# Evolution of Hybrid LiFi-WiFi Networks: A Survey
## 2 Framework for HWLNet
### Modulation Schemes  
LiFi standards for modulation schemes are defined by IEEE 802.15.7 [11] with VPPM/OOK/CSK modulation techniques presented in Figure 4. The Variable Pulse Position Modulation (VPPM) is used to achieve variable range and constant data rate by adjusting the pulse width. This modulation's largest benefit is protection from intra frame flicker. This is achieved by constant pulse amplitude and variable pulse width to control dimming. Logic '0' and '1' are mapped using positive pulse at the beginning followed by negative pulse and negative pulse at the beginning followed by positive pulse, respectively. This type of modulation is efficient only if the time span that contains pulses is long enough to distinguish pulse positions. Figure 3: Different types of LiFi AP deployments. In the On Off Keying modulation (OOK), data are represented by turning the LED diode on and off. In simple terms, light states such as 'ON' constitutes logic '1' and 'OFF' constitutes logic '0'. OOK uses the Manchester code to represent digital information with edge transition concepts, where low to high represent logic '1' and vice versa for logic '0'. This technique achieves a variable data rate at constant range with the insertion of compensation time. Colour Shift Keying modulation (CSK) is used to represent bits of information in the form of different colour wavelengths. Red, green, and blue LEDs are used on the transmit side to produce different colours with different wavelengths to encode the information bits. The diagram maps different wavelengths denoted as 'blue' with respective colour counterparts. This type of modulation is most expensive to manufacture because it uses a  
Figure 4: Representation of VPPM and OOK and CSK modulations: (**a**) VPPM, (**b**) OOK, and (**c**) CSK. complex design of transmitters and receivers, which is the only drawback of this scheme. The advantages of CSK far outnumber the disadvantage and make this modulation very popular. The most notable advantages are:  
* colour co-ordinates represent information, which in turn is represented in the shape of binary codes for programming clarity;
* information is rendered by the light's colour; amplitude is maintained, thus the sum of the average light source power is constant;
* CSK assists in realizing variable high bit rate because it uses high order modulation support such as 4-CSK, 8-CSK, and 16-CSK. Another LiFi standard is ITU-T G.vlc with the DCO-OFDM modulation technique, as presented in Figure 5. The direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) technique is applied to LOS (line of sight) and non-LOS models to achieve high-speed data transmission and maximizes the channel capacity by applying power and bit loading. This DCO version of OFDM is used only in VLC because of the need for real-valued nonnegative symbols. This is achieved by applying Hermitian symmetry to the information frame before the inverse fast Fourier transformation (IFFT). The information is transmitted by modulating the amplitude of the LED signal at a sub-carrier frequency. The sub-carrier frequencies are selected to be orthogonal to each other, so they do not mutually interfere. This technique helps in reducing the interference between the sub-carriers and increasing the system's spectral efficiency. However, the LED-based transmitter has a non-linear response, which distorts the transmitted signal. This distortion can be reduced by introducing a direct current (DC) offset in the signal to linearize the LED's response and improve the system's performance. On the receiver side, a PD receives the modulated signal and then converts the optical signal into the electrical one. The received signal is then demodulated using fast Fourier transform (FFT) to recover the original signal. DCO-OFDM has several advantages over other modulation techniques for VLC systems. It is a simple and efficient technique that can achieve high data rates with low complexity.