In the OMCR, you’ll find parameters related to half rate, called U_*_SV1 and L_*_SV1 (“*” is a string of letters, that i forgot 🙂 )
H is the upper threshold, and L is the lower threshold. SV1 means those are the thresholds for half rate and full rate. SV3 is related to AMR HR and AMR FR.
Set the upper threshold at 80% and the lower threshold at 60%. Like this, you’ll have between 20% to 40% of half rate in your cell. It’s not possibe to choose exactly 20%, because the mechanism is based on the traffic load.
If traffic > 80%, then all new calls are in half rate
If traffic was > 80% but is now < 60%, the new calls are set in full rate. Don't choose lower and upper thresholds to close to each other, because when you translate the percentage into number of timeslots, you see they are actually quite close. For instance, 3 TRX = 21 TCH 80% = 16.8 60% = 12.6 if 17TCH are allocated, the load > 80%
if 12 TCH are allocated, the load < 60%. It's only 5 TCH difference between 80% and 60%, and since one call lasts about 40s, there can be some statistical effect.