## 4 Evaluation and Result
### Results  
The access tests for the internal registers the QSFP28 were conducted through the assessment board to verify the host interface implementation. Figure 12 shows the access result from the QSFP28 optical transceiver internal register using a self-produced memory control program. Figure 12a shows the process of writing and reading a value to the first address of upper page 02h, which is used as an EEPROM to store user data. Figure 12b shows the result of reading both the default lower and upper pages, and Figure 12c shows the result of reading upper page 02h. We can verify that the value written in Figure 12a is read in Figure 12a. Based on the register access results, we confirmed that the host interface implemented using the proposed firmware design scheme and register access algorithm function normally according to the standard. Figure 13 shows an oscilloscope measurement of the 50G QSFP28 PAM4 optical transmission signal. An 86100C Infiniium digital communication analyzer (DCA) oscilloscope mainframe and 86105D Optical/Electrical (O/E) module were used for measurement. The detailed PAM4 signal parameters could not be determined owing to the limitations of the available measuring equipment, but four levels were clearly measured. The amplitude ratio at each level is evenly maintained and constant (level 1 = 940 uW, level 2 = 2.92 mW,  
Figure 11: QSFP28 test-bed configuration images (self-loopback). Figure 10: QSFP28 transmission test-bed configuration diagram. level 3 = 4.94 mW, and level 4 = 6.94 mW). This means that the proposed design scheme operates the PAM4 DSP IC and optical transmitter normally. It was also confirmed that the transmission and reception were performed through the self-loopback test and that the BER could be measured. Figure 14 shows a receiving BER graph of the QSFP28 PAM4 optical transceiver. The measurement results show that, in all temperature environments, the receive sensitivity was 8.6 \(\times 10^{-5}\) when the received optical power was \(-13.99\) dBm. Figure 12: QSFP28 register access test result. Measurements of the QSFP28 PAM4 optical transceiver parameters produced by the proposed design scheme are summarized in Table 4. The tests were conducted in low, ordinary, and high temperature environments and compared with the measurements required by the standard. The measurement results met all the standard figures. The transmission optical power and extinction ratio showed improved performance over that of the standard figures. This is analyzed as a result of optimizing the PAM4 signal of the transmitter output to overcome the limitations of the PIN-PD receiver for long-distance transmission. In addition, the power consumption also improved compared to that of the specification and transmission distance is up to 57 km. To verify the performance of the DDM function, we measured the received optical power DDM of the QSFP28 optical transceiver. To adjust the input optical power input into the transceiver, we used a variable optical attenuator. The received optical power DDM accuracy was measured in low, room, and high temperature environments. Figure 15 shows the received optical power DDM value of the QSFP4 optical transceiver measured for input optical powers ranging from \(-3\) dBm to \(-15\) dBm. All results are within the margin of error 2 dB, and the accuracy is over 92%. Figure 14: Optical PAM4 bit error rate (BER) curves obtained for the three different temperatures  
Figure 13: QSFP28 Optical PAM4 eye pattern. Furthermore, we compared the performance between the existing methods for long-distance transmission of PAM4 signals described in the Introduction section and the proposed method. One existing method is to use an APD receiver for reception, and another method is to amplify signals using SOA on the PIN-PD receiver. The proposed method uses a PIN-PD receiver only for reception. We experimented on our own using a commercial APD and SOA for performance comparison.