# Performance Analysis of Cooperative Non-Orthogonal Multiple Access in Visible Light Communication
## 1 Introduction  
Visible light communication (VLC) is an attractive complementary wireless communication technique to conventional radio frequency (RF), especially in indoor environments due to its high data rate capability. VLC exploits light-emitting diodes (LEDs) lighting infrastructure which provides illumination and data transmission simultaneously [1]. VLC is an economical and secure system that can provide a high quality of service (QoS) with no electromagnetic interference, particularly in indoor environments [2, 3]. One of the main advantages of LEDs is their ability to transmit data by varying the light intensity at very high frequency without being perceived by the human eye. This process is called intensity modulation (IM). At the receiver end, in a process known as direct detection (DD), a photodetector (PD) is used to generate an electrical current proportional to the variation in the received optical power [4]. The most commonly used IM technique is the on-off keying (OOK) modulation scheme due its implementation simplicity [5]. In spite of the above remarkable advantages, the major limitation to developing a high data rate link in VLC is the narrow modulation bandwidth of LEDs (several megahertz), which is a hardware issue [2]. Nevertheless, to overcome this constraint, multiple access techniques are used [6]. In [7], the authors discuss several multiple-access techniques to improve the downlink VLC data rate, including orthogonal multiple-access (OMA) techniques such as time division multiple access (TDMA), code division multiple access (CDMA) and orthogonal frequency division multiple access (OFDMA). On the other hand, power domain non-orthogonal multiple access (PD-NOMA) has an advantage over other conventional OMA techniques because different users can share the same time and frequency resources in the power domain, and thereby increase the spectral efficiency [8; 9]. At the transmitter end, superposition coding is used to superimpose the signals of different users in the power domain by allocating a certain power level to each user based on its corresponding channel condition. PD-NOMA allocates low power levels to users with good channel conditions, and high power levels to users with bad channel conditions. Therefore, at the receiver end, the user with bad channel conditions (highest power allocation) can directly decode its signal and treats other signals as noise. Meanwhile, other users have to apply successive interference cancellation (SIC) for multiuser signals separation before decoding their own signals [8]. Several researchers report on the PD-NOMA-VLC performance in comparison to other OMA techniques. The authors in [10; 11] investigate the ergodic sum rate of PD-NOMA-VLC. Their results show that PD-NOMA outperforms TDMA in terms of system capacity. The authors in [12] illustrate the superior performance of PD-NOMA-VLC over OFDMA scheme in terms of achievable data rate. Thus, one can conclude that PD-NOMA is an attractive multiple-access technique in VLC systems. The authors in [13; 14] propose cooperative relaying as a promising candidate that improves the link reliability, and that extends the communication systems range. The authors in [15] apply PD-NOMA to multiple-antenna relaying networks to study the outage behavior of mobile users. Their results show that PD-NOMA improves spectral efficiency and fairness of the system compared to OMA schemes. Consequently, the authors in [16] propose a cooperative PD-NOMA for 5G systems where the user with good channel condition acts as a relay that retransmits data of the users with bad channel conditions by exploiting the prior information acquired from the SIC process. In addition to the downlink transmission phase in PD-NOMA, the user with good channel condition acts as decode and forward (DF) relay for other users with bad channel conditions to enhance the signal reception reliability. Users cooperate and transmit data by using short-range communication technologies, like ultra-wideband and Bluetooth.