WO2014179351A1

WO2014179351A1 - Method and system for interference coordination in a wireless lte-tdd system

Info

Publication number: WO2014179351A1
Application number: PCT/US2014/035940
Authority: WO
Inventors: Huaming Wu; Wenfeng Zhang; Peng Hao; Lu Ren; Yu Ngok LI
Original assignee: Zte (Usa) Inc.
Priority date: 2013-05-03
Filing date: 2014-04-29
Publication date: 2014-11-06

Abstract

This application provides a method of information exchange among cells via backhaul by conveying the interference information indicating the interference caused by different TDD D/U allocations between neighboring base stations. In this method, a first cell sends a message to a second cell, the message including information of the first cell's subframe allocation that indicates the subframe type such as downlink, uplink or special subframe in the first cell. The second cell (the recipient of this message) then provides feedback in the form of a Subframe Interference Indicator to the first cell, which indicates the interference observed in those subframes possibly caused by downlink transmission in the neighboring first cell. The first cell then takes this feedback message into account when scheduling its own UEs by, e.g., adjusting its transmission power or changing the type of certain subframes from downlink to uplink for interference mitigation purpose.

Description

METHOD AND SYSTEM FOR INTERFERENCE COORDINATION IN A

WIRELESS LTE-TDD SYSTEM

TECHNICAL FIELD

[0001] The disclosed implementations relate generally to wireless communication, and in particular, to method and system for exchanging interference information between different base stations in a wireless Long-Term-Evolution (LTE) Time-Division Duplexing (TDD) system.

BACKGROUND

[0002] One benefit of the LTE-TDD system is the flexibility of bandwidth allocation in the unpaired frequency bands and the choice of downlink-to-uplink resource allocation (also known as "D/U allocation" in this application) at different base stations (also known as "cell" or "eNB" in this application). The latter one becomes more attractive nowadays because of the emerging traffic service types and traffic volume turbulence, both of which result in the wide range of D/U allocation. Once the D/U allocation is determined, all the user equipments (also known as "UE" in this application) served by a base station are informed of the D/U allocation via broadcast signaling in the current wireless system, e.g., the LTE-TDD system. Any subsequent changes of the existing D/U allocation are done by broadcasting the new D/U allocation through broadcast signaling.

SUMMARY

[0003] This application provides a method of information exchange via backhaul among base stations by conveying the interference information among cells to indicate the interference caused by different TDD D/U allocations between neighboring base stations. In this method, a first cell sends a message to a second cell, the message including information of the first cell's subframe allocation that indicates the subframe type such as downlink, uplink or special subframe in the first cell. The second cell (the recipient of this message) then provides feedback in the form of a Subframe Interference Indicator (SII) to the first cell, which indicates the interference observed in those subframes possibly caused by downlink transmission in the neighboring first cell. The first cell then takes this feedback message into account when scheduling its own UEs by, e.g., adjusting its transmission power or changing the type of certain subframes from downlink to uplink for interference mitigation purpose.

[0004] In some implementations, a method of mitigating interference between base stations within a wireless communication network is performed at a first base station and a second base station, the method comprising: the first base station generating a subframe interference indicator (SII) report, the SII report including an array of T elements corresponding to respective subframes of a radio frame and each element including a value indicating whether the first base station is subject to possible interference in one or more subframes caused by downlink transmission from the second base station, and sending the SSI report to the second base station; and the second base station, upon receipt of the SSI report, adjusting scheduling of downlink transmission for a respective subframe whose corresponding element value in the array of T elements indicates that the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station.

[0005] In some other implementations, a system for mitigating interference within a wireless communication network includes multiple base stations including a first base station and a second base station. The first base station is configured to generate a subframe interference indicator (SII) report, the SII report including an array of T elements

corresponding to respective subframes of a radio frame and each element including a value indicating whether the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station, and send the SSI report to the second base station. The second base station is configured to receive the SSI report and adjust scheduling of downlink transmission for a respective subframe whose corresponding element value in the array of T elements indicates that the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station.

BRIEF DESCRIPTION OF DRAWINGS

[0006] The aforementioned implementation of the invention as well as additional implementations will be more clearly understood as a result of the following detailed description of the various aspects of the invention when taken in conjunction with the drawings. Like reference numerals refer to corresponding parts throughout the several views of the drawings. [0007] FIG. 1 is a block diagram illustrating the frame structure of a LTE-TDD system.

[0008] FIG. 2 is a block diagram illustrating one exemplary deployment of the LTE-TDD system including multiple base stations of different types.

