Patent Application
20160211955
The present invention relates to methods of interference management in a heterogeneous network (HetNet) system with densely deployed small cells, and in particular, exchanging information of subframe utilization between enhanced Node B's (eNBs) for interference coordination of user equipments.
A traditional high power base station can create a relatively large area of coverage that can be referred to as a macro cell. Several different types of low power nodes (LPN) have been developed that have a smaller area of coverage than a macro cell and that are intended to improve the quality and reliability of the service UE received in this area. These low power base station nodes include remote radio heads, pico eNBs, home eNBs (HeNBs), and similar components. The term eNB, in LTE networks, refers to an “evolved Node B” which is based off of the concept of a “Node B,” an element of a 3G UMTS cellular network. A radio access base station that creates a macro cell might be referred to herein as a macro eNB. The area of coverage created by a low power base station might be referred to as a pico cell, a femto cell, a hotzone cell, a small cell, or a similar term. To be general, the term small cell is used herein to refer the coverage created by those LPNs. The term low power node or small cell eNB is used interchangeably herein.
Heterogeneous networks are one of many features in LTE Advanced (LTE-A). Heterogeneous network deployments consist of deployments where low power nodes are placed throughout a macro-cell layout. However, it's also possible to deploy standalone low power nodes without macro base stations. Heterogeneous network deployments are being increasingly utilized to address the limitations of traditional homogeneous network deployments, particularly the occurrence of blind spots in coverage or limited network capacity. Employing a HetNet system can provide coverage in indoor areas such as shopping malls or subways, which would receive little or no cellular coverage in a traditional homogeneous macro cell network.
One difference between low power nodes and macro eNBs is the transmit power. For example, the maximum allowable transmit power of a macro eNB is typically 46 dBm, a remote radio head/pico is typically 30 dBm, and a HeNB is typically 20 dBm, for a 10 MHz carrier bandwidth. Another difference between low power nodes is the connection to the core network (i.e. backhaul) and the connection to neighbor cells. For example, a pico cell is a small cellular base station with a dedicated backhaul connection to the service provider's core network (e.g, through S1 interface in 3GPP) and other pico/macro cells (e.g., through X2 interface in 3GPP). A femto cell, on the other hand, is a small cellular base station, typically designed for use in a home or small business. It connects to the service provider's network via broadband (such as DSL or cable). Typically, the HeNB has no direct interface (e.g, X2 interface in 3GPP) to other femto cells or any Macro eNodeB (MeNB).
Although HetNet offers opportunities to enable large-scale, low-cost deployment of small base stations below rooftops to improve user experience at hotspots or to improve the network coverage, it introduces new challenges to inter-cell interference coordination within the same channel as well as load balancing and user mobility in idle and active mode.
In accordance with some implementations, a method of reducing interference between different cells in a densely populated cell network at an ABS allocation controller includes sending an interference estimation request to at least one of a plurality of cells communicatively coupled with the ABS controller to report estimated interference from its neighboring one or more cells. The ABS controller receives an interference estimation response from the one or more cells of the plurality of cells. The ABS controller receives traffic load information from each cell communicatively coupled with the ABS controller. The ABS controller analyzes all the received interference estimates and traffic load information to determine almost blank subframe allocation at one or more interfering cells of the plurality of cells, and sends an ABS allocation command to one or more interfering cells of the plurality of cells.
In accordance with some implementations, a method of managing communication interference in a heterogeneous network includes an interfered cell receiving information from one or more user equipments reporting interference from at least two other cells. In some embodiments, in response to receiving the interference reports, the interfered cell transmits to the two or more interfering cells that there is interference being experienced by user equipments of the interfered cell. A first of the two or more interfering cells determines a first set of one or more data structures, each representing an almost blank subframe pattern of downlink subframes available for the interfered cell to schedule its user equipments, based on the interference information reported by the interfered cell. A second of the two or more interfering cells determines a second set of one or more data structures, each representing an almost blank subframe pattern of downlink subframes available for the interfered cell to schedule its user equipments, based on the interference information reported by the interfered cell. The interfered cell receives from the first interfering cell, the first set of one or more data structures, and receives from the second interfering cell the second set of one or more data structures. The interfered cell compares the almost blank subframe patterns of the first set and second set and selects at least one of the first set of almost blank subframe patterns and selects at least one of the second set of almost blank subframe patterns. The interfered cell transmits to the first interfering cell the selection from the first set of almost blank subframe patterns and transmits to the second interfering cell the selection from the second set of almost blank subframe patterns, and the interfered cell schedules downlink subframes for user equipments of the interfered cell, in accordance with the selected ABS patterns.
