# Effects of Interference and Mitigation Using Notch Filter for the DVB-S2 Standard
## 4 Proposed Solution and Efficiency Analysis
### Effects of Notch Filtering and CWI Removal  
The DVB standard is a narrow band signal, which means that a lot of information is compressed to a narrow range of frequencies [35]. A notch filter is utilized as a simple solution to attenuate the frequency that the CW interference is compromising. Frequency removal can improve the system's performance. However, it should be considered that it also removes a part of the useful received signal. Clearly, interference cancelation for powerful interference demands a wider gap or a greater attenuation from the notch in the frequency domain, but there is a tradeoff between the amount of interference power reduction and the removed bandwidth from the signal. To study the effect of bandwidth removal on the system's performance, first, consider that there is no interference and a notch filter removes a part of the pure signal spectrum, in other words, the input of the notch filter is a QPSK signal with AWGN noise at variable power. The performance degradation can be expressed based on the comparison of the SNR measurement before and after filtering. This section explains the effect of the notch filter on a QPSK signal with AWGN noise is. Figure 20 shows the block diagram of this test analysis. This test tries to clarify the negative effect of a notch filter on the system's performance based on the SNR measurement. Only the AWGN noise is added to the signal. The input and output SNR are measured and analyzed in Figure 21. This figure shows the variation of the output SNR for different \(r\) values. The results show that there is performance degradation related to the bandwidth of a notch filter. Clearly, a wider notch removes more information from the desired signal, therefore it will cause more SNR reduction. For a larger input SNR, the output SNR  
Figure 19: Notch filters response with different r parameters. (**a**) Magnitude (**b**) Phase. Figure 20: Structure used to measure the effects on system performance. decreases much faster with the increase in the r parameter. Again here, a system with higher QoS (larger SNR) experiences more distortion. When the r increases towards a value of 1, the output SNR is approximately equal to the input SNR. Figure 22 shows the relation between the input and output SNR for different values of \(r\). As the notch bandwidth decreases, the output SNR increases as well for the same amount of input SNR. For a specific bandwidth, the SNR after filtering cannot exceed a certain level, even if the input SNR is increasing. These curves display the maximum available SNR at the output of the notch filters with different bandwidth. Based on this figure, it can be concluded that for a special notch bandwidth, the obtained SNR is under the threshold level, and large bandwidth will cause the residual error in the receiver. The following test explains the amount of residual interference in the received signal after notch filtering. To study the performance of the notch filter in JSR reduction, the next simulation is performed under a similar situation to the block diagram in Figure 23. In this test, a CWI with variable power is added to a QPSK signal, then, the notch filter reduces the interference power. JSR measurement before and after filtering is utilized to express the filter performance with different bandwidth. Figure 24 shows the influence of the notch filter on the JSR reduction for the QPSK signal under the effect of a CWI. The JSR is reduced greatly after filtering (almost as much as a 10 dB improvement). The relation between the input and output of the filter is almost linear for a given r.  
Figure 21: Relation between output SNR and r parameter for different input SNR. Figure 22: Relation between output and input SNR for different r values. The final step combines the SNR and the JSR as shown in Figure 25. The next test and figure reveal how much the notch filter can improve the SNR in the presence of interference. Figure 25 displays the utilized situation for the next measurement.