## 1 Introduction
Furthermore, using wider beams, which recalls that it is required for providing a uniform illumination (see Figure 2) and leads to a poor spectral efficiency as it can be seen in Figure 4, because most of the locations of the scenario are subject to overlapping from several optical transmitters, i.e., to inter-cell interference. Figure 3: Spectral efficiency without cooperation among transmitters. Beam angle of 20\({}^{\circ}\). Figure 2: Illuminance in an indoor scenario. Beam angle of 50\({}^{\circ}\). At this point, the two main requirements for VLC are presented: providing satisfactory illumination and data transmission simultaneously, which involves a trade-off between overlapping and interference. This way, as the primary functionality of lighting fixtures is to provide a standard compliant illumination, overlapping areas are unavoidable and interference management techniques must be invoked. In this context, cooperation among transmitters widely analyzed for RF cellular networks [30] have recently been proposed for VLC [31]. Besides, enabling cooperation among several optical APs inherently generates a multiple-input multiple-output (MIMO) network [32], allowing us to implement the well-known precoding schemes derived for RF systems [33, 34]. Both cooperation and preceding techniques can be combined in order to create clusters of optical APs from a user-centric perspective [35]. This paradigm involves VLC networks more resilient to LoS-link blocking effects. However, the step of implementing cooperation and precoding schemes originally derived for RF systems to VLC networks is not straightforward. In this sense, this work presents a comprehensive analysis and evaluation of the state of the art by considering the need for overcoming the LoS-link blockage. The organization of this work is described as follows:  
* In Section 2, the system model for VLC is analyzed, presenting a brief overview of the single and multi carrier modulation schemes. The network requirements for implementing cooperative and precoding schemes are presented considering the transmitter, receiver and network architectures, which involve parameters such as dynamic range, noise, field of view (FoV) or the need for backhaul links between transmitters, among others. * Section 3 motivates the need for employing cooperative schemes in VLC scenarios and presents the state of the art cooperative techniques for VLC published in the literature. Fundamentals of every cooperative technique are described and compared to its counterparts. * Section 4 is devoted to introducing the concept of precoding schemes from the signal processing perspective and for analyzing their application to VLC systems. First, the fundamentals of precoding schemes and its particularities for VLC are presented. After that, the differences between the capacity for RF and VLC systems are highlighted. Specifically, for VLC, the typical Shannon capacity equation [36, 37], i.e., \(\log_{2}(1+\text{SNR})\), where SNR is the signal to noise ratio, does not hold, since the optical channel is amplitude-constrained. In this sense, the lower and upper bounds of the capacity for VLC derived in [38, 39, 40] are presented. Moreover, the capacity bounds assuming  
Figure 4: Spectral efficiency without cooperation among transmitters. Beam angle of 50\({}^{\circ}\). MIMO precoding in VLC systems derived in [34] are analyzed. Finally, we present an evaluation of the state of the art for precoding applied to VLC networks. * In Section 5, a review of the intra-cluster and inter-cluster interference management in VLC networks is presented, considering the combination of cooperative and precoding schemes. After that, the concept of user-centric clustering is analyzed reviewing the recent works that apply this approach in VLC networks [35]. Furthermore, VLC systems are usually deployed in indoor scenarios, where an umbrella RF system, e.g., WiFi or femtocells, is already available.