## 3 Cooperative Schemes
### Cooperative Schemes for VLC
#### 3.2.3 Pre-Defined Jt-Comp  
In general terms, the use of CoMP in VLC has demonstrated a higher level of reliability. However, most of the CoMP techniques proposed in the literature so far for VLC are optimized ad hoc for the set of active users and, due to that, they have a computational complexity that increases with the number of users. Besides, these solutions are not static, making the communication unstable and unreliable when LoS-link blockages with relative short duration happen regularly, e.g., when obstacles pass by quickly. These techniques demand to sense the optical wireless channel permanently and to reallocate resources accordingly if a blockage event is detected; in addition, since the blockage also affects the feedback channel, the users may not have the chance to inform the serving AP for fully optical VLC systems about this situation. This way, pre-defined JT-CoMP systems are proposed in the literature, whose fundamentals are to keep the same transmission resources over time, and to pre-define the allocation of transmission resources in an efficient way depending on the characteristics of the scenario. Firstly, a simple pre-defined JT-CoMP scheme is proposed in [104] and applied to a corridor scenario. A time division multiple access (TDMA) is employed, where the time domain is divided into three time slots and each of them assigned to a different AP. Each user is served by three different APs, which shows a robust and reliable coverage against LoS-link blocking elements. Later on, the authors of [46] extend the previous work to an O-OFDM system with a hexagonal cell deployment, which comprises large indoor scenarios such as conference rooms or laboratories. Notice that TDMA is no longer used for resource allocation as it decreases the system performance notably. Therefore, frequency sub-bands and I-Q signals may be defined as resources to be allocated. General guidelines for creating pre-defined JT-CoMP schemes are published in [31], where multi-chip LEDs and O-OFDM are proposed in order to increase the degrees of freedom, i.e., the transmission resources. LED color and frequency sub-bands are the two variables defining the allocation resources. Note that I-Q signals are no longer considered independently in order not to decrease the system performance. The number of resources is defined as \(\mathcal{G}=|\mathcal{C}|\cdot|\mathcal{F}|\), where \(|\mathcal{C}|\) is the number of chip colors of an LED and \(|\mathcal{F}|\) is the number of frequency sub-bands per color. The cluster size is \(Q=\mathcal{G}/|\mathcal{S}|\), where \(|\mathcal{S}|\) is the number of sectors per cell. The number of optical APs that can cooperate to serve users simultaneously by means of JT-CoMP is also defined, and according to all these concepts, a JT-CoMP network is pre-defined. An example of pre-defined JT-CoMP network is presented in Figure 15, where a two-tier VLC multi-cell network is represented. RGB LEDs are used (\(|\mathcal{C}|=3\)), cells are virtually divided into 3 sectors (\(|\mathcal{S}|=3\)) and the electrical bandwidth of every color chip is divided into 3 sub-bands (\(|\mathcal{F}|=3\)) that are represented with different brightness levels. Thus, depending on the location, the user must decode the coming signal at color and sub-band that correspond to such sector. It is worthy to mention that the authors of [31] provide the guidelines for the creation of pre-defined JT-CoMP guaranteeing a homogeneous signal-to-interference-plus-noise ratio (SINR), i.e., the spatial SINR distribution at every sector of the network is the same, with and without cooperation among neighbouring APs. It is demonstrated that JT-CoMP increases the mean cell data rate with respect to single-AP transmission in both cases, with and without LoS-link blockages. One of the presented pre-defined JT-CoMP schemes is thoroughly studied in [111] for a scenario where the orientation of the receiver, i.e., the PD, and the density of obstacles vary.