A Dimmable OFDM Scheme With Dynamic Subcarrier Activation for VLC
IEEE Photonics Journal
Dimmable optical orthogonal frequency division multiplexing (D-OFDM) is challenging to design for visible light communications, due to the high peak-to-average power ratio (PAPR) of OFDM signals and limited dynamic range of LEDs. Current D-OFDM schemes focused on the time domain and spatial domain designs of OFDM signals. This paper proposes a novel dynamic subcarrier activation based OFDM (DSA-OFDM) scheme. In DSA-OFDM, the number of activated subcarriers, as well as the signal's DC bias and
... nal's DC bias and the number of activated LEDs, is adjusted to provide both flexible dimming control and reliable communication. A generalized index modulation in both spatial domain and frequency domain is designed to select the activated subcarriers and activated LEDs. To show the influence of frequency domain design analytically, the closed-form expressions of the PAPR and the clipping noise of the DSA-OFDM signals are derived, and the optimized signal form is obtained accordingly. Simulation and numerical results show that the proposed scheme can outperform conventional dimming control schemes at various illumination levels in terms of the bit-error rate performance. Index Terms: Dimming control, orthogonal frequency division multiplexing, peak-toaverage power ratio, visible light communication. IEEE Photonics Journal Dimmable OFDM Scheme With Dynamic Subcarrier Activation and non-negative, several O-OFDM schemes have been proposed, such as direct current biased O-OFDM (DCO-OFDM)  and asymmetrically clipped O-OFDM  . Besides, to support dimming control, several types of dimmable optical OFDM (D-OFDM) schemes have been proposed. Analogue dimming (AD) schemes, such as enhanced DCO-OFDM , achieve dimming control by varying the DC bias of the transmit signals. However, the dimming range of AD needs to be small to avoid excessive clipping noise caused by the high peak-to-average power ratio (PAPR) of OFDM signals. To avoid high PAPR, digital dimming (DD) schemes achieve dimming control by adjusting the duty cycle of digital signals. In  , the authors proposed a composite signal form of OFDM and pulse width modulation (PWM) for high-speed communication, where the duty cycle of PWM was used to achieve dimming control. In addition, spatial dimming (SD) was proposed in , and it achieves dimming control by adjusting the number of activated LEDs in an LED array. In this way, the DC bias of OFDM signals is always equal to the middle of the dynamic range of LEDs, so that a relatively high PAPR can be tolerated. However, the data rate of DD is constrained by the duty cycle of the PWM signal, while the dimming range and dimming accuracy of SD is constrained by the total number of LEDs in the LED array. Recently, a hybrid dimmable OFDM (HD-OFDM) scheme that combines AD and SD was proposed in  to achieve the advantages of the both. However, when the dimming level is sufficiently high, all the LEDs should be activated to satisfy the required dimming level, and the high PAPR issue of OFDM signals still degrades the performance of HD-OFDM. Different from the RF based PAPR reduction scheme, the optical power constraint of OFDM signals should also be considered in the PAPR reduction schemes for VLC. This is particularly important for D-OFDM schemes, in which the optical power changes dynamically. This is because the optical power is mainly determined by the DC bias, while the DC bias has a direct impact on the PAPR of D-OFDM signals. However, the conventional PAPR reduction methods in RF  and VLC -  do not take the optical power constraints into consideration. This motivates us to combine the PAPR reduction technology with dimming control in D-OFDM for VLC. The existing studies have focused on the time domain design of OFDM signals, while the frequency domain design also matters for a D-OFDM scheme. More specifically, the number of activated subcarriers can significantly affect the PAPR of OFDM signals ,  , thus further affecting the performance of a D-OFDM scheme. Hence, even though the frequency design of OFDM signals may not directly determine the optical power, it is necessary to be considered in a D-OFDM scheme. The main contribution of this paper is to propose a novel dynamic subcarrier activation based OFDM (DSA-OFDM) scheme for VLC to achieve flexible dimming control and reliable communication. Since the dynamic subcarrier activation is based on frequency domain design, DSA-OFDM can be combined with existing D-OFDM schemes that focused on the time and spatial design of OFDM signals. Therefore, in DSA-OFDM, the number of activated subcarriers, as well as the signal's DC bias and the number of activated LEDs, is adjusted for a target illumination level. Since only parts of subcarriers and LEDs are activated, a generalized index modulation in both spatial and frequency domains is employed to select the indices of activated subcarriers and the indices of activated LEDs. To investigate the performance of DSA-OFDM, we derive the closed-form expressions of the PAPR and the clipping noise of DSA-OFDM signals, and then obtain the optimized signal form of DSA-OFDM accordingly. Simulation and numerical results verify that the proposed DSA-OFDM can outperform conventional dimming control schemes without frequency domain design in terms of BER performance at various illumination levels. The remainder of this paper is organized as follows. In Section 2, we introduce a typical dimmable multiple-imput multiple-output (MIMO) OFDM based VLC system model. Section 3 presents the detail of the proposed DSA-OFDM. In Section 4, simulation and numerical results are presented and analyzed. Finally, we draw some important conclusions in Section 5.