## 3 Results and Discussion
### Guard Band  
For twin-SSB signal recovery, the LSB and RSB signals need to be separated by a pair of OBPFs. To model its non-ideal nature in practice, the OBPF was modeled as a 4 th order Gaussian filter with 3 dB bandwidth of \(Bw=2(1+\beta)Rs+GB\) in this work. The bandwidth of the guard band is an important parameter for the system as it affects both the magnitude of sideband crosstalk and the spectrum efficiency. In order to find an optimum \(GB\), Figure 2 shows the receiver's signal-to-noise ratio (SNR) variation for ONU1, ONU2, ONU3 and ONU4 as a function of guard band for the LSB and RSB signals in a B2B scenario. As the performance of LSB sub-bands and RSB sub-bands were almost the same, we chose ONU1 and ONU2 (belonging to LSB) for the following discussion only. From Figure 2, it was found that the guard band parameter had much larger effects on the central sub-band for ONU2 than the outer sub-band for ONU1. For ONU2, the SNR changed with the guard band in three different stages as shown in the insets (I)-(III) in Figure 2. When a small \(GB\) of less than 3 GHz was used for high spectrum  
Figure 2: Signal-to-noise ratio (SNR) versus Guard Band for 12.5 Gbaud per sub-band 16 quadrature amplitude modulation (QAM) modulation in a back to back scenario. Insets: (I)–(III) represent the signal spectra before OBPF, after OBPF and after PD detection corresponding to stage (I)–(III) marked in the above curves figure. efficiency, the SNR performance increased rapidly with increasing guard band as shown in stage (I) of Figure 2 because, in this stage, the SNR for the central sub-band was mainly dominated by the crosstalk from the other sideband due to non-ideal optical filtering. As the non-ideal filtering-induced crosstalk was very sensitive to the \(GB\) parameter, a sharp SNR increasing was observed as the \(GB\) increased in this stage. Due to the fact that KK receiver has no effect on this type of crosstalk, a close SNR performance was found for both cases without and with a KK receiver in stage (I), especially for small \(GB\). However, if a \(GB\) between 3 GHz and 25 GHz was selected as in stage (II) of Figure 2, the other sideband could be removed almost completely by the OBPF. In this case, the SNR performance of ONU2 was then dominated by the SSBI term from square-law detection and the KK-receiver was found to be able to improve about 1 dB SNR compared with the case without the KK-receiver. However, it was also observed that the rate of the SNR improvement got much slower in stage (II) compared to stage (I) as the \(GB\) increased. Moreover, the SNR performance of ONU2 eventually saturated when \(GB\) went beyond 25 GHz as shown in stage (III) of Figure 2. In this stage, the SSBI term stayed at a different frequency location from the expected signals and could be efficiently removed by a digital filter. As a result, the KK-receiver showed no SNR improvement. Compared with ONU2, the outer sub-band for ONU1 showed much smaller SNR variation as \(GB\) varies. For ONU1, the KK-receiver showed no improvement compared to the conventional receiver except when \(GB\) was very small because the SSBI products appear over a bandwidth equal to that of the original modulated SSB signal and the level of SSBI product appearing around a low frequency is much larger than that of high frequency. Furthermore, when \(GB\) was lower than 13 GHZ, the outer sub-band for ONU1 only overlapped with the SSBI partially while the inner sub-band for ONU2 overlapped with the SSBI fully. As a result, ONU1 outperformed ONU2 in this case. When \(GB\) was larger than 13 GHz, the SSBI would not overlap with the outer sub-band for ONU1 but its performance was still mainly limited by the filter edge roll off of the OBPF. Note that, in this case, the inner sub-band for ONU2 outperformed the outer sub-band for ONU1 because that ONU1 was located much closer to the edge of the OBPF. Based on the results shown in Figure 2, a 3 GHz guard band was selected in our system for a good compromise between the system performance and spectrum efficiency.