Patent Publication Number: US-2007097900-A1

Title: Apparatus and method for controlling transmission/reception power by measuring propagation delay and processing time in a wireless communication system

Description:
PRIORITY  
      This application claims priority under 35 U.S.C. §119 to an application filed in the Korean Intellectual Property Office on Oct. 27, 2005 and assigned Serial No. 2005-101586, 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 multi-hop relay broadband wireless communication system, and in particular, to an apparatus and method for controlling transmission/reception power by measuring propagation delays and processing times among a Base Station (BS), a Relay Station (RS), and a Mobile Station (MS) and estimating distances between them using the propagation delays and processing times in a multi-hop relay broadband wireless communication system.  
      2. Description of the Related Art  
      One of the most critical requirements for deployment of a 4 th  Generation (4G) mobile communication system under recent active research is to build a self-configurable wireless network.  
      A self-configurable wireless network is a wireless network configured in an autonomous or distributed manner without being under the control of a central system to provide mobile communication services. For the 4G mobile communication system, cells of very small radii are defined for the purpose of enabling high-speed communications and accommodating a larger number of calls. In this case, conventional centralized wireless network design is not feasible. Rather, the wireless network should be built to be under distributed control and to actively cope with an environmental change, such as the addition of a new BS. For this reason, the 4G mobile communication system requires a self-configurable wireless network configuration.  
      For real deployment of the self-configurable wireless network, techniques used for an ad hoc network should be introduced to the mobile communication system. A major example of these techniques is a multi-hop relay cellular network configured by applying a multi-hop relay scheme used for the ad hoc network to a cellular network with fixed BSs. In general, since a BS and an MS communicate with each other via a direct link in the cellular network, a highly reliable radio link can be established easily between them.  
      However, due to the fixedness of the BSs, the configuration of the wireless network is not flexible, making it difficult to provide efficient service in a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.  
      This drawback can be overcome by a relay scheme which delivers data over multiple hops using a plurality of neighbor MSs or neighbor RSs. The multi-hop relay scheme facilitates fast network reconfiguration adaptive to environmental changes and renders the overall wireless network operation efficient. Also, a radio channel in a better channel status can be provided to an MS by installing an RS between the BS and the MS and thus establishing a multi-hop relay path via the RS. In addition, since high-speed data channels can be provided to MSs in a shadowing area or an area where communications with the BS are unavailable, cell coverage is expanded.  
       FIG. 1  illustrates the configuration of a typical multi-hop relay cellular network.  
      Referring to  FIG. 1 , an MS  110  within the BS service area  101  of a BS  100  is connected to the BS  100  via a direct link. On the other hand, an MS  120 , which is located outside the BS service area  101  of the BS  100  and thus in a poor channel state, communicates with the BS  100  via a relay link of an RS  130  in RS coverage area  111 .  
      The RS  130  provides better-quality radio channels to the MSs  110  and  120  when they communicate with the BS  100  in a poor channel state as they are located at a boundary of the service area  101 . Thus, the BS  100  can provide a high-speed data channel to the cell boundary area using a multi-hop relay scheme and thus expand its cell coverage. As illustrated in  FIG. 1 , there are a BS-MS link, a BS-RS link, and an RS-MS link in the multi-hop relay cellular network.  
      As described above, the BS sends a downlink signal to the MS via the relay link established by the RS or via the direct link in the above multi-hop relay broadband wireless communication system. Unless the BS controls transmission power taking into account the distances to the RS and the MS, the signal may interfere with other MSs or RSs.  
      Conventionally, the BS controls power by sending a signal at a predetermined power level and receiving information about a reception power level from the RS or the MS receiving the signal. This conventional power control requires establishment of a plurality of signal links between the BS and the RS or the MS in the multi-hop relay broadband wireless communication system. As a consequence, resources are wasted and protocol complexity increases.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an object of the present invention is to provide an apparatus and method for measuring propagation delay in a multi-hop relay broadband wireless communication system.  
      Another object of the present invention is to provide an apparatus and method for controlling transmission/reception power by estimating the distance between a transmitter and a receiver according to a propagation delay measurement between the transmitter and receiver in a multi-hop relay broadband wireless communication system.  
      The above objects are achieved by providing an apparatus and method for controlling power in a multi-hop relay broadband wireless communication system.  
