Inter-Cell Interference Coordination(ICIC)

• As the term ICIC stands for, it is a type of technology which is designed to reduce the interference created by  two or more cells.

• As a UE moves away from the cell-center, SINR degrades due to two factors. Firstly, the received signal strength goes down as the path-loss increases with distance from the serving eNB. Secondly, the inter-cell interference (ICI) goes up because when a UE moves away from one eNB.

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 through using Inter-Cell Interference Coordination Schemes . 

• 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 th
• e cell center, having the one at the center use low Tx power in communicating.

How eNB reduce interference using Inter-Cell Interference Coordination Schemes ?

• Depending upon resource usage status and traffic load situation, three types of schemes have been defined to reduce interference.
  •  Hard Frequency Reuse
  •  Fracational Frequency Reuse
  •  Soft Frequency Reuse

Hard Frequency Reuse

• In this ICIC scheme, frequency is distributed among cell in multiple disjoint sets of frequency and each of these sets are assigned to individual eNBs in such a way that neighboring cell does not use the same set of frequencies. This reduces the interference at the cell edge of neighboring cell. While user interference is maximally reduced,  the spectrum efficiency drops by a factor equal to the reuse factor.

Fractional Frequency Reuse

• In Fractional frequency reuse (FFR) scheme, frequency resources are divided into three groups cell center, cell edge and blocked resources. Cell center resource is used as cell cell center frequency resource in neighboring cell but cell edge resources are blocked in neighboring cell. This scheme  is based on the concept of reuse partitioning. In reuse partitioning, the users with the highest signal quality use a lower reuse factor while users with low SINR use a higher reuse factor. Below is an example of FFR where the cell-center users have a reuse factor of 1 and cell-edge users have a reuse factor of
  • Example:f1 = 0-50 RBs, f2 = 51-60 RBs ,f3 = 61-80 RBs ,f4 =81-100 RBs and Remaining RBs of cell edge(Total Block RBs=(50-10)+(50-20)+(50-20)=30+30+40=100 RBs ) for particular eNB is blocked.
• RBs Allocation……

Soft Frequency Reuse

• In soft frequency reuse (SFR) scheme, all the frequency resources can be used in all the cells. The basic idea is that the frequency band used in the cell-center of one cell is used as the cell-edge frequency for neighboring cell. Below is an example of SFR where the cell-edge users have a reuse factor of 3.
  
  Example: f1 =0-25 ,f2 = 75-100 ,f3 = 26-74
• Since all the frequency resource is used in all the cells, the cell-edge users experience high interference. To reduce this interference, the cell-center users are allotted lower transmission power and the cell-edge users are allotted higher transmit power.

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: 

• 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).  
• 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. 
• 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.  
• 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.

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