Abstract:
Disclosed is a radio resource reuse method, a base station device, a terminal device, and a radio communication system. The system at least comprises two adjacent cells C i  (i=1, 2), using time-frequency resource, denoted as S for communication, which is divided into at least two resource units, each of the resource units being one time-frequency resource block. The method comprises: dividing the time-frequency resource S into N non-overlapping resource groups, denoted as RG n  (n=1, 2, . . . , N), N being an integer larger than 1, and each of the resource groups comprising at least two of the resource units; and dividing RG n  (n=1, 2, . . . , N), in cell C i  (i=1, 2), into two non-overlapping portions, of which, one portion is configured with an upper power density limit h (i)   n  and termed a primary sub-resource group of RG n , and the other portion is configured with an upper power density limit l (i)   n  and termed a secondary sub-resource group of RG n , wherein the sub-resource groups have the following relationships in terms of upper power density limit thereof: l (i)   1 /h (i)   1 &lt;l (i)   2 /h (i)   2 &lt; . . . &lt;l (i)   N /h (i)   N ≦1, wherein the primary sub-resource groups of the two adjacent cells C i  (i=1, 2) do not overlap with each other in each resource group.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the priority of Chinese patent application CN 201310735224.2, entitled “Multilevel soft time-frequency reuse and resource allocation method, device, and system” and filed on Dec. 28, 2013, and the priority of Chinese patent application CN 201410082468.X, entitled “Multilevel soft time-frequency reuse and resource allocation method, device, and system” and filed on Mar. 8, 2014, the entireties of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of radio communications, and in particular, to a radio resource reuse method, a base station device, a terminal device, and a radio communication system. 
       BACKGROUND OF THE INVENTION 
       [0003]    When the LTE standard began, the requirement on improvement of cell edge data rate was proposed. To fulfill such a requirement, a variety of solutions have been put forward in the industry, including the well-known soft frequency reuse and fractional frequency reuse, among others. Currently, these technologies are being widely studied and applied, and have been developed into a key LTE feature as inter cell interference coordination (ICIC). 
         [0004]    The frequency band used in fractional frequency reuse is divided into two portions, each portion using a different reuse factor. As shown in  FIG. 1 , the B 1  (=B 11 +B 12 +B 13 ) portion uses reuse factor 3, while the B 2 +B 3  portion uses reuse factor 1. The curve in  FIG. 1  illustrates the upper power density limit (UPDL) of a transmitter on each portion. The power density of the transmitter can be smaller than or equal to such UPDL, which, however, cannot be exceeded. This curve is also termed power density mask (PDM for short). 
         [0005]    The B 1  portion with reuse factor of 3 uses the frequency band with a relatively high UPDL, which can cover the entire cell; while the B 2 +B 3  portion with reuse factor 1 use a relatively low UPDL, which can cover an inner region of the cell only. The coverage areas of fractional frequency reuse are illustrated in  FIGS. 2 and 3 , of which,  FIG. 2  shows the scenario of omnidirectional cells, and  FIG. 3  shows the scenario of three-sector cells. The two scenarios are similar to each other. For the sake of convenience, the omnidirectional cell and the three-sector cell are both termed cell in the present invention. 
         [0006]    The symbols of frequency band in  FIGS. 2 and 3 , i.e., B 11 , B 12 , B 13 , B 2 , and B 3  are placed at the inner sides of the farthest boundaries of their corresponding coverage areas. For example, B 2  and B 3  are placed at inner positions of the cells, which indicates coverage of only the inner cell region; while B 11 , B 12 , and B 13  are placed at the edges of the cells, indicating coverage of entire cells, including, of course, inner regions of the cells, although not marked in the figures. 
         [0007]      FIGS. 2 and 3  show regular hexagonal cells, wherein the inner region of a cell is the area inside a regular circle or ellipse. In practice, due to the complex radio environments, a cell is usually in an irregular shape, with a complex boundary between the inner region and the cell edge. 
         [0008]    The cell edge users are subjected to relatively severe interferences. Reuse factor 3 can be used to prevent interferences from adjacent cells, thereby improving signal-to-interference-plus-noise ratio (SINR). Interferences at the cell center are relatively slight, and thus reuse factor 1 can be used to make full use of the bandwidth. Therefore, fractional frequency reuse has attracted much attention. 
         [0009]    The fractional frequency reuse can be applied in both uplink (signals travel from a terminal to a base station) and downlink (signals travel from a base station to a terminal). 
         [0010]    The major defects of fractional frequency reuse lie in that a cell cannot use the whole bandwidth, and even fewer bandwidth is available at the cell edge. 
         [0011]    The soft frequency reuse scheme was disclosed in CN 1783861, entitled “Method for realizing soft frequency reuse in radio communication system,” and invented by Yang Xuezhi. The power density mask thereof is shown in  FIG. 4 .  FIGS. 5 and 6  show coverage areas of all frequency bands. 
         [0012]    In the soft frequency reuse scheme as shown in  FIG. 4 , the whole bandwidth is divided into three sub-bands, referred to as B 1 , B 2 , and B 3 , respectively. One sub-band is selected as primary sub-band for each cell with a relatively high UPDL, which can cover the entire cell. Other sub-bands are selected as secondary sub-bands with a relatively low UPDL, which can only cover an inner region of the cell. Users at the cell edge can use the primary sub-band of the cell only, while users at the inner region can use both the primary and the secondary sub-bands. 
         [0013]    As shown in  FIG. 5 or 6 , in cell  1 , B 1  is the primary sub-band, which has a relatively high UPDL and can cover the entire cell; while B 2  and B 3  are secondary sub-bands with a relatively low UPDL and can cover the inner region of the cell only. Users at the cell edge can merely use the primary sub-band B 1 , while users at the inner region can use both the primary sub-band B 1  and the secondary sub-bands B 2  and B 3 . 
