Abstract:
A method and apparatus for selecting a Relay Station (RS) for a Mobile Station (MS) of a communication system are provided. The communication system is provided with the MS, a serving Base Station (BS) for providing the MS with a service and RSs for relaying signals of the MS and the BS. The MS receives RS information for increasing system capacity from the serving BS, measures reference signal strength of an RS indicated in the RS information, and selects an associated RS as a target RS for relaying a signal when the measured reference signal strength satisfies a condition.

Description:
PRIORITY 
     This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Dec. 27, 2005 and assigned Ser. No. 2005-130853, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a communication system using a multihop relay scheme, and more particularly to a method and system for selecting a relay station by a mobile station. 
     2. Description of the Related Art 
     Wireless mobile communication systems are currently evolving from Third-Generation (3G) to Fourth-Generation (4G) mobile communication systems. Research is being conducted to increase system capacity and extend a wireless transmission range, i.e., service coverage, in the 4G mobile communication systems. To do so, a multihop scheme has been proposed. When a channel state between a Base Station (BS) and a Mobile Station (MS) is poor, the multihop scheme can form a multihop relay path through a Relay Station (RS) by installing the RS between the BS and the MS. The MS can receive a radio channel having a good state through the multihop relay path. Further, when the multihop relay scheme is used for the MS located inside/outside a cell boundary at which a channel state is poor, the MS can receive a data channel at a higher rate and can extend cell service coverage in terms of the system. 
       FIG. 1  illustrates a communication system using the conventional multihop relay scheme. 
     Referring to  FIG. 1 , the multihop relay communication system has a multicell structure including a cell  100  covered by a BS  110 , a cell  140  covered by a BS  150 , a cell  130  covered by an RS  120  and a cell  170  covered by an RS  160 . Multiple MSs are present in the cells  100 ,  130 ,  140  and  170 . Herein, the RSs  120  and  160  can be located inside/outside the cell areas of the BSs  110  and  150  and can relay signals of the BSs  110  and  150  to MSs  121 ,  123 ,  161  and  163 . If an area in which a signal of an RS such as the RS  160  can be received partially overlaps with a cell area of a specific BS even when the RS is not located inside the cell area of the specific BS, the RS can receive a signal of the specific BS. 
     As described above, an RS can relay a signal between a BS and an MS located outside the cell area of the BS to extend the cell coverage or can relay the signal between a BS and an MS located inside the cell area of the BS to increase the system capacity. That is, the RS not only can relay a signal of the MS located outside the cell area, but also can relay a signal of the MS located inside the cell area, thereby increasing the system capacity. Thus, the MS should be able to select the proper RS to extend cell coverage and increase system capacity. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a method and system in which an MS receives RS information and selects an RS to increase system capacity in a communication system using a multihop relay scheme. 
     In accordance with the present invention, there is provided a method for selecting an RS by an MS of a communication system provided with the MS, a serving BS for providing the MS with a service and RSs for relaying signals of the MS and the BS, including receiving RS information for increasing system capacity from the serving BS, measuring reference signal strength of an RS indicated in the RS information, and selecting an associated RS as a target RS for relaying a signal when the measured reference signal strength satisfies a condition. 
     In accordance with the present invention, there is provided a signal transmission method for selecting an RS by a serving BS cell area of a communication system provided with the MS, the serving BS for providing the MS with a service and RSs for relaying signals of the MS and the BS, including detecting the number of RSs for increasing system capacity, and providing the MS with a message by generating the message including identifier information and preamble index information of each RS mapped to the detected number of RSs. 
     In accordance with the present invention, there is provided a method for selecting an RS by an MS of a communication system provided with the MS, a serving BS for providing the MS with a service and RSs for relaying signals of the MS and the BS, including receiving a message including first RS information for increasing system capacity and second RS information for extending cell coverage, extracting only the first RS information from the message when the MS selects a target RS for increasing the system capacity, measuring reference signal strength of an RS indicated in the extracted first RS information, and selecting an associated RS as the target RS for relaying a signal when the measured reference signal strength satisfies a condition. 
     In accordance with the present invention, there is provided a communication system for selecting an RS, including an MS and a serving BS for providing the MS with a service, wherein the serving BS provides the MS with RS information for increasing system capacity, and the MS receives the RS information, measures reference signal strength of an RS indicated in the RS information, and selects an associated RS as a target RS for relaying a signal when the measured reference signal strength satisfies a condition. 