      According to one aspect of the present invention, in a power controlling method for a BS in a multi-hop relay broadband wireless communication system, the BS sends a signal to an RS and an MS to estimate distances between the BS, the RS and the MS. Upon receipt of signals from the RS and the MS, the BS measures Time of Arrivals (ToAs) of the received signals and detects time information included in the received signals, and calculates propagation delays among the BS, the RS and the MS based on the ToAs and the time information. The BS estimates the distances among the BS, the RS and the MS using the propagation delays, and controls transmission power according to the estimated distances.  
      According to another aspect of the present invention, in a power controlling method for an RS in a multi-hop relay broadband wireless communication system, the RS activates a timer, upon receipt of a signal from a BS. Upon receipt of a signal from an MS, the RS detects time information of the timer and time information of the MS included in the received signal, and sends a signal including the time information of the timer and the time information of the MS to the BS. Upon receipt of distances estimated using propagation delays and processing times from the BS, the RS controls the power of a transmission signal according to the estimated distances.  
      According to a further aspect of the present invention, in a power controlling method for an MS in a multi-hop relay broadband wireless communication system, the MS activates a timer, upon receipt of a signal from a BS. The MS sends a signal including time information of the timer to the BS and the RS. Upon receipt of estimated distances, the MS controls the power of a transmission signal according to the estimated distances.  
      According to still another aspect of the present invention, in a power controlling apparatus for a BS in a multi-hop relay broadband wireless communication system, upon receipt of signals from an RS and an MS, a time measurer measures ToAs of the received signals and detects time information included in the received signals. A distance estimator estimates the distances between the BS, the RS and the MS using the ToAs and the time information. A power controller controls the power of a transmission signal according to the estimated distances. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  illustrates multiple links in a typical multi-hop relay broadband wireless communication system;  
       FIG. 2  illustrates a signal flow through signal links for estimating distances in a multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 3  is a diagram illustrating signal timings in the multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 4  is a block diagram of a BS for power control in the multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 5  is a flowchart illustrating a power control operation in the BS in the multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 6  is a flowchart illustrating a power control operation in an RS in the multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 7  is a flowchart illustrating a power control operation in an MS in the multi-hop relay broadband wireless communication system according to the present invention;  
       FIG. 8  illustrates a signal flow through signal links in the multi-hop relay broadband wireless communication system according to the present invention; and  
       FIG. 9  illustrates a signal flow through signal links in the multi-hop relay broadband wireless communication system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
      The present invention provides a technique for controlling transmission/reception power by estimating the distance between a transmitter and a receiver in a multi-hop relay broadband wireless communication system. The distance between the transmitter and the receiver is estimated using propagation delays and system signal processing times between a BS, an RS and an MS, by way of example. While the following description is made in the context of an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system, it is to be clearly understood that the present invention is also applicable to any other multiple access scheme.  
       FIG. 2  illustrates a signal flow through signal links for estimating distances in a multi-hop relay broadband wireless communication system according to the present invention, and  FIG. 3  illustrates signal timer durations in the BS, the RS and the MS.  
      Referring to  FIG. 2 , a BS  201  sends a signal to an MS  203  and an RS  205  to estimate the distances between them based on propagation delays in steps  211  and  212 . Simultaneously, the BS  201  activates a BS timer at time  301  in  FIG. 3 . The transmission signal is a control signal sent for channel estimation or system synchronization before data transmission, or a general data signal.  
      The MS  203  detects the received signal and activates an MS timer at time  321  in  FIG. 3 . In steps  213  and  214 , the MS  203  sends a control signal to the BS  201  and the RS  205 . The control signal includes channel status information between the BS  201  and the MS  203  and a processing time Δ x  in the MS  203 . The processing time Δ x  spans from time  321  to time  323  in  FIG. 3 , counted by the MS timer.  
      Upon receipt of the control signal from the MS  203  in step  213 , the BS  201  counts the Round Trip Delay (RTD) between the BS  201  and the MS  203  by the BS timer. The RTD spans from time  301  to time  303  in  FIG. 3 . Because the RTD includes the processing time Δ x  of the MS  203 , the propagation delay between the BS  201  and the MS  203  is calculated by Equation (1):  
               T     S   →   D       ≈       (       T     S   ↔   D       -     Δ   x       )     2             (   1   )             
 
