Capacity Max: Understanding Fixed and Flexible Channel Plans

DMR Tier 3 trunked systems support two different Channel Plans: Fixed and Flexible. For those of you, who are familiar with MPT1327, these two words should be nothing new since the concept is the same as MPT1327.

When designing a Capacity Max system, one of the questions you will need to ask yourself is, when do I use a fixed channel plan and when do I use a flexible channel plan?

Well, the answer is dependent on what frequencies have been allocated to you by the Telecommunications Authority, or what frequencies the existing analogue system (that Capacity max will replace) uses. To illustrate this, consider the two example frequency plans below:
Click on the image to enlarge.
As you can see, the frequencies in Example A all have the same TX-RX separation (5MHz). All the TX frequencies and all the RX frequencies are within 13MHz of each other so in this case, the Fixed Channel Plan could be used.

Although all the TX and RX frequencies are within 13MHz of each other, in example B the Flexible Channel Plan should be used. This is because the TX-RX separation for each channel pair is different.

The DMR Tier 3 standard uses channel numbers, rather than frequencies, in its call management routines - much like other trunking technologies. The standard mentions channel numbers in the range of 0 to 1023 so 1024*12,5kHz=13MHz hence the need for frequencies to be within 13MHz.

With MOTOTRBO, it is possible to have frequencies outside this 13MHz range - up to 4094 channels. There may be a technical limitation on the RF conditioning hardware on the sites so this needs to be checked first.

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