         [0014]    In the soft frequency reuse scheme, the primary sub-bands of adjacent cells do not overlap with each other, and users at the edges of the adjacent cells therefore do not interfere with each other. At the cell edge, although there are less bandwidth available compared to reuse 1, the SINR is improved significantly enough to compensate the loss in bandwidth. As a result, the channel capacity at cell edge is largely improved. At the inner region of the cell, the expected signals are relatively strong, while the interferences from neighboring cells are relatively weak. Therefore all bandwidth is used, leading to high spectrum efficiency. Soft frequency reuse, which overcomes the defects of fractional frequency reuse, has now been intensively studied and applied, and become the mainstream technology in the ICIC field. 
         [0015]    It can be seen that soft frequency reuse is essentially a network planning technology, providing a framework for resource allocation in each cell, under which an optimized interference pattern is achieved, improving performances of the entire network. 
         [0016]    Soft frequency reuse divides communication resource in the frequency dimension. In parallel, communication resource can be divided in the time dimension. Time can be divided into several time slots, each of which is configured with a UPDL. This will lead to the soft time reuse scheme. More generally, communication resource can be divided in the time-frequency plane into many time-frequency blocks, thus forming the soft time-frequency reuse scheme. 
         [0017]    The defect of soft frequency reuse lies in that the UPDL of each cell has only two levels, which merely provides a relatively rough restriction on resource allocation in each cell. Thus, there is a still relatively large room for performance improvement. 
         [0018]    In two adjacent cells as shown in  FIG. 7 , the primary sub-band and secondary sub-band of cell  1  are B 1  and B 2  respectively. At the edge of cell  1 , there are two user terminals u 11  and u 12 , respectively using f 1  and f 2 , which both belong to B 1 , for communication. As u 12  is closer to the cell edge than u 11 , it uses a larger transmit power. The primary sub-band and secondary sub-band of cell  2  are B 2  and B 1  respectively, and there are two user terminals u 21  and u 22  at the inner region of cell  2 , using f 1  and f 2  for communication respectively. As u 21  is closer to the cell center than u 22 , it uses a smaller transmit power. Situations are similar in the uplink and downlink. Thus, the interference pattern here is, u 11  and u 21  interfere with each other, and u 12  and u 22  interfere with each other. 
         [0019]    Such an interference pattern is inferior to that shown in  FIG. 8 . Since u 12  is closer to the cell edge than u 11 , it is more likely to be interfered by an adjacent cell. Therefore, in a better interference pattern, u 12  is placed on the same frequency with u 21 , while u 11  is placed on the same frequency with u 22 . In such a pattern, user u 12  with the poorest transmission condition will be least interfered, thus achieving higher rates; while the largest interference produced by u 12  is exerted on user u 21  with the best transmission condition. As compared with the interference pattern in  FIG. 7 , the interference pattern in  FIG. 8  improves the lowest rate at the cell edge at the cost of lowering the highest rate in the cell center. This can improve users&#39; experience and reduce complaints, thereby facilitating operations. 
         [0020]    However, the existing soft frequency reuse scheme with only two UPDLs cannot provide more accurate restriction to achieve such a superior interference pattern. This is especially true in relatively complex networks. The performance of the system thus still has a relatively large room for improvement. 
       SUMMARY OF THE INVENTION 
       [0021]    In order to further optimize the interference pattern of soft frequency reuse, and improve the network performance, the present invention provides a method, a device, and a system for achieving multilevel soft time-frequency reuse and resource allocation in a radio communication system. 
         [0022]    The radio communication system of the present invention comprises at least two adjacent cells C i  (i=1, 2), which use time-frequency resource S for communication. According to the present invention, a cell refers to a geometric area, and uses all the time-frequency resource S. In an actual system, two different carrier frequencies in the same geographic area may form two logical cells, with different cell identities. According to the present invention, these two logical cells belong to the same cell, which can be either an omnidirectional cell or a sectorized cell. In the present invention, two cells are adjacent means that the two cells have overlapping coverage regions. Because different time-frequency resources are to be configured with different power densities, it is required that the radio communication system can divide the time-frequency resources S into a plurality of (e.g., at least two as will be mentioned in the detailed embodiments) resource units. Each resource unit is one time-frequency resource block for which a separate power density can be set. A technical solution of the multilevel soft time-frequency reuse according to the embodiments of the present invention is as follows. 
         [0023]    The time-frequency resource S is divided into N non-overlapping resource groups, denoted as RG n  (n=1, 2, . . . , N) with N being an integer larger than 1. Each of the resource groups comprises a plurality of (e.g., at least two as will be mentioned in detailed embodiments) resource units. 
         [0024]    In cell C i  (i=1, 2), RG n  (n=1, 2, . . . , N) is divided into two non-overlapping portions, of which, one portion is configured with a UPDL h (i)   n  and termed primary sub-resource-group (SRG) of RG n , and the other portion is configured with a UPDL l (i)   n  and termed secondary SRG of RG n , wherein the primary SRG and the secondary SRG, termed SRG in general, have the following relationship in terms of UPDL thereof: l (i)   1 /h (i)   1 &lt;l (i)   2 /h (i)   2 &lt; . . . &lt;l (i)   N /h (i)   N ≦1, and if N=2, then l (i)   2 ≠h (i)   2 . Each SRG is configured with a UPDL, in such a manner that the transmit power density of a transmitter in the SRG does not exceed the UPDL of the SRG. 
         [0025]    The primary SRGs of two adjacent cells do not overlap with each other in each resource group. 
