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The signal strength E_if of the intermodulation product is given by

                                                                          (Eq. 101)

with some constant k to be determined. For signal levels (measured in dB), E_if becomes  and the equation above reads

                                             (Eq. 102)

From the measurement procedure which is described e.g. in the ETSI standard ETSI 300-113, clause 8.8, we can derive the calculation algorithm. The method is similar to the contribution for blocking interference. ETSI 300-113 defines via the intermodulation response L_imr the interfering signal levels iRSS_i1 = iRSS_i2 at which bit errors due to intermodulation just start to be recorded.

This means, for iRSS_i1 and iRSS_i2, we have an intermodulation product  relative to the noise floor (0 dB), due to the noise augmentation (N+I)/N of 3 dB corresponds to an I/N of 0 dB. Transferring Figure 371 into a formula gives the relation

                                                          (Eq. 103)

from which we can derive the calculation algorithm for this example

                               (Eq. 104)

where for interferer i-th, at frequency x:

                  (Eq. 105)

where

• P^output_ILT      : power supplied to the ILT antenna (before power control);
• g^PC_ILT          : power control gain for the ILT with the power control function,
  see ‎ANNEX 14:;
• PL_ILT->VLR  : path loss between the interfering link transmitter i and the victim link receiver;
• G_ILT->VLR    : ILT antenna gain in the direction of the VLR;
• G_VLR->ILT    : VLR antenna gain in the direction of the ILT;
• MCL         : Minimum coupling loss given by the system parameter definition.

For any other consistent combination of (N+I)/N and I/N this becomes

        (Eq. 106)

For the computation of the intermodulation products in the victim link receiver two different interfering systems (i-th and j-th) are required and the total intermodulation product is

                   (Eq. 107)

where:

• iRSS^(i,j)_intermod: intermodulation product at the frequency f0;
•  L_inter           : the attenuation given by the input parameter 'Intermodulation rejection'. This attenuation applies inside the bandwidth of the VLR. It is therefore advisable to set a constant value as intermodulation rejection;
• L_sens            : the sensitivity of the VLR given by the system parameter definition;
•    (N+I)/N           : Noise augmentation given by the system parameter definition;
•    I/N              : interference to noise ratio given by the system parameter definition.

In case the intermodulation product does not fit the frequency condition  , a value of -1000 dBm is returned in SEAMCAT.

The EPP Demo9 (Figure 372) allows you to see the behaviour of this equation.