Indoor visible light communication systems generally need to provide both communication and lighting services, and in order to meet the standard requirements for indoor lighting, indoor visible light communication systems must have brightness control (also called dimming control). Therefore, it is imperative to design indoor visible light signal modulation methods that can perform dimming control. Indoor high-speed visible light communication often uses orthogonal frequency division multiplexing (OFDM) modulation to achieve high-speed transmission, and traditional optical OFDM research mainly focuses on improving data transmission rate, which cannot effectively support dimming control, resulting in poor user experience. Therefore, dimmable OFDM modulation schemes adapted to communication-lighting integrated applications need to be designed. In OFDM dimming design, a balance between transmission efficiency and dimming capability is required. Direct current biased optical OFDM (DCO-OFDM) controls the brightness level directly by controlling the DC bias level, but it may limit and thus corrupt the signal waveform. By superimposing layered asymmetrically clipped optical OFDM (LACO-OFDM) with multiple positives and negatives, it achieves both dimming and improved spectral efficiency by using the multi-layer transmission. However, the multi-layer superposition characteristic of LACO-OFDM leads to a relatively high peak-to-average-power ratio and deteriorating the bit error rate (BER) performance at specific dimming levels.

For the lighting demand and dimming requirement of visible light communication, this paper proposes a hybrid superimposed LACO-OFDM (HSLACO-OFDM) modulation method based on LACO-OFDM modulation. According to the principle of LACO-OFDM signal formation and subcarrier occupancy in each layer, it can be seen that after the superposition of signals in the Lth layer, there will still be some subcarriers that are not used. The superposition of the adjusted signals on these subcarriers does not interfere with the signals of the LACO-OFDM layers. Therefore, we can design a time domain amplitude adjustment of the LACO-OFDM signal after superimposing the signals using these subcarriers to generate the superimposed LACO-OFDM (SLACO-OFDM) signal. For SLACO-OFDM signals, the receiver can detect the signal layer by layer using the same method of successive interference cancellation as for LACO-OFDM signal detection. By combining SLACO-OFDM and its negative signal NSLACO-OFDM through time division multiplexing, the HSLACO-OFDM signal is formed. The proposed HSLACO-OFDM makes full use of the entire dynamic range of light emitting diodes (LEDs) with adjustable mixed signal ratios to achieve the desired brightness. We propose an optimal setting of the HSLACO-OFDM interlayer power scaling factor using a Lagrangian function under a certain constraint of electrical power. It is also proved that the optimal ratio should be the same in each layer because the parameters are the same in different layers, when the signal transmission rate reaches the maximum. We also investigate the HSLACO-OFDM hybrid scaling factor determination method under dimming constraints. The two degrees of freedom of the dimming factor are analyzed, and the method of taking values under different dimming levels is given.

Simulations are performed to evaluate the performance of the proposed HSLACO-OFDM. It is also compared with DCO-OFDM, reconstructed LACO-OFDM (RLACO-OFDM), and adaptively biased layered optical OFDM (ABLO-OFDM). The BER performances of 4-layer HSLACO-OFDM and LACO-OFDM with 16-ary quadrature amplitude modulation (QAM), 32-ary QAM (32QAM) and 64-ary QAM (64QAM) modulation are simulated (Fig.6). At the same QAM modulation order, the BER decreases and then increases as the dimming factor changes from 1 to 6.5. The main reason is that as the dimming factor increases, the limiting noise of the signal decreases and the BER improves. However, too large dimming factor can also lead to a smaller signal power, resulting in a deterioration of the BER performance. And the dimming factor can be taken as the value corresponding to the smallest BER. From the results, it can be seen that the proposed HSLACO-OFDM signal has better BER performance compared with LACO-OFDM under the same simulation parameters. The spectral efficiency of HSLACO-OFDM under the condition of dimming constraint is simulated (Figs.7 and 8). For the same number of layers, the proposed scheme can achieve higher spectral efficiency than the conventional RLACO-OFDM and ABLO-OFDM for intermediate dimming levels. From the simulation results, it can be summarized that for HSLACO-OFDM, the optimal number of stacked layers varies with the noise power and dimming level. In low brightness and high brightness or high noise environments, a lower number of stacked layers should be used. When working in medium dimming level and low noise environment, more stacked layers can be used.

In order to realize the dimming control of visible light communication, this paper designs the HSLACO-OFDM dimming modulation based on SLACO-OFDM low peak-to-average-power ratio multilayer superposition modulation by combining SLACO-OFDM and NSLACO-OFDM signals. The hybrid ratio calculation method under the minimum BER and dimming constraints is given. The proposed HSLACO-OFDM makes full use of the entire dynamic range of LEDs with adjustable mixing signal ratio to achieve the desired brightness. Since HSLACO-OFDM has a lower peak-to-average-power ratio, a relatively stable transmission efficiency can be obtained over a wide adjustable range of light brightness. Simulation results show that this scheme has advantages over other commonly used multilayer modulation and dimming OFDM schemes in terms of BER performance and spectrum utilization.