         [0026]    In one embodiment of the present invention, the primary SRG and the secondary SRG are termed SRG in general. The way how a time-frequency resource S is divided into resource groups and SRGs, and the UPDLs of all SRGs are termed PDM information. 
         [0027]    As can be seen, in the multilevel soft time-frequency reuse scheme, a soft time-frequency reuse scheme is used on each resource group among the N resource groups. The ratio between the UPDL of the secondary SRG and the UPDL of the primary SRG in each resource group, i.e., l (i)   n /h (i)   n , increases with the index number of the resource group in each of two adjacent cells. If the partitioning of the resource groups is maintained, with only the index numbers of the resource groups being re-specified, and the ratios l (i)   n /h (i)   n  of two adjacent cells are sorted from small to large to obtain two sequences, the indexes of which will be the same. 
         [0028]    In practice, it is reasonable that a relatively high UPDL corresponds to a relatively large coverage area, and thus typically further specifies: l (i)   1 ≦l (i)   2 ≦ . . . ≦l (i)   N ≦h (i)   N ≦ . . . ≦h (i)   2 ≦h (i)   1  (i=1, 2). Such configuration enables the pair and mutual interference between the resource with the highest/second highest UPDL in a cell and the resource with the lowest/second lowest UPDL in its adjacent cell, thereby achieving an optimized interference pattern. In an interference-limited scenario, i.e., interference is much higher than noise, communication quality will not be influenced if the power density of a certain resource is amplified by a same factor in all cells, i.e., user signal and interference are increased by the same factor. Therefore, based on the relationship l (i)   1 ≦l (i)   2 ≦ . . . ≦l (i)   N ≦h (i)   N ≦ . . . ≦h (i)   2 ≦h (i)   1  (i=1, 2), l (i) n/h (i)   n  will keep unchanged if l (i)   n  and h (i)   n  (i=1, 2) are both multiplied by a same factor, and so does the communication quality. The order of UPDLs, however, might be destroyed, and an SRG with a higher UPDL might unexpectedly cover a smaller area, thus causing the waste of power. 
         [0029]    It can be further specified that l (i)   1 &lt;a (i) &lt;b (i) &lt;h (i)   1  (i=1, 2), wherein a (i)  and b (i)  refer to any two different UPDLs among h (i)   n  (n=2, 3, . . . , N) and l (i)   n  (n=2, 3, . . . , N). That is, at least four different values of UPDL are configured. 
         [0030]    There are only two UPDLs in the soft frequency reuse scheme, and a cell is accordingly divided into two regions, a cell edge and a cell center. In the multilevel soft time-frequency reuse scheme, however, if N=2, the cell edge will be further divided into two regions, which may be termed a farthermost edge and a second farthermost edge. Besides, the cell center will also be divided into two regions, which may be termed a most central region and a sub-central region. A terminal at the farthermost edge of a cell will be allocated a resource with the highest UPDL, and such a resource has the lowest UPDL in the adjacent cell, and thus can be allocated to a terminal at the most central region of the adjacent cell. As can be seen from the above discussion, this is a superior interference pattern, which can prevent the mutual interference between the farthermost region and the sub-central region, thereby improving the performance of the system. 
         [0031]    Such an analysis further indicates that the larger number N of resource groups, and the more levels of UPDL can provide more accurate constraints and achieve better interference pattern. 
         [0032]    The multilevel soft time-frequency reuse scheme can be used in both uplink and downlink. 
         [0033]    The base station of the present invention, which refers to a system device or function for formation and control of a cell, can be either a device node, or distributed among a plurality of device nodes. 
         [0034]    The base station can modify the PDM parameter of a cell if necessary. 
         [0035]    When the scheme is used in uplink, the base station broadcasts the PDM information in the cell. In uplink, a user terminal needs the PDM information to control transmit power density. Therefore, it is necessary for the base station to transmit the PDM information to the user terminal. The semi-static PDM information can be regarded as parameter of a cell, and it would be appropriate to transmit the PDM information as system information through a broadcast channel. 
         [0036]    Similar to soft frequency reuse, the PDM determined by the multilevel soft time-frequency reuse scheme provides a framework for resource allocation in each cell. In resource allocation in each cell, each SRG is specified with a coverage area (in other words, a separate coverage area is specified for each SRG), and the SRG with a higher UPDL is configured with a larger coverage area. 
         [0037]    The base station continuously transmits a reference signal with a constant power, which is received by the user terminal. The received power is denoted as RSRP. An RSRP threshold is determined for each SRG, in such a manner that the SRG with a higher UPDL is configured with a smaller RSRP threshold, while the SRG with a lower UPDL is configured with a larger RSRP threshold. 
         [0038]    The user terminal measures the RSRP, and reports it to the base station. An RSRP offset is configured for each terminal, and is used to modify the RSRP reported by the terminal, to obtain a modified RSRP for the user terminal. The base station compares the modified RSRP with the RSRP threshold of each SRG. The SRG with an RSRP threshold smaller than or equal to the modified RSRP of the user terminal is the one covering said user terminal. So a list of SRGs covering the user terminal can be established. More specifically, if the modified RSRP of the user terminal is equal to or larger than the RSRP threshold of an SRG, then the user terminal is judged to be within the coverage area of the SRG. The modified RSRP is compared with the RSRP thresholds of all SRGs, so as to establish a list of SRGs covering the user terminal. 
         [0039]    The RSRP offset is a parameter of a user terminal, and will be reported to the base station when the terminal gets access to a cell. The base station can modify the RSRP offset of the user terminal. 
         [0040]    The base station sorts all the SRGs in the order of the RSRP thresholds thereof. The SRGs with their RSRP thresholds smaller than or equal to the modified RSRP of the user terminal will be added into the list of SRGs covering the terminal. When such a sorting procedure is employed, the list of SRGs covering the user terminal can be quickly established. 
