Abstract:
A scheduling method for a wireless multi-hop relay communication system, wherein the communication system includes a base station dominating a plurality of relay stations, the scheduling method including separating the plurality of relay stations into N groups, N being a natural number, dividing a period for providing a service by the base station into N phases, wherein N is the number of the groups of the relay stations, serving the relay stations in a j th  group during an i th  phase by the base station, wherein 1≦i, j≦N, and serving a user or a subordinate relay station within service areas of the relay stations not in the j th  group during the i th  phase by the relay stations not in the j th  group.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/614,982, filed Dec. 22, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method and a system for grouping relay stations in a wireless multi-hop relay communication system. More particularly, the present invention relates to a method for scheduling a wireless multi-hop relay communication system so as to improve the transmission efficiency and capacity of the wireless multi-hop communication system. 
     Next generation mobile communication systems may be envisioned to provide high-speed, high link quality, and high security transmissions, and may also be expected to support various communication services. An effective resource schedule/allocation method may have to be established to meet different quality of service (QoS) requirements from different users. Users located at cell boundary may have worse link quality due to the long transmission distance to a base station, and users in a cell with severe shadowing effect may also have worse link quality, thereby the foregoing users may not perform high-speed data transmissions. To resolve the foregoing problem, the deployment density of base stations may be increased to shorten the propagation distances between the base stations and the users so as to improve the link quality, or more base stations may be deployed at those areas with severe shadowing for improving the link quality of users in the areas. However, the cost of the base stations and the cost of the backhaul network connections may be substantially increased by the aforementioned method. On the other hand, the transmission power of the base station may be increased to improve the link quality and to reduce the cost of the base station. However, if the transmission power is increased, not only the transmission cost but also the interference level may be increased. 
     Multi-hop relay cell architecture may be a good solution when considering all factors such as QoS, deployment cost, transmission power, and coverage area of the cell. Relay stations may be deployed within a cell to relay information from a base station to mobile stations with worse link quality, and vise versa. It has been shown that using relay stations may improve cell coverage, user throughput and system capacity. 
     Relay stations may be deployed at areas with severe shadowing or near the cell boundary, the users who may not be directly served by base station may be served by the relay stations, therefore the effective coverage area of the base station may be extended. 
     A single link with worse quality may be divided into a plurality of links with better quality so that each of the links may provide higher transmission rate. However, since the same data may be duplicated and relayed over the air multiple times for multi-hop transmissions, it may consume the radio resources. 
     Moreover, since there may be a base station and several relay stations in a cell, to improve the spectrum efficiency, multiple serving stations may be active simultaneously if the potential interference is tolerant. 
     To obtain benefits for multi-hop relay communication systems, an efficient scheduling mechanism may require arranging the transmissions of base stations and relay stations. 
     To improve the performance of a wireless communication system, a method of relay stations deployment in a Manhattan-like environment was provided in the Wireless World Initiative New Radio (WINNER) program. The Manhattan-like environment is a grid environment wherein the width of blocks is about 200 meters (m) and the width of streets is about 30 m. 
       FIG. 2  is a diagram illustrating a first layout of a base station  205  and a plurality of relay stations  201  to  204  of a single cell in a Manhattan-like environment in a conventional communication system. Referring to  FIG. 2 , the base station  205  and the relay stations  201  to  204  are disposed in the single cell, and the base station  205  and the relay stations  201  to  204  may all communicate with users through omni-directional antennas. However, since the relay stations  201  to  204  may be disposed outside a coverage area  206  of the base station  205 , each of the relay station  201  to  204  may require an additional directional antenna pointing at the base station  205  for communicating with the base station  205 , and thus may increase the hardware cost of the relay stations. 
       FIG. 3  is a diagram illustrating a transmission scheduling for a frame structure applicable to the first layout shown in  FIG. 2  within a single cell in the Manhattan-like environment. Referring to  FIG. 3 , a frame S 301  may be divided into two sub-frames S 302  and S 303 . The first sub-frame S 302  may further be divided into 5 time slots S 304  to S 308 , wherein a base station  305  may serve four relay stations  301  to  304  during the first four time slots S 304  to S 307 , respectively, and the base station  305  may serve users within an area  306  which may be directly connected to the base station during the fifth time slot S 308 . The second sub frame S 303  may be divided into two time slots S 309  and S 310 , and with the characteristics of spatial separation of the environment, the relay stations  301  and  302  may serve users within areas  307  and  308  connected thereto during the same time slot S 309 , and the relay stations  303  and  304  may serve users within areas  309  and  310  connected thereto during another time slot S 310 . 
