As mobile communication technology has evolved dramatically, from LTE (10 MHz) to LTE-A (10+10 MHz), and then to wideband LTE (20 MHz), South Korea's mobile market is hotter than ever with its big 3 operators competing fiercely in speed and quality (see Netmanias Report, LTE in Korea UPDATE - May 1, 2014). Operators can offer different maximum speeds depending on how wide frequency bandwidths they can actually use.  All three, with pretty much same amount of LTE frequency bandwidths obtained, practically support the same maximum speeds. 

However, these theoretical maximum speeds are not available to users in real life.  What users experience, i.e., Quality of Experience (QoE) is affected by various factors, and so the actual QoE is far from the maximum speeds. One of the biggest factors that causes such quality degradation is Inter-cell Interference.  

In 2G/3G networks, it was base station controllers, i.e., upper nodes of base stations, that control inter-cell interference.  In 4G networks like LTE/LTE-A, however, inter-cell interference can be controlled through coordination among base stations. This was made possible because now LTE networks have X2 interfaces defined between base stations. By exchanging interference information over these X2 interfaces, base stations now can schedule radio resources in a way that avoids inter-cell interference.¹ 

There are several Interference Coordination technologies in LTE and LTE-A: 

• LTE: Inter-Cell Interference Coordination (ICIC) 
• LTE-A: Enhanced ICIC (eICIC) which is an adjusted version of ICIC for HetNet, and Coordinated Multi-Point (CoMP) which uses Channel Status Information (CSI) reported by UE 

In this and next few posts, we will learn more about these Interference Coordination technologies. First, let's find out ICIC, the most basic interference coordination technology.

Inter-Cell Interference Coordination (ICIC)

What causes inter-cell interference? 
The biggest cause of lower mobile network capacity is interference. Interference is caused when users in different neighbor cells attempt to use the same resource at the same time. Suppose there are two cells that use the same frequency channel (F, e.g., 10MHz at 1.8GHz band), and each cell has a UE that uses the same frequency resource² (f_i, f_i∈F). As seen in the figure below, if the two UEs are located in cell centers like A2 and B2, no interference is caused because they use low power to communicate. However, if they are at cell edges like A1 and B1, their signals cause interference for each other because the two use high power to communicate. 

Interference is caused because cells only know what radio resources their own UEs are using, and not what other UEs in the neighbor cells are using. For example, in the figure above, Cell A knows what resources A1 is using, but not about what B1 is using, and vice versa. And the cells independently schedule radio resources for their own UEs. So, to the UEs at cell edges (A1 in Cell A and B1 in Cell B), same frequency resource can be allocated.

ICIC Concept

ICIC is defined in 3GPP release 8 as an interference coordination technology used in LTE systems. It reduces inter-cell interference by having UEs, at the same cell edge but belonging to different cells, use different frequency resources. Base stations that support this feature can generate interference information for each frequency resource (RB), and exchange the information with neighbor base stations through X2 messages. Then, from the messages, the neighbor stations can learn the interference status of their neighbors, and allocate radio resources (frequency, Tx power, etc.) to their UEs in a way that would avoid inter-cell interference. 

For instance, let's say a UE belonging to Cell A is using high Tx power on frequency resouce (f₃) at the cell edge. With ICIC, Cell B then allocates a different frequency resource (f₂) to its UE at the cell edge, and f₃ to its other UE at the cell center, having the one at the center use low Tx power in communicating. 

Interference Information used in ICIC 

Basic ICIC Behavior