LTE: Carrier Aggregation based ICIC

In a Heterogeneous network (HetNet), terminals in Cell Range Extension (CRE), zone would experience severe interference from the aggressor cells. Aggressor cell could be a macro cell in case of macro-pico or a femto cell in case of macro-femto scenario. A number of features added to the 3GPP LTE specifications 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 discussed in detail in the earlier post. It is introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. Moreover, only frequency domain ICIC was prioritized which manages PRBs, such that multiple cells coordinate use of frequency domain resources. The major problem here is the interference introduced by downlink control channels.

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. 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). For the detailed discussion, check the post eICIC.

Another approach is based on carrier aggregation (CA) with cross-carrier scheduling which is mainly frequency domain ICIC. The main difference as compared to frequency domain ICIC introduced in Release-8 is that the CA based ICIC would work with control channels (PCFICH, PHICH, and PDCCH) as well.

Carrier Aggregation based ICIC

Carrier aggregation (CA) is one of the most important LTE-Advanced features introduced in Release-10. With CA, two or more component carriers (CCs) are aggregated in order to support wider transmission bandwidths up to 100MHz. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities.

When CA is configured, the UE only has one RRC connection with the network. The serving cell managing the UE’s RRC connection is referred to as the Primary Cell (PCell). Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells.

In CA, a UE may be scheduled via PDCCH over multiple serving cells simultaneously. Cross-carrier scheduling with the Carrier Indicator Field (CIF) allows the PDCCH of a serving cell to schedule resources on another serving cell. In other words, a UE receiving a downlink assignment on one CC may receive associated data on another CC.

Cross-carrier scheduling is an important feature in HetNets where inter-cell interference is significant when the cells within HetNet are deployed on the same carrier frequency. It is discussed in detail in the post cross-carrier scheduling.

A number of HetNet deployment scenarios are presented by means of CA based ICIC. A promising approach is explained below by using a macro-pico example. The basic idea is to split the available spectrum into two downlink CCs denoted as CC1 (f1) and CC2 (f2), both CCs are available in both Macro and Pico layers. Macro layer configures PCell on f1 and SCell on f2 whereas, pico cell configures PCell on f2 and SCell on f1.

As shown in the figure above, three regions are of interest for control signalling (PCFICH, PDCCH, and PHICH); macro cell-center region, pico cell’s CRE region, and pico cell-center region. In the macro cell-center region, both f1 and f2 can carry control signalling. In the CRE region, macro-cell wouldn’t transmit control signalling on f2 i.e., scheduling assignments for SCell are carried by PCell on f1. So, interference caused by control signalling is minimized on f2 in CRE region.

Now, let us look at how pico cell is transmitting. Similar to macro cell, in the pico cell-center region, control signalling is transmitted on both f1 and f2. In the pico cell’s CRE region, pico cell would be transmitting control signalling only on f2 (PCell) and no transmission of control signalling on f1. i.e., scheduling assignments for SCell are carried by PCell on f2. This minimizes the interference in CRE zone which is caused by control signalling from pico cell on f1.

The downside with cross-carrier scheduling is that it will increase the load in the control region of the cell that is scheduling for another cell. This is due to the fact that the scheduling cell has to accommodate resource allocation for PDCCH for both PCell and SCell. In Release-11, a new channel known as Enhanced Physical Downlink Control Channel (EPDCCH) is introduced. This increases the control channel capacity as EDPCCH uses the same resources as PDSCH instead of control region. The EPDCCH could be used for resource allocation within each SCell without using cross-carrier scheduling. Moreover, by applying frequency domain ICIC, the reliability of receiving EPDCCH could be increased as in the case of PDSCH.

So far, the discussion was about control channels only. For data channel (PDSCH) both carriers (f1 and f2) are available in all three regions discussed above. The interference between macro and pico layers is handled by conventional ICIC method which is based on X2 signaling of RNTP between macro and pico eNBs as discussed in the post ICIC.