       FIG. 4  is a diagram illustrating a layout of base stations  405 ,  415  and relay stations  401  to  404 ,  411  to  414  in a multi-cell structure in the Manhattan-like environment illustrated in  FIG. 2 . Referring to  FIG. 4 , a coverage area  406  of a single cell A and a coverage area  416  of a single cell B are arranged in a staggered way. Moreover, the base stations  405  and  415  in  FIG. 4  respectively represent the positions of the base stations in the single cell A and the single cell B, the relay stations  401  to  404  belong to the single cell A, and the relay stations  411  to  414  belong to the single cell B. 
       FIG. 5  is a diagram illustrating a transmission scheduling for a frame structure applicable to the layout shown in  FIG. 4  within the multi-cell structure in the Manhattan-like environment. Referring to  FIG. 5 , an arrangement of transmission frames between adjacent cells may be used to permute the operation orders of sub-frames S 502  and S 503  in a frame S 501  so that interference between cells may be prevented. The main purpose of the relay stations may be to extend the coverage area of the base station. However, the link quality of users at the boundary of the service range of the base station may not be improved. Moreover, all of the base stations may be idle for some time durations in the frame structure. Since base stations may be the only serving stations connected to the backhaul networks and carrying the effective data, the transmission efficiency of the base station in this design may not be desirable. 
       FIG. 6  is a diagram illustrating a second layout of a base station  605  and four relay stations  601  to  604  with omni-directional antennas in a Manhattan-like environment. Referring to  FIG. 6 , the base station  605  and the relay stations  601  to  604  may all communicate with users by using omni-directional antennas. Since the relay stations  601  to  604  are disposed within a coverage area  606  of the base station  605 , no additional directional antenna may be required by each of the relay station  601  to  604  for communicating with the base station  605 . With the design, the link quality of users in the cell boundary may be improved. 
       FIG. 7  is a diagram illustrating a transmission scheduling for a frame structure applicable to the second layout shown in  FIG. 6  with all serving stations equipped with omni-directional antennas in the Manhattan-like environment. Referring to  FIG. 7 , the base station  705  may serve four relay stations  701  to  704  sequentially during first four time slots S 701  to S 704 , and at the same time, the base station  705  may serve users directly connected to the base station  705 . The relay stations  701  and  703  may serve users connected thereto during the time slot S 705 . After that, the relay stations  702  and  704  may serve users during the next time slot S 706 . The main purpose of such a layout may be to improve the link quality of users at cell boundary. However, a complete transmission within a single cell may require at least 6 phases to be completed. When considering the multi-cell structure, because of the use of omni-directional antennas, the reuse factor of at least 2 may be required to avoid the severe inter-cell interference, and thus decreases the overall system capacity. 
     Regardless of the first layout or the second layout that all serving stations are equipped with omni-directional antennas, all the base stations and the relay stations may be idle for some time in the frame structure. Accordingly, the transmission efficiency thereof may not be desirable. It may therefore be desirable to have a scheduling method for a wireless multi-hop relay communication system for improving the transmission efficiency and capacity of the system. 
     BRIEF SUMMARY OF THE INVENTION 
     Examples of the present invention may provide a scheduling method for a wireless multi-hop relay communication system, wherein the communication system comprises a base station dominating a plurality of relay stations, the scheduling method comprising separating the plurality of relay stations into N groups, N being a natural number, dividing a period for providing a service by the base station into N phases, wherein N is the number of the groups of the relay stations, serving the relay stations in a j th  group during an i th  phase by the base station, wherein 1≦i, j≦N, and serving a user or a subordinate relay station within service areas of the relay stations not in the j th  group during the i th  phase by the relay stations not in the j th  group. 
     Some examples of the present invention may provide a scheduling method for a wireless multi-hop relay communication system, wherein the communication system includes a plurality of cells and each of the plurality of cells includes a base station and a plurality of relay stations, the scheduling method comprising separating the plurality of relay stations in each of the plurality of cells into N groups, N being a natural number, dividing a period for providing a service by the base station to each of plurality of cells into N phases, wherein N is the number of the groups of the relay stations in a cell, and the plurality of cells comprises two adjacent cells A and B, in the cell A, the base station serving the relay stations in a j th  group during an i th  phase, wherein 1≦i, j≦N, in the cell B, the base station serving the relay stations in a j group during an i th  phase, wherein 1≦k≦N, in the cell A, relay stations not in the j th  group serving a first user within the service areas of the relay stations not in the j th  group during the i th  phase, and in the cell B, relay stations not in the k th  group serving a second user within the service areas of the relay stations not in the k th  group during the i th  phase, wherein the interference between the relay stations of the j th  group in the cell A and the relay stations of the k th  group in the cell B is within an interference threshold. 
