## 6 VLC-IoT: Existing and Proposed Approaches
### Multiple Access Technique in VLC-IoT
#### 6.1.1 Existing Approaches  
The existing multiple access techniques that are usually implemented for VLC-IoT include the following and we provide a detailed account of the techniques below.  
1. Orthogonal frequency division multiple access (OFDMA) scheme  
OFDMA is an orthogonal multiple access (OMA) scheme used to facilitate the efficient deployment of bandwidth [42]. The OFDMA scheme achieves high data rates and mitigates mutual interference by allocating multiple devices to separate narrow-band orthogonal sub-channels to guarantee the QoS requirements of devices [16]. However, the implementation of OMA-aided techniques for massive connectivity with diverse QoS requirements is a challenge due to the scarcity of wireless resources. Although VLC mitigates the scarcity of wireless resources, nevertheless, OMA schemes cannot be applied directly for intensity modulation and direct detection (IM/DD) systems. This is because most of these techniques are developed for RF-based systems and have been adopted for VLC without considering the unique characteristics of VLC systems [4].  
Figure 2: Layered model.  
Code division multiple access (CDMA) scheme  
In the CDMA scheme, multiple VLC-IoT devices can transmit concurrently in a shared time slot [33; 41]. The optical CDMA (O-CDMA) was developed specifically to mitigate multipath interference challenges experienced in VLC systems [16]. The O-CDMA utilizes optical orthogonal codes (OOC) comprising of a sequence 0 s and 1 s. However, in massive IoT, it is a challenge to generate OOC for each device. As a result, the random optical code (ROC) was developed for massive connectivity VLC-IoT systems. Another approach has been considered for massive connectivity in VLC-IoT systems is to combine color shifting key (CSK) modulation with OCDMA.  
Space division multiple access (SDMA) scheme  
By exploiting spatial diversity, frequency-time resources can be shared by multiple devices in the SDMA scheme [17; 33]. Furthermore, since the SDMA scheme is unaffected by the number of devices it can be deployed for massive IoT [16]. However, in order to apply SDMA directly for VLC-IoT systems, directional light beams have to be generated via the limited field-of-view of the LEDs. Similarly, a higher aggregate uplink data rate can be achieved through SDMA by using a CMOS sensor at the VLC-IoT access [38; 40].  
Non orthogonal modulation access (NOMA)-aided VLC (in indoor down link VLC networks)  
NOMA is an emerging multiple access (MA) technique with the capacity to provide higher data rates than OMA due to its superior spectral efficiency [43]. NOMA exploits the power domain at the transmitter and continuous interference cancellation (SIC) at the receiver to enable multiple devices to transmit simultaneously on the same bandwidth or in the same time slot without mutual interference [42]. NOMA is a suitable MA technique for VLC-IoT systems because the technologies implemented in NOMA support an increasing number of connections, improve network capacity, and reduce network latency. Furthermore, the short distance between the transmitter and receiver in VLC systems enhances high-performance gain in NOMA-aided VLC-IoT. Again, the power allocation and decoding order in NOMA, which exclusively depends on the channel state information, can accurately be estimated in VLC systems due to its constant channel gain making VLC-IoT easily compatible with the NOMA technique [16; 33; 38].