LTE: Inter-Cell Interference Coordination: ICIC

LTE uses frequency reuse factor of 1 in order to maximize spectrum efficiency. This inherently means that transmissions carried out on the same time-frequency resource will cause interference between different cells at cell-edges.

As shown in the figure below, in a macro cell network, a UE receives interference from the neighboring cell (red line) in addition to the desired signal from its serving cell (blue line). In addition, the same UE will create interference to the neighboring cell in the uplink. This interference has both control and traffic channel components.

 

In order to keep the inter-cell interference (ICI) under the control of radio resource management (RRM) methods, Inter-Cell Interference Coordination (ICIC) is introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells.

In Release-8 the frequency domain ICIC is prioritized which manages radio resources, notably the radio resource blocks, such that multiple cells coordinate use of frequency domain resources. More specifically, the focus was to define X2 signalling that could be used for the co-ordination between cells that belongs to two different eNBs. The X2AP message used for ICIC purpose is called LOAD INFORMATION.

An eNB transmitting LOAD INFORMATION message to eNBs controlling intra-frequency neighbouring cells includes CellInformation for 1 or several cells. The CellInformation structure is given below.

Uplink Interference Coordination

In Release-8, two IEs are defined in LOAD INFORMATION message to assist uplink ICIC; the Overload Indication (OI) and High Interference Indication (HII) as shown in the figure below.

The UL Interference Overload Indication IE received in the LOAD INFORMATION message indicates the interference level experienced by the indicated cell on all resource blocks, per PRB. For each PRB, one of three levels of interference (High, Medium, Low-interference) is indicated. The receiving eNB would take this OI information into account when setting its scheduling policy to improve the interference situation for the eNB which has sent this OI.

The uplink High Interference Indication IE received in the LOAD INFORMATION message indicates, per PRB, the possibility of high sensitivity to interference as seen by the sending eNB. The receiving eNB should try to avoid scheduling cell edge UEs in its own cells for the concerned PRBs. This reduces Uplink interference to cell-edge transmissions in its own cells as well as in the cells of eNB from which HII was received. The Target Cell ID IE received within the HII IE indicates the cell for which the corresponding uplink HII is meant.

Downlink Interference Coordination

The IE Relative Narrowband Tx Power (RNTP) is defined in the LOAD INFORMATION message for interference coordination in the downlink.

RNTP indicates, per PRB, whether downlink transmission power is lower than the value indicated by the RNTP Threshold IE i.e., the sending eNB indicates if Downlink Tx power is higher or lower than a set threshold value. As shown in the figure above, the receiving eNB may take such information into account in scheduling its own cell-edge terminals and try not to schedule on the same PRBs to avoid interference.

Is it enough?

As discussed already, ICIC is limited to data channels and does not reduce interference on control channels. Moreover, Release-8/9 ICIC works well for homogeneous networks but it doesn’t provide significant gain in Heterogeneous Network (HetNet). This is due to the fact that ICIC has a limited Range Extension as it applies only to data channels and not to control channels where interference can remain significant.

ICIC has evolved to better support HetNets, especially interference control for downlink control channels. Enhanced ICIC (eICIC) was introduced in LTE Release-10 and Further enhanced ICIC (FeICIC) in Release-11. The major change is the addition of time domain ICIC. eICIC and FeICIC will be thoroughly discussed in a future post.

Reference: 3GPP TS 36.423, 36.300, and HetNet