         [0041]    In a mobile communication system, the user terminal may be in movement, such that the SRGs covering it will change. Therefore, the user terminal can measure the RSRP periodically, and report the measured results to the base station. The base station can thus update the list of SRGs covering the user terminal according to the RSRP measurement reports. 
         [0042]    A user terminal can use the SRG that covers the user terminal, i.e., the SRG in the list of SRGs covering the user terminal, and will not use any SRG that is not in the list of SRGs covering the user terminal. 
         [0043]    A user of the present invention refers to a data channel between the terminal and the base station at the application level. The data channel can be either uplink or downlink, e.g., a logical channel in the 3GPP medium access control (MAC) protocol. 
         [0044]    The present invention further discloses a resource allocation method based on the multilevel soft time-frequency reuse scheme, comprising the steps of: establishing, for each user terminal participating in resource allocation, a list of SRGs covering the user terminal; allocating resources, for each user in data transmission, first in the SRG that covers the user terminal and has the smallest coverage area; if resources are insufficient in the SRG with the smallest coverage area, allocating resources in the SRG with the second smallest coverage area, and so on; and repeating, if the user acquires the required resources, the above steps for the next user, until all users acquire the required resources, or no resources are available. 
         [0045]    From a functional perspective only, resources can be allocated to one user in any SRG covering the user terminal. In terms of performance of the system, however, if an SRG with a large coverage area is prematurely allocated to a cell center user, no resources might be left for cell edge users, while SRG with small coverage area might be in spare. Therefore, the SRG with smallest coverage area will be first allocated, enabling efficient utilization of resources, especially in heavy-loaded network. 
         [0046]    According to the present invention, the result of resource allocation is termed resource allocation information. 
         [0047]    The resource allocation method based on the multilevel soft time-frequency reuse scheme can be used in both uplink and downlink. When the method is used in downlink, resource allocation and data transmission are both performed at the base station, and the resource allocation information can be directly used by the transmitter. When the method is used in uplink, the resource allocation is performed at the base station, while the transmitter is at the user terminal. Therefore, the base station will transmit the resource allocation information to the user terminal, so as to enable the user terminal to transmit user data on the allocated resources. 
         [0048]    The power density mask can be further configured to let at least two primary SRGs in at least one cell of C 1  and C 2  have consecutive index numbers, which are used to identify the resources during resource allocation. In this way, when a user needs to be allocated resources in these two primary SRGs, the resources can be conveniently identified. Particularly, if the index numbers of all the primary SRGs are consecutive, the consecutiveness of the index numbers of secondary SRGs will also be favorable, thereby rendering it more convenient to identify the resources. 
         [0049]    The present invention further discloses a base station device, termed BS 0 , which can realize multilevel soft time-frequency reuse scheme in a radio communication system. BS 0  comprises the following modules: 
         [0050]    a transmitter module: continuously transmitting a reference signal with a constant power; 
         [0051]    a receiver module: receiving RSRP and RSRP offset transmitted by a user terminal, and transmitting them to a terminal coverage SRG list generation module; 
         [0052]    a PDM configuration module: dividing time-frequency resource S into N non-overlapping resource groups, denoted as RG n  (n=1, 2, . . . , N), N being an integer larger than 1, and each of the resource groups including at least two resource units; dividing RG n  (n=1, 2, . . . , N) into two non-overlapping portions, of which, one portion is configured with a UPDL h n  and termed primary SRG of RG n , and the other portion is configured with a UPDL l n  and termed secondary SRG of RG n , wherein the SRGs have the following relationships in terms of UPDL thereof: l(i) 1 /h(i) 1 &lt;l(i) 2 /h(i) 2 &lt; . . . &lt;l(i) N /h(i) N ≦1, and l(i) 1 ≦l(i) 2 ≦ . . . ≦l(i) N ≦h(i) N ≦ . . . ≦h(i) 2 ≦h(i) 1 , and if N=2, then l (i)   2 ≠h (i)   2 ; and transmitting PDM information to a coverage threshold generation module and the transmitter module; 
         [0053]    the coverage threshold generation module: determining one RSRP threshold for each SRG, in such a manner that the SRG with a higher UPDL is configured with a smaller RSRP threshold, while the SRG having a lower UPDL is configured with a larger RSRP threshold; and transmitting RSRP threshold information to the terminal coverage SRG list generation module; and 
         [0054]    the terminal coverage SRG list generation module: modifying the RSRP transmitted by each user terminal according to the RSRP offset transmitted by the user terminal, so as to obtain a modified RSRP; comparing the modified RSRP of the user terminal with the RSRP threshold of each of the SRGs; and determining the SRGs with RSRP thresholds smaller than or equal to the modified RSRP of the user terminal to be the SRGs covering said user terminal, so as to establish a list of SRGs covering the user terminal. 
         [0055]    The base station device BS 0 , while being used in downlink, is termed BS 1 . As shown in  FIG. 9 , BS 1  can further comprise the following module: 
         [0056]    a resource allocation module: first allocating resources, for a user in data transmission, in the SRG which covers the user terminal and has the smallest coverage area, and if resources are insufficient in the SRG with the smallest coverage area, allocating resources in the SRG which covers the user terminal and has the second smallest coverage area, and so on; repeating, if said user acquires required resources, the above step for the next user, until all users acquire required resources, or no resources are available; and transmitting the resource allocation information to the transmitter module, under which circumstance, the aforementioned transmitter module is further used for: scheduling resources for user according to the resource allocation information, transmitting user data, and controlling the transmission power density of a transmitter on each SRG, so as to enable it not to exceed the UPDL of the SRG. 