     Other examples of the present invention may provide a system for reusing radio resources, the system comprising at least one relay station, and at least one base station capable of separating the relay stations into N groups based on the intensity of a potential interference level between one of the at least one base station and each of the relay stations, N being a natural number, wherein the one base station divides a service period into N phases, wherein N is the number of groups of the relay stations, and wherein the one base station serves the relay stations in the j th  group during the i th  phase, wherein 1≦i, j≦N, and the relay stations not in the j th  group serve users within the service areas of the relay stations not in the j th  group during the i th  phase. 
     Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
       In the drawings: 
         FIG. 1  is a flow diagram illustrating a scheduling method of a wireless multi-hop relay communication system in accordance with an example of the present invention; 
         FIG. 2  is a diagram illustrating a first layout of a base station and a plurality of relay stations of a single cell in a Manhattan-like environment in a conventional communication system; 
         FIG. 3  is a diagram illustrating a transmission scheduling for a frame structure applicable to the first layout shown in  FIG. 2  within a single cell in the Manhattan-like environment; 
         FIG. 4  is a diagram illustrating a layout of base stations and relay stations in a multi-cell structure in the Manhattan-like environment illustrated in  FIG. 2 ; 
         FIG. 5  is a diagram illustrating a transmission scheduling for a frame structure applicable to the layout shown in  FIG. 4  within the multi-cell structure in the Manhattan-like environment; 
         FIG. 6  is a diagram illustrating a second layout of a base station and four relay stations with omni-directional antennas in a Manhattan-like environment; 
         FIG. 7  is a diagram illustrating a transmission scheduling for a frame structure applicable to the second layout shown in  FIG. 6  with all serving stations equipped with omni-directional antennas in the Manhattan-like environment; 
         FIG. 8  is a diagram illustrating a layout of a base station and a plurality of relay stations in a Manhattan-like environment in accordance with an example of the present invention; 
         FIG. 9  is a diagram illustrating a first phase of a transmission scheduling for an uplink transmission and a downlink transmission within a single cell in accordance with another example of the present invention; 
         FIG. 10  is a diagram illustrating a second phase of a transmission scheduling for an uplink transmission and a downlink transmission within a single cell in accordance with still another example of the present invention; 
         FIG. 11  is a diagram illustrating a first phase of a transmission scheduling for an uplink transmission and a downlink transmission between adjacent cells in accordance with yet another example of the present invention; 
         FIG. 12  is a diagram illustrating a second phase of a transmission scheduling for an uplink transmission and a downlink transmission between adjacent cells in accordance with yet still another example of the present invention; and 
         FIG. 13  is a diagram illustrating operations of a transmission scheduling during various phases of a single cell in accordance with an example of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the present examples of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The following examples will be described with a Manhattan-like environment, and those skilled in the art should be able to implement the present invention in any other environment according to the spirit of the present invention and the descriptions of the following examples. In following examples, interference level is weakened by spatial separation produced by the shadowing effect of surrounding buildings in the Manhattan-like environment. 
       FIG. 8  is a diagram illustrating a layout of a base station  805  and a plurality of relay stations  801  to  804  in a Manhattan-like environment in accordance with an example of the present invention. Referring to  FIG. 8 , a microcell may cover 690*690 square meters, and the base station  805  may be disposed at a crossroad and the four relay stations  801 ,  802 ,  803  and  804  may be disposed at intersections of two crossed streets (not numbered) with other streets in four directions (not numbered). That is, the relay stations  801  to  804  may be disposed at the intersections of the line of sight (LOS) and have non line of sight (NLOS) of the base station  805 . 
     The base station  805  may use four directional antennas or a four-sector antenna for transmitting data to users in the streets in four directions and the relay stations  801  to  804 , and the relay stations  801  to  804  may use two directional antennas or two-sector antennas for data transmission with users within the NLOS of the base station  805 . In other words, the base station  805  and the relay stations  801  to  804  may serve all users within a coverage area  811  of a cell. Wherein users within the LOS of the base station  805  may have single-hop links to the base station  805 , while users outside the LOS of the base station  805  may establish multi-hop links to the base station  805  through the relay stations  801  to  804 . 