In accordance with some implementations, a method of managing communication interference in a heterogeneous network includes a first of two or more interfering cells determining a first set of one or more almost blank subframe patterns of downlink subframes available for an interfered cell to schedule its user equipments. A second of the two or more interfering cells, also determines a second set of one or more almost blank subframe patterns of downlink subframes available for the interfered cell to schedule its user equipments. The interfered cell receives from the first interfering cell, the first set of one or more data structures, each representing an almost blank subframe pattern, and receives from the second interfering cell the second set of one or more data structures, each representing an almost blank subframe pattern. The interfered cell compares the almost blank subframe patterns of the first set and second set and selects at least one of the first set of almost blank subframe patterns and selects at least one of the second set of almost blank subframe patterns in accordance with interference reports from one or more user equipments that communicate with the interfered cell. The interfered cell transmits to the first interfering cell the selection from the first set of almost blank subframe patterns and transmits to the second interfering cell the selection from the second set of almost blank subframe patterns; and the interfered cell schedules downlink subframes for user equipments of the interfered cell, in accordance with the selected ABS patterns.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated herein and constitute a part of the specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. Like reference numerals refer to corresponding parts throughout the several views of the drawings.
Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous non-limiting specific details are set forth in order to assist in understanding the subject matter presented herein. It will be apparent, however, to one of ordinary skill in the art that various alternatives may be used without departing from the scope of the present invention and the subject matter may be practiced without these specific details. For example, it will be apparent to one of ordinary skill in the art that the subject matter presented herein can be implemented on many types of radio communication systems.
A typical high power node, which has a maximum allowable transmit power of 46 dBm for a 10 MHz carrier bandwidth, can create a relatively large coverage area. Several types of low power nodes (LPN), which have a smaller coverage area than the high power node but may improve the quality and reliability of the coverage provided by the high power node, have been introduced into the large coverage area. These low power nodes include remote radio heads, pico eNBs, home eNBs (HeNBs), and the like. The maximum allowable transmit power of a remote radio head/pico eNB or a HeNB is typically 30 dBm or 20 dBm for a 10 MHz carrier bandwidth. In the present application, terms like “high power cell,” “high power node,” “macro cell,” and “micro eNB” are used interchangeably. As such, the coverage area of a low power node is referred to as a pico eNB (or cell), a femto eNB (or cell), a hotzone eNB (or cell), a low power eNB (or cell), or the like, any of which might be used interchangeably herein.
Besides the different transmit powers, these low power nodes have different connections to the core network (i.e., the EPC 20) and to their neighboring cells. For example, a pico cell is a small cellular base station with a dedicated backhaul connection to the service provider's core network (e.g., through S1 interface in 3GPP) and other pico/macro cells (e.g., through X2 interface in 3GPP). A femto cell is also a small cellular base station designed for use at a home or small business. The femto cell connects to the service provider's network via broadband internet (such as DSL or cable). The HeNB may have no direct interface to other femto cells or any macro cell.
When high power nodes and low power nodes are deployed in the same data/control channel, there may be interference problems for high power and low power cell User Equipments (UEs). For illustrative purposes,
One method of solving this inter-cell interference problem is to use the so-called “Almost Blank Subframe” (ABS) at the interfering cell and inform the victim cell of this ABS configuration. The basic unit of downlink transmission in 3GPP-LTE is a downlink subframe. The eNB of the victim cell then schedules downlinks for its UEs during the subframes defined by the ABS of other interfering cell(s).
In short, the ABS Pattern Info 220 identifies the subframes designated as almost blank subframes by the sending eNB (typically, of the aggressor cell) for the purpose of interference coordination. The receiving eNB (typically, of the victim cell) may consider such information when scheduling its own serving UEs. In some implementations, the aggressor cell also sends a request for feedback to the victim cell in order to know the victim cell's used ABS pattern and evaluate the need for modification of the ABS Pattern Info 220. The victim cell's pattern represented by the bitmap is a subset of, or the same as, the corresponding “ABS Pattern Info” message from the aggressor cell.
