Required OSNR and maximum transmission distance

64 Table 4.5 presents the EVM of each band of the 11 2Gb/s MB-OFDM signal when DPostD is used to mitigate the SSBI, when it is not and difference between the two cases. It can be seen that the improvement in system performance varies between 3.18 and 3.8 dB when DPostD is implemented.

Table 4.5: EVM of each band of the 112 Gb/s MB-OFDM signal when DPostD considering the MP[2-4-1-4-5-4-1-4] is used, without SSBI mitigation and the difference between both for

an OSNR of 40 dB.

65 First, it can be seen that the band 12 is the one that presents the best results, since it is the one that requires the lowest OSNR for the three MP structures and for all distances. The required OSNR for the system is imposed by the band 9 for 50 km of fiber for the three MP structures. The required OSNR for the 112 Gb/s MB-OFDM system is 40.5 dB. In section 4.2, it was shown that the distortion after the EAM is almost the same for all the bands. So, it was expected that the bands had the same performance. However, the bands have different set of symbols, which will implicate some difference in system performance after DPostD.

Moreover, the distortion due to crosstalk is different for the bands.

Fig. 4.5 Required OSNR for the 12 bands of the 112 Gb/s MB-OFDM signal when DPostD considering the MP[5-3-1-4-5-0-0-0] is used.

Fig. 4.6 Required OSNR for the 12 bands of the 112 Gb/s MB-OFDM signal when DPostD considering the MP[5-3-0-0-0-1-4-5] is used.

66 Fig. 4.8 shows the spectrum of the band 1, 6 and 12 after the BS. Since an ideal rectangular filter is not being used, the adjacent bands will interfere with the band that is being selected creating some distortion in the photodetection process. Fig. 4.8 b) shows that the band 6 suffers interference from band 5 and 7 after photodetection. Fig. 4.8 a) and c) show that bands 1 and 12 only have one band interfering with them. So, it is expected that they had a better performance compared to the ba nd 6 and similar performances compared to each other. However, when comparing the spectrum of this two bands, it can be seen that the first band has part of the band 2 while the band 12 has the virtual carrier belonging to the band 11.

After photodetection, the distortion caused in band 12 by the virtual carrier of the band 11 does not affect the performance of the band. However, the distortion caused by band 2 affects the performance of band 1. This is the reason way the band 12 presents the best performance.

Table 4.6 shows the required OSNR for 50 and 300 km of fiber when the BS is an ideal rectangular filter, when the BS is a 2- nd order super Gaussian filter and the difference between the two when DPostD considering the MP[5-3-1-4-5-0-0-0] is used to mitigate the

Fig. 4.7 Required OSNR for the 12 bands of the 112 Gb/s MB-OFDM signal when DPostD considering the MP[2-4-1-4-5-4-1-4] is used.

Fig. 4.8 Spectrum of the a) band 1, b) band 6 and c) band 12 after the BS.

67 SSBI. From Table 4.6, it can be seen that band 12 presents the lowest penalty due to the BS shape and that the penalty is different for all bands and for different fiber lengths.

Table 4.6: Required OSNR for 50 and 300 km of fiber when the BS is an ideal rectangular filter, when the BS is a 2-nd order super Gaussian filter and the difference between the two

when DPostD considering the MP[5-3-1-4-5-0-0-0] is used.

The OSNR required for the 112 Gb/s MB-OFDM system employing EAMs and DPostD based on MP is 40.5 dB and transmission distances of 400 km are still accepted. In order to assess the penalty due to the transmission of the twelve bands, the required OSNR to achieve a BER of 4×10^-3 in a single band transmission was evaluated. The parameters of the system are the same as the ones used to study the 112 Gb/s MB-OFDM system, this includes, for example, the BS and VBPR used. However, since the signal comprises only one band the bit rate of the signal is equal to the bit rate of one band of the 112 Gb/s MB-OFDM signal (9.33 Gb/s). Fig. 4.9 shows the required OSNR has a function of the fiber length considering the MP structures previous studied for two situations: i) signal comprising 1 band; ii) band 9 of the 112 Gb/s MB-OFDM signal. The fifteen sequences of QAM symbols used to estimate the BER, in a single band transmission, and the noise of the system are the same for all three MP structures and for different distances. First, it can be concluded that the required OSNR of the system is around 20.5 dB. The required OSNR is imposed by the MP [5-3-1-4-5-0-0-0] at 400 km .

Band number

OSNRre q for 50 km[dB]

ΔOSNR [dB]

OSNRre q for 300

km[dB] ΔOSNR

[dB]

Rect.

filter

2-nd Gaussian filter

Rect.

filter

2nd Gaussian filter

1 32.9 39.6 6.7 32 37.1 5.1

2 31.8 37.4 5.6 31.9 37.3 5.4

3 31.3 35.4 4.1 31.6 35.6 4.0

4 31.7 36.9 5.2 31.7 35.9 4.2

5 31.8 36.3 4.5 31.8 36.6 4.8

6 31.6 36.7 5.1 31.3 35.9 4.6

7 34.0 38.4 4.4 33.7 37.8 4.1

8 33.1 37.2 4.1 33.3 37.3 4.0

9 34.0 40.3 6.3 32.5 37.4 4.9

10 31.9 37.7 5.8 31.8 37.1 5.3

11 32.6 39.1 6.5 32.1 38.1 6.0

12 31.5 33.6 2.1 31.2 33.3 2.1

68 Comparing the OSNR required in a single band transmission with the transmission of the twelve bands, it can be seen that the penalty is around 20 dB. 11 dB of the 20 dB of penalty are due to the power of the signal being divided by the twelve bands and, from Table 4.6, it can be seen that around 6 dB are due to crosstalk and the shape of the BS. The remaining 3 dB can be attributed mainly to the EAM distortion and fiber dispersion.

It is important to have in mind that these results are for a set of symbols. It was demonstrated, in section 3.4, that 15 iterations were enough to estimate the average EVM.

However, it was also shown that the average EVM can vary 0.5 dB. Appendix C.4 shows the required OSNR for two different set of symbols when the BS is a n ideal rectangular filter. It can be seen that the required OSNR for both cases is different. However, this difference is less than 0.5 dB.