A24.3 ASSUMPTIONS

Starting from the characteristics provided above, some “reference” hypothesis might be used to obtain a first set of results.

A24.3.1     Distribution of the Mobile Stations (MS) and propagation model

It is assumed that all mobile stations are located uniformly distributed in the areas around their base stations. For worst case consideration a rural area may be assumed.

A24.3..2     TETRA network 

A 2 MHz band is available for TETRA systems of 25 kHz channel spacing, therefore a total of 80 channels are available. It is supposed that only one TETRA MS terminal may use the same frequency at a given time.

The frequency cluster is taken equal to 9, this leads to have 80/9 = 8.88 frequency per cell available. Then up to 9 terminals may be deployed in a TETRA cell.

If the TETRA cell is a traffic limited network and the density of terminals is equal to 5 terminals per km², then the coverage radius is given by the following formula (see ERC Report 68 or User Documentation):

  (Eq. 261)

The radius of the cell of the interfering system is given by

R = sqrt( (80*1) / (3.14*5*9) ) = 0.7523 km.                                                         

Figure 534 describes a TETRA network based on the above assumptions.

Figure 542: TETRA network based on these assumptions

A24.3.3    Interference situation

As a worst case assumption, we may suppose that, the FM BS station will be based in the TETRA cell where the channel (411.9875 MHz) operating close to the edge of the TETRA band is used. The same probability is assigned to each channel available in the TETRA cell. Then the frequency plan in this cell starts from 410.0125 MHz up to 411.9875  MHz with a step of 25 kHz x 9. A discrete uniform distribution for frequency can be selected, e.g., between 410.0125 MHz and 411.9875 MHz having a step of 25 kHz (see ‎ANNEX 10: for a description of the discrete uniform distribution).

If there is no spatial correlation between the elements of the victim system and the elements of the interfering system then the simulation radius of the simulation is given by the formula:

   = radius of the area where interferers are uniformly spread

                (Eq. 262)

We have to consider 9 active interferers with a density of 5 terminals/km². Then the radius of simulation area results in:

                                                R = sqrt( 9 / (3.14 x 5) ) = 0.7569 km.

The FM Base Station, which is the victim link receiver, is supposed to be operating at 412.00625 MHz, that is the centre of the first channel, i.e. no guard interval is assumed.

A24.3.4    Emission mask

The total emission of the interefering link transmitter consists of

• The wanted signal within the defined bandwidth corresponding to a frequency offset of ± 50 % of the emission bandwidth It_BW;
• The out-of-band emission caused by the modulation within, e.g. ± 50...250 % of It_BW;
• The spurious emission outside these ranges.

This means the mask to be defined has to cover this whole range. This allows co-channel consideration as well.

It should be noted that the TETRA MS unwanted emissions mask is not defined for frequency offsets below 12.5 kHz. Considering the channel spacing of 25 kHz and assuming 0 dBc/18 kHz in that range, then in the undefined range between 12.5 and 25 kHz, a linear interpolation between 0 and 60 dBc attenuation will be performed. It should also be taken into account that the reference bandwidth (18 kHz) used for the mask is identical to the bandwidth of the interfering system (see ‎ANNEX 6:).