In this example, the aggressor cell periodically mutes the transmission that inflicts interference onto other victim cells for each designated subframe defined in the ABS Pattern Info message, so that the victim cells have a chance to serve their own UEs otherwise suffering the interference from the aggressor cell in these subframes. In some implementations, this muting is incomplete. Certain signals such as the common reference symbols, the primary and secondary synchronization signals (PSS and SSS), the physical broadcast channel (PBCH), and the system information block (SIB) and paging with their associated PDCCH, have to be transmitted even in the otherwise muted subframes, e.g., to avoid radio link failure or for reasons of backwards compatibility. Such muted subframes are thus referred to as almost blank subframes (ABS). In some implementations, the time domain muting (TDM) patterns are configured semi-statically and exchanged between different eNBs over the X2 interface.
As shown in
(i) UEs that are exposed to severe interference from another cell using TDM muting and hence should be scheduled in subframes muted by the corresponding aggressor cell (e.g., the edge UE 110 in
(ii) UEs that are indifferent to the interference from another eNB using the TDM muting (e.g., the center UE 120 and the macro UE 195 in
In reality, a victim cell is often the victim of interference from more than one interfering cell, especially in a densely deployed network. In such case, the above mentioned concept of a victim cell coordinating its downlink subframes for one or more UEs based on the ABS information received from one interfering cell may still result in interrupted service for the UE due to interference from one or more other interfering cells.
To illustrate this problem,
In contrast, a UE (e.g., the edge UE 370) located in the edge area 170 of pico eNB 100 and edge area 330 of femto eNB 300, may receive useful signal 340 from femto eNB 300 and strong interference 350 from pico eNB 100 and strong interference 150 from macro eNB 140 that may affect the quality of the useful signal 340. Similarly, when macro cell UE 195 moves close to the femto eNB 300, the signal from the femto eNB 300 may also cause interference to the signal the macro cell UE 195 receives from the macro eNB 140. If macro cell UE 195 moves into an area where pico edge area 170 and femto edge area 330 overlap, both pico eNB 100 and femto eNB 300 may cause interference to the signal macro UE 195 receives from macro eNB 140. In both cases, the interference may be so strong that the affected UE cannot maintain a normal service from its serving cell.
In current 3GPP LTE specifications, the traffic load in terms of physical resource block (PRB) utilization may be exchanged on the X2 interface among cells. This PRB utilization information reveals the resource usage in frequency domain and may approximately represent the average traffic load for a cell. In current 3GPP LTE specifications, there is a Relative Narrow Band Transmit Power (RNTP) message exchanged on the X2 interface among cells. However, such RNTP information only reveals the relative download transmit power in frequency domain from an interfering cell to a victim cell. It does not represent the actual interference over time domain. ABS pattern allocation is a time-domain based methodology for inter-cell coordination in LTE-A.
Due to the potential for interference from more than one cell in a dense cellular network, a solution is required to allow for interference-free communication of user equipments. The above mentioned concept of “ABS Pattern Info” is designed for the case of a single aggressor cell and a single victim cell. Such a system is much simpler to design for managing interference, but it is insufficient to rely on single-aggressor to single-victim ABS communication in a dense cellular network with multiple aggressor cells to one victim cell. In such a network, while one interfering cell may provide almost blank subframes for the victim cell to schedule its user equipments, if a second aggressor cell is still transmitting in those almost blank subframes of the first aggressor cell, edge user equipments of the victim cell will still experience interference.
One solution is to employ a centralized ABS allocation scheme, where an ABS controller, communicatively coupled to a plurality of cells, can receive information regarding traffic loads and interference from any and every cell of the plurality of cells, and send appropriate ABS allocation commands to aggressor cells. The ABS controller would balance the competing interests of victim cells that wish to allocate subframes for their user equipments, and aggressor cells that may have high traffic loads and an inability to give up subframes. Cells are not restricted to any particular type, and therefore can be macro cells or small cells in this centralized ABS allocation scheme.
