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 (fi, fi∈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

eNBs exchange interference information of their cells with the neighbor eNBs by sending an X2 message (Load

Information message3) after each ICIC period. At this time, the message includes information like Relative Narrowband Tx

Power (RNTP), High-Interference Indicator (HII), and Overload Indicator (OI).

RNTP: Indicates frequency resources (RBs) that will be using high Tx power for DL during the next ICIC period. Power

strength of each RB is measured over the current ICIC period and shown in bits (0: low, 1: high). For example, the

strength can be averaged over the current ICIC period.

HII: Indicates frequency resources (RBs) that will be using high Tx power for UL during the next ICIC period, just like

RNTP, but for UL this time. RBs with high allocated power are used by UEs at cell edges, and thus are very likely to cause

interference for neighbor cells. The power strength of each RB is measured and shown in bits (0: low, 1: high).

OI: Indicates frequency resources (RBs) that have experienced most interference during the last ICIC period. Degree of

interference caused to each RB is measured and marked as Low, Medium or High.

RNTP and HII are information about interference to be caused by a cell to its neighbor cell. However, OI is information

about interference that has already been caused by the neighbor cell to the cell during the last ICIC period.

HII information is mandatory and serves as the most important information.

Basic ICIC Behavior

eNBs send interference information to neighbor eNBs after each ICIC period. In general, an ICIC period (ranging tens ~

hundreds of ms) is longer than a scheduling period, TTI (1 ms). Below is the illustration of an example that shows how

ICIC works. Here, the ICIC period of both Cell A and Cell B is 20 ms.

Basic operations of ICIC are:

❶ Generate interference information (ICIC period #11)

Cell A and B measure signal/interference strength during an ICIC period, and generate interference information

(RNTP, HII, OI).

❷ Share interference information (ICIC period #12)

Cell A and B share the interference information with neighbor cells through X2 message. X2 delay between neighbor

cells must be shorter than the ICIC period.

❸ Resource Coordination: ICIC calculation (ICIC period #12)

Both cells run an ICIC algorithm based on the neighbor cell's interference information received, and determine

frequency resources (RBs or sub-carriers) that will be used at cell edges, and thus will be using high Tx power.

❹ Coordinated resource-based local scheduling (ICIC period #13)

The results of ICIC calculation are applied to local schedulers. Based on coordinated resources, cells perform local

scheduling (i.e. allocating radio resources to the UEs accessed to them) depending on the channel status of each

UE.

With ICIC, each cell can carry out local scheduling based on resources coordinated with neighbor cells, consequently

reducing inter-cell interference. Next time, we will discuss eICIC, an adjusted version of ICIC for HetNet environment.

Footnotes

1. Over X2 interfaces, not only interference information, but also information on handover, resource status, neighbor

cells, etc., can be exchanged. However, only interference information is discussed here in this post.

2. Frequency resources are allocated in resource blocks (RBs). In this post, RBs (or sub-carriers) that are allocated to

UEi are referred to as fi.

3. 3GPP TS 36.423