## 5 Precoding as Cooperative and Hybrid Networks
### Clustering for VLC. Intra-Cluster and Inter-Cluster Interference  
For RF systems, clustering is a well-known technique for both increasing the achievable DoF through precoding schemes and improving the spectral efficiency by reusing the frequency, i.e., a specific bandwidth, among the clusters [170; 171]. Following a network-centric approach, the deployment of APs, e.g., base stations or attocells, is divided into clusters according to a predefined methodology, as is shown in Figure 19. Thus, in these multi footprint schemes, the intra-cluster interference is managed by implementing precoding schemes as described in Section 4. For instance, UE 1 and UE 2 in Figure 19 can be served simultaneously, i.e., avoiding orthogonal resource allocation such as TDMA, while minimizing or even cancelling the intra-cluster interference through any signal processing technique such as the cooperative or precoding schemes described in this work. However, UE 3 on the cluster edge may be subject to inter-cluster interference. In Section 3, the use of cooperative schemes such as FFR is proposed for managing the interference at the cell/cluster edge.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Technique** & **CSI Requirements** & **Interference Mitigation** & **LoS-Link BLOCKAGE Avoidance** & **Complexity** & **Source** \\ \hline OMIMO & Receiver & No & No & Low & [32; 154; 155] \\ \hline DPC & Transmitters and Receivers & Yes & Medium & Very High & [160] \\ \hline Linear ZF & Transmitters & Yes & Medium & High & [161; 162; 167] \\ \hline BD & Transmitters and Receiver & Yes & High (ADRs) & High & [167] \\ \hline max-min SINR & Transmitter & Yes & Medium & Medium & [76; 166] \\ \hline MMSE & Transmitter & Yes & Medium & High & [163] \\ \hline CoMP-MMSE & Transmitters & Yes & High & Medium & [118] \\ \hline BIA & Receiver & Yes & Medium (ADRs) & Medium & [29] \\ \hline \hline \end{tabular}
\end{table}
Table 4: VLC precoding techniques.  
From the signal processing perspective, which can be applied to both RF and VLC systems, the fundamental limits of cooperation are analyzed in [88]. Theoretically, the spectral efficiency grows linearly as the transmitted power and the size of the network increase. In [88], it is demonstrated that this is not true in practice because of the need for pilot transmission associated to CSI and the constraints given by the capacity of the backhaul links, which are described in Section 4.1.2. As a consequence, cooperation is only possible within clusters of limited size, and therefore, it is required to manage both intra-cluster and inter-cluster interference. This way, the use of index coding by exploiting the network topology instead of CSI is proposed in [172].  
Moving to VLC, each optical AP provides a small and confined area of coverage usually referred to as attocell. Therefore, VLC systems lay into the framework of the heterogeneous networks when they are considered as an element of the cellular network. In this sense, a comparison of the area spectral efficiency between femtocells and attocells composed of a single optical APs is carried out in [173]. It is shown that the area spectral efficiency achieved by VLC outperforms the performance of the femtocell system. Similarly, the energy efficiency, i.e., the costs in Joules for transmitting 1 bit, in VLC networks, is analyzed in [174].  
As described in Section 3, several optical APs can be grouped to form an optical cell from a network centric perspective. Thus, the following question may be arisen; Are the well-known clustering strategies derived for RF suitable for VLC? The general answer is not due to the small and confined area of coverage of each optical APs. In the following, the concepts of user-centric clustering applied to VLC networks and hybrid RF/VLC networks are presented, taking into consideration the blocking effects that may appear in VLC networks.