In TCH interval grouping, the three cells of the same BTS use the same HSN while different BTS use different HSN, and different MAIO are used by carriers in the same layer of the same BTS.
The HSN of BTS A is 1, the MAIO of two carriers TCH1 and TCH2 in Group 1 cell are 0 and 1 respectively, the MAIO of two carriers TCH1 and TCH2 of Group 2 cell are 2 and 3 respectively, the MAIO of two carriers TCH1 and TCH2 of Group 3 cell are 4 and 0 respectively, the HSN of BTS B is 2, and so on.
In this way, adjacent frequency is avoided between the three different cells of the same BTS, the possibility of adjacent frequency conflict between opposite cells of different BTS is reduced in comparison with TCH consecutive grouping, but there is an extra possibility of adjacent frequency conflict between cells of different BTS on parallel direction in comparison with TCH consecutive grouping.
As to which TCH grouping mode produces less 1*3 frequency hopping interferences, both consecutive and interval grouping modes have their disadvantages. For the downtown with dense BTS distribution, adjacent frequency influence from opposite cell is more than that from adjacent cells on parallel direction, which makes consecutive grouping more appropriate.
But in sub-urban areas, interval grouping helps to average out the interferences due to the irregular distribution of BTS. Therefore, actual local situation should be considered when choosing grouping mode. After the new channel distribution arithmetic under close reuse has been realized, it is recommended to adopt consecutive grouping scheme to ensure better service quality over the whole network.