# 1.4.4  Interference criteria relationship

C/I may vary typically from 9 dB (e.g. for <span data-highlighted="true" data-vc="highlighted-text"><span class="_kqswh2mm"><span class="_5pioz8co _189e1dm9 _1il9buyh _19lc184f _d0altlke" data-testid="definition-highlighter">QPSK</span></span></span>) to 26 dB or higher (e.g. for 64QAM…). By introducing artificial noise iRSS on top of the noise floor (I/N), C/I is then desensitised by (N+I)/N resulting in C/(N+I). Note that the desensitisation is exactly the factor (N+I)/N (also = 1+I/N).

Further details of the relationship are given in [ANNEX 3:](https://ecowiki.atlassian.net/wiki/spaces/SH/pages/494652/A3+Relationship+between+C+I+C+I+N+I+N+N+and+I+N#A3RelationshipbetweenC/I,C/(I+N),(I+N)/NandI/N-A "https://ecowiki.atlassian.net/wiki/spaces/SH/pages/494652/A3+Relationship+between+C+I+C+I+N+I+N+N+and+I+N#A3RelationshipbetweenC/I,C/(I+N),(I+N)/NandI/N-A").

Considering that

[![image.png](https://wiki.cept.org/uploads/images/gallery/2026-04/scaled-1680-/GTYtMv0kzr6RlavK-image.png)](https://wiki.cept.org/uploads/images/gallery/2026-04/GTYtMv0kzr6RlavK-image.png) (Eq. 12)

and assuming a C/I of 19 dB, the following examples may be considered:

- I/N = 0 dB, results in (N+I)/N = 3 dB and considering C/I = 19 dB, then C/(N+I) = C/I - 3 dB = 16 dB
- I/N = -6 dB, results in (N+I)/N » 1 dB and considering C/I = 19 dB, then C/(N+I) = C/I - 7 dB = 12 dB
- I/N = -10 dB, results in (N+I)/N » 0.4 dB and considering C/I = 19 dB, then C/(N+I) = C/I - 10 dB = 9 dB
- I/N = -20 dB, results in (N+I)/N = 0.04 » 0.1 dB and C/I = 19 dB, then C/(N+I) = C/I - 20 dB = -1 dB

 **Note:**

*In case C/(I+N) is chosen as the protection criterion:*

*if I/N ≤ -20 dB, the impact of the interferer is negligible compared to the noise floor (i.e. C/(I+N) ≈ C/N);*

*if I/N &gt; 10 dB, then C/(I+N) ≈ C/I (i.e. the interferer is more dominant than the noise).*

![image.png](https://wiki.cept.org/uploads/images/gallery/2026-04/scaled-1680-/7eWemeMzkhjkoUVd-image.png)