         [0057]    The base station BS 0 , while being used in uplink, is termed BS 2 . As shown in  FIG. 10 , BS 2  further comprises the following modules: 
         [0058]    a terminal coverage SRG list generation module: transmitting, for a user terminal, the list of SRGs covering the user terminal to a resource allocation module; and 
         [0059]    the resource allocation module: first allocating resources, for a user in data transmission, in the SRG which covers the user terminal and has the smallest coverage area, and if resources are insufficient in the SRG with the smallest coverage area, allocating resources in the SRG which covers the terminal and has the second smallest coverage area, so on and so forth; and repeating, if said user acquires required resources, the above step for a next user, until all users acquire required resources, or until no resources are available; and transmitting the resource allocation information to the transmitter module, 
         [0060]    wherein the transmitter module is further used for: broadcasting PDM information in a cell, and transmitting the resource allocation information determined by the resource allocation module to the user terminal. 
         [0061]    The present invention still further discloses a terminal device that achieves multilevel soft time-frequency reuse in a radio communication system. As illustrated in  FIG. 11 , the terminal device comprises the following modules with respective functions: 
         [0062]    a receiver module, which receives a reference signal transmitted by a base station, transmits the reference signal to a measurement module, receives resource allocation information and PDM information broadcasted by the base station, and transmits the PDM information and the resource allocation information to a transmitter module; 
         [0063]    the measurement module, which measures RSRP, and generates an RSRP measurement report comprising the measured RSRP, and transmits the RSRP measurement report to a transmitter module; and 
         [0064]    the transmitter module, which transmits the RSRP measurement report and RSRP offset to the base station, schedules resources for user data according to the resource allocation information, transmits user data, and control the transmission power density of a transmitter on each SRG, so as to enable it not to exceed the UPDL of the SRG. 
         [0065]    The present invention further discloses a radio communication system that realizes multilevel soft time-frequency reuse scheme, comprising at least two base station devices (BS 0 , BS 1 , or BS 2 ) according to the technical solution as explained above that form two adjacent cells, wherein the primary SRGs of the two adjacent cells do not overlap with each other in each resource group. The multilevel soft time-frequency reuse scheme as provided in the present invention can thus be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0066]    The accompanying drawings are provided for further understanding of the present invention, and constitute one part of the description. They serve to explain the present invention in conjunction with the embodiments, rather than to limit the present invention in any manner. In the drawings: 
           [0067]      FIG. 1  shows power density mask of fractional frequency reuse; 
           [0068]      FIG. 2  shows coverage areas of frequency bands in fractional frequency reuse in the scenario of omnidirectional cells; 
           [0069]      FIG. 3  shows coverage areas of frequency bands in fractional frequency reuse in the scenario of three-sector cells; 
           [0070]      FIG. 4  shows power density mask of soft frequency reuse; 
           [0071]      FIG. 5  shows coverage areas of frequency bands in soft frequency reuse in the scenario of omnidirectional cells; 
           [0072]      FIG. 6  shows coverage areas of frequency bands in soft frequency reuse in the scenario of three-sector cells; 
           [0073]      FIG. 7  shows an inferior interference pattern in soft frequency reuse; 
           [0074]      FIG. 8  shows a superior interference pattern in soft frequency reuse; 
           [0075]      FIG. 9  shows a schematic diagram of a base station device that realizes multilevel soft time-frequency reuse in downlink; 
           [0076]      FIG. 10  shows a schematic diagram of a base station device that realizes multilevel soft time-frequency reuse in uplink; 
           [0077]      FIG. 11  shows a schematic diagram of a terminal device that realizes multilevel soft time-frequency reuse in a radio communication system; 
           [0078]      FIG. 12A  shows a time-frequency plane divided into four groups of resource units in the frequency dimension; 
           [0079]      FIG. 12B  shows the time-frequency plane divided into four groups of resource units in the time dimension; 
           [0080]      FIG. 12C  shows the time-frequency plane divided into four groups of resource units in time and frequency dimensions; 
           [0081]      FIG. 13  shows power density mask of multilevel soft time-frequency reuse in the scenario of two resource groups and two adjacent cells; 
           [0082]      FIG. 14  shows coverage areas of SRGs in multilevel soft time-frequency reuse in the scenario of two resource groups and two adjacent cells; 
           [0083]      FIG. 15  shows coverage areas of SRGs in multilevel soft time-frequency reuse in the scenario of two resource groups and hierarchical cells; 
           [0084]      FIG. 16  shows power density mask of multilevel soft time-frequency reuse in the scenario of three resource groups and two adjacent cells; 
           [0085]      FIG. 17  shows coverage areas of SRGs in multilevel soft time-frequency reuse in the scenario of three resource groups and two adjacent cells; 
           [0086]      FIG. 18  shows power density mask of multilevel soft time-frequency reuse in the scenario of two resource groups and three adjacent cells; 
           [0087]      FIG. 19  shows coverage areas of SRGs in multilevel soft time-frequency reuse in the scenario of two resource groups and three adjacent omnidirectional cells; and 
           [0088]      FIG. 20  shows coverage areas of SRGs in multilevel soft time-frequency reuse in the scenario of two resource groups and three adjacent sector cells. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0089]    The present invention is described in detail in conjunction with the accompanying drawings to explain the purpose, technical solutions, and advantages of the present invention more explicitly. 
