LTE: Enhanced Inter-Cell Interference Coordination: eICIC

Cell Range Extension (CRE) and Inter-Cell Interference Coordination (ICIC) were discussed in detail in previous posts.

Introduction to ICIC

ICIC was introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. Only frequency domain ICIC was prioritized which manages radio resource, notably the physical resource blocks (PRBs), such that multiple cells coordinate use of frequency domain resources.

More specifically, focus at that time was to define X2 signalling that could be used for the co-ordination between cells that belongs to two different eNBs. The X2AP message LOAD INFORMATION carriers the information required for the purpose of ICIC. For uplink ICIC, two IEs Overload Indication (OI) and High Interference Indication (HII) are defined whereas for downlink ICIC purpose the IE Relative Narrowband Tx Power (RNTP) is defined.

The frequency domain ICIC feature 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.

Enhanced Inter-Cell Interference Coordination (eICIC)

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. eICIC is especially important when Carrier Aggregation (CA) is not used.

While ICIC coordinates inter-cell interference in the frequency and power domains, eICIC coordinates inter-cell interference in time domain in addition to frequency and power domains.

The major change in eICIC is the addition of time domain ICIC. With time domain ICIC, a CRE UE may continue to be served by a victim cell (i.e., the weaker cell) even while under strong interference from aggressor cells (i.e., the stronger cell) i.e., time domain ICIC significantly improves CRE thereby helping for traffic offloading from a macro cell to a pico cell and increasing HetNet system efficiency.

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. ABS does not carry any data (PDSCH) and thus no corresponding control information (PDCCH, EPDCCH, PCFICH, and PHICH). In order to ensure backward compatibility with 3GPP Release-8/9 UEs, all the necessary signals have to be transmitted even in ABS. So, ABS contains the necessary signals with low power. These signals include cell reference signals (CRS), synchronization signals (PSS/SSS), broadcast (SIB1) and paging messages.

An aggressor cell (macro cell) will transmit ABS to protect resources in subframes in the victim cell (small cell) receiving strong inter-cell interference i.e., if the victim cell schedules its UEs in subframes that overlap with aggressor cell’s ABS, the victim cell “protects” its UEs from strong inter-cell interference.

As shown in the figure below, a macro-eNB will transmit ABS according to a semi-static pattern. During these subframes, UEs at the cell-edge, typically in the CRE region, can receive downlink information, both control and user data from victim cells. In other words, the UEs in aggressor cell won’t get any data during ABS and the UEs in victim cell CRE region may not be scheduled during aggressor cell's non-ABS. Note that the UEs in victim cell's central region (not in CRE) may may still receive data in all the subframes irrespective of whether or not aggressor cell is transmitting ABS, as the interference is not that significant.

 

The macro-eNB will inform the eNB in the small cell about the ABS pattern as shown in the figure above. The X2AP message LOAD INFORMATION is used by the macro-eNB to inform the eNB of small cell about the ABS pattern being used. The IE ABS Information included in the LOAD INFORMATION message mainly consists of ABS Pattern Info IE and it provides information about which subframes the macro eNB is configuring as ABS. The small cell may take such information into consideration when scheduling its UEs. ABN Pattern Info is a 40-bit string, each bit indicating a subframe, for which value ‘1’ indicates ABS and ‘0’ indicates normal subframe. This pattern repeats every 40 subframes in the case of FDD, whereas for TDD, periodicity depends upon the UL/DL configuration.

The macro-eNB also includes the IE Measurement Subset in ABS Information.  The eNB of small cell may use the Measurement Subset information for the configuration of specific measurements towards its UEs. This is 40 bit string indicating a subset of the ABS pattern explained above. This subset is a recommendation from macro cell to pico cell for configuring measurement resource restrictions for RRM/RLM/CSI measurements towards a UE.

The message structure of ABS Information is shown below


The eNB cannot configure MBSFN subframes as ABSs when these MBSFN subframes are used for other usages (e.g.,MBMS, LCS).

Time domain measurement resource restrictions for the UE

Obviously, the interference experienced by a UE connected to the victim cell in CRE region may vary significantly between protected (aggressor cell’s ABS) and non-protected subframes. A UE may add measurements from the protected to the measurements from non-protected subframes for averaging purpose. So, it is important to restrict the UE’s measurements in specific subframes which is called as measurement resource restrictions.