       FIG. 1  is a flow diagram illustrating a scheduling method of a wireless multi-hop relay communication system in accordance with an example of the present invention. Referring to  FIG. 1 , after the base station  805  and the relay stations  801  to  804  are started up in step S 101 , the relay stations  801  to  804  may respectively measure the intensities of an interference level from other relay stations and base stations in step S 102 , wherein the potential interference level may be measured by measuring a data signal or a reference signal transmitted by the relay stations and base stations respectively. Furthermore, the data signal or the reference signal may include a preamble with a preamble index and a least signal strength of the wireless multi-hop relay communication system. Moreover, the intensities of the potential interference level may be measured by measuring a signal-to-interference-and-noise-ratio (SINR), a carrier-to-interference-and-noise-ratio (CINR) or a received signal strength indicator (RSSI) of the data signal or the reference signal. 
     In step S 103 , the relay stations  801  to  804  may report the measurement results thereof back to the base station  805 . Next, the base station  805  may separate the relay stations  801  to  804  into groups based on the measurement results from the relay stations  801  to  804 . The base station  805  may separate relay stations that may potentially go beyond a tolerable interference threshold into different groups. For example, the relay stations  801  and  803  may be put into a group A, while the relay stations  802  and  804  may be put into a group B. Alternatively, if the transmission target of one of the relay stations  801  to  804  is another relay station and the target relay station not capable of receiving and transmitting data at the same time, the two relay stations are put into different groups. Moreover, since the number of groups may be related to the number of phases in a transmission scheduling, and may therefore influence the efficiency of utilization of the communication system, the number of groups may be kept as small as possible. 
     In step S 104 , the base station  805  may arrange a transmission scheduling for the relay stations  801  to  804  after the relay stations  801  to  804  are grouped, wherein the number of groups may be regarded as the number of phases in a service period for the transmission scheduling. Subsequently, in step S 105 , the base station  805 , the relay stations  801  to  804  and the users may start to communicate with one another. 
     In one example, if the number of groups is N, then a service period of a complete transmission scheduling may be divided into N phases, and a downlink transmission and an uplink transmission may be contained in each phase. The service period may be the length of a frame and the frame is divided into N phases. Also, the service period may be the length of a plurality of frames and the frames altogether are divided into N phases. The downlink and the uplink transmissions during various phases in a frame may be arranged accordingly to the definition of the frame. For example, the downlink and the uplink transmissions during various phases may be arranged alternatively, or the downlink transmission of various phases are arranged before the uplink transmissions. Skilled persons in the art will understand that other examples of arrangement for the downlink and the uplink transmissions may be possible. In one example, the relay stations  801  to  804  may be separated into 2 groups and thus a service period may be divided into 2 phases. 
       FIG. 9  is a diagram illustrating a first phase of a transmission scheduling for an uplink transmission and a downlink transmission within a single cell in accordance with another example of the present invention. Referring to  FIG. 9 , during the first phase, the base station  905  may serve the relay stations  901  and  903  in a first group (referred to as the group A hereinafter) and users within LOS  906  and  907  of a base station  905  in a direction of the group A. The base station  905  may serve the group A through, for example, a downlink transmission and/or an uplink transmission. 
     The downlink transmission refers to a transmission that the base station  905  transmits data to the relay stations  901  and  903  in the group A and to the users within the LOS  906  and  907  of the base station  905  in the direction of the group A. During the same phase, the relay station  902  in a second group (referred to as the group B hereinafter) may relay the data received from the base station  905  during the previous phase to users within an NLOS of the base station  905  and within the LOS  908  and  909  of the group B, and the relay station  904  in the group B may relay the data received from the base station  905  during the previous phase to the users within the NLOS of the base station  905  and within the LOS  910  and  911  of the group B. Moreover, depending on applications, the base station  905  may be configured to serve users within service areas  912  and  913  around the base station  905  and in the direction of the group B with appropriate power control at a relatively low transmission power during the first phase. Such lower transmission power may reduce the interference in the relay stations  901  to  904  caused by the base station  905  to a level lower than a tolerable threshold. 