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

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

LTE: Inter-Cell Interference Coordination: ICIC

LTE uses frequency reuse factor of 1 in order to maximize spectrum efficiency. This inherently means that transmissions carried out on the same time-frequency resource will cause interference between different cells at cell-edges.

As shown in the figure below, in a macro cell network, a UE receives interference from the neighboring cell (red line) in addition to the desired signal from its serving cell (blue line). In addition, the same UE will create interference to the neighboring cell in the uplink. This interference has both control and traffic channel components.

 

In order to keep the inter-cell interference (ICI) under the control of radio resource management (RRM) methods, Inter-Cell Interference Coordination (ICIC) is introduced in 3GPP Release-8 specifications to mitigate interference on traffic channels only. ICIC is inherently a multi-cell RRM function that needs to take into account information (e.g. the resource usage status and traffic load situation) from multiple cells.

In Release-8 the frequency domain ICIC is prioritized which manages radio resources, notably the radio resource blocks, such that multiple cells coordinate use of frequency domain resources. More specifically, the focus was to define X2 signalling that could be used for the co-ordination between cells that belongs to two different eNBs. The X2AP message used for ICIC purpose is called LOAD INFORMATION.

An eNB transmitting LOAD INFORMATION message to eNBs controlling intra-frequency neighbouring cells includes CellInformation for 1 or several cells. The CellInformation structure is given below.

Uplink Interference Coordination

In Release-8, two IEs are defined in LOAD INFORMATION message to assist uplink ICIC; the Overload Indication (OI) and High Interference Indication (HII) as shown in the figure below.

The UL Interference Overload Indication IE received in the LOAD INFORMATION message indicates the interference level experienced by the indicated cell on all resource blocks, per PRB. For each PRB, one of three levels of interference (High, Medium, Low-interference) is indicated. The receiving eNB would take this OI information into account when setting its scheduling policy to improve the interference situation for the eNB which has sent this OI.

The uplink High Interference Indication IE received in the LOAD INFORMATION message indicates, per PRB, the possibility of high sensitivity to interference as seen by the sending eNB. The receiving eNB should try to avoid scheduling cell edge UEs in its own cells for the concerned PRBs. This reduces Uplink interference to cell-edge transmissions in its own cells as well as in the cells of eNB from which HII was received. The Target Cell ID IE received within the HII IE indicates the cell for which the corresponding uplink HII is meant.

Downlink Interference Coordination

The IE Relative Narrowband Tx Power (RNTP) is defined in the LOAD INFORMATION message for interference coordination in the downlink.

RNTP indicates, per PRB, whether downlink transmission power is lower than the value indicated by the RNTP Threshold IE i.e., the sending eNB indicates if Downlink Tx power is higher or lower than a set threshold value. As shown in the figure above, the receiving eNB may take such information into account in scheduling its own cell-edge terminals and try not to schedule on the same PRBs to avoid interference.

Is it enough?

As discussed already, ICIC is limited to data channels and does not reduce interference on control channels. Moreover, Release-8/9 ICIC works well for homogeneous networks but it doesn’t provide significant gain in Heterogeneous Network (HetNet). This is due to the fact that ICIC has a limited Range Extension as it applies only to data channels and not to control channels where interference can remain significant.

ICIC has evolved to better support HetNets, especially interference control for downlink control channels. Enhanced ICIC (eICIC) was introduced in LTE Release-10 and Further enhanced ICIC (FeICIC) in Release-11. The major change is the addition of time domain ICIC. eICIC and FeICIC will be thoroughly discussed in a future post.

Reference: 3GPP TS 36.423, 36.300, and HetNet

LTE: Cell Range Extension (CRE)

In a Heterogeneous Network (HetNet) introduced in 3GPP Release 10, low power nodes (LPNs) such as RRUs/RRHs, Pice cells, femto cells, and relay nodes are deployed in a macro cell.

