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.
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).
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.
It’s essentially time-domain-based ICIC in 3gpp Rel-10 (LTE-A) specifications & the overall objective of eICIC is to mute certain sub frames of one layer of cells so that the interference becomes less in the other layer.
Interference Scenarios in LTE-A HetNets
Heterogeneous network layouts introduce basically two predominant inter-cell interference scenarios. One is the macro-pico scenario with cell range expansion (CRE) depicted in left of Figure 1.0 the other is the macro-femto scenario with closed subscriber groups (CSG) depicted in right of Figure 1.0.
- Macro-pico deployments with UEs operating in cell range expansion
- Nominally, a UE associates with a base station with strong DL SINR
- With cell range expansion, a UE can associate with a low power eNB
- The DL SINR can be much lower than 0 dB in cell range expansion
- UEs in close proximity to CSG femto cells if UE is not allowed to connect to
- This results in strong interference from the CSG cell
Interference Scenarios in LTE-A heterogeneous networks (HetNets).
The range expansion bias (REB) allows the heterogeneous networks (HetNets) to offload more number of users from a macrocell to its overlaid picocells. However, the offloaded users that are now in the range expansion region of picocell would experience a high interference from the macro basestation (MBS). Enhanced inter-cell interference coordination (eICIC) mechanism is introduced in LTE Rel.-10 which is illustrated in the Fig. 1.1.
The MBS does not transmit data during certain subframes. (For example, MBS subframes 2, 6 and 9 in Fig. 1). The picocell users does not experience interference from the MBS during these subframes (coordinated subframes). Therefore, the picocell users (PUEs) in the range expanded region (which are prone to the high interference from MBS) can be scheduled to the CSFs, while the other PUEs can be scheduled to the USFs.The main objective of this research topic is to develop the analytic model using the stochastic geometry tools and derive expressions for
eICIC Techniques
These techniques can be classified mainly as
- Frequency domain &
- Time domain
Frequency Domain Solution:
- Carrier Aggregation (CA): The idea here is to divide the spectrum into two parts to be used by the macro cell and small cell, each of which uses its part of the spectrum to transmit its control channels to avoid interference. The standard allows each cell layer to use either part of the spectrum to communicate with the mobile through a fast cross-carrier scheduling process. The mobile unit must support Release 10 features to take advantage of this technique.Implementation of carrier aggregation can take on several forms. For instance, there’s the choice of intra-band CA where both carriers are in the same band and are either contiguous (lowest complexity and flexibility) or non-contiguous, and inter-band CA where the two carriers can be in two completely different bands (highest complexity and flexibility option). Evolution of Carrier Aggregation also allows for un-symmetric allocation of carriers which allows for instance higher downlink bandwidth.
- It’s essentially a Cross Carrier Scheduling which is used in 3gpp Rel-10 CA. Cross-carrier scheduling is only used to schedule resources on an Scell without PDCCH.
- PCell cannot be cross scheduled; it is always scheduled through its own PDCCH
Time-Domain Multiplexing (TDM):
- While carrier aggregation aims at isolating interference by segmenting spectrum, TDM techniques are targeted at using the same frequency channel on all layers of the HetNet while leveraging the time domain to manage interference between the layers.
- TDM techniques feature the capability to suppress transmissions in certain sub-frames of the aggressor base station. This reduces interference and allows the victim base station to schedule transmissions during these quiet sub-frames. However, transmission suppression is incomplete; rather some control signaling will continue to be broadcast for backward compatibility. So, this technique is referred to as ‘Almost Blank Subframe’ (ABS) and will take full form in LTE Release 11 when mobiles will be able to apply interference suppression to better receive control signaling from the victim (low-power) base station. ABS results in a base station losing capacity, but this is acceptable as ABS allows more small cells to be deployed.
- The overall objective here is to mute certain sub frames of one layer of cells so that the interference becomes less in the other layer.
- These muted sub frames are called Almost Blank Sub frames (ABS)
- This approach involves periodically muting the transmissions of entire sub-frames from nodes that cause harmful interference onto others; in this way nodes affected by high interference can serve their subscribers in these sub-frames.
- In a ABS, no unicast PDSCH and PDCCH is transmitted
- Since most REs are blank (zero power), interference is reduced.
- To ensure backward compatibility, following signals are transmitted
- CRS (pilot signal)
- PSS/SSS (synchronization signals)
- SIB1/MIB (broadcast information)
- CRS/PSS/SSS/SIB1/MIB can still cause strong interference in certain PRBs/REs
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.
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.
Activation of ABS MODE
Once the macro station knows which femto nodes are generating high interferences on the victims UEs, the ABS transmission mode in these femto nodes must be activated. In this process, coordination between the macro station and the femto nodes of the network is essential. The macro station not only has to notify the femto nodes selected as aggressors that they must operate in ABS mode, but also must specify which ABS pattern should be followed.
