# Mixed-Carrier Communication for Technology Division Multiplexing
## 1 Introduction
However, it combines both ACO-OFDM and pulse amplitude modulated discrete multitone (PAM-DMT) signals, thus a more complex transmitter and receiver design is required to generate and process both signals separately. For SEE-OFDM, the spectral efficiency is decided based on the waveform design. While SEE-OFDM can achieve a comparable spectral efficiency to DCO-OFDM, the receiver design requires pre-conditioning before FFT where reconstruction or iterative subtracting is used to eliminate intermodulation interference due to signal clipping. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Network Requirements** & **5G** & **6G** \\ \hline Spectrum & mmWave & THz \\ \hline Bandwidth & Up to 1 GHz & Up to 3 THz \\ \hline Downlink Data rate & 20 Gbps & 1 Tbps \\ \hline Latency & 10 ms & 1 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 5G VS 6G network requirements expectations based on state-of-the-art research. \begin{table}
\begin{tabular}{c c} \hline \hline
**Modulation** & **Spectral Efficiency** \\ \hline DCO-OFDM & \(\eta_{\text{DCO}}=(\frac{N}{N+N_{\text{cyc}}})\,log_{2}M\) \\ \hline ACO-OFDM & \(\eta_{\text{ACO}}=(\frac{N}{N+N_{\text{cyc}}})\,log_{2}M\) \\ \hline HACO-OFDM & \(\eta_{\text{HACO}}=(\frac{N}{N+N_{\text{cyc}}})\,log_{2}M\) \\ \hline LACO-OFDM & \(\eta_{\text{LACO}}\leq 2\eta_{\text{ACO}}\) \\ \hline SEE-OFDM & \(\frac{3}{4}\eta_{\text{DCO}}\leq\eta_{\text{SEE}}\leq\eta_{\text{DCO}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Spectral efficiency of common VLC OFDM modulation techniques. Despite the decent amount of research on several VLC-OFDM modulation schemes, to the best of our knowledge, mixed-carrier communication (MCC) is the first unified transmission scheme that embeds both high- and low-speed modulation schemes within the same waveform design. The novelty about MCC is that it can offer connectivity for low- and high-end devices, simultaneously. Thus, MCC enables transmission by design to high-speed receivers based on multi-carrier OFDM, as well as limited power IoT sensors without trading off spectral efficiency. This means MCC offers simultaneous broadband access, low-rate IoT connectivity, device-free sensing, and device-based localization [7; 19; 20]. Similar to reverse polarity optical OFDM (RPO-OFDM) [21], MCC is built upon a pulse width modulation (PWM)-like envelope to support dimming control as well. In MCC, OFDM samples are conditioned to shape the PWM-like envelope based on the requested dimming level. Within the PWM-like envelope, beacons are embedded and modulated using single carrier modulation techniques such as PPM and phase-shift keying (PSK) to transmit the low-stream bits and localization information. A similar design to RPO-OFDM is proposed in [22], where OFDM is reshaped in an OOK envelope to enhance the spectral efficiency of the system; however, the system does not offer dimming control compatibility or multiple services as MCC offers. The authors first introduced MCC in [7; 19; 20], where the concept, simulation, and experimental validation, and an interference management study among different embedded services are investigated. However, the previous research did not present the varying parameters impacting the MCC design and performance. In this manuscript, a more focused analytical framework of the MCC transmission rather than end-to-end system simulation and experimental proof-of-concept is proposed. The main contributions of this work are as follows:  
* The design process of the MCC transmitter and both the low-speed and high-speed receivers;
* The computational complexity analysis to evaluate the effectiveness of MCC for resource-limited devices;
* The derivation of the bit-error-rate (BER) formulas per embedded modulation within MCC to investigate design trade-offs and performance optimization. The manuscript is organized as follows: in Section 2, a detailed transmitter and receiver design is proposed, followed by an arithmetic complexity analysis in Section 3, which is essential to define MCC advantage for low-end receivers.