     The uplink transmission refers to a transmission that the relay stations  901  and  903  in the group A and the users within the LOS  906  and  907  of the base station  905  in the direction of the group A transmit data to the base station  905 . During the same phase, the relay station  902  in the group B may receive uplink data from users within the areas  908  and  909 , and the relay station  904  in the group B may receive uplink data from the users within the areas  910  and  911 . Moreover, depending on applications, the users within the service areas  912  and  913  around the base station  905  and in the direction of the group B may be allowed to transmit uplink data to the base station  905  during the first phase. 
       FIG. 10  is a diagram illustrating a second phase of a transmission scheduling for an uplink transmission and a downlink transmission within a single cell in accordance with still another example of the present invention. Referring to  FIG. 10 , during the second phase, the base station  905  may serve the group B and users within LOS  1006  and  1007  of the base station  905  in the direction of the group B. The base station  905  may serve the group B through, for example, a downlink transmission and/or an uplink transmission. 
     The downlink transmission during the second phase may refer to a transmission that the base station  905  transmits data to the relay stations  902  and  904  in the group B and the users within the LOS  1006  and  1007  of the base station  905  in the direction of the group B. During the same phase, the relay stations  901  and  903  in the group A may respectively relay data received from the base station  905  during the previous phase to users within the NLOS of the base station  905  and within the LOS  1008  to  1009  and  1010  to  1011  of the group A. Moreover, the base station  905  may be configured to serve users in the service areas  1012  and  1013  around the base station  905  and in the direction of the group A with appropriate power control at a relatively low transmission power during the second phase. 
     The uplink transmission during the second phase refers to a transmission that the relay stations  902  and  904  in the group B and the users within the LOS  1006  and  1007  of the base station  905  in the direction of the group B may transmit data to the base station  905 . During the same phase, the relay station  901  in the group A may receive uplink data from users in areas  1008  and  1009 , and the relay station  903  in the group A may receive uplink data from users within areas  1010  and  1011 . Moreover, the users within the areas  1012  and  1013  may be allowed to transmit uplink data to the base station  905  during the second phase. 
       FIG. 11  is a diagram illustrating a first phase of a transmission scheduling for an uplink transmission and a downlink transmission between adjacent cells in accordance with yet another example of the present invention. Referring to  FIG. 11 , in a multi-cell structure, service orders of transmission scheduling of two adjacent cells may be permuted with interferences between cells and signal quality of users at cell boundary. Wherein the cells adjacent to a cell A (with a coverage area  1106 ) in four directions include a cell B (with a coverage area  1116 ), a cell C (with a coverage area  1126 ), a cell D (with a coverage area  1136 ) and a cell E (with a coverage area  1146 ). A base station  1115  and relay stations  1111  to  1114  may be disposed in the coverage area  1116  of the cell B, a base station  1125  and relay stations  1121  to  1124  may be disposed in the coverage area  1126  of the cell C, a base station  1135  and relay stations  1131  to  1134  may be disposed in the coverage area  1136  of the cell D, and a base station  1145  and relay stations  1141  to  1144  may be disposed in the coverage area  1146  of the cell E. In one example, the service orders of the cells B to E may be assumed to be the same. Accordingly, only the cell B will be described by way of an example below. 
     Within the coverage area  1106  of the cell A, when the base station  1105  serves the relay stations  1101  and  1103  in the group A and users within the LOS of the base station  1105  in the direction of the group A (i.e., the group A which performs single cell transmission scheduling), the adjacent base stations in four directions, for example, the base station  1115  in the coverage area  1116  of the cell B, may serve the relay stations  1112  and  1114  in the group B and users in the LOS of the base station  1115  in the direction of the group B (i.e., the group B which performs single cell transmission scheduling). Meanwhile, the relay stations  1102  and  1104  in the group B within the coverage area  1106  of the cell A and the relay stations  1111  and  1113  in the group A within the coverage area  1116  of the cell B may perform data transmission (serving users). In another example, the base stations  1105  and  1115  may respectively transmit data to users within areas  1107  to  1108  and  1117  to  1118  at a relatively low transmission power. 
       FIG. 12  is a diagram illustrating a second phase of a transmission scheduling for an uplink transmission and a downlink transmission between adjacent cells in accordance with yet still another example of the present invention. Referring to  FIG. 12 , within the coverage area  1106  of the cell A, when the base station  1105  serves the relay stations  1102  and  1104  in the group B and users within the LOS of the base station  1105  in the direction of the group B, the adjacent base stations in four directions, for example, the base station  1115  in the coverage area  1116  of the cell B, may serve the relay stations  1111  and  1113  in the group A and users within the LOS of the base station  1115  in the direction of the group A. Meanwhile, the relay stations  1101  and  1103  in the group A within the coverage area  1106  of the cell A and the relay stations  1112  and  1114  in the group B within the coverage area  1116  of the cell B may perform data transmission (serving users). In another example, the base stations  1105  and  1115  respectively transmit data to users within areas  1207  to  1208  and  1217  to  1218  at a relatively low transmission power. 
