## 3 Performance Evaluation of Combinations of Modulation and Spreading Code Techniques
### Performance Evaluation of Combined Techniques  
The experimental environment was the same as that for the prototype LIDAR system, and a modulation technique and a spreading code technique were used. Experiments were conducted using various parameters in Section 3.1 with a 2 m \(\times\) 2 m white paper wall, as shown in Figure 3. We evaluated the performance of the following elements based on combinations of various modulation and spreading code techniques, as well as the operating characteristics of the prototype LIDAR system. As listed below, several operating conditions were specified according to the characteristics of the prototype LIDAR system [16; 17; 32] and simplified in order to compare only the performance of combinations:  
* A nine-bit block was used to identify each measurement point, and the first bit was always '1';
* Combination of modulation and spreading code: Tables 11-13;
* Up to five measurement points could be measured simultaneously;
* Pulse width was fixed at 5 ns and pulse transmission was completed within 67 \(\upmu\)s;
* The maximum  
illustrated in Equation (3), we estimated the received power based on the distance (\(R\)). Since detection is possible when the received power according to the distance is larger than the TNR, the longest distance having a received power that was larger than the TNR was regarded as the maximum distance [17]. Accuracy and precision were determined using the American Society for Photogrammetry and Remote Sensing (ASPRS)'s positional standards for digital elevation data [68; 69]. We calculated the ground truth distance from the geographical relationship between the wall and the prototype LIDAR system. The distance error was calculated by comparing the measured results with the prototype LIDAR and the ground truth. In a non-vegetated terrain, the corresponding estimates of accuracy at the 95 % confidence level were computed using ASPRS positional accuracy standards such that it was approximated by multiplying the root-mean-square-error (RMSE) by 1.96 to estimate the positional accuracy. The precision was equal to the standard deviation of the measurements. The maximum measurement distance of the pulse was proportional to the number of transmitted pulses, as were accuracy and precision. Among the LIDAR performance indices, maximum distance, accuracy, and precision were most affected by the number of pulses used at one measurement point. Therefore, the maximum distance, accuracy, and precision were very similar when the same number of pulses was used at a measurement point due to the combination of modulation scheme and spreading code. Therefore, PPM, DPPM, and DPIM showed similar results, and MPPM and DH-PIM showed similar results. If the number of slots corresponding to '1' was large, the pulse peak power was small, and the maximum measurement distance was shortened, but the accuracy and precision were improved. DH-PIM changes the number of '1's according to the block. If the pulse peak power can be changed dynamically according to the number of '1's, a longer pulse distance can be measured using the pulse peak power when the number of '1's is small. However, as in the case of DH-PIM, the number of '1's needed to represent a symbol was increased, so the pulse peak power was reduced. As a result, the maximum measurement distance was shortened, but accuracy and precision were improved. MPPM has the advantage that the average and maximum block sizes were the smallest among the modulation schemes. Compared