[0009] FIG. 3 is a flow chart illustrating how two base stations exchange interference information indicating with each other according to some implementations of the present application.

[0010] FIG. 4 is a block diagram illustrating another exemplary deployment of the LTE- TDD system including multiple base stations of different types according to some

implementations of the present application.

[0011] FIGS. 5A-5C are block diagrams of data structures used for storing overload indicators and subframe interference indicators according to some implementations of the present application

DETAILED DESCRIPTION

[0012] In LTE system, the minimum transmission time interval (TTI) is called "subframe", whose time duration is one-tenth of one radio frame. FIG. 1 is a block diagram illustrating the frame structure of a LTE-TDD system. As shown in the figure, one radio frame of 10 ms in length consists of ten subframes, each subframe being 1 ms in length. For the LTE-TDD system, a subframe can be a downlink subframe (marked as "D"), an uplink subframe (marked as "U") or a special subframe (marked as "S"). Each special subframe includes three fields: a Downlink Pilot Timeslot (DwPTS), a Guard Period (GP), and an Uplink Pilot Timeslot (UpPTS). The DwPTS is in the downlink direction and the UpPTS is in the uplink direction; the GP is located between the DwPTS and the UpPTS and it has no transmission. A combination of choices on {D, U, S} in every one of the ten subframes (Subframe #0, Subframe #9) per radio frame constructs one TDD downlink-uplink allocation (also known as "D/U Configuration" in this application).

[0013] For example, Table 1 below illustrates seven downlink-uplink allocations used by the LTE-TDD system at two different switch-point periodicities.

0 5 ms D S u u u D S u u u

1 5 ms D S u u D D S u u D

2 5 ms D s u D D D s u D D

3 10 ms D s u u U D D D D D

4 10 ms D s u u D D D D D D

5 10 ms D s u D D D D D D D

6 5 ms D s u u U D S U U D

Table 1 Downlink-Uplink Allocations in LTE-TDD

[0014] Different D/U configurations, numbered zero to six, are defined in the standard for the subframe number allocated for the uplink and downlink transmission. Note that subframe 1 in all the seven configurations and subframe 6 in configurations 0, 1 , 2 and 6 are special subframes, each special subframe consisting of DwPTS, GP and UpPTS. Each of the other subframes is defined as two slots.

[0015] As shown in the table above, switch-point periodicities of 5 ms and 10 ms are supported by the LTE-TDD system. In the case of a 5-ms switch-point periodicity, UpPTS and subframes 2 and 7 are reserved for uplink transmission. In the case of a 10-ms switch- point periodicity, UpPTS and subframe 2 are reserved for uplink transmission and subframes 7 to 9 are reserved for downlink transmission.

[0016] Subframe 0 and 5 are always for the downlink (DL). The subframe following the special subframe is always for the uplink (UL). The DwPTS field carries synchronization and user data as well as the downlink control channel for transmitting scheduling and control information. The UpPTS field is used for transmitting the Preamble Random Access Channel (PRACH) and the Sounding Reference Signal (SRS).

[0017] In the conventional LTE-TDD system, neighboring cells are configured to have the same TDD D/U allocation; otherwise, there may be interference between the downlink transmission in one cell and the uplink transmission in another neighboring, resulting in so- called Base-to-Base (or eNB-to-eNB) interference and Mobile-to-Mobile (or UE-to-UE) interference. But the recent LTE standard development proposes to further enhance the LTE- TDD system performance by adapting the TDD D/U allocation to the traffic variation. For example, due to the rapid and independent changes of traffic volume at each cell, neighboring cells may be able to choose different D/U allocations in Table 1 to better match the traffic pattern in their own cells. Consequently, the inter-cell interference may become more severe than in the conventional LTE-TDD system. In order to mitigate such interference, it is important that the base stations (or eNBs) should cooperate with each other by exchanging information between eNBs regarding to the interference characteristics experienced by each eNB.