     In accordance with the present invention, there is provided a communication system for selecting an RS, including an MS and a serving BS for providing the MS with a service, wherein the serving BS provides the MS with a message including first RS information for increasing system capacity and second RS information for extending cell coverage, and the MS receives the message, extracts only the first RS information from the message when selecting a target RS for increasing the system capacity, measures reference signal strength of an RS indicated in the extracted first RS information, and selects an associated RS as the target RS for relaying a signal when the measured reference signal strength satisfies a condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a communication system using a conventional multihop relay scheme; 
         FIG. 2  illustrates a communication system using a multihop relay scheme in which a BS transmits signals to and receives signals from an RS and an MS in accordance with the present invention; 
         FIG. 3  illustrates a process for selecting an RS for an MS that can increase system capacity in the communication system using the multihop relay scheme in accordance with a first embodiment of the present invention; 
         FIG. 4  illustrates a process for selecting an RS for an MS that can increase system capacity in the communication system using the multihop relay scheme in accordance with a second embodiment of the present invention; 
         FIG. 5  illustrates a process for selecting an RS for an MS in the communication system using the multihop relay scheme in accordance with the present invention; and 
         FIG. 6  illustrates a structure of an MS (or RS or BS) in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. In the following description, detailed descriptions of functions and configurations incorporated herein that are well known to those skilled in the art are omitted for the sake of clarity and conciseness. 
     The present invention discloses a method and system in which an MS receives Relay Station INFOrmation (RS-INFO) and selects an RS in a communication system using a multihop relay scheme. The RS-INFO is used to extend cell coverage and/or increase system capacity. 
     For this, a BS broadcasts the RS-INFO message to all nodes located inside its cell area. Herein, the nodes include all nodes in which a relay function is performed or not performed. Further, it is noted that a signal of the BS can be received even when a node is located outside a cell area such as the RS  160  of  FIG. 1 . A node for performing the relay function is an RS. The RS may be an infrastructure of a system or a client RS serving as a specific node for performing the relay function. Further, the RS may be a fixed RS without mobility, a nomadic RS in which moving and settling are repeated, or a mobile RS. 
       FIG. 2  illustrates the communication system using the multihop relay scheme in which a BS transmits signals to and receives signals from an RS and an MS in accordance with the present invention. 
     Referring to  FIG. 2 , the system includes a BS  210 , multiple MSs  211 ,  213 ,  221 ,  223 ,  231  and  233 , and RSs  220  and  230  for performing relay functions. 
     The MSs  211 ,  213 ,  221 ,  223 ,  231  and  233  can directly transmit signals to and receive signals from the BS  210 , and can transmit signals to and receive signals from the BS  210  using the relay functions of the RSs  220  and  230 . 
     If an arbitrary MS is located at a cell edge of the BS  210  which has a poor channel state, the RSs  220  and  230  relay signals, thereby improving the poor channel state. The RSs provide the MS with a high-speed stable data transmission path, thereby increasing a valid signal transmission rate and system capacity. 
       FIG. 3  illustrates a process for selecting an RS in an MS that can increase system capacity in the communication system using the multihop relay scheme in accordance with a first embodiment of the present invention. 
     Referring to  FIG. 3 , an MS  350  can directly communicate with a serving BS  300  by performing a network entry or re-entry procedure with the serving BS  300  (step  302 ). 
     The serving BS  300  collects information about an RS located inside its cell area. Herein, the information is used to increase the system capacity, and can be an RS IDentifier (RSID) and a preamble index among information elements constructing a first format RS-INFO message as shown below in Table 1. The serving BS  300  should know in advance whether the RS is designed to increase the system capacity or to extend the cell coverage. 
     The serving BS  300  checks a transmission period of the RS-INFO message containing the collected information (step  304 ). If a period in which the RS-INFO message is transmitted is reached, the serving BS  300  broadcasts the RS-INFO message to the MS  350  (step  306 ). 
     The MS  350  acquires RS-INFO by receiving the RS-INFO message (step  308 ). Thus, the MS  350  measures the strength of a preamble signal of at least one RS using the RS-INFO and selects an RS with a highest signal strength value as a target RS for relaying a signal of the BS (step  310 ). The MS  350  compares the signal strength of the serving BS  300  with that of the selected RS. When the signal strength of the RS is higher, relay communication is performed using the RS. Further, the MS  350  can select an RS having a signal strength that is equal to or greater than a reference signal strength as the target RS for performing the relay function. 
     The RS-INFO message is to be broadcast and also can be received by not only the MS  350  but also RSs. Thus, the RSs receive the RS-INFO message and can use information contained in the message. For example, in a system considering relay communication of at least two hops, the RSs can select other RSs for relaying a signal of the BS through a procedure for acquiring information about a neighbor RS from the received RS-INFO message and measuring the preamble signal strength of the neighbor RS. 
     Table 1 shows the first format RS-INFO message in accordance with the first embodiment of the present invention. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Syntax 
                 Size 
                 Notes 
               
