## 3 Performance Evaluation of Combinations of Modulation and Spreading Code Techniques
### Combinations of Modulation and Spreading Code Techniques
All these block partitionings are used to evaluate the performance (e.g., the maximum distance, accuracy, and precision). In these block partitionings, **1**:4:4, **1**:5:3, **1**:6:2, and **1**:7:1 have equal numbers of '1's and different numbers of '0's, and then they have very similar performance. Therefore, block partitioning **1**:4:4 and **1**:5:3 were selected as two representatives of four three-small-piece block partitionings. The prototype LIDAR system is a non-directional non-line of sight (NLOS) optical wireless communication system that uses Lambertian diffusion. Eye safety is a critical issue in optical wireless systems because optical signals can penetrate the human cornea and potentially cause thermal damage to the retina. Optical transmitters must comply with the class 1 of the International Electrotechnical Commission (IEC) standard. The MPE is the highest power or energy density of a light source considered safe (i.e., less likely to cause damage). The combination of modulation and spreading code techniques was determined to satisfy all operating conditions of the prototype LIDAR. The following operating characteristics were determined according to the combinations:  
* Symbol stream;
* Block size and partitioning;
* Pulse peak power;
* Number of time slots;
* Number of pulses; \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Block Size (\(M\))** & \(M\)**-OOK** & \(M\)**-PPM** & \(M\)**-DPPM** & **2-\(n\)MPPM** & \(M\)**-DPIM** & \(M\)**-DH-PIM\({}_{2}\)** \\ \hline
1-bit & 1, 1 & 1, 2 & 1, 2 & 2, 3 & 1, 2 & 2, 3 \\
2-bit & 2, 2 & 1, 4 & 1, 4 & 2, 4 & 1, 4 & 2, 4 \\
3-bit & 3, 3 & 1, 8 & 1, 8 & 2, 5 & 1, 8 & 2, 6 \\
4-bit & 4, 4 & 1, 16 & 1, 16 & 2, 6 & 1, 16 & 2, 10 \\
5-bit & 5, 5 & 1, 32 & 1, 32 & 2, 9 & 1, 32 & 2, 18 \\
6-bit & 6, 6 & 1, 64 & 1, 64 & 2, 12 & 1, 64 & 2, 34 \\
7-bit & 7, 7 & 1, 128 & 1, 128 & 2, 17 & 1, 128 & 2, 66 \\
8-bit & 8, 8 & 1, 256 & 1, 256 & 2, 24 & 1, 256 & 2, 129 \\
9-bit & 9, 9 & 1, 512 & 1, 512 & 2, 33 & 1, 512 & 2, 258 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Possible modulation schemes according to the size of the bit input block. Each cell expresses the modulated results as a 2-tuple \((N_{p},L_{max})\), where \(N_{p}\) is the maximum number of pulses and \(L_{max}\) is the maximum number of time slots. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source Symbol** & **OOK** & **8-PPM** & **8-DPPM** & **2-SMPPM** & **8-DPIM** & **8-DH-PIM\({}_{2}\)** \\ \hline
0 & 000 & 1000000 & 1 & 10001 & 1 & 100 \\
1 & 001 & 01000000 & 01 & 01100 & 10 & 1000 \\
2 & 010 & 00100000 & 001 & 01001 & 100 & 10000 \\
3 & 011 & 00010000 & 0001 & 10010 & 10000 & 100000 \\
4 & 100 & 00001000 & 000001 & 11000 & 100000 & 110000 \\
5 & 101 & 00000100 & 000001 & 00101 & 100000 & 1100 \\
6 & 110 & 00000010 & 0000001 & 00011 & 1000000 & 1100 \\
7 & 111 & 00000001 & 0000001 & 10100 & 10000000 & 110 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Three-bit block representation according to modulation technique. * Leading '**1'** or trailing '**1'**. The operating environment of the prototype system was simplified for the validation and performance evaluation. The length of the optical spreading code was proportional to the square of the cardinality so that the number of pulses used at one measurement point also increased in proportion to the square. To measure dozens or hundreds of measurement points at the same time, the cardinality of the spreading code must be very large. Therefore, to reduce the number of pulses used at one measurement point, the cardinality was lowered, and the number of simultaneous measurement points was reduced to shorten the spreading code length. If the bit input block were divided into partitions of various sizes and the optical spreading code with a cardinality of five were applied, the transmission characteristics would be as shown in Tables 12 and 13. The number of time slots needed for transmission is the greatest, and each pair relates the number of slots, the number of pulses, and the maximum pulse output.