First of all, a UE (Release-10) should signal its support of time domain ICIC in Feature Group Indicator (FGI) #115 which is included in UE Capability Information message. This FGI informs the eNB that the UE is capable of the following;

- Time domain ICIC RLM/RRM measurement subframe restriction for the      serving cell

- Time domain ICIC RRM measurement subframe restriction for neighbor      cells

- Time domain ICIC CSI measurement subframe restriction

It is the job of victim cell to identify all those UEs in CRE region and inform those UEs via dedicated RRC signalling about protected subframes. For this purpose, a MeasSubframePattern similar to ABS Pattern (40-bit pattern in case of FDD) is sent to the UE. The IE MeasSubframePattern is used to specify time domain measurement resource restriction.

There are 3 different kinds of measurement restriction patterns as explained below;

Firstly, a single pattern for RRM/RLM measurements for the PCell. A release-10 IE measSubframePatternPCell (40-bit pattern) represents "time domain measurement resource restriction pattern for the PCell measurements (RSRP, RSRQ and the radio link monitoring)”.  In a macro-pico scenario for example, the pico UE may be configured with this resource restriction so that RRM/RLM measurements performed by the pico UE is limited to ABS of the macro cell. Otherwise, RLM measurements on the pico cell subframes that coincide with macro-cell’s non-ABS subframes which contain high interference could cause the pico UE to trigger unnecessary radio link failure procedure.

Secondly, a single pattern for RRM measurements in the neighbor cells operating in the same carrier frequency as the PCell. The Release-10 IE measSubframePatternNeigh represents “time domain measurement resource restriction pattern applicable to neighbor cell RSRP and RSRQ”. The eNB may also include a list of cells for which measSubframePatternNeigh is applicable in the IE MeasSubframeCellList. In a macro-pico scenario for example, these restrictions are configured with a list of pico neighbor cells so that the UE measures that pico cells accurately in the non-interfering subframes. For neighboring cells that do not belong to this list (MeasSubframeCellList) the UE could choose any subframe for measurement.

Last but not least, resource restriction for CSI measurements of the PCell which is discussed in detail in the following section.

Resource Restricted CSI measurements

Channel state information (CSI) measurement feedback which consists of CQI, PMI, PTI, and/or RI from the UE is used by the eNB for scheduling and link adaptation. The UE may average the channel and interference estimates over multiple subframes to derive CSI.

The interference levels experienced by the UEs in CRE in protected and non-protected subframes are significantly different. So average channel estimates across protected and non-protected subframes doesn’t provide the eNB with accurate channel status. To overcome this issue, the pico eNB may configure a UE with two subframe subsets so that the UE is forced to perform CSI measurements in specific subframes.

As shown the figure below, two subframe subsets (csi-MeasSubframeSet1 and csi-MeasSubframeSet2) are configured per UE. The UE reports CSI for each configured subframe subsets seperately. It is up to the network how to choose the two subframe subsets but typically the two subframe subsets are chosen such that one subset selects the subframes from the ABSs and the other one from the non-ABSs.

The UE reports CSI for each subset separately; hence the UE should only average measurements from subframes belonging to the same subset. 3GPP 36.213 mandated that any given subframe should only belong to one subset but not to both. Also, the UE is not expected to perform CSI measurements in a subframe that doesn’t belong to either subframe set.

For periodic CSI reporting, the configuration is done for each subset. So, based on the subframe where CSI report is received, the eNB can understand the corresponding subset to which the received CSI report is concerned. The existing Release-8/9 IEs cqi-pmi-ConfigIndex and ri-ConfigIndex are used for configuring periodic CSI reports for setset1. A couple of new IEs cqi-pmi-ConfigIndex2 and ri-ConfigIndex2 are defined for configuring periodic CSI reporting for subset2. It should be noted that the eNB sends this configuration only if csi-SubframePatternConfig is configured (see the picture above). For aperiodic CSI reports, the UE reports CSI based on the subframe subset containing the CSI reference resource.

CSG Scenario

In a macro-femto scenario, dominant interference condition may happen when non-member UE is in close proximity of a CSG cell. Sometimes, the signal from CSG cell may be stronger than the serving macro cell.

Time domain ICIC may be used to allow such non-member UEs to remain served by the macro cell on the same frequency layer. Such interference may be mitigated by the CSG cell's ABS to protect the corresponding macro cell’s subframes from the interference. A non-member UE may be signaled to utilize the protected resources for RRM/RLM/CSI measurements for the serving macro cell.

As there is no X2 interface in the case of macro-femto scenario, OAM configuration is used to configure ABS to the CSG cells.

Further enhanced ICIC (FeICIC)

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.

ICIC and eICIC 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 for control signals, enabling even further cell range extension.

FeICIC will be thoroughly discussed in a future post.

Reference: 3GPP TS 36.423, 36.300, 36.331, 36.213 and HetNet