## 1 Introduction  
Visible light communication (VLC) utilizing commercially available light emitting diodes (LEDs) has become more appealing to the sixth generation (6G) and Internet of Things (IoT) systems, due to its enormous merits of abundant and unregulated spectrum resources, no electromagnetic radiation and high security [1, 2]. However, the achievable capacity of practical VLC systems is greatly restrained by the small modulation bandwidth and the severe nonlinearity of off-the-shelf LEDs [3].  
Many techniques have been proposed to expand the available bandwidth for VLC systems. For one thing, analog or digital equalization techniques can be applied to break the limited bandwidth of LEDs [4, 5]. For another thing, various spectrally efficient modulation and multiple access techniques, including carrierless amplitude and phase (CAP) modulation, orthogonal frequency division multiplexing (OFDM) with high-order modulation formats, multiple-input multiple-output (MIMO) and nonorthogonal multiple access (NOMA), can also be considered to enhance the achievable data rate of VLC systems [6, 7, 8, 9]. Moreover, CAP has the benefits of a lower implementation cost and lower peak-to-average power ratio (PAPR) in comparison to OFDM. To increase the link performance, multiband CAP (\(m\)CAP) has been further proposed, which splits the available signal bandwidth into \(m\) subbands so as to improve the tolerance against the low-pass effect of LEDs [10].  
Recently, a novel \(m\)CAP with index modulation (\(m\)CAP-IM) technique has been proposed to obtain better bit error rate (BER) performance than classical \(m\)CAP. In \(m\)CAP-IM,the subbands of classical \(m\)CAP are divided into two parts: the active and inactive subbands. The active subbands can be modulated by constellation data symbols while the inactive subbands are nulled [11]. To further enhance the spectral efficiency (SE) of \(m\)CAP-IM, \(m\)CAP with dual-mode index modulation (\(m\)CAP-DM) has been proposed in [12], where all subbands are modulated to transmit data symbols. Moreover, the detection of signals plays an important role in optical wireless communication systems. In [13], a novel high-dimensional (HD) noncoherent detection scheme was proposed for multi-intensity-modulated ultraviolet communication (UVC) systems to address the intersymbol interference (ISI) issue. In [14], a sparse signal detection scheme exploiting the sparse reconstruction algorithms in compressed sensing (CS) was proposed for indoor VLC systems using generalized space shift keying (GSSK). For VLC systems employing \(m\)CAP-DM, the low-complexity near-optimal log-likelihood rate (LLR) detector is generally adopted [11; 12]. For practical \(m\)CAP-DM VLC systems, the signal-to-noise ratio (SNR) of high-frequency subbands imposes inevitable degradation compared with that of low-frequency subbands. Hence, all subbands are in the condition of unbalanced SNR, which results in the degradation of the overall BER performance. In order to resolve the issue of unbalanced SNR, pairwise coding (PWC) can be introduced to improve the overall system performance, which requires no overhead and exhibits low computation complexity [15]. Although VLC has inherent security against eavesdroppers outside its coverage, the confidential information might still be eavesdropped by unintended or unauthorized users when they are located within the system coverage [16].  
In this paper, we propose and investigate a novel PWC-based \(m\)CAP-DM scheme with two-dimensional (2D) chaotic encryption for practical VLC systems. The superiority of the proposed \(m\)CAP-DM scheme was successfully verified by both numerical simulations and hardware experiments.