         [0090]    Time and frequency are resources of radio communication. The time-frequency resource used in the present invention is denoted as S. Typically, S can be a time-frequency plane B×T, wherein B is the frequency bandwidth, and can be either a continuous frequency band, or several discontinuous segments of spectrum; while T is a period of time, generally continuous. Under rather rare conditions, T can be discontinuous as well. 
         [0091]    In most radio communication systems, the time resource is divided into equally long time periods (termed frame) for communication. For example, the frame length of the LTE system is 10 ms, and one frame can be further divided into 10 sub-frames, each of which lasts for 1 ms. Thus, in the time dimension, a sub-frame can be regarded as the granularity for resource partition. In the frequency dimension, each carrier has a certain bandwidth. For example, WCDMA system has a carrier bandwidth of 5 MHz, and LTE system has 6 carrier bandwidths including 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. Therefore, a carrier can be regarded as the granularity for resource partition. 
         [0092]    In OFDM based radio communication system, one carrier can be divided into even smaller sub-carriers in the frequency dimension. For example, the width of a sub-carrier of the LTE system is 15 kHz. In order to prevent fragmentation of resources, the basic unit for resource allocation in LTE system is 12 sub-carriers, i.e., 180 kHz, such that 180 kHz can be regarded as the granularity for resource partition in the frequency dimension. 
         [0093]    One resource unit can be selected as one partition granularity in the frequency dimension×partition granularity in the time dimension, the UPDL of which can be in dependently configured. 
         [0094]    Typically, S is a time-frequency plane B×T. Under some circumstances, however, a portion of the time-frequency plane B×T will be used for some special purposes. In the LTE system, for example, a portion of the time-frequency resources are used as common reference signals (CRS). In this case, S can be a part of the time-frequency plane B×T. In the example of the LTE system, S may be the remaining part of the time-frequency plane apart from the resources occupied by the CRS. 
         [0095]    Time-frequency resource S can be divided in different manners. As illustrated in  FIGS. 12A, 12B, and 12C , the time-frequency plane B×T is divided into four groups of resource units B 1 , B 2 , B 3 , and B 4  in the frequency dimension, time dimension, time-frequency dimensions, respectively, wherein each resource group can comprise one or more resource units. 
         [0096]    Each of the four groups of resource units B 1 , B 2 , B 3 , and B 4  can be either a block of continuous resources or several blocks of discontinuous time-frequency resources. 
         [0097]    According to one embodiment of multilevel soft time-frequency reuse, the time-frequency plane B×T is divided into two resource groups, wherein a first resource group comprises B 1  and B 2 , and a second resource group comprises B 3  and B 4 . In cell  1 , B 1  and B 3  are the primary SRGs, and B 2  and B 4  are the secondary SRGs. In cell  2 , B 2  and B 4  are the primary SRGs, and B 1  and B 3  are the secondary SRGs. In the two cells, the SRGs are generally configured to have different coverage areas, with different UPDLs satisfying the following relationships: l (i)   1 &lt;l (i)   2 &lt;h (i)   2 &lt;h (i)   1 , (i=1, 2). With h (i)   1  taken as the reference, relative gains of the UPDLs can be selected as −12 dB, −8 dB, −4 dB, and 0 dB, respectively, and the value of h (i)   1  can be selected according to cell coverage, to be 45 dBm/20 MHz, for instance.  FIG. 13  shows power density mask of such an embodiment, and  FIG. 14  shows coverage areas of the SRGs in a scenario of two adjacent cells. Under some special occasions, equality can be taken, for example, l (i)   1 &lt;l (i)   2 &lt;h (i)   2 =h (i)   1 , or l (i)   1 =l (i)   2 &lt;h (i)   2 &lt;h (i)   1 . If l (i)   1 &lt;l (i)   2 =h (i)   2 &lt;h (i)   1 , (i=1, 2), and then the solution will be fractional frequency reuse (l (i)   1 =0) or the combination of soft frequency reuse and reuse 1 (l (i)   1 ≠0). 
         [0098]    The time-frequency plane B×T can be divided into resource groups in other way. For example, the first resource group can comprise B 1  and B 3 , while the second resource group comprises B 2  and B 4 . In cell  1 , B 1  and B 2  may be selected as the primary SRGs, and B 3  and B 4  may be selected as the secondary SRGs; while in cell  2 , B 3  and B 4  may be selected as the primary SRGs, and B 1  and B 2  may be selected as the secondary SRGs. Such an embodiment may enable SRGs with similar UPDLs to be grouped together. When allocating resources in two or more SRGs with similar UPDLs for one user, continuous resources can be conveniently identified. For example, if the range of index numbers of B 1  is 1-50, and the range of index numbers of B 2  is 51-100, then a sign like 40-80 can identify the resources across two SRGs. If the index numbers of B 1  and B 2  are discontinuous, however, it will be necessary to identify the allocated resources in the two SRGs separately, which is less convenient. 
         [0099]    The combination mode of the power density masks of the two cells can be extended in a plurality of cells along a curve, so as to form coverage in a linear region. That is, the power density masks of the two cells can be repeated in such a manner as cell  1 , cell  2 , cell  1 , cell  2 , cell  1 , and cell  2 . 
         [0100]    The power density mask shown in  FIG. 13  can be also used in a hierarchical cell structure illustrated in  FIG. 15 . In such a structure, cell  1  is a macro-cell used for wide coverage, while cell  2  is a microcell used to cover blind spots or hot spots. The microcell has a relatively small radius, and is completely or partially located within the coverage area of cell  1 . 