               
                   
               
             
             
               
                 RS INFO message format( ) { 
                   
                   
               
               
                 Management   Message 
                 8 bits 
                 To be determined 
               
               
                 Type=TBD 
                   
                   
               
               
                 N_Type2_RS 
                 8 bits 
                 The count of the unique 
               
               
                   
                   
                 type 2 RS information 
               
               
                 For(i=0; i&lt;N_Type2_RS; i++) { 
                   
                   
               
               
                 RSID 
                 48 bits  
                 ID of type 2 RS in 
               
               
                   
                   
                 serving cell 
               
               
                 Preamble index 
                 TBD 
                   
               
               
                 } 
                   
                   
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, the RS-INFO message includes a “management message type” field for indicating a type of message to be transmitted, an “N_Type&#39; 2 _RS” field for indicating the number of RSs for increasing system capacity, an “RSID” field for indicating IDs of the RSs, and a “preamble index” field for indicating preamble indices of the RSs. Herein, preambles used by the RSs can use a preamble of a serving BS without modification. An unique preamble can be used for each RS. 
       FIG. 4  illustrates a process for selecting an RS in an MS that can increase system capacity in the communication system using the multihop relay scheme in accordance with a second embodiment of the present invention. 
     Referring to  FIG. 4 , an RS  480  can perform multihop communication or direct communication with a serving BS  400  (step  402 ). An MS  440  can directly communicate with the serving BS  400  through a network entry or re-entry procedure (step  404 ). Herein, the RS  480  transmits a signal to and receives a signal from an MS located outside a cell area of the serving BS  400 . 
     The serving BS  400  collects information about an RS located inside its cell area. Herein, the information includes information for both increasing system capacity and extending cell coverage, which is different from that of the first embodiment. As shown below in Table 2, a second format RS-INFO message contains the above-described information. 
     The serving BS  400  checks a transmission period of the RS-INFO message containing the collected information (step  406 ). If a period in which the RS-INFO message is transmitted to the MS  440  and the RS  480  is reached, the serving BS  400  broadcasts the RS-INFO message to the MS  440  and the RS  480  (step  408 ). 
     Table 2 shows the second format RS-INFO message in accordance with the second embodiment of the present invention. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Size 
                   
               
               
                 Syntax 
                 (bits) 
                 Notes 
               
               
                   
               
             
             
               
                 ServingRS INFO message 
                   
                   
               
               
                 format( ) { 
                   
                   
               
               
                 Management  Message 
                 8 
                 TBD(To be determined) 
               
               
                 Type=TBD 
                   
                   
               
               
                 N_Type2_RS 
                 8 
                 The count of the unique type 2 RS 
               
               
                   
                   
                 information 
               
               
                 For(i=0; i&lt;N_Type2_RS; 
                   
                   
               
               
                 i++) { 
                   
                   
               
               
                  RSID 
                 48  
                 ID of type 2 RS in serving cell 
               
               
                  Preamble index 
                 TBD 
                   
               
               
                 } 
                   
                   
               
               
                 N_Type1_NRS 
                 8 
                 The count of the unique RS 
               
               
                   
                   
                 information 
               
               
                 For(j=0; j&lt;N_NRS; j++) { 
                   
                   
               