       FIG. 13  is a diagram illustrating operations of a transmission scheduling during various phases of a single cell in accordance with an example of the present invention. Referring to  FIG. 13  and also  FIGS. 9 and 10 , operations S 1311  and S 1312  during a first phase S 1310  of a single cell transmission scheduling may include the fact that the base station  905  serves the relay stations  901  and  903  in the group A and the users within areas  906  and  907 . During the same phase, operations S 1313  and S 1314  of a single cell transmission scheduling S 1310  may include the fact that the relay stations  902  and  904  in the group B respectively serve the users within areas  908  to  909  and areas  910  to  911 . Moreover, based on actual requirements, the operations S 1315  and S 1316  during the first phase S 1310  of a single cell transmission scheduling may include the fact that the base station serves users within areas  912  and  913 . 
     Operations S 1323  and S 1324  during the second phase S 1320  of a single cell transmission scheduling may include the fact that the base station  905  serves the relay stations  902  and  904  in the group B and the users within areas  1006  and  1007 . During the same phase, operations S 1321  and S 1322  of the single cell transmission scheduling may include the fact that the relay stations  901  and  903  in the group A respectively serve the users within areas  1008  to  1009  and areas  1010  to  1011 . Moreover, based on actual requirements, operations S 1325  and S 1326  during the second phase S 1320  of a single cell transmission scheduling may include the fact that the base station  905  serves the users within areas  1012  and  1013 . 
     In a multi-cell structure, service orders of the transmission scheduling in the frame structures of two adjacent cells are permuted with interferences between cells and the signal quality of users at cell boundary in consideration. 
     Table 1 shows related comparisons between the present invention and the conventional technique in the communication system. In Table 1, the “frequency reuse factor” refers to the ratio of usable frequency of a single cell to the usable frequency of the system. Furthermore, since a base station is the only serving station connected to the backhaul network in a cell, the “effective frame” refers to the number of frames a base station receives and sends during a service period. Moreover, the “capacity gain” is the gain obtained with the “frequency reuse factor” and the “effective frame” in consideration. The present invention is compared to the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas of the same coverage areas. “First design example of the present invention” is a design example wherein the base station does not serve users around the base station at a relatively low transmission power, and “Second design example of the present invention” is a design example wherein the base station serves users around the base station with appropriate power control at a relatively low transmission power. 
     In the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas, data have to be transmitted between adjacent cells at different frequencies to prevent interference between adjacent cells. Accordingly, the “frequency reuse factor” thereof is ½. In this design, 6 phases are needed to complete a downlink transmission and/or an uplink transmission. The actual number of frames transmitted by the base station is 4, and thus the “effective frame” is ⅔ (= 4/6). 
     According to the first design example of the present invention, data may be transmitted at the same frequency between adjacent cells. Accordingly, the “frequency reuse factor” thereof is 1. And during the two phases of a complete downlink transmission, the base station actually transmits 4 frames, and thus the “effective frame” thereof is 2. The uplink transmission is similar to the downlink transmission. Furthermore, if it is assumed that the “capacity gain” of the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas is 1, then the first design of the present invention may exceed 2 times in the usage of frequency spectrum. The “effective frame” of the first design of the present invention is 3 times that of the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas, resulting in a “capacity gain” of “6.” 
     In the second design example of the present invention, since data may be transmitted at the same frequency between adjacent cells, the “frequency reuse factor” thereof is 1. During the 2 phases of a complete downlink transmission, the base station actually transmits 8 frames, and thus the “effective frame” is 4. The uplink transmission is similar to the downlink transmission. Furthermore, if the “capacity gain” of the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas is assumed to be 1, then the first design of the present invention may exceed 2 times in the usage of frequency spectrum. The “effective frames” of the first design of the present invention is 6 times that of the second setup in the WINNER&#39;s design with all serving stations equipped with omni-directional antennas, resulting in a “capacity gain” of “12.” 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparisons between examples of the present invention and 
               