         [0101]    The time-frequency resource S may be divided into more resource groups. For example, in the power density mask illustrated in  FIG. 16 , three resource groups are formed, i.e., a first resource group including B 1  and B 6 , a second resource group including B 2  and B 5 , and a third resource group including B 3  and B 4 . In cell  1 , B 1 , B 2 , and B 3  may be selected as the primary SRGs, B 6 , B 5 , and B 4  may be selected as the secondary SRGs. In cell  2 , B 6 , B 5 , and B 4  may be selected as the primary SRGs, B 1 , B 2 , and B 3  may be selected as the secondary SRGs. In the two cells, the UPDLs satisfy the following relationships: l (i)   1 &lt;l (i)   2 &lt;l (i)   3 &lt;h (i)   3 &lt;h (i)   2 &lt;h (i)   1 , (i=1, 2). With h (i)   1  taken as the reference, the relative gains of the UPDLs can be selected as −15 dB, −12 dB, −9 dB, −6 dB, −3 dB, and 0 dB, respectively. As can be seen, the UPDL of the SRG in the power density mask varies monotonically with the index number of resource unit. The coverage areas of the SRGs in a scenario of two adjacent cells are shown in  FIG. 17 . 
         [0102]    The number of resource groups can be further increased. The more the number of resource groups, the more levels of UPDL there will be, and interference pattern will be more optimized, thereby improving performances of the system. However, an excessive amount of resource groups would lead to fragmentation of resources, which is unfavorable for resource allocation. 
         [0103]    Multilevel soft time-frequency reuse is more typically used in coverage of a plane.  FIG. 18  shows power density mask of multilevel soft time-frequency reuse scheme used in three adjacent cells in a plane. The time-frequency resource S is divided into six resource units B 1 , B 2 , B 3 , B 4 , B 5 , and B 6 . These resource units constitute two resource groups, i.e., a first resource group including B 1 , B 2 , and B 3 , and a second resource group including B 4 , B 5 , and B 6 . In cell  1 , B 1  and B 4  are primary SRGs, and others are secondary SRGs. In cell  2 , B 2  and B 5  are primary SRGs, and others are secondary SRGs. In cell  3 , B 3  and B 6  are primary SRGs, and others are secondary SRGs. In the three cells, the UPDLs satisfy the following relationships: l (i)   1 &lt;l (i)   2 &lt;h (i)   2 &lt;h (i)   1 , (i=1, 2, 3).  FIG. 19  shows coverage areas of the SRGs in a scenario of omnidirectional cells, and  FIG. 20  shows coverage areas of the SRGs in a scenario of three-sector cells. 
         [0104]    In covering a plane, although the UPDL cannot monotonically increase or decrease along with the index number of resource unit, the primary SRGs can still be continuous, so as to facilitate identification of resources. For example, the first resource group can comprise B 1 , B 3 , and B 5 , and the second resource group then comprises B 2 , B 4 , and B 6 . In cell  1 , B 1  and B 2  are primary SRGs, and others are secondary SRGs. In cell  2 , B 3  and B 4  are primary SRGs, and others are secondary SRGs. In cell  3 , B 5  and B 6  are primary SRGs, and others are secondary SRGs. 
         [0105]    The combination mode of the power density masks of the three cells can be extended in the plane, so as to cover the entire plane. 
         [0106]    In the present embodiment, the primary SRG accounts for ⅓ of each resource group. Other proportions, such as ¼, can also be selected for irregular cells or 3 dimensional network. 
         [0107]    The base station can change the PDM configuration of a cell, so as to be adapted to variations in traffic distribution, radio propagation environment, etc. 
         [0108]    Because different SRGs have different UPDLs, coverage areas thereof will be different also. Therefore, it is necessary for the base station to determine whether a user terminal is within the coverage area of an SRG, so as to determine which SRG the user terminal can use. 
         [0109]    The base station continuously transmits a reference signal with a constant power, which can be realized in different ways. For example, in the LTE system, common reference signal (CRS) can be selected as the reference signal, and in some other systems, a broadcast channel or synchronization channel can be selected as the reference signal. 
         [0110]    Reference signal receiving power (RSRP) of a user terminal is the received power of the reference signal at the user terminal, generally in dBm. As the reference signal has a constant transmission power, the closer the user terminal is to the cell edge, the smaller the RSRP will be. Therefore, RSRP can be regarded as a metric of the distance of the user terminal to the base station. Because the uplink is relevant with downlink in terms of large-scale fading, such an approach is applicable for both uplink and downlink, possibly with different specific numerical values though. 
         [0111]    Each SRG is configured with an RSRP threshold, which geographically corresponds to a curve surrounding a base station antenna. The region surrounded by the curve is the coverage area of the SRG. An SRG with a relative high UPDL has a relatively large coverage area. At a far end of the coverage area, the RSRP of the user terminal will be relatively small, and thus a relatively small RSRP threshold should be configured. 
         [0112]    As different user terminals might have different capabilities of reception or transmission, an offset, usually indicated by dB, can be additionally configured on the basis of the RSRP measured by the terminal. The RSRP of a user terminal, after being modified by the offset, is compared with the RSRP threshold of the SRG. The RSRP of a user terminal with strong capability can be increased, such that the terminal can use an SRG in a larger region; while the RSRP of a user terminal with weak capability can be reduced, such that the user terminal can use the SRG in a smaller region. If the RSRP is not modified, the offset of the RSRP is 0 dB, which is a special example. 
         [0113]    The RSRP offset can be stored in a user terminal as a parameter, and reported to the base station when the user terminal is connected to a cell. If the user terminal fails to report the RSRP offset, the base station can configure the offset to be 0 dB by default. The base station can modify the RSRP offset of the user terminal if necessary. 
         [0114]    For each user in communication, a list of SRGs covering the user terminal is established. Resources in the list can be allocated to the user, while resources not in the list cannot be allocated to said user. The list of SRGs can be established by comparing the modified RSRP of the terminal with the RSRP threshold of each SRG. 
         [0115]    As the UPDL and RSRP threshold of an SRG are generally static or semi-static, the list of SRGs covering a user terminal can be established in a relatively rapid manner through a sorting procedure. For example, if there are six SRGs in a system, their RSRP thresholds can be sorted as follows in ascending order. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Index number of SRG 
               
