# A Review of Hybrid VLC/RF Networks: Features, Applications, and Future Directions
## 2 Hybrid VLC/RF Networks in Indoor Scenarios under Ideal Conditions
### Hybrid Channel Model  
In an indoor scenario, the impulse response of a VLC channel is modeled through a Geometric Based Model, which can be approximated for simplicity to a Lambertian model, which depends on half the beamwidth angle of the transmitting LED [38]. The total channel impulse response is provided by the sum of the line-of-sight (LOS) impulse response and the non-line-of-sight (NLOS) impulse response [39] using recursive methods. When the LEDs are very close to the receiver, a LOS link is achieved, promoting a channel model without temporal dispersion. When there are multiple signal reflections due to walls or obstacles within the room or the distance between the emitter and receiver is very large, NLOS links are created. To calculate the impulse response of these links, it is necessary to take into account the signal tails caused by time dispersion and multiple paths. This is because the information transmitted by the LED may reflect off stage walls or other blocking object before reaching the receiver [40]. Because LEDs produce light with non-coherent characteristics and have a limited dynamic range, the most conducive modulation scheme for VLC is intensity modulation (IM)/direct detection (DD) [41]. When an optical OFDM signal is used in an IM/DD system, intensity modulation of the carrier signal is performed. Therefore, the transmitted signal must be real and of positive value in the time domain. Consequently, it is necessary to use real transformations such as the discrete cosine transform (DCT) or the fast Walsh Hadamard transform (FWHT), among others, to obtain a real signal. If the optical OFDM signal cannot be matched to the linear region of the LED voltage-current relationship, clipping of the signal occurs, causing nonlinear distortions. To overcome the distortions induced by this nonlinearity, peak-to-average power ratio (PAPR) reduction techniques or predistortion techniques must be used [42]. An electromagnetic wave incident on an object can be reflected, refracted, scattered, diffracted, or absorbed by it. This depends on the frequency of the incident wave and the physicochemical properties of the object. This interaction determines the level of opacity or transparency of the object at different wavelengths. When white light falls on a surface, certain wavelengths are absorbed while others are reflected, determining the object's color. Opaque objects such as wood, metal, and plastic restrict the propagation of visible light [43]; therefore, when designing a VLC communication channel the behavior of visible light wavelengths interacting with such materials strongly influences the decision-making process. Conversely, an object can be transparent at certain wavelengths, allowing light to pass through it easily. On the other hand, the RF communication channel model consists of the path loss model and the spatial channel model, usually composed of the lumped channel impulse responses and angular dispersion statistics [44]. In an indoor scenario, the received signal arrives through multiple simultaneous paths that are vectorially combined to obtain a resulting oscillatory signal. This multi-path can be caused by phenomena such as reflection, refraction, diffraction, or scattering. The most common way to include these phenomena is by means of the channel impulse response model, which is presented in a temporal way, as it can have discrete values or be a stochastic variable [45]. Due to the symmetry properties of the RF channel, the RF channel model can be Hermitian. The ratio between the imaginary part and the real part of the channel impulse response defines the statistical distribution in which the signal fades follow at each coefficient, that is, each delay can be considered a different variable, typically Rayleigh, Rice, or simply Gaussian depending on whether or not there is a predominant component [46]. The possibility of Doppler effects due to mobility between transmitter and receiver must be taken into account.