Hi Jane,
3 TRX, 1 BCCH and if you declared 2 SDCCH then remains 21 TCH.
25 Erl with 2% blocking needed about 34 TCHs. So, you need more 2 TRXs. If you declare 2 TRX in HalfRate and assume first TRX has 1 SDCCH and second 1 SDCCH then second and third TRX together have 8+7=15 TCHs which would be in Half Rate. 6 TCH in BCCH TRX that’s all together 6+15*2=36 TCHs, enough to avoid congestion.
Now, (15 TCH in Hal Rate/(15+6))*100=71.42%. So, THR_FR_LOAD_U_SV1=71 (aprox 70) and THR_FR_LOAD_L_SV1=30 (because you have 3 TRXs)
I hoppe it helps,
Rex.