 where T S→D  denotes the propagation delay from the BS  201  to the MS  203 , T S→D  denotes the RTD between them, measured by the BS  201 , and Δ x  denotes the processing time of the MS  203 . Assuming that the downlink and the uplink between the BS  201  and the MS  203  are identical, the downlink and the uplink experience an equal propagation delay. 
 
      Upon receipt of the signal from the BS  201  in step  212 , the RS  205  activates an RS timer at time  311  in  FIG. 3 . When the RS  205  receives the control signal from the MS  203  in step  214 , it counts the time span from the reception of the signal from the BS  201  (time  311 ) and the reception of the control signal from the MS  203  (time  313 ) by the RS timer. The time span is expressed as Equation (2): 
 
 T   R ≈( T   D→R +α)+Δ x    (2) 
 
 where T R  denotes the time from the reception of the signal from the BS  201  to the reception of the control signal from the MS  203 , T D→R  denotes the propagation delay from the MS  203  to the RS  205 , a denotes the time between the reception of the signal from the BS  201  in the RS  205  and the reception of the signal from the BS  201  in the MS  203 , and Δ x  denotes the processing time of the MS  203 . 
 
      Then, the RS  205  forwards the signal received from the BS  201  to the MS  203  in step  216  and forwards the signal received from the MS  203  to the BS  201  in step  215 . These signals contain T R  and the processing time Δ y  of the RS  205 , i.e. from time  311  to time  315  in  FIG. 3 . T R  and Δ y  are carried in the form of integers in a frame-like structure, for example, in a preamble or reserved bits of a frame.  
      Upon receipt of the signal from the RS  205 , the BS  201  calculates the RTD (time  301  to time  305  in  FIG. 3 ) between the BS  201  and the RS  205  using the BS timer by Equation (3): 
 
 T   S   ≈T   R +Δ y   +T   R→S −( T   D→R +α)+Δ x +Δ y   +T   R⇄S    (3) 
 
 where T S  denotes the time between the signal transmission to the RS  205  and the signal reception from the RS  205 , T R  denotes the time from the signal reception from the BS  201  and the signal reception from the MS  203 , T R⇄S  denotes the RTD between the BS  201  and the RS  205 , T D→R  denotes the propagation delay from the MS  203  to the RS  205  denotes the time between the reception of the signal from the BS  201  in the RS  205  and the reception of the signal from the BS  201  in the MS  203 , Δ y  denotes the processing time of the RS  205 , and Δ x  denotes the processing time of the MS  203 . 
 
      Using T R  and Δ y  included in the signal received from the RS  205 , the BS  201  calculates the propagation delay between the BS  201  and the RS  205  by Equation (4):  
               T     R   →   S       ≈       (       T   S     -     (       T   R     +     Δ   y       )       )     2             (   4   )             
 
 where T R→S  denotes the propagation delay from the RS  205  to the BS  201 , T S  denotes the time between the signal transmission from the BS  201  to the RS  205  and the signal reception from the RS  205  in the BS  201 , T R  denotes the time from the signal reception from the BS  201  in the RS  205  and the signal reception from the MS  203  in the RS  205 , and Δ y  denotes the processing time of the RS  205 . 
 
      The propagation delay T S→D  from the MS  203  to the BS  201  is given as Equation (5): 
 
 T   S→D   =α+T   S→R   =α+T   R→S    (5) 
 
 where α denotes the time between the reception of the signal from the BS  201  in the RS  205  and the reception of the signal from the BS  201  in the MS  203 , T S→R  denotes the propagation delay from the BS  201  to the RS  205 , and T R→S  denotes the propagation delay from the RS  205  to the BS  201 . 
 
