# Mixed-Carrier Communication for Technology Division Multiplexing
## 1 Introduction  
Enhanced Mobile BroadBand (eMBB), massive-Machine Type Communications (mMTC), and Ultra-Reliable and Low-Latency Communications (URLLC) have gained the most significant interest in the fifth-generation (5G) networks era. The requirements of these generic categories of service include high data rates, a huge number of connected devices and critical reliability, and low latency. For instance, eMBB aims to offer peak data rates on the order of gigabits per second, while mMTC tends to provide connectivity for a large number of sensors with limited power capabilities [1; 2; 3]. However, 5G does not satisfy the latency and reliability requirements for URLLC time-sensitive applications such as remote surgery. Although tactile Internet is becoming an interesting cellular application, it exhausts the network resources by a huge amount of sensors and control data [4]. Thus, recent research envisions sixth-generation (6G) wireless communication networks to introduce new performance metrics and use cases [5]. 6G has stricter requirements, compared to 5G, such as Terabits per second (Tbps) data rates, 10 times lower latency, and sub-centimeter geo-location accuracy [6]. Though mmWave is expected to provide data rates of Gbps in 5G, Tbps is needed for 6G applications. Hence, different frequency bands are to be explored to support higher data rates and more coverage density including terahertz (THz), and optical frequency bands [5]. To support the strict 6G requirements and the newly introduced frequency bands, a new air interface and transmission technologies are required. Novel network architecture is essential including new physical layer (PHY) design, multiple access techniques, and heterogeneity of different spectrum bands [7; 8; 9]. Additionally, PHY security and enhanced spectral efficiency become essential for connecting billions of sensors and devices within the Internet of Things (IoT)6G network [10]. In the following Table 1, a comparison between 5G and 6G network requirements is illustrated. Motivated by the requirement of new spectrum bands for futuristic networks, visible light communication (VLC) has gained a significantly increasing interest from academia and industry. For instance, low-complexity single sub-carrier VLC modulation schemes have been proposed including on-off-keying (OOK) and pulse position modulation (PPM) [11]. Additionally, orthogonal frequency division multiplexing (OFDM) has been widely used for VLC systems to avoid inter-symbol interference (ISI) [12; 13]. To satisfy the positive and real constraints on an OFDM VLC signal, unipolar real modulation schemes are introduced. From the popular techniques, DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are the most common. However, DCO-OFDM is limited in terms of power efficiency while ACO-OFDM has less spectral efficiency [14; 15]. Therefore, several techniques aimed at achieving better spectral efficiency such as hybrid ACO-OFDM (HACO-OFDM), layered ACO-OFDM (LACO-OFDM), enhanced ACO-OFDM (eACO-OFDM), enhanced unipolar OFDM (eU-OFDM), and spectral and energy-efficient OFDM (SEE-OFDM) [16; 17; 18]. In Table 2, the achievable spectral efficiencies for the most common VLC OFDM modulation techniques are highlighted. \(M\) represents the quadrature amplitude modulation (QAM) order, \(N\) represents the fast Fourier transform (FFT) length, and \(N_{\text{cyc}}\) represents the cyclic prefix length. However, achieving a higher spectral efficiency has a penalty on both power efficiency and computational complexity. For instance, DCO-OFDM offers double the spectral efficiency compared to ACO-OFDM; however, ACO-OFDM is more power-efficient than DCO-OFDM, where DCO-OFDM requires a higher DC-bias to modulate a positive-valued waveform. Another example is the HACO-OFDM which achieves the same spectral efficiency as DCO-OFDM.