               
                  RSID 
                 48  
                 ID of neighbor RS 
               
               
                  Preamble Index 
                 8 
                   
               
               
                   
                 (TBD) 
                   
               
               
                  RS PHY Profile ID 
                 8 
                 Aggregated IDs of Co-located FA 
               
               
                   
                   
                 Indicator, FA Configuration 
               
               
                   
                   
                 Indicator, RS EIRP Indicator,  
               
               
                   
                   
                 DCD/UCD Reference Indicator,  
               
               
                   
                   
                 RS FA Index Indicator, Trigger 
               
               
                   
                   
                 Reference Indicator 
               
               
                  If(RS FA Index 
                   
                   
               
               
                 Indicator==1) { 
                   
                   
               
               
                   RS FA Index 
                 8 
                 This field, Frequency Assignment 
               
               
                   
                   
                 Index, is present only the RS FA  
               
               
                   
                   
                 Index Indicator in RS PHY Profile 
               
               
                   
                   
                 ID is set. Otherwise, the neighbor 
               
               
                   
                   
                 RS has the same FA Index as the 
               
               
                   
                   
                 serving RS. 
               
               
                  } 
                   
                   
               
               
                  If(RS EIRP Indicator==1) 
                   
                   
               
               
                 { 
                   
                   
               
               
                   RS EIRP 
                 8 
                 Signed Integer from −128 to 127  
               
               
                   
                   
                 in unit of dBm. This field is present  
               
               
                   
                   
                 only if the RS EIRP Indicator is set 
               
               
                   
                   
                 in RS PHY Profile ID. Otherwise, 
               
               
                   
                   
                 the neighbor RS has the same EIRP 
               
               
                   
                   
                 as the serving RS. 
               
               
                 } 
                   
                   
               
               
                  RS DCD Configuration 
                 4 
                 This represents the 4 LSBs of the  
               
               
                 Change Count 
                   
                 RS current DCD configuration 
               
               
                   
                   
                 change count. 
               
               
                  RS UCD Configuration 
                 4 
                 This represents the 4 LSBs of the 
               
               
                 Change Count 
                   
                 RS current UCD configuration 
               
               
                   
                   
                 change count. 
               
               
                  TLV Encoded RS 
                 Variable 
                 TLV specific 
               
               
                 Information 
                   
                   
               
               
                 } 
                   
                   
               
               
                 } 
               
               
                   
               
             
          
         
       
     