[0018] In some implementations, one method of the eNB-level cooperation is to use the overload indicator (OI) that was already defined in the current LTE specification. In the current release of LTE, OI is a data structure sent by one eNB to other eNBs over the X2 interface. A source eNB uses the OI to inform a destination eNB of the uplink interference that are observed by the source eNB on all Physical Resource Blocks (PRB) in the frequency domain. As shown in FIG. 5 A, if there are N resource blocks, the OI report contains a data structure of Nxl array, with each array element including an interference indicator corresponding to a resource block. For example, "1" indicates that there is at least a predefined level of inter-cell uplink interference at that PRB and "0" indicates that there is no uplink interference above the predefined level at that PRB. Once receiving the OI from the source eNB, the destination eNB adjusts its scheduling of the uplink transmission in terms of resource block allocation in the frequency domain based on the interference sensitivity reported in the OI. In some embodiments, this OI reporting mechanism intends to indicate the uplink interference only by, e.g., taking the statistical average of measured interference over an OI exchange time window and it cannot indicate the subframe-level interference. One extension to the mechanism is to replace the OI data structure to a matrix of size NxT as shown in FIG. 5B, where T is the dimension in the time domain to indicate the number of subframes. For example, one of the T arrays in this matrix (each array size being Nxl) provides information about the interference statistics on all resource blocks within a specific set of subframes that occur periodically. According to this solution, once receiving the OI data structure from the source eNB, the destination eNB adjusts its scheduling on either downlink transmission or uplink transmission, based on the interference sensitivity and the subframe-level interference information reported in the OI data structure.

[0019] Although the time-domain extended OI method mentioned above can inform an eNB in both time (per subframe) and frequency (per PRB) domain of when the interference is severe and when it is not, the overhead of conveying such information over backhaul is much (e.g., T times) larger than the first 01 method described above.

[0020] As mentioned above, the Ol-based mechanism measures the uplink interference only by, e.g., taking the statistical average of interference over the 01 exchange time window and it cannot indicate the interference per subframe. As shown in FIG. 2, the interfered picol eNB sends 01 messages to the macro eNB, pico2 eNB and pico3 eNB, which schedule both downlink and uplink transmission during the 01 reporting time window. In this example, the D/U allocations at the four cells are shown in Table 2 below:

Table 2 D/U Allocations at different base stations in FIG. 2

[0021] In other words, the four neighboring cells shown in FIG. 2 are configured with different D/U allocations. If the 01 indicates a high uplink interference in subframe #2, the eNBs at the macro site and pico2/pico3 sites can tell that the interference at the picol eNB is caused by uplink transmission because all D/U allocations in Table 1 mark subframe #2 as uplink subframe. On the other hand, a statistical averaged 01 for some particular PRBs may not reveal the true interference level in subframe #3, because the corresponding interference can be caused by either the downlink transmission in the macro eNB or the uplink

transmission in the pico2 eNB and/or pico3 eNB or both. In this case, the macro and pico2/pico3 eNBs may not be able to adjust their scheduling properly based on the 01 message from the picol eNB and without further information.

[0022] According to some implementations, described below is a new mechanism that not only overcomes the above-mentioned drawbacks in the existing Ol-based method but also has less overhead compared to the time-domain extended 01 report. To be more specific, the mechanism provides the following information in a new data structure, which is referred to as "Subframe Interference Indicator" (SII) to be sent over X2 interface:

The information contained in the SII that indicates the interference level in each subframe possibly caused by downlink transmission from neighboring cells.

[0023] According to some implementations of this application, a method of interference information exchange via backhaul among base stations is provided by conveying the interference information among cells to indicate the interference caused by different TDD D/U allocations between neighboring base stations. In this method, a first cell sends a message to a second cell, the message including the first cell's D/U subframe allocation, e.g., information about the type of a subframe being one of downlink, uplink or special subframe in the first cell. The second cell (the recipient of this message) then provides feedback in the form of an SII to the first cell, the SII indicating the interference observed in those subframes possibly caused by downlink transmission in the neighboring first cell. A more detailed explanation of the SII reporting mechanism is given below. For illustrative purposes, it is assumed that every eNB provides its D/U subframe configuration to its neighboring cells via backhaul. For example, as shown in FIG.2, the macro, pico2 and pico3 eNBs all provide their corresponding D/U subframe configurations to the picol eNB.

[0024] As shown in FIG 3, the interfered eNB (e.g., the pico l eNB) measures (300) the received signals across the whole reception bandwidth and during the SII reporting time window, and derives (310) the interference level statistics for each subframe in which the picol eNB actually receives a signal in the uplink. Because of higher transmit power from a base station eNB compared to UE (e.g., 46 dBm for a macro eNB, 30 dBm for a pico eNB and 23 dBm for UE), the interference strength received at the interfered eNB and caused by the neighboring cells' downlink transmission are usually significantly stronger than the received interference strength caused by uplink transmission from the neighboring cell's UEs. In other words, the interfered eNB can tell statistically whether the interference in a subframe is primarily caused by other eNB's downlink signals or other cell UEs' uplink signals by, e.g., comparing the interference instance in that subframe with the average interference level in time domain or the average interference level in a particular subframe (e.g., subframe #2 shown in FIG. 2) in which there is no downlink signal transmission in any neighboring cells. For example, if the interference instance in that subframe is higher than the average interference level in the time domain, the interference at the subframe is more likely caused by the other eNB's downlink signals. Conversely, if the interference instance in that subframe is similar to or even lower than the average interference level in a particular subframe in which there is no downlink signal transmission in any neighboring cells, the interference at the subframe is more likely caused by the uplink transmission from the neighboring cell's UEs.

