## 4 Evaluation
### Error Distribution  
Another interesting aspect is the distribution of bit errors in the raw data transmission. In the experimental setup any environmental errors or errors caused by movements of the transmitting car or the receiving camera were tried to be omitted. The only errors to analyze are the ones caused by the modulation itself, due to ambiguous states as previously shown in Figure 9. Figure 18 shows the error distribution for three data transmissions  
Figure 17: BER depending on accuracy of loose synchronization.  
over approx. 4.5 min with different synchronization qualities. Each of those sub-figures shows two graphs. The top graph depicts the errors in detail by comparing the received bit stream with the expected message taking bit offsets into account. At the start of the data transmission, expected and received bit stream are aligned to get a bit offset of 0. If two bits encoded into one frame are duplicated because the sampling of the receiving camera was faster than the modulation, the bit offset is increased by 2 bits. The bottom graph shows the actual bit errors that occurred during the transmission with adjustment of the bit offset if bits are duplicated.  
Figure 18a shows that even with the best possible synchronization the system runs into error bursts, because even here the stripe pattern is slowly moving. In this transmission one can see two error bursts. The first one lasts from bit index 5831 to bit index 5960, i.e., 2.15 s, the second one from bit index 12,972 to bit index 13,077, i.e., 1.75 s. The time between the two error bursts is approx. 2 min. However, within the time of this transmission the bit offset does not change because no bit is missing or duplicated.  
Figure 18: _Cont._  
With a slightly imperfect synchronization, where the modulation of one bit takes 10 us too long, the frequency of error bursts increases; however, the duration of a single error burst is shorter, as seen in Figure 18b. During the transmission, equidistant error bursts occur after every 17.3 s on average with an average duration of 392 ms. What can also be seen is that after 8 equidistant error bursts the bit offset is increased by 2, which means the two encoded bits from one frame are duplicated. This causes one additional error burst. Those error bursts always occur when a transition of the stripe pattern, representing the rising and falling edges of the modulation signal, is visible at the vertical position of the taillights. For each rising and falling edge in the modulation signal an error burst occurs. As seen in the previously shown Figure 7, the modulation signal in \(\pi/2\) mode uses 9 edges to modulate one bit. The original modulation mode signal depicted in Figure 6 only uses 8 edges to modulate one bit and hence with the original modulation mode, the additional error burst would not occur. However, this is the only and just a minor downside of the \(\pi/2\) modulation mode.  
In the last transmission example shown in Figure 18c, the modulation of one bit takes 20 us too long. Here the average time between two error bursts is only 5.5 s where each of them lasts about 250 ms. Within the transmission time of approx. 4.5 min, the bit offset is increased 5 times, which causes additional error bursts. However, as in the previous example, 8 equidistant error bursts occur before the bit offset is increased which causes an additional error burst.