     As shown in Table 2, the RS-INFO message can be divided into an “N_Type  1 _NRS” field for indicating the number of RSs for extending cell coverage, information fields related to the “N_Type  1 _NRS” field, an “N_Type  2 _RS” field for indicating the number of RSs for increasing system capacity, and information fields related to the “N_Type  2 _RS” field. The “N_Type  2 _RS” field and its related information fields are shown in Table 1 and therefore its description is omitted here. 
     The “N_Type  1 _NRS” field and its related information fields are required when MSs located outside the cell area of the serving BS  400  select RSs. The information includes, for example, the number of RSs for supporting the cell coverage extension, RSID information, a preamble index of each RS, information required to acquire synchronization with each RS, Effective Isotropic Radiated Power (EIRP) information related to each RS and Type/Length/Value (TLV) information related to each RS. The EIRP indicates the strength of transmission power in a Radio Frequency (RF) stage. 
     Thus, when receiving the RS-INFO message from the serving BS  400 , the RS  480  extracts RS INFO for extending the cell coverage contained in the RS-INFO message (step  410 ). That is, the RS  480  discards the “N_Type  2 _RS” field and its related information fields and extracts only the “N_Type  1 _NRS” field and its related information fields. The RS  480  transmits the extracted RS INFO to the MS located outside the cell area of the serving BS  400  (step  412 ). 
     The MS  440  receives the RS-INFO message from the serving BS  400 , extracts only the “N_Type  2 _RS” field and its related information fields and discards the “N_Type  1 _NRS” field and its related information elements (step  414 ). Using the extracted information, the MS  440  measures the strength of a preamble signal of at least one RS and selects an RS with a highest signal strength value as a target RS for performing a relay function with the BS (step  416 ). The MS  440  compares the signal strength of the serving BS  400  with that of the selected RS. When the signal strength of the RS is higher, relay communication is performed using the RS. 
       FIG. 5  illustrates a process for selecting an RS for an MS in the communication system using the multihop relay scheme in accordance with exemplary embodiments of the present invention. 
     In  FIG. 5 , the MS can directly communicate with a serving BS since it is located inside a cell area of the serving BS. However, the MS can use relay communication rather than direct communication with the BS when a channel state is less than a threshold level. For this, the MS should select an RS for the relay communication. 
     Referring to  FIG. 5 , the MS receives an RS-INFO message from the serving BS in step  502  and then proceeds to step  504 . In step  504 , the MS determines whether the received RS-INFO message is a first format RS-INFO message only containing RS INFO for increasing system capacity. If the received RS-INFO message is determined to be the first format RS-INFO message, the MS proceeds to step  508 . However, if the received RS-INFO message is a second format RS-INFO message containing both information for increasing system capacity and information for extending cell coverage, the MS proceeds to step  506 . 
     The MS extracts only RS INFO for increasing the system capacity from the received RS-INFO message, i.e., the second format RS-INFO message, in step  506  and then proceeds to step  508 . The MS acquires the RS INFO extracted from the first or second format RS-INFO message in step  508  and proceeds to step  510 . In this case, the RS-INFO has the format as shown in Table 1. In step  510 , the MS selects an RS with the highest preamble signal strength as a target RS for performing a relay function when transmitting a signal to and receiving a signal from the serving BS. 
       FIG. 6  illustrates a structure of the MS (or RS or BS) in accordance with the present invention. 
     Referring to  FIG. 6 , the MS includes a message processor  611 , a message generator  613 , an RS-INFO processor  615 , a memory  617 , a controller  619  and an interface module  621 . 
     The controller  619  controls the overall operation of the MS. For example, the controller  619  controls an operation for processing and controlling voice and data communication and an operation for processing RS INFO for increasing system capacity. That is, the controller  619  provides the message processor  611  with an RS-INFO message received from a serving BS, receives a message to be transmitted to the serving BS from the message generator  613  and provides the interface module  621  with the received message. 
     The message processor  611  interprets a control message received from the RS or BS and then reports an interpretation result to the controller  619 . That is, when the MS receives the RS-INFO message as shown in Tables 1 and 2, the message processor  611  extracts various information elements contained in the message and provides the controller  619  with the extracted information. The controller  619  controls the RS-INFO processor  615  on the basis of the information input from the message processor  611 . 
     The message generator  613  generates a message to be transmitted to the RS or BS under control of the controller  619 . The message generated from the message generator  613  is output to the interface module  621  through the controller  619 . 
     The RS-INFO processor  615  provides the controller  619  with a signal for performing a communication procedure using RS INFO for increasing system capacity under the control of the controller  619 . 
     The memory  617  stores a program for controlling the overall operation of the MS and stores a message and temporary data generated in operation of the program. 
     The interface module  621  communicates with the RS or BS, and includes an RF processor and a baseband processor. The RF processor converts a signal received through an antenna into a baseband signal and then provides the baseband processor with the baseband signal. The RF processor converts a baseband signal received from the baseband processor into an RF signal to be transmitted over the air and then transmits the RF signal through the antenna. For example, when the communication system uses an Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) scheme, the baseband processor provides the control section  619  with original information/data by performing a Fast Fourier Transform (FFT) process and a channel decoding process for the signal from the RF processor. In contrast, the baseband processor performs a channel encoding process and an Inverse Fast Fourier Transform (IFFT) process for data from the control section  619  and then provides the RF processor with the processed data. 
     As described above, the present invention discloses a scheme in which an MS receives RS INFO from a serving BS and selects an RS using the received RS INFO. However, the RS can be selected by the serving BS rather than the MS. That is, the serving BS can select the RS for the MS by considering RS-by-RS Channel Quality Information (CQI) reported from the MS. 
     As is apparent from the above description, the present invention can increase system capacity when a BS transmits RS INFO to an MS and the MS performs multihop relay communication with the BS through an RS with a good channel state. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope of the present invention. Therefore, the present invention is not limited to the above-described embodiments, but is defined by the following claims, along with their full scope of equivalents.