[0025] Next, the interfered eNB constructs (320) a new data structure for SII reporting. As shown in FIG. 5C, this new data structure contains an array of T elements, each element corresponding to an individual subframe in the time domain and indicating whether the interference is possibly caused by the downlink transmission in the cell of the recipient eNB of this SII report message. Note that each element in this array is a 2-level (i.e. binary) report on the interference level, for which value "Ί " indicates 'high interference' (highly possible caused by the neighbouring eNB's downlink transmission) and value "0" indicates 'low interference' (less likely caused by the neighbouring eNB's downlink transmission). In some other embodiments, each of the T elements corresponds to a set of subframes, each set including one or more subframes.

[0026] Once the interfered eNB constructs the SII report, it sends (330) the report to the neighboring eNBs which may be the source of interference. Note that existing high- interference indicator (HII) and OI reports are assumed to be exchanged between eNBs as defined by the LTE standard. In other words, the SII report does not replace the existing OI report but is exchanged among eNBs in addition to the OI report if the TDD dynamic D/U ratio change feature is enabled. Upon reception (340) of the SII report from other eNBs, the potentially interfering eNB may compare (350) the received OI and SII reports with its scheduling history per subframe. If the SII report element contains information indicating that a possible interference caused by downlink transmission at the interfering eNB for a subframe, the interfering eNB then schedules (360) downlink transmission on the

corresponding subframe and adjusts its scheduling policy by taking this SII report into account when operating its interference mitigation algorithm.

[0027] This procedure can be illustrated by the following examples. As shown in FIG. 4, the picol eNB obtains the D U configurations information from the macro, pico2 and pico3 eNBs as each of these eNBs sends a message to the picol eNB containing its corresponding LTE-TDD D/U configuration information. As shown in the figure, it is assumed that the old special subframes are used for uplink/downlink transmission. For example, the special subframe #1 is used in the macro, picol , pico2 and pico3 eNBs for uplink transmission and the special subframe #6 is used in the pico3 eNB for downlink transmission, but used as uplink transmission in the macro, picol and pico2 eNBs; and the pico l eNB measures uplink interference in subframes #1 , #2, #3, #4, #6, #7, #8, #9, which are actual uplink transmissions in the picol eNB.

[0028] In one example, T is 10, corresponding to a full radio frame. Note that for subframes #0 and #5 which are downlink subframes at the pico l eNB, no interference is measured by the pico l eNB and thus the corresponding element values of the SII reports to neighboring cells for subframe #0 and #5 are '0' while the other elements being '0' or Ί ' as shown in FIG. 5C. Table 3 below provides SII report at the macro, pico2, and pico 3 eNB, respectively, each element in the SII report corresponding to subframe #0 to #9 in order.

Table 3 Exemplary Subframe Interference Indicator Report for T=10

[0029] Note that for subframes #4 and #9, due to multiple possible interference sources, the pico l eNB indicates a high interference level in multiple SII reports sent to different eNBs. Then the macro eNB may adjust its scheduling policy in subframes #3, #4, #8 and #9 to mitigate the inference to the picol cell caused by the downlink transmission at the macro eNB. Similarly, the pico2 eNB may adjust its scheduling policy in subframes #4 and #9 to mitigate the inference to the pico 1 eNB caused by the downlink transmission at the pico2 eNB. Similarly, the pico3 eNB may adjust its scheduling policy in subframes #6 and #9 to mitigate the inference to the pico l eNB caused by the downlink transmission at the pico3 eNB.

