# Orthogonal Subblock Division Multiple Access for OFDM-IM-Based Multi-User VLC Systems
## 2 Osdma with DFT-Spreading and Interleaving for Bandlimited MU-VLC
### OSDMA with DFT Spreading and Interleaving  
To efficiently apply OFDM-IM techniques in practical bandlimited MU-VLC systems, we proposed an OSDMA scheme with DFT spreading and interleaving. In conventional OFDMA-based MU-VLC, multiple access is achieved via subcarrier allocation among multiple users. However, in the proposed OSDMA-based MU-VLC, multiple access is enabled through subblock allocation. Figure 1a,b show the block diagrams of the transmitter and receiver of the \(K\)-user OSDMA-MU-VLC system with DFT spreading and interleaving,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Index Bit** & **Index** & **Subblocks** \\ \hline
0 & 1 & \([S_{m},0]\) \\ \hline
1 & 2 & \([0,S_{m}]\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Mapping table of OFDM-SM-IM with \(N=2\) and \(k=1\).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Index Bit** & \(\mathcal{M}_{A}\)**Index** & \(\mathcal{M}_{B}\)**Index** & **Subblocks** \\ \hline
0 & 1 & 2 & \([S_{i}^{A},S_{j}^{B}]\) \\ \hline
1 & 2 & 1 & \([S_{j}^{B},S_{i}^{A}]\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mapping table of OFDM-DM-IM with \(N=2\) and \(k=1\).  
respectively. Assuming the OSDMA-MU-VLC system consists of totally \(G\) subblocks for valid data transmission and there subblocks are equally allocated to all the \(K\) users, the number of subblocks for each user is \(G/K\). Hence, letting \(B\) denote the signal bandwidth of the system, the achievable bandwidth of the \(k\)-th user is given by \(B_{k}=B/K\) (\(k=1,\ldots,K\)).  
At the transmitter, as shown in Figure 1a, the input bits of the \(k\)-th user are first partitioned into \(G/K\) groups through a bit splitter. In order to generate a subblock with length \(N\), the input bits of each subblock are further divided into index bits and constellation bits, which are fed into the index selector and the SM/DM constellation mapper, respectively. Subsequently, the \(G/K\) subblocks of the \(k\)-th user are combined by a subblock combiner and DFT spreading is then executed to reduce the PAPR of the signal. After that, all the subblocks for \(K\) users are further concatenated by a OFDM block creator to create a complete OFDM block. Furthermore, interleaving is conducted to mitigate the adverse effect of the low-pass frequency response of the LED. The detailed principle of interleaved subcarrier multiplexing will be discussed in the next subsection. Finally, the transmitted signal is generated after performing inverse fast Fourier transform (IFFT) with Hermitian symmetry (HS) and the parallel-to-serial (P/S) conversion.  
At the receiver, as can be seen from Figure 1b, the received signal of the \(k\)-th user is first converted to a parallel signal via serial-to-parallel (S/P). Then, FFT and frequency-domain equalization (FDE) are executed, and de-interleaving is also carried out. Subsequently,  
Figure 1: Block diagrams of the \(K\)-user OSDMA-MU-VLC system with DFT-spreading and interleaving: (**a**) transmitter and (**b**) receiver.  
the OFDM block is divided via a OFDM block splitter and inverse DFT (IDFT) is performed accordingly. The \(G/K\) subblocks of the \(k\)-th user can be obtained through a subblock splitter. In each subblock, a low-complexity LLR detector is adopted for signal detection. The index bits and the constellation bits can be recovered through the index decoder and the SM/DM constellation demapper, respectively. The final output bits can be generated by combining the index bits and the constellation bits together via a bit combiner.