what is ICIC( Inter-cell interference coordination) and eICIC (enhanced Inter-cell interference coordination)

Background

• LTE is designed for frequency reuse 1 (To maximize spectrum efficiency), which means that all the neighbor cells are using same frequency channels and therefore there is no cell-planning to deal with the interference issues
• There is a high probability that a resource block scheduled to cell edge user, is also being transmitted by neighbor cell, resulting in high interference, eventually low throughput or call drops (see figure) 
• Traffic channel can sustain upto 10% of BLER in low SINR but control channels cannot. Neighbor interference can result in radio link failures at cell edge.
• Heterogeneous networks require some sort of interference mitigation, since pico-cells/femto cells and macro-cells are overlapping in many scenarios

ICIC (Inter-cell interference coordination)

• Inter-cell interference coordination is introduced in 3GPP release 8
• ICIC is introduced to deal with interference issues at cell-edge
• ICIC mitigates interference on traffic channels only
• ICIC uses power and frequency domain to mitigate cell-edge interference from neighbor cells 
• One scheme of ICIC is where neighbor eNBs use different sets of resource blocks through out the cell at given time i.e. no two neighbor eNBs will use same resource assignments for their UEs. This greatly improves cell-edge SINR. The disadvantage is decrease in throughput throughout the cell, since full resources blocks are not being utilized.
• In the second scheme, all eNBs utilize complete range of resource blocks for centrally located users but for cell-edge users, no two neighbor eNBs uses the same set of resource blocks at give time
• In the third scheme (probably the preferred scheme), all the neighbor eNBs use different power schemes across the spectrum while resource block assignment can be according to second scheme explained above. For example, eNB can use power boost for cell edge users with specific set of resources (not used by neighbors), while keeping low signal power for center users with availability of all resource blocks (see the figure)
• X2 interface is used to share the information between the eNBs

eICIC (enhanced Inter-cell interference coordination)

• eICIC introduced in 3GPP release 10
• eICIC introduced to deal interference issues in Heterogeneous Networks (HetNet)
• eICIC mitigates interference on traffic and control channels
• eICIC uses power, frequency and also time domain to mitigate intra-frequency interference in heterogeneous networks
• eICIC introduces concept of "Almost blank subframe" (ABS). ABS subframes do not send any traffic channels and are mostly control channel frames with very low power. If macro cell configure ABS subframes then UEs connected to pico/femto cells can send their data during such ABS frames and avoid interference from macro cell (see the figure)
• ABS configuration is shared via OAM or x2 interface