               
                 conventional technique 
               
             
          
           
               
                   
                 Frequency reuse 
                 Effective 
                 Capacity 
               
               
                   
                 factor 
                 frames 
                 gain 
               
               
                   
               
             
          
           
               
                 The second setup in the 
                 ½ 
                 ⅔ 
                 1 
               
               
                 WINNER&#39;s design with 
                   
                   
                   
               
               
                 all serving stations 
                   
                   
                   
               
               
                 equipped with omni- 
                   
                   
                   
               
               
                 directional antennas. 
                   
                   
                   
               
               
                 First design example of the 
                 1 
                 2 
                 6 
               
               
                 present invention. 
                   
                   
                   
               
               
                 Second design example of 
                 1 
                 4 
                 12 
               
               
                 the present invention. 
               
               
                   
               
             
          
         
       
     
     In summary, according to examples of the present invention, in a wireless multi-hop relay communication system, the service areas of the base station and relay stations may be divided into a plurality of regions by using the shadowing effect of the surroundings. The intensity of an interference level may be measured by each of the relay stations and sent to the base station, based on which the base station may separate the relay stations into different groups so that the base station may serve the groups sequentially in time domain. With desirable isolation from interference signals due to shadow effect, the same radio resources may be reused and scheduled for different relay stations, thereby improving the system capacity with insignificant interference increment. In a multi-cell structure, universal frequency reuse may be achieved by permuting the group service orders of transmission scheduling of adjacent cells. Through the mechanism of grouping and permutation of transmission scheduling, interference inside a single cell and between adjacent cells may be prevented and high spectrum efficiency may be achieved through aggressive radio frequency reuse. Furthermore, in the transmission scheduling structure provided by the present invention, the base station may transmit data during various phases so that the effective cell/system capacity may be improved considerably. 
     It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 
     Further, in describing representative examples of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.