## 3 Cooperative Schemes
### Cooperative Schemes for VLC
#### 3.2.2 Joint Transmission--Coordinated Multipoint (Jt-Comp)  
CoMP transmission schemes were firstly proposed to multi-cell RF communication systems subject to strong inter-cell interference. The main difference with respect to FR techniques is that there are no divisions in the frequency bands for allocating them to APs, and then, the area is not divided in advance for serving users over specific frequency bands. The fundamentals of JT-CoMP is the coordination of multiple APs in order to serve users in the same time and frequency resources in a cooperative manner. This way, neighboring APs are no longer interference sources, since using JT-CoMP converts the inter-cell interference coming from different optical APs into useful signals [112; 113]. These methods can be grouped into the single footprint scenario. Transmission techniques based on CoMP are originally proposed in long term evolution(LTE) [114] and LTE advanced [115] for improving the cell-edge throughput in multi-cell scenarios. Early studies employed directional antennas in order to improve the performance of JT-CoMP in RF [116] and VLC [117] systems, where, in the latter case, the APs are equipped with non-conventional directional LEDs pointing to the cell-edge areas. Similarly to the approach considered in FFR, CoMP techniques are also proposed to cooperatively serve users only located at cell borders [117]. To provide such a network in VLC, sophisticated APs must be used, where some LEDs are pointing downwards to the center and some other are facing to the cell borders. This may be difficult to deploy, especially guaranteeing homogeneous illumination. In this context, JT-CoMP techniques have gained importance in VLC because, in addition to managing the inter-cell interference, they also mitigate the LoS-link blockage problem that the technology experiences [118; 119]. Now, the LoS-link blockage with one AP is not as dramatic as when users are served by one only AP. An illustration of JT-CoMP is given in Figure 14, where multiple APs are serving the same users. Note that the user equipment (UE) 2 is served only by AP 2 as its location allows it to get a great coverage by receiving information from its nearest AP. However, UE 1 and UE 3 are located at cell edge, where cooperation among neighbouring APs is considered in order to improve their performance. In these cases, cooperation among 2 or 3 APs is required, respectively. Figure 14: Toy example of JT-CoMP. In this case, CoMP techniques have also been demonstrated to be helpful for enabling smoother handover operation among neighbouring VLC APs [120]. In [121], the authors present an analytical model of a multi-user VLC scenario where JT-CoMP techniques are employed. It is demonstrated that JT-CoMP also improves the coverage performance of the network. Moreover, in this work, two ways of invoking JT-CoMP techniques are proposed; non-coherent and coherent. In the former, multiple APs transmit the same information to the users without any correction or synchronization, and therefore, the signal power levels received from multiple optical APs are added at the receiver [113; 122]. On the contrary, in the coherent technique, the cooperating APs share the CSI with users. Satisfying this condition allows us to implement precoding techniques, which are described in detail in the following section together with synchronization signaling in order to provide multiplexing gain, i.e., transmitting several symbols over the same transmission resource; either time of frequency, canceling or minimizing the interference among them [113; 122]. It is demonstrated that JT-CoMP coherent technique provides higher performance gain in comparison with JT-CoMP non-coherent techniques, at the expense of a higher complexity and feedback. When the VLC system is fully optical, the LoS-link blockage may affect both uplink and downlink channels, without having the option of communicating feedback information from a blocked user for the whole duration of the blockage event.