LTE: Further enhanced Inter-Cell Interference Coordination: FeICIC

Cell Range Extension (CRE), Inter-Cell Interference Coordination (ICIC) and Enhanced Inter-Cell Interference Coordination (eICIC) were discussed in previous posts. FeICIC which is introduced in Release-11 will be discussed in detail in this post.

Introduction

The main idea behind CRE is to offload more traffic from macro cells to small cells and also to increase the HetNet efficiency. The range of the small cell is expanded by implementing a selection offset (UEs in idle mode) or handover threshold in measurement configuration (UEs in connected mode) in the favor of the small cell. A major problem is that, the UEs in CRE zone are forcibly being served by the small cell, but in reality, the downlink received power from the macro cell is higher than the small cell. So, there will be a severe downlink interference from the macro cell to UEs being served by the pico cell.

A number of features added to the 3GPP LTE specifications can be used to mitigate the above-mentioned interference problem in HetNets with small cells. Inter-cell interference coordination (ICIC) has the task to manage radio resources such that inter-cell interference is kept under control.

ICIC is introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. Only frequency domain ICIC was prioritized which manages radio resources, notably the physical resource blocks (PRBs), such that multiple cells coordinate use of frequency domain resources. Support for X2 signalling is added for the co-ordination between cells that belongs to two different eNBs. The frequency domain ICIC doesn’t provide significant gain in Heterogeneous Networks (HetNets). This is due to the fact that with ICIC the provided Range Extension is limited as it applies only to data channels and not to control channels where interference can remain significant.

The enhanced ICIC (eICIC) is introduced in 3GPP LTE Release-10 to deal with interference issues in HetNets, and mitigate interference on traffic and control channels. eICIC feature increases the coverage area of the victim cells without boosting downlink power. While ICIC coordinates inter-cell interference in the frequency and power domains, eICIC coordinates inter-cell interference in time, frequency and power domains. The major change in eICIC is the addition of time domain ICIC. Time domain ICIC is realized through the use of Almost Blank Subframes (ABS). ABSs are subframes with reduced transmit power including no transmission on some physical channels and/or reduced activity.

Further enhanced Inter-Cell Interference Coordination (FeICIC)

The eICIC scheme introduced in Release-10 did not address interference caused by cell-specific reference signals (CRS), synchronization signals, broadcast and paging messages as these signals are still transmitted by aggressor cell even during ABS. These signals and messages were necessary even during ABS in order to ensure backward compatibility with Release-8/9 UEs.

The major issue with eICIC is that, in CRE region the interference from CRS of aggressor cell (even during ABS). This makes demodulation of data (PDSH) and control channels/signals (PSS/SSS, PBCH, CRS) from victim cell (pico cell) very difficult.

ICIC is evolved in LTE 3GPP Release-11 to Further enhanced ICIC (FeICIC). The focus here is interference handling by the UE through inter-cell interference cancellation. With FeICIC the cell expansion is spread even further by increasing the biasing level from approximately 6dB to more than 9dB. Increase in bias, further extends small cell’s CRE therefore HetNet efficiency is improved.

FeICIC is mainly implemented at the UE side. The UE’s receiver first estimates the interfering signal and then removes/subtracts the interference from the received signal. In order for the UE’s receiver to apply interference cancellation, the UE needs to be assisted by the serving eNB with some information pertaining to the aggressor cell. For each interfering cell, the following CRS assistance information is signalled to the UE.

-   Physical Cell ID

-   Number of Antenna ports

-   MBSFN subframe configuration

With the help of CRS assistance information for each interfering cell, the UE can determine the location and the sequence of interfering CRS. MBSFN configuration is also important as there is no CRS transmitted in the data region in MBSFN subframes.

Furthermore, the UE may use CRS assistance information to mitigate CRS interference while performing RRM/RLM/CSI measurements. These measurements are discussed in detail in eICIC post.

RRC signaling support

In Release-11, a couple of new UE capabilities (shown below) are introduced to inform the eNB that the UE is capable of performing interference cancellation.

The UE’s support of interference handling for CRS is indicated by the parameter crs-InterfHandl whereas ss-CCH-InterfHandl indicates support for synchronization signal and common channel interference handling.

As discussed already, the eNB may send CRS assistance information of the aggressor cells to the UE to aid the UE to mitigate the interference from CRS of the aggressor cells. In Release-11, RadioResourceConfigDedicated may optionally include neighCellsCRS-Info-r11 which is used to send CRS assistance information of one or several aggressor cells as shown in the figure below.

SIB1 via dedicated RRC signalling

With FeICIC, the UE in CRE with 9dB bias, it is expected that the UE may not be able to decode SIB1 as the interference from aggressor cell is very high. One could argue that the interference is equally applicable for MIB, SIB1 and SIB2 which is the most important system information in LTE.

MIB is transmitted on PBCH in subframe#0 of every radio frame (including repetitions) so it encounters a strong interference from aggressor cell’s MIB and other signals. A UE supporting interference cancellation of common channels (UE indicates with ss-CCH-InterfHandl) could easily mitigate this interference problem.

SIB2 and all other SIBs (except SIB1) are scheduled within periodically occurring SI-windows using dynamic scheduling. So, the serving cell can easily schedule these SIBs in protected subframes (aggressor cell’s ABS).

SIB1 uses a fixed schedule. SIB1 and its repetitions are usually transmitted on PDSCH in subframe#5 of a radio frame for which SFN mod 2 = 0 (even radio frames). It is really difficult to avoid or cancel the interference when the subframe overlaps with a non-protected subframe. For this reason, in Release-11, the eNB may provide SIB1 to the UE in the CRE region by a dedicated RRC signaling. A new IE (shown below) systemInformationBlockType1Dedicated-r11 is introduced for this purpose.

Reference: 3GPP TS 36.300, 36.331, 36.101, 36.133, HetNet