In order to effectively allocate almost blank subframes for one or many cells, the ABS allocation controller requests information about the traffic load for each cell it is communicatively coupled with and requests information regarding interference from aggressor cells to victim cells. In some embodiments, the ABS allocation controller 400 makes a periodic request for traffic load information and interference estimation information, and in some embodiments the ABS allocation controller 400 can be triggered to assess the interference management needs of the network by one or more of the cells it is communicatively coupled with.
In
After receiving the reported information of the interference estimates from the requested cell or cells, the ABS allocation controller analyzes the interference information and the traffic load information for the plurality of cells it is communicatively coupled with. In some embodiments all the cells communicatively coupled with the ABS controller report their traffic load, and all the cells report their estimate interference from neighboring cells. In some embodiments only a subset of cells report traffic load or interference estimates. The ABS allocation controller uses the information it receives, to determine an ABS allocation pattern for some or all of the interfering cells in the plurality of cells. In some embodiments, some of the interfering cells in the plurality of cells may also be victim cells. The ABS allocation controller is able to determine an efficient allocation of subframes among the various cells because it has a large scope of interference and traffic load information for the plurality of cells it is communicatively coupled with. The ABS allocation controller typically operates to minimize the number of UE's that experience interference in the coverage area of the plurality of cells communicatively coupled with the ABS allocation controller. The ABS allocation controller may use other factors beyond traffic load and interference estimates to govern its decision to allocate resources among the plurality of cells. The ABS allocation controller governs the allocation of almost blank subframes by sending one or more ABS allocation commands to one or more interfering cells.
Another solution to the problem of allocating almost blank subframes in a multiple aggressor cell network, is to employ a distributed ABS allocation scheme. In a distributed ABS allocation scheme there is no dedicated ABS controller, but rather a plurality of cells communicatively coupled to each other manage ABS allocation amongst themselves to minimize interference at user equipments. In contrast to the centralized ABS allocation scheme, in the distributed ABS allocation scheme there is no hierarchical relationship among the elements in the network, therefore each victim cell and each aggressor cell must negotiate ABS allocation. In some embodiments, victim cells may have to request ABS allocation from more than one neighboring aggressor cell, and aggressor cells may receive ABS allocation requests from more than one victim cell. As in the centralized ABS allocation scheme, some victim cells may also be aggressor cells, particularly in a very dense network. Cells are not restricted to any particular type, and therefore can be macro cells or small cells in this centralized ABS allocation scheme.
Aggressor cells eNB A 800 and eNB C 860 respond to notification from eNB B 810 that it is experiencing interference in some portion of its cell coverage area. In some embodiments, victim cell eNB B 810 sends a notification to merely indicate the presence of interference. In some embodiments, the victim cell transmits actual interference reports received from the UE's to the aggressor cells, and in some embodiments the victim cell transmits some subset or interpretation of the information contained in the interference reports, such as the SNR of the victim cell signal and the aggressor cell signal. In response, eNB A 800 sends ABS pattern candidate set 820 to eNB B 810, and eNB C 860 sends ABS pattern candidate set 870 to eNB B 810. This demonstrates two distinctions from existing ABS allocation deployments. In the distributed ABS allocation scheme, one victim cell can receive and process ABS patterns sent from more than one aggressor cell. Another distinction is that aggressor cells can each send more than one candidate ABS pattern to the victim cell.
In some embodiments, the victim cell can return more than one ABS pattern in the preferred ABS pattern message to an aggressor cell. In some embodiments an ABS pattern message with more than one ABS pattern is interpreted by the aggressor cell to indicate equal preference by the victim cell for all the returned ABS patterns. In some embodiments, the victim cell can return ABS patterns received from other aggressor cells, to a particular aggressor cell. In some embodiments, the victim cell can return some or all of the patterns from ABS pattern candidate set in a ranked order of preference. In some of these embodiments, the preferred ABS pattern message sent from a victim cell to an aggressor cell may require additional fields for additional information.
In some embodiments, the victim cell will receive ABS allocation messages from all of its corresponding aggressor cells, providing at least one common almost blank subframe. Upon receiving these ABS allocation messages, the victim cell can schedule its user equipments in those almost blank subframes. In some embodiments the victim cell can return information to some or all of its aggressor cells regarding the actual subframes utilized by the victim cell for its user equipments. This feedback could provide the aggressor cells with more information and may free up resources in the aggressor cells for their own user equipments, or for other victim cells.
