in the alu system, you can get more details about your problem :
– check the tch efficiency rate per trx
– check the path balance avg per trx
– check the idle band measurements per trx (if available)
– check the dl rxlev avg & the ul rxlev avg per trx
– check the timing adv avg per trx
– check the noise calls due to abnormal FER per trx
based on those indicators, you can know whether your problem is coverage related (when adding more trx, you add more combining losses, therefore decrease the dl coverage of your cell) ,or interference related, or hardware related.