The macro station decides which ABS pattern will be used taking into account some parameters such as the number of victim UEs, the level of interference at the victim UEs, their locations in the cell and their requested services,
or in other words, their input load. It is also important to consider the impact that the selected pattern will have on the performance of the aggressors femto nodes, since the throughput of the femto cells selected to operate in the ABS mode will be degraded due to the blanked sub-frames.
Once the macro station knows which femto nodes are generating high interferences on the victims UEs, the ABS transmission mode in these femto nodes must be activated. In this process, coordination between the macro station and the femto nodes of the network is essential. The macro station not only has to notify the femto nodes selected as aggressors that they must operate in ABS mode, but also must specify which ABS pattern should be followed.
The macro station decides which ABS pattern will be used taking into account some parameters such as the number of victim UEs, the level of interference at the victim UEs, their locations in the cell and their requested services,
or in other words, their input load. It is also important to consider the impact that the selected pattern will have on the performance of the aggressors femto nodes, since the throughput of the femto cells selected to operate in the ABS mode will be degraded due to the blanked sub-frames.
eICIC Concept: Problems with ICIC solved by having cells use radio resources in different time :
As seen above, in a HetNet, even ICIC cannot completely prevent control information of UEs in a small cell from being significantly interfered with by its neighbor macro cell. To avoid this inter-cell interference, ICIC effect through inter-cell cooperation should be obtained through taking advantage of time domain, instead of frequency domain. That’s why eICIC was introduced. The basic idea of eICIC is that it allows a macro cell and its neighbor small cells to deliver data, subframe by subframe, by using different time ranges. So, when communicating with cell-edge UEs, small cells use the subframes that are not used by their neighbor macro cell, avoiding interference by the macro cell. Of course, when communicating with UEs at cell centers, the small cells can use any subframe available whether the macro cell is delivering data or not at the time.
eICIC Operation: Delivering ABS pattern information over X2 interface
A base station and UE exchange data in radio frames, and one radio frame consists of 10 subframes. The decision on how many ABS subframes are to be included in a radio frame is made based on traffic load and network operators’ policy. In the figure below, a macro base station decides which subframes will carry data (“0”) and which ones will not (“1”), and then saves it as ABS pattern information (e.g. “0011000110”). Then, it prepares a Load Information message,2 and send it to a small cell base station over X2 interface. Upon receiving this message, the small cell learn from the pattern which subframes are to be used by the macro cell, and deliver data to cell-edge UEs through ABS subframes only.
Below is the result of LTE-A eICIC field test conducted by Softbank in March 2014. The figure shows a case where ABS ratio of macro cell to small cell is 5:5. The graph in the upper right displays radio resource allocation (macro cell in blue and small cell in purple), and the one in the bottom shows cell throughputs of macro cell (in blue), small cell with eICIC (in yellow), and small cell without eICIC (in purple). From this, we can tell the small cell throughput has been improved when eICIC was employed. Increased number of small cells or higher ratio of ABS in a macro cell (e.g. 7:3) will result in even higher throughput in small cells.
Deactivation of ABS MODE
When the SINR level of the victim UE rises to the target SINR level during the non-ABS; that is to say, when a victim UE reports a CQI feedback above the CQI threshold, the macrostation has to trigger the femto node, which was causing interference in the UE, to deactivatethe ABS transmissions. This procedure is symmetrical as the described for activating the ABS mode.
When the SINR level of the victim UE rises to the target SINR level during the non-ABS; that is to say, when a victim UE reports a CQI feedback above the CQI threshold, the macrostation has to trigger the femto node, which was causing interference in the UE, to deactivatethe ABS transmissions. This procedure is symmetrical as the described for activating the ABS mode.
The X2AP protocol provides the following functions :
X2 Application Protocol (X2-AP) is used to co-ordinate handovers and perform load management between eNodeB (Evolved Node B) network elements – Source eNodeB and Target eNodeB and here i will discuss over Load Management and know the behaviour of ABS pattern during eICIC in detail.
Mapping between X2AP functions and know the detail how ABS pattern is enabled and map to another eNB through Load Management.
a)Resource Status Reporting Initiation
When first time ABS Status IE is set or in which situation eNB1 indicate ABS pattern to eNB2. ?
This procedure is used by an eNB to request the reporting of load measurements to another eNB.
RESOURCE STATUS REQUEST message sent from eNB1 to eNB2. Upon receipt, eNB2 shall initiate the requested measurement according to the parameters given in the request in case the Registration Request IE set to “start” and shall stop all cells measurements and terminate the reporting in case the Registration Request IE is set to “stop”.
If the Registration Request IE is set to “start” then the Report Characteristics IE shall be included in RESOURCE STATUS REQUEST message.The Report Characteristics IE indicates the type of objects eNB2 shall perform measurements on.
