## 3 Description of the Proposed VLC Prototype
### Hardware Design and Implementation
#### 3.2.1 Modulation and Coding Techniques  
Outdoor low data rate applications should use OOK modulation and Manchester coding, achieving data rates between 11 and 100 kb/s, according to the IEEE 802.15.7 standard for short wireless optical communications using visible light [7]. The performances of OOK and of Manchester coding are well established in the literature. Manchester's code advantages are given by its DC balanced characteristics, simple clock and data recovery, and by the decent BER performances. Furthermore, in VLC applications, the Manchester code is suitable due to its ability to prevent light flickering. In the Manchester code, a binary 1 is represented as a high to low mid-bit transition, whereas a binary 0 is represented as a low to high mid-bit transition. Thus, while transmitting a Manchester encoded message, the light of each data bit is constant, which prevents any intra-frame flickering. On the down-side, the Manchester code has the disadvantage of larger bandwidth requirements as compared to other common codes. Accordingly, in addition to the classical Manchester code, the performances of the Miller code [70,71] are also investigated. The Miller code has rather similar advantages as the Manchester code, but it is also bandwidth-efficient. Consequently, the use of the delay modulation could be a solution that is suitable for MIMO applications. This characteristic is given by the fact that the energy of the Miller encoded data is concentrated at a lower frequency and within a narrower bandwidth. Therefore, bandpass filters become more efficient in separating adjacent channels.  
DSSS modulation with SIK coding is the second modulation technique under consideration. DSSS is a robust modulation technique that uses spread spectrum and sequence inverse keying [72,72]. This modulation increases the communication robustness to noise and to potential multipath dispersion at the cost of a much larger bandwidth requirement. Basically, the information is expanded on a much wider spectrum, increasing the robustness to light interferences. In this case, the data are transmitted using sequence inverse keying, which means that a binary 1 is represented as a pseudo-noise sequence of zeros and ones, whereas binary 0 is represented by its complement. The length of the pseudo-noise sequence is established as a compromise between robustness to noise and bandwidth. In this specific case, a 7-bit noise sequence has been selected. Thus, a binary 1 is represented as 1010101, whereas a binary 0 is represented as 0101010. In such a case, the information can still be extracted when up to three bits of the pseudo noise sequence are affected by noise.  
In conclusion, the prototype is designed to work with two types of modulations and three types of coding techniques. The OOK modulation along with Manchester coding represents the standard solution for such applications, whereas the Miller code is considered for its improved bandwidth efficiency and its potential use in MIMO applications. Additionally, for the case of low SNR conditions, DSSS modulation along with SIK coding is considered. In this last case, the data rate is sacrificed in order to maintain the link active.