As far as I know, at the high level two types of charging mechanisms named as "offline" and "online" are supported in LTE technology. Rf and Ro interfaces are used for offline and online charging respectively. Charging Trigger Function (CTF) is integrated in NE (network elements like PGW, SGSN, SGW) and would be responsible to trigger "charging events" towards Rf and Ro interfaces.
In case of online charging system (OCS), charging events are received by OCF (online charging function) which takes decision on further usage of resource with the help of RF (Rating function) and ABMF (Account balance management function).
In case of offline charging system (OFCS), charging events are received by CDF (charging data function) which generates charging data records i.e. CDRs and send to charging gateway function (CGF). CGF acts as a gateway between 3GPP network and Billing domain. CGF sends this CDRs to billing domain (BD).
This is all, I know about offline and online charging in LTE network.
3GPP TS 32.240