The ABS Status IE, if the fifth bit, “ABS Status Periodic” of the Report Characteristics IE included in the RESOURCE STATUS REQUEST message is set to 1 and eNB1 had indicated the ABS pattern to eNB2.
If the Reporting Periodicity IE is included in the RESOURCE STATUS REQUEST message, eNB2 shall use its value as the time interval between two subsequent measurement reports.
If eNB2 is capable to provide all requested resource status information, it shall initiate the measurement as requested by eNB1, and respond with the RESOURCE STATUS RESPONSE message.
Below is the IE of RESOURCE STATUS REQUEST or Resource Status Reporting Initiation
Message Type = 9
eNB1 Measurement ID INTEGER (1..4095,…)
eNB2 Measurement ID INTEGER (1..4095,…) (C-ifRegistrationRequestStop – This IE shall be present if the Registration Request IE is set to the value “stop”.)
Registration Request IE : ENUMERATED(start, stop,…) (A value set to “stop”, indicates a request to stop all cells measurements.)
If the Registration Request IE is set to “start” then the Report Characteristics IE
shall be included in RESOURCE STATUS REQUEST message.Upon receipt, eNB2 shall
initiate the requested measurement according to the parameters given in the request in case the
Registration Request IE set to “start” and shall stop all cells measurements and terminate the
reporting in case the Registration Request IE is set to “stop”.
Report Characteristics: BITSTRING(SIZE(32)) [ Each position in the bitmap indicates measurement object the eNB2 is requested to report.
First Bit = PRB Periodic,
Second Bit= TNL load Ind Periodic,
Third Bit = HW Load Ind Periodic,
Fourth Bit = Composite Available Capacity Periodic,
Fifth Bit = ABS Status Periodic.
Other bits shall be ignored by the eNB2 ]
Cell To Report : [Cell ID list for which measurement is needed]
>Cell To Report Item = 1 .. <maxCellineNB>
>>Cell ID : UTRAN Cell Global Identifier (ECGI) = [PLMN Identity, > Macro eNB ID ,>Home eNB ID]
Reporting Periodicity : ENUMERATED(1000ms, 2000ms, 5000ms,10000ms, …)
Partial Success Indicator: ENUMERATED(partial success allowed, …) Included if partial success is allowed.
b) Resource Status Reporting or RESOURCE STATUS UPDATE
This procedure is initiated by eNB2 to report the result of measurements admitted by eNB2 following a successful Resource Status Reporting Initiation procedure.
If the eNB1 receives a RESOURCE STATUS UPDATE message which includes the ABS Status IE, and all bits in the Usable ABS Pattern Info IE are set to ‘0’, the eNB1 shall ignore the DL ABS Status IE.
The admitted measurements are the measurements that were successfully initiated during the preceding Resource Status Reporting Initiation procedure,
This message is sent by eNB2 to neighbouring eNB1 to report the results of the requested measurements.
Note:
Radio Resource Status:
Abnorma conditions:
If the eNB1 receives a RESOURCE STATUS UPDATE message which includes the ABS Status IE, and all bits in the Usable ABS Pattern Info IE are set to ‘0’, the eNB1 shall ignore the DL ABS Status IE
c)Load Indication
The purpose of the Load Indication procedure is to transfer load and interference co-ordination information between eNBs controlling intra-frequency neighboring cells.
UL Interference Overload Indication it indicates the interference level experienced by the indicated cell on all resource blocks, per PRB . value valid until reception of a new LOAD INFORMATION
UL High Interference Indication , it indicates, per PRB, the occurrence of high interference sensitivity, as seen from the sending eNB. The receiving eNB should try to avoid scheduling cell edge UEs in its cells for the concerned PRBs.
Relative Narrowband Tx Power (RNTP) it indicates, per PRB, whether downlink transmission power is lower than the value indicated by the RNTPThreshold IE. The receiving eNB may take such information into account when setting its scheduling policy
ABS Information IE IE indicates the subframes designated as almost blank subframes by the sending eNB for the purpose of interference coordination
Measurement Subset IE for the configuration of specific measurements towards the UE
ABS pattern info IE the receiving eNB shall consider that the subframe is designated as almost blank subframe by the sending eNB
the Invoke Indication IE , it indicates which type of information the sending eNB would like the receiving eNB to send back.
Invoke Indication IE is set to “ABS Information”
If the Invoke Indication IE is set to “ABS Information”, it indicates the sending eNB would like the receiving eNB to initiate the Load Indication procedure, with the LOAD INFORMATION message containing the ABS Information IE indicating non-zero ABS patterns in the relevant cells.
This message is sent by an eNB to neighbouring eNBs to transfer load and interference co-ordination information.
Direction: eNB1 -> eNB2.
Call flow of ABS pattern exchange between Aggressor and Victim node
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