             
          
           
               
                   
                 1 
                 3 
                 6 
                 5 
                 4 
                 2 
               
               
                   
                   
               
             
          
           
               
                 RSRP threshold (dBm) 
                 −100 
                 −90 
                 −80 
                 −70 
                 −60 
                 −50 
               
               
                   
               
             
          
         
       
     
         [0116]    If the RSRP of a user terminal is −68 dBm, and its RSRP offset is 0 dB, all SRGs with their RSRP thresholds lower than −68 dBm in the sequence will be added into the list of SRGs covering the user terminal, i.e., SRGs 1, 3, 6, and 5. 
         [0117]    If the user terminal is configured with an RSRP offset of −3 dB, i.e., the modified RSRP of the user terminal is −71 dBm, the list of SRGs covering the user terminal will be 1, 3, and 6. If the RSRP offset of a user terminal is 10 dB, the modified RSRP of the user terminal will be −58 dBm. The list of SRGs covering the terminal will then be 1, 3, 6, 5, and 4. 
         [0118]    The RSRP thresholds can also be sorted in descending order. 
         [0119]    In a mobile communication system, a user terminal may be in movement, and list of SRGs covering the user terminal may also change. A user terminal may periodically measure its RSRP, and report the results to the base station. The base station will, according to the RSRP measurement report from the user terminal, update the list of SRGs covering the user terminal. The period at which the terminal reports RSRP depends. For example, if the user terminal moves at a fast speed, a relatively short report period will be adopted; while if the user terminal moves relatively slowly, the report period will then be relatively long. 
         [0120]    The general principle for resource allocation is, a user can use the SRGs that cover the user terminal, but cannot use the SRGs that do not cover the user terminal. The following approach can be used for better performance, especially in heavy loaded network. 
         [0121]    For each user participating in resource allocation, a list of SRGs covering the user terminal is established. For a user in data transmission, resources are first allocated in an SRG that covers the user terminal and has the smallest coverage area. If the resources are insufficient, resources will then be allocated in the SRG having the second smallest coverage area, so on and so forth. When the user acquires necessary resources, the above steps will be repeatedly performed on a next user, until all users acquire necessary resources, or no resources are available for allocation. 
         [0122]    From a functional perspective only, resources can be allocated for one user in any SRG covering the user terminal. In terms of performance of the system, however, if an SRG with a large coverage area is prematurely allocated to a cell center user, no resources might be available for cell edge users, while SRGs with small coverage areas might be unused. Therefore, the SRGs with small coverage areas will be first allocated, thereby enabling efficient use of resources. 
         [0123]    It should be noted that, resource allocated to a user is not limited to time and frequency. It could be other resource such as a code in CDMA system. 
         [0124]    The resource allocation approach based on multilevel soft time-frequency reuse can be used in both uplink and downlink. When the approach is used in uplink, resource allocation is performed at the base station, while the transmitter is at the user terminal. Therefore, the base station will transmit resource allocation information to the user terminal, such that the user terminal can transmit user data on the allocated resources. In downlink, as resource allocation and data transmission are both performed at the base station, the resource allocation information can be directly used by the transmitter. However, it is still necessary for the base station to transmit the resource allocation information to the user terminal, so as to enable the user to receive data on the allocated resources. 
         [0125]    Resource allocation can be realized in different ways. If the communication system has a plurality of carriers, each of which is regarded as an SRG and form a logical cell. Under such circumstances, selection of one SRG means selection of one logical cell, which can be realized by a cell selection and reselection procedure, and then resources can be requested in the resided cell. In a broadband OFDM system, such as the LTE system, multiple users share one channel, which can be divided into a plurality of SRGs. Resource allocation can be realized through a scheduling procedure. 
         [0126]    It is necessary for the transmitter to know the PDM information to control the actual transmission power density on each SRG below the configured UPDL. In downlink, the PDM is configured by the base station, and the transmitter can be directly informed of such information. In uplink, the PDM information is configured by the base station also, and the user terminals should be informed of such information somehow. It is proper to regard the PDM information as the configuration parameter of a cell, and broadcast it as system information to be received by all terminals in the cell. Such system information can be packaged into RRC system information in the 3GPP protocol. 
         [0127]    For example, the following data structure can be used to indicate PDM information, comprising the following fields: 
         [0128]    Field 1: number of SRGs; 
         [0129]    Field 2: index range of each SRG, wherein continuous resources can be expressed with start/end, and discontinuous resources can be expressed with a plurality of starts/ends; 
         [0130]    Field 3: highest UPDL, as a reference value; and 
         [0131]    Field 4: gain of each SRG relative to the highest UPDL, expressed in dB. 
         [0132]    Each field can be expressed by a certain number of bits for multiple values. For example, if two bits are used for Field 1, four types of SRG numbers can be indicated: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 00 
                 1 
               
               
                   
                 01 
                 2 
               
               
                   
                 10 
                 4 
               
               
                   
                 11 
                 8 
               
               
                   
                   
               
             
          
         
       
     
         [0133]    The list of these values can be specified in a protocol. Other fields can be similarly treated. 
         [0134]    In some circumstances, the base station can modify the RSRP offset of a user terminal. For example, if the power control function frequently seeks to improve the power density of the transmitter on a SRG to exceed its UPDL due to various reasons, the base station may reduce the RSRP offset of the user terminal, and remove the SRG from the list of SRGs covering the user terminal. As a result, the terminal is switched onto an SRG with a higher UPDL. On the contrary, if the actual transmission power density of a user terminal is much lower than the UPDL of the SRG with the smallest coverage area for a while, the base station can increase the RSRP offset of the user terminal, so as to add an SRG having an even smaller UPDL into the list of SRGs covering the user terminal. As a result, the terminal will be switched to an SRG with a smaller UPDL. In another situation, a user terminal is relatively far from the base station, but has high need for data rate that cannot be satisfied even all the SRGs in the list of SRGs covering it are allocated to it. If this user has a high priority, and the base station decides to allocate more resources to it, the RSRP offset can then be increased, so as to enable the list of SRGs covering the user terminal to have more resources. 
         [0135]    While the embodiments of the present invention are described above, the description should not be construed as limitations of the present invention, but merely as embodiments for readily understanding the present invention. Anyone skilled in the art, within the spirit and scope of the present invention, can make amendments or modification to the implementing forms and details of the embodiments. Hence, the scope of the present invention should be subject to the scope defined in the claims.