[0030] In another example, T is 8, corresponding to the maximal number of possible subframes for uplink transmission (as shown in Table 1 , subframes #0 and #5 are always downlink for all LTE-TDD D/U configurations). Note that for subframes #0 and #5 which are downlink subframes in the picol eNB, no interference is measured by the picol eNB and thus the corresponding element values of the SII reports to the neighboring cells for subframes #0 and #5 can be skipped because they are always 'Ο'. As a result, the SII reports shown in Table 3 can be simplified as Table 4 below:

Table 4 Exemplary Subframe Interference Indicator Report for T=8

[0031] Note that for subframes #4 and #9, due to multiple possible interference sources, the picol eNB indicates a high interference level in multiple SII reports sent to different eNBs. Then the macro eNB may adjust its scheduling policy in subframes #3, #4, #8 and #9 to mitigate the inference to the picol cell caused by the downlink transmission at the macro eNB. Similarly, the pico2 eNB may adjust its scheduling policy in subframes #4 and #9 to mitigate the inference to the pico 1 eNB caused by the downlink transmission at the pico2 eNB. Similarly, the pico3 eNB may adjust its scheduling policy in subframes #6 and #9 to mitigate the inference to the picol eNB caused by the downlink transmission at the pico3 eNB.

[0032] Note that the size of T may be determined by the length of the LTE-TDD D/U configuration bitmap or pattern sent from other eNBs. For instance, if the macro eNB sends a message to the picol eNB containing its D/U configuration at a frame level, the SII report from the picol eNB to the macro eNB would be the same length as the macro eNB's D/U bitmap or pattern. In another instance, if the pico2 eNB only indicates flexible subframe allocation (i.e. those subframes could be used for either downlink or uplink), the SII report from the picol eNB to the pico2 eNB would be the same length as the flexible subframe allocation from the pico2 eNB.

[0033] As noted above, the T elements in the SII report may correspond to T subframe sets, instead of T individual subframes, and each set may include one or multiple subframes within one radio frame. Whether or not those multiple subframes in a subframe set are contiguous in the time domain, the value of the corresponding element in the SII report indicates the same level of observed interference or equivalently the same chance that the interference is caused by neighbouring eNB downlink transmission. For the example described above, T can be further reduced from 8 to 2, i.e., one element in the SII report indicating the interference level in the uplink subframes #2 and #7 in the neighboring cells, and the other element in the SII report indicating the interference level in other uplink subframes since the other uplink subframes observe the same downlink interference level. The value of T can be larger than 2 if more subframe sets are needed to distinguish among different tracks of interference level variation.

[0034] In implementation, the above described mechanisms and signaling and their variations may be implemented as computer software instructions or firmware instructions. Such instructions may be stored in an article with one or more machine-readable storage devices connected to one or more computers or digital processors such as digital signal processors and microprocessors. In a communication system, the interference mitigation and its process may be implemented in form of software instructions or firmware instructions for execution by a processor in the transmitter or its transmission controller. In operation, the instructions are executed by one or more processors to cause the transmitter or its transmission controller and receiver or receiver controller to perform the described functions and operations. Other variations and enhancements are possible based on what is mentioned here.

[0035] Although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, first element could be termed second element, and, similarly, second element could be teraied first element, without departing from the scope of the present invention. First element and second element are both elements, but they are not the same element.

[0036] The terminology used in the description of the invention herein is for the purpose of describing particular implementations only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises," and/or "comprising," when used in this specification, specify the presence of stated features, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, operations, elements, components, and/or groups thereof.

[0037] As used herein, the term "if may be construed to mean "when" or "upon" or "in response to determining" or "in accordance with a determination" or "in response to detecting," that a stated condition precedent is true, depending on the context. Similarly, the phrase "if it is determined [that a stated condition precedent is true]" or "if [a stated condition precedent is true]" or "when [a stated condition precedent is true]" may be construed to mean "upon determining" or "in response to determining" or "in accordance with a determination" or "upon detecting" or "in response to detecting" that the stated condition precedent is true, depending on the context.

[0038] Although some of the various drawings illustrate a number of logical stages in a particular order, stages that are not order dependent may be reordered and other stages may be combined or broken out. While some reordering or other groupings are specifically mentioned, others will be obvious to those of ordinary skill in the art and so do not present an exhaustive list of alternatives. Moreover, it should be recognized that the stages could be implemented in hardware, firmware, software or any combination thereof.

[0039] The foregoing description, for purpose of explanation, has been described with reference to specific implementations. 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

implementations were chosen and described in order to best explain principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various implementations with various modifications as are suited to the particular use contemplated. Implementations include alternatives, modifications and equivalents that are within the spirit and scope of the appended claims. Numerous specific details are set forth in order to provide a thorough understanding of the subject matter presented herein. But it will be apparent to one of ordinary skill in the art that the subject matter may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the implementations.

