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In SEAMCAT, it is called the intermodulation rejection, and in the presence of more than one interfering system applying its own frequency distribution, a theoretically unlimited number of intermodulation products exist, caused by the non-linearity of the VLR. In practice, just the products close to the frequency of the VLR are of importance, of which the products of the so called 3rd order[1] are the most dominant and therefore considered by SEAMCAT. The frequency conditions for the intermodulation products of the 3rd order are
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with
f_VLR-b/2≤f_0≤f_VLR+b/2 (Eq. 99)
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Figure 372: Illustration of intermodulation product Eif of third order at the frequency f0Anchor F372 F372
The signal strength Eif of the intermodulation product is given by
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with some constant k to be determined. For signal levels (measured in dB), Eif 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 Limr the interfering signal levels iRSSi1 = iRSSi2 at which bit errors due to intermodulation just start to be recorded.
Figure 373: Illustration of intermodulation product from ETSI 300-113Anchor F373 F373
This means, for iRSSi1 and iRSSi2, 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
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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.
Figure 374: EPP Demo 9 – intermodulation internalsAnchor F374 F374
[1] the order is given by the sum of the absolute values of the coefficients, here 2 + |1| = 3