MIMO）spatial multiplexing（SMP）techniques can significantly increase system capacity without adding additional spectrum resources,so it has been widely applied to VLC systems.However,the use of intensity modulation and direct detection techniques（IM/DD）may create a correlated channel in VLC systems,especially when the transmitters and receivers are closely placed.Thus,the bit error rate（BER）performance deteriorates dramatically as the receivers fail to separate the parallel data streams when the channel links between transmitters and receivers are similar to each other.Recently,superposed constellation combined with spatial multiplexing MIMO techniques has attracted the attention of researchers,as multiplexing gains can be achieved regardless of the correlation extent of the VLC channel.Based on this,we carried out thorough research in this paper,and the main work is as follows:（1）In order to solve the problems of nonlinear distortion and power competition in VLC system,we propose a novel superposed 64-quadrature amplitude modulation（QAM）constellation scheme for 2×2 MIMO VLC systems.Two geometrically shaped 8QAM constellations are designed at the transmitter to superpose a square-shaped 64QAM constellation in the receiver,which is in an interleaved manner.The superiority of the proposed scheme can be explained as follows:first,the selected QAM format of the same order ensures that both transmitted signals are modulated in a low order.Meanwhile,the PAPR of the proposed 8QAM constellation is much lower than that of the traditional8QAM constellation.Consequently,the nonlinear distortion caused by the non-ideal transmission characteristics of LEDs can be reduced significantly.Secondly,the optimal64QAM constellation with uniformly distributed constellation points is achieved when two 8QAM signals are equally allocated,which avoids power competition caused by power imbalance and reduces the risk of nonlinear distortion of the system.In order to further verify the performance superiority of the proposed superposed 64QAM constellation scheme,an experimental investigation was carried out comprehensively,where the system performance was evaluated under different transmitted powers,direct current（DC）bias currents,and driving peak-to-peak voltage（Vpp）.Compared with the traditional superposed 64QAM schemes,the dynamic range of the driving Vpp of the two LEDs improved from 0.06 to 0.4 V and 0.23 to 0.4 V under a 7%pre-forward error correction（pre-FEC）BER threshold of 3.8×10^-3.（2）Although the above scheme meets the requirement of an equal power ratio and achieves a much lower PAPR of the transmitted signals However,the adjacent constellation points are more likely to overlap because of interleaved superposition.Consequently,the BER performance decreased dramatically when the power ratio was not equal to 1.In this chapter,a flipped superposed constellation scheme is proposed for2×2 MIMO VLC systems,which is valid for a superposed 2ⁿ-order QAM constellation of any order with n greater than 2.In this scheme,a 2ⁿ-order quadrature amplitude modulation（QAM）signal is obtained by superposing 4QAM and processed 2^n-2-order QAM signals.Based on the original 2^n-2-order QAM signal,the processed signal is generated by adding an optimal offset,power normalization,and flipping according to the value of the 4QAM signal.Thus,the required power ratio between the two superposed signals is equal to 1,which not only avoids the power competition in the receiver,but also reduces the risk of the nonlinear distortion of the light-emitting diode at the transmitter.In addition,the scheme has two additional performance gains:first,thorough Gray coding of the superposed QAM constellation is realized in the receiver.Second,benefitting from the correlation of the transmitted signals owing to the flipped superposition,the received power increases,which further improves the BER performance.The experimental results confirm that the proposed scheme not only obtains a much lower BER,but also realizes a larger dynamic range of the driving peak-to-peak voltage（Vpp）compared with the existing schemes.Compared with the interleaved superposed-64QAM-constellation scheme,the dynamic range of the driving Vpp of the flipped superposed constellation scheme improved from 0.09 to 0.36V under a 7%pre-forward error correction（pre-FEC）BER threshold of 3.8×10^-3.