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Interference Coordination in LTE/LTE-A (2): eICIC (enhanced ICIC)
August 06, 2014 | By Dr. Michelle M. Do and Dr. Harrison J. Son (tech@netmanias.com)
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Enhanced Inter-Cell Interference Coordination (eICIC)

 

As noted in the previous post about ICIC, we will find out about enhanced Inter-Cell Interference Coordination (eICIC), an interference control technology in LTE-A in this post. In LTE/LTE-A, one key challenge for operators is that they have to increase network capacity to keep up with fast-growing traffic. Especially, crowded areas in metropolitan cities have hotspots with extremely high traffic. For these hotspots, just reducing the size of macro cells is not quite enough to handle the high traffic. So, network operators want to increase the network capacity in a more economical way - by installing small cells.

 

Networks consisting of the same type of cells (e.g. existing macro networks), as presented in the previous post, are called homogeneous networks while ones with different types of cells are called heterogeneous networks (HetNet). So, HetNet is a network where small cells are deployed within a macro cell coverage. From Release 10 on, HetNet environments are also considered when discussing LTE-A standards.

 

Figure 1. Homogeneous network and heterogeneous network (HetNet)

 

What is eICIC?
eICIC is an interference control technology defined in 3GPP release 10. It is an advanced version of ICIC, previously defined in 3GPP release 8, evolved to support HetNet environments. To prevent inter-cell interference, ICIC allows cell-edge UEs in neighbor cells to use different frequency ranges (RBs or sub-carriers). On the other hand, eICIC allows them to use different time ranges (subframes) for the same purpose. That is, with eICIC, a macro cell and small cells that share a co-channel can use radio resources in different time ranges (i.e. subframes).

 

Two main features of eICIC are: Almost Blank Subframe (ABS) technology defined in Release 10 and Cell Range Expansion (CRE) technology defined in Release 11. ABS can prevent cell-edge UEs in small cells from being interfered with by the neighboring macro cell by having both cells still use the same radio resources, but in different time ranges (subframes). CRE expands the coverage of a small cell so that more UEs near cell edge can access the small cell. In this post, we will discuss ABS only.

 

Figure 2. eICIC technology: ABS

 

Problems with ICIC

First, you may wonder what issues ICIC had that made HetNet choose eICIC over ICIC. ICIC enables cell-edge UEs to use different frequency resources (RBs) in communicating, by having neighboring base stations exchange interference information with each other over X2 interface. This is effective in reducing inter-cell interference in an existing macro cell-based homogeneous network, but causes interference between control channels in a HetNet.   

 

When a base station communicates with a UE, each DL subframe of 1 msec consists of two periods - one for delivering control channel and the other for delivering data channel. ICIC can allocates different frequency resources to cell-edge UEs only when delivering data channels (Physical Downlink Shared Channel; PDSCH). Resource information allocated to UEs is delivered through control channels (Physical Downlink Control Channel; PDCCH). Here the thing is, unlike data channels, control channels are not delivered through different frequency ranges, but distributed across the entire channel bandwidth first and then delivered. This may cause UEs in neighbor cells to share the same frequency resources.

 

Figure 3. Control channel (PDCCH) and data channel (PDSCH)  

 

In a homogeneous network, this is not a big problem because there isn't much difference in Tx power from neighbor cells' antenna, and hence no significant inter-channel interference by control channels is caused between neighbor cells at cell edge. On the other hand, in HetNet where a macro cell has much higher Tx power than a small cell1, the small cell's control channel is inevitably interfered with by the macro cell's, making ICIC applied to the data channel ineffective.

 

Figure 4. Issues with ICIC in HetNet: Interference by macro cell's control channel

 

 

 

eICIC Concept: Problems with ICIC solved by having cells use radio resources in different time   

eICIC Operation: Delivering ABS pattern information over X2 interface

 

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ahmedbasim 2015-06-12 00:26:07

A nice article, starting from principles and giving a good summery on eICIC.

I wish you would make further article about FeICIC with more details on this new solution.

Artur Menezes 2015-11-03 11:38:18

Hi Netmanias...

This is an excellent article regarding Inter-Cell Interference. I wonder if you will deliver another post where would be discussed about the Cell Range Expansion (CRE).

 

BR, 

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