          Small cells are primarily added to increase capacity in hot spots with high user demand and to fill in areas not covered by the macro network – both outdoors and indoors. They also improve network performance and service quality by offloading from the large macro-cells. The result is a HetNets with large macro-cells in combination with small cells providing increased bitrates per unit area.

Cell Range Extension

A major issue in Hetnet planning is to ensure that the small cells actually serve enough users. One way to do that is to increase the area served by the small cell, which can be done through the use of a positive cell selection offset to the the received power of the small cell. Extending the coverage of a cell by means of connecting a UE to cell that is weaker than the strongest detected cell is referred to as cell range extension.

In a network, with all cells using the same frequency (frequency reuse = 1), the UE normally camps on a cell with strongest received DL signal (SS_DL­­­); hence the border between two cells is located at the point where SS_DL is the same for both the cells. In homogeneous networks, this also typically coincides with the point of equal path loss for the UL (PL_UL) in both cells.

In a Hetnet, macro cell transmits at a very high power as compared to LPN. So, as shown in the figure below, there are two borders, a Downlink cell border which is defined by SS_DL­­­ and an Uplink cell border which is defined by PL_UL. In the Downlink, SS_DL observed from the macro cell and the pico cell are equivalent at a location that is closer to the pico cell, which forms the Downlink cell border. On the other hand, the location where path loss to macro and pico cells is equivalent is far from the pico cell. This is due to the fact that the UE transmission power is same whether the UE is being served by macro cell or pico cell.

 

In a normal case without CRE feature, the serving cell choice for the UE is based on the DL received power. With CRE, a positive cell selection offset is added to the SS_DL of the small cell to increase the range served by small cell. So, more UEs are attracted by small cell as compared to the case without CRE which greatly increases HetNet efficiency.

          In CRE (gray region) in the figure above, the UE does not necessarily be served by the eNB that has the strongest DL received power.

In addition to the advantages of increasing HetNet efficiency, offloading macro cell’s traffic, CRE has another advantage of reduced Uplink interference in the system. A UE without CRE, served by the macro cell closer to the pico cell border, would be transmitting at a higher power and interfering with the uplink of small cell. With CRE, the same UE is likely to be served by closer cell (small cell) with lower path loss reducing the need for the UE to transmit with such a high power.

As discussed already, 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.

UEs in Connected Mode

A connected mode UE may be configured with cellIndividualOffset as defined within measObjectEUTRA corresponding to the frequency of the neighbor cell. Measurement events that are influenced by this parameter are A3, A4, A5 and A6.

This parameter is applied individually to each neighbor cell with load management purposes. The higher the value allocated to a neighbor cell, the “more attractive” the cell will be.

As an example for event A3, (not considering other offsets and hysteresis), the event entering condition is fulfilled when;

 where M_n and M_{s ­­­}are measurement result of the neighboring and serving cells respectively. So, the more the value of cellIndividualOffset, the early the UE sends measurement report indicating neighbor cell (small cell) is better than the serving.

UEs in Idle Mode

In Idle mode, mainly, parameter q-OffsetCell within IntraFreqNeighCellInfo (in SIB4) and within InterFreqNeighCellInfo (in SIB5) used for cell range extension purpose. This parameter is mainly used in cell-ranking criterion for neighbor cell.

Drawbacks

Having discussed the benefits of CRE feature, a major drawback of this feature is that in the CRE zone, the UEs 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 severe downlink inter-cell interference from the macro cell to UEs receiving transmissions from a pico cell i.e., the SINR at the UEs in CRE which is being served by small cell is below 0 dB. This may impact the reception of the DL control channels in particular.

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. These features will be discussed in detail in further posts. Usually CRE is jointly designed with ICIC/eICIC/FeICIC schemes/features to mitigate this interference problem.

Reference: 3GPP TS 36.300, 36.331, HetNet