# Evolution of Hybrid LiFi-WiFi Networks: A Survey
## 3 Performance of HWLNets
### SINR Coverage  
The SINR for each user must be above a certain margin to provide a stable connection. RF devices use omnidirectional antennas and the signal is dependent on distance and obstacles. For VLC, user's orientation has to be considered; if the wave-front is parallel to an interface, signal power is at maximum, otherwise if the user's equipment is beyond the line of sight of the LED transmitters, no signal will be received. Because of light's physical property, user orientation can influence the coverage area of APs [26]. There are two basic ways of solving the coverage problem:  
* using a large photo diode (PD), which in turn expands a user's ability to receive more light but also more interference;
* using multiple narrow PDs with an angle diversity receiver (ADR) as shown in [27].  
Researchers have demonstrated that HWLNets can improve SINR over standalone networks with the use of single PD receivers and an angle of view below 45\({}^{\circ}\)[28].  
In [27], researchers derive the cumulative distribution function (CDF) of received SINR to determine performance. This study uses a simplified non-line-of-sight (NLOS) model because the conventional model is unmanageable. In Figure 8a, two categories of a single PD receiver, Scenario I and Scenario II, are presented. Both depend on the user's position and are statistically analysed. In the first scenario, all users are located around the cell centre with no LOS interference, whereas in the second scenario, LOS interference is present.  
Researchers [27] have shown that in the Scenario I, with limited field of view (FOV), a PD LOS neighbouring interference can be rejected so that NLOS remains, thus resulting in high SINR. At the cell edge, low SINR is present, as shown in Scenario II, due to LOS.  
Multiple PDs [27] use signal-combining schemes such as select best combining (SBC), equal gain combining (EGC), maximum ratio combining (MRC), and optimum combining (OPC). Tools and analyses of PD receivers can be extrapolated for use in angle diversity receivers (ADRs) with the assumption that each PD points to a different location (no overlap) and can establish one LOS link with AP.  
Combining schemes for ADR with 9 or 20 PDs, as presented in Figure 8b,c [27], are as follows:  
* SBC: user in centre selects upward pointing PD to get the best SINR;
* EGC: user's received signal is similar to SBC in the centre of the cell due to the fact that only one PD can receive the LOS signal. In this scheme, weights of PDs are the same and NLOS will increase with the increase in PD numbers;
* MRC: different PDs have different weights by their SINR. This model can establish two LOS links with APs;
* OPC: LOS signals are captured by two PDs that establish LOS to APs similarly to MRC.  
Authors in [27] have demonstrated that SINR performance with a nine-PD receiver is significantly improved compared to that of a single PD due to SBC choosing PD that is best located to obtain high channel gain. This scheme has a narrow FOV and no LOS interference. Compared to SBC and MRC, OPC has the best SINR performance because it uses an interference-plus-noise correlation matrix to generate optimized weights, thus suppressing NLOS interference and achieving 20 dB improvement. For the 20-PD receiver, SINR performance of EGC is similar to the nine-PD EGC. In SBC and MRC, the 20-PD receiver has 5 dB improvement, and in OPC, with even narrower FOV and higher concentration of PDs, 20-PD can receive stronger light signals, resulting in better SINR performance over the nine-PD receiver.  
Figure 8: CDF performance of received SINR for different PD configurations [27].