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5.2 Engineering guidelines (to be included in PIB 38)
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a. Coverage area
The coverage area of a base station is the area of service within which the licensee has a reasonable expectation of protection from harmful interference. This is defined by a continuous contour equivalent to a signal level of -106 dBm or a theoretical coverage area with a radius of 58 km*, whichever is the smaller. See Figures 2 and 3.
Figure 3 - With terrain limitation, the smaller -106 dBm area applies
b. Co-channel re-use distance
The re-use distance is the distance required to ensure a suitable grade of service between co-channel services. Generally, the re-use distance is constrained by topographic obstructions. See Figure 4.
Co-channel services isolated by topography
In areas where the local topography may not provide sufficient levels of obstruction loss between co-channel services, the proposed service must protect existing services to a minimum C/I of 18 dB* at the edge of the existing coverage area. See Figure 5.
Figure 5 - Re-use distance to protect existing co-channel services.
Insufficient topographic obstruction.
To comply with the minimum C/I of 18 dB at the edge of coverage of existing services, the proposed service must constrain the interfering impact of its transmission by other means. In the example above, a modified horizontal radiation pattern (HRP) is used to achieve the required C/I ratio.
c. Protecting first adjacent channel services
In non co-sited situations – with services sharing a substantial overlapping coverage area – it is recommended that between analogue FM, APCO and TETRA base stations, the new base station be spaced at least 5 km away from the edge of the coverage area of the existing service. In situations where the local terrain and clutter provide sufficient obstruction loss, this measure can be relaxed provided that an equivalent co-channel C/I of 18 dB is maintained by the proposed service at the edge of the coverage area of existing services. Where the new service is TETRA, and has an adjacent channel leakage ratio of -55 dB, the C/I for the first and second adjacent channel is -37 dB, and where the proposed service is APCO P25, with an ACLR of -60 dB, the first and second adjacent channel C/I is -42 dB. See Figure 6.
Co-channel, first and second adjacent channels, etc, in this document refer to the 25 kHz raster, not the 12.5 kHz sub-channels.
d. Protected C/I
In some instances, the above guidelines may not be optimal when considering specific characteristics of the local topography, especially in large and flat terrains. In such cases, the above distances can be adapted, provided that the proposed service complies with the minimum C/I of 18 dB at the edge of the coverage area of the existing service.
Explanatory notes:
* The C/I level of 18 dB has been obtained considering dynamic conditions (fading) for analogue, APCO and TETRA systems. This figure takes into account the C/I requirements to maintain a Delivered Audio Quality (DAQ) grade of 3.4 (as specified in TSB88.1, table A-1). This value is also used in ITU-R SM.337-4 (table 1), where a calculation example is given for determining frequency and distance separation between land mobile services in the UHF band.
Detail of C/I specifications (extract from TSB88, table A-1):
| Modulation type(channel spacing) | Static ref / C N |
DAQ 3.4 BER% / C (I+N) |
| Analogue FM ± 5kHz (25kHz) | 12 dB SINAD / 4 dB | 20 dB |
| Analogue FM ± 4kHz (25kHz) | 12 dB SINAD / 5 dB | 22 dB |
| C4FM (IMBE) (12.5kHz) | 5% / 7.6 dB | 2% / 17.7 dB |
| CQPSK (IMBE) (12.5kHz) | 5% / 7.6 dB | 2% / 17.7 dB |
| CQPSK (IMBE) (6.25kHz) | 5% / 7.6 dB | 2% / 17.7 dB |
| TETRA (25kHz) | 5% / 8 dB | 2% / 16 dB |