Claims

WHAT IS CLAIMED IS:

1. A method of mitigating interference between base stations within a wireless communication network, the method comprising:

a first base station generating a subframe interference indicator (SII) report, the SII report including an array of T elements corresponding to respective subframes of a radio frame and each element including a value indicating whether the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station, and sending the SSI report to the second base station; and

the second base station, upon receipt of the SSI report, adjusting scheduling of downlink transmission for a respective subframe whose corresponding element value in the array of T elements indicates that the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station.

2. The method of claim 1 , further comprising:

the first base station and the second base station exchanging their downlink-uplink allocation information before the first base station generates the SII report.

3. The method of claim 2, wherein the first base station and the second base station have different downlink-uplink allocations.

4. The method of claim 1 , further comprising:

before generating the SII report:

the first base station measuring received signals in an uplink across a predefined bandwidth and during a predefined time window; and

the first base station deriving interference level statistics for each subframe in which the first base station receives a signal in the uplink, wherein the interference level statistics is uses for constructing the SII report.

5. The method of claim 4, further comprising:

for a given subframe:

setting a corresponding element in the array of T elements to the value indicating that the first base station is subject to possible interference caused by downlink transmission from the second base station if an interference instance in that subframe is higher than an average interference level across the predefined bandwidth and during the predefined time window; and setting the corresponding element in the array of T elements to the value indicating that the first base station is not subject to possible interference caused by downlink transmission from the second base station if the interference instance in that subframe is similar to or lower than the average interference level in a subframe in which there is no downlink transmission in any base stations near the first base station.

6. The method of claim 1 , further comprising:

after receiving the SII report:

the second base station comparing the elements in the SII report with its scheduling history per subframe; and

the second base station avoiding scheduling downlink transmission on the subframe whose corresponding element indicates a possible interference to the first base station.

7. The method of claim 1 , wherein the first base station is a pico base station and the second base station is one of a pico base station and a macro base station.

8. The method of claim 1, wherein the SII report is sent from the first base station to the second base station via an X2 interface.

9. The method of claim 8, wherein the second base station receives an overload indicator (01) report from the first base station via the X2 interface and the second base station uses both the SII report and the 01 report for adjusting scheduling of downlink transmission for a respective subframe within a radio frame.

10. The method of claim 1 , wherein each of the T elements corresponds to one and only one subframe within a radio frame.

1 1. The method of claim 1 , wherein the number T of the T elements is less than or equal to the number of subframes within a radio frame.

12. The method of claim 1 , wherein each of the T elements corresponds to multiple subframes belonging to a subframe set within a radio frame.

13. The method of claim 12, wherein the multiple subframes are contiguous within the radio frame.

14. The method of claim 12, wherein the multiple subframes are discontinuous within the radio frame.

15. A system for mitigating interference within a wireless communication network comprising:

multiple base stations including a first base station and a second base station;

wherein:

the first base station is configured to generate a subframe interference indicator (SII) report, the SII report including an array of T elements corresponding to respective subframes of a radio frame and each element including a value indicating whether the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station, and send the SSI report to the second base station; and

the second base station is configured to receive the SSI report and adjust scheduling of downlink transmission for a respective subframe whose corresponding element value in the array of T elements indicates that the first base station is subject to possible interference in one or more subframes caused by downlink transmission from a second base station.

16. The system of claim 15, wherein the first base station and the second base station exchange their downlink-uplink allocation information before the first base station generates the SII report.

17. The system of claim 16, wherein the first base station and the second base station have different downlink-uplink allocations.

18. The system of claim 15, wherein the first base station is configured to, before generating the SII report:

measure received signals in an uplink across a predefined bandwidth and during a predefined time window; and

derive interference level statistics for each subframe in which the first base station receives a signal in the uplink, wherein the interference level statistics is uses for constructing the SII report.

19. The system of claim 18, wherein the first base station is further configured to:

for a given subframe:

set a corresponding element in the array of T elements to the value indicating that the first base station is subject to possible interference caused by downlink transmission from the second base station if an interference instance in that subframe is higher than an average interference level across the predefined bandwidth and during the predefined time window; and

set the corresponding element in the array of T elements to the value indicating that the first base station is not subject to possible interference caused by downlink transmission from the second base station if the interference instance in that subframe is similar to or lower than the average interference level in a subframe in which there is no downlink transmission in any base stations near the first base station.

20. The system of claim 15, wherein the second base station is further configured to: after receiving the SII report:

compare the elements in the SII report with its scheduling history per subframe; and

avoid scheduling downlink transmission on the subframe whose corresponding element indicates a possible interference to the first base station.