The ABS allocation controller then receives (904) an interference estimation response from the one or more cells of the plurality of cells. In some embodiments, the interference estimation response comprises the cell ID of the reporting cell, and a list of at least one neighboring cell ID and a corresponding value of estimated interference from that neighboring cell. In some embodiments, the value of estimated interference can be one of the set [<−10, −10, −8, −6, −4, −2, 0, 2, 4, 6, 8, 10, >10] (dB). The ABS allocation controller also receives (906) traffic load information from each cell communicatively coupled with the ABS controller. The ABS allocation controller analyzes (908) all the received interference estimates and traffic load information to determine almost blank subframe allocation at one or more interfering cells of the plurality of cells. The ABS allocation controller then sends (910) an ABS allocation command to one or more interfering cells of the plurality of cells. In some embodiments, the ABS allocation command comprises the cell ID of the cell receiving the command and ABS pattern information.
A first of the two or more interfering cells determines (1002) a first set of one or more data structures, each representing an almost blank subframe pattern of downlink subframes available for the interfered cell to schedule its user equipments, based on the interference information reported by the interfered cell. A second of the two or more interfering cells determines (1004) a second set of one or more data structures, each representing an almost blank subframe pattern of downlink subframes available for the interfered cell to schedule its user equipments, based on the interference information reported by the interfered cell. In some embodiments, an interfering cell determining a set of one or more almost blank subframe patterns of downlink subframes available for the interfered cell to use for its user equipments, further comprises determining one or more almost blank subframe patterns based on the amount of traffic being experienced at the respective interfering cell and requests for almost blank subframe patterns from other interfered cells.
The interfered cell receives (1006) from the first interfering cell, the first set of one or more data structures, and receives from the second interfering cell the second set of one or more data structures. The interfered cell compares (1008) the almost blank subframe patterns of the first set and second set and selects at least one of the first set of almost blank subframe patterns and selects at least one of the second set of almost blank subframe patterns. In some embodiments, comparing the almost blank subframe patterns of the first set and second set, further comprises determining one or more combinations of first set and second set almost blank subframe patterns, and determining one of the patterns of the first set and determining one of the patterns of the second set that results in the minimum possible interference at the interfered cell.
In some embodiments, comparing the almost blank subframe patterns of the first set and second set, further comprises determining a plurality of combinations of first set and second set almost blank subframe patterns, ranking the combinations of first set and second set almost blank subframe patterns on the basis of interference, determining the most to least preferred of the patterns of the first set and determining the most to least preferred of the patterns of the second set based on the ranking of combinations, and adding a value representing the preference of the first interfered cell to the data structure of at least one almost blank subframe pattern of the first set and adding a value representing the preference of the first interfered cell to the data structure of at least one almost blank subframe pattern of the second set.
The interfered cell transmits (1010) to the first interfering cell the selection from the first set of almost blank subframe patterns and transmits to the second interfering cell the selection from the second set of almost blank subframe patterns. In some embodiments, in response to receiving from the interfered cell the selection from the set of almost blank subframe patterns, the interfering cell adopts one of the selected almost blank subframe patterns and transmits information to the interfered cell indicating the almost blank subframe pattern adopted by the first interfering cell. In some embodiments, the interfered cell determines a third pattern of downlink subframes, wherein the third pattern of downlink subframes represents the overlapping almost blank subframes from the almost blank subframe pattern adopted by the first interfering cell and the almost blank subframe pattern adopted by the second interfering cell. Then the interfered cell transmits the third pattern to the first interfering cell and the second interfering cell. The interfered cell schedules (1012) downlink subframes for user equipments of the interfered cell, in accordance with the selected ABS patterns.
In some embodiments, the interfered cell determines a third pattern of downlink subframes, wherein the third pattern of downlink subframes represents the pattern of actual scheduling of the user equipments of the interfered cell. Then, the interfered cell transmits the third pattern to the first interfering cell and the second interfering cell.
Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific examples of the embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/051738 | 8/19/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61874271 | Sep 2013 | US |
