Inter cell Interference Coordination

In LTE, due to orthogonal nature of OFDM signal, there is no intra cell interference. However, as the signal coming from different cells is not orthogonal in nature, cell edge user may experience interference from adjacent cell. LTE uses frequency reuse of one, meaning adjacent cells  may transmit on same frequency, resulting in inter-cell interference.

Inter-cell Interference Coordination (ICIC) is the method used in LTE to manage the interference arising due to signal coming from adjacent cell sites. The basic principle is to coordinate the scheduling of cell edge user in a way that users are not scheduled on same frequency time resources as users in other cell.

The coordination between cell sites is achieved by exchanging messages between eNodeBs over X2 interface. Two messages are defined for uplink interference coordination: High Interference Indicator (HII) and overload indication (OI).

High Interference Indicator HII) is used to communicate, on which frequency time resources and eNodeB is going to schedule cell edge users. By listening to this message, a neighbor eNodeB can avoid scheduling cell edge users in the indicated resources. This can, therefore, result in reduced uplink interference for both of the cells. The action to be taken by an eNodeB when it receives HII message is implementation specific.

An eNodeB send Overload Indicator message to indicate the level of interference experience in different frequency time resources to neighbor eNodeB. Three levels of interference are defined : Low, Mid and High. When an eNodeB receives overload indicator message, it can change the scheduling pattern to free the resources indicated in the overload indicator message, therefore, reducing the interference for cell edge users.

In the downlink, interference coordination can be achieved by controlling the downlink cell power for resources.  This is achieved by sending Relative Narrowband Transmit Power (RNTP) message. This message contains information whether or not the frequency time resource is limited by transmit power.  When a neighbor eNodeB listens to this message, it can avoid scheduling on the indicated resources.