      Since a is obtained by Equation (2) or Equation (3), the propagation delay between the MS  203  and the RS  205  is determined by Equation (6): 
 
 T   D→R   =T   R −(α+Δ x )   (6) 
 
 where T R  denotes the time from the signal reception from the BS  201  in the RS  205  and the signal reception from the MS  203  in the RS  205 , a denotes the time between the reception of the signal from the BS  201  in the RS  205  and the reception of the signal from the BS  201  in the MS  203 , and Δ x  denotes the processing time of the MS  203 . 
 
      Using the propagation delays of signals among the BS  201 , the MS  203 , and the RS  205  via the links as illustrated in  FIG. 2 , the distances between them are calculated by Equation (7): 
 
 R   S→D   =C×T   S→D  
 
 R   D→R   =C×T   D→R  
 
 R   R→S   =C×T   R=S    (7) 
 
 where C denotes a propagation velocity, 3×10 8  m/sec, T S→D  denotes the propagation delay from the BS  201  to the MS  203 , T D→R  denotes the propagation delay from the MS  203  to the RS  205 , and T R→S  denotes the propagation delay from the RS  205  to the BS  201 . 
 
      The BS can control its transmission power according to the distances between the BS, the MS and the RS calculated by Equation (7). Thus, interference between the RS or the MS communicating with the BS and other systems is minimized. Also, since the BS sends distance information to the RS and the MS, the RS and the MS control their transmit power, thereby efficiently saving power.  
       FIG. 4  is a block diagram of the BS for power control in the multi-hop relay broadband wireless communication system according to the present invention.  
      Referring to  FIG. 4 , the BS includes a distance estimator  401 , an encoder  403 , a modulator  405 , a subcarrier mapper  407 , an Orthogonal Frequency Division Multiplexing (OFDM) modulator  409 , a Radio Frequency (RF) processor  411 , and a power controller  413 .  
      The distance estimator  401  calculates propagation delays and processing times using time information received from the RS and the MS by Equation (1) to Equation (7), and estimates the distances among the BS, the MS and the RS using on the propagation delays and the processing times. The distance estimator  401  provides the distance information to the encoder  403 , for transmission of the distance information to the MS and the RS, and to the power controller  413 , for controlling transmission power to the MS and the RS.  
      The encoder channel-encodes input information data at a predetermined coding rate to achieve robustness against a radio channel. The information data includes the distance information. The modulator  405  modulates the coded data in a predetermined modulation scheme such as Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-ary Quadrature Amplitude Modulation (16 QAM), or 64 QAM. The subcarrier mapper  407  maps the modulated data to subcarriers.  
      The OFDM modulator  409  converts the mapped data to time sample data by Inverse Fast Fourier Transform (IFFT).  
      The RF processor  411  upconverts the baseband signal received from the OFDM modulator  409  to an RF signal and sends the RF signal to the RS or the MS through an antenna, controlling the power of the RF signal under the control of the power controller  413 .  
      The power controller  413  outputs a power control signal for controlling the transmission power of the signal to be sent through the RF processor  411  based on the estimated distances between the BS, the RS and the MS received from the distance estimator  401 .  
       FIG. 5  is a flowchart illustrating a power control operation in the BS in the multi-hop relay broadband wireless communication system according to the present invention.  
      Referring to  FIG. 5 , the BS sends a signal to the RS and the MS to calculate propagation delays to the RS and the MS in step  501 . Simultaneously, the BS activates its timer. The transmitted signal includes one of a control signal, a control frame such as a preamble, and a data frame.  
      In step  503 , the BS monitors reception of signals from the MS and the RS. Upon receipt of the signals from the MS and the RS, the BS checks the Time of Arrivals (ToAs) of the signals from the MS and the RS using the timer in step  505 .  
      The BS then checks time information included in the received signals and calculates propagation delays in step  507 . The time information contains information about time measurements counted by the MS timer and the RS timer.  
      The BS calculates the propagation delays between the BS, the MS and the RS using the ToAs and the time information by Equation (1) to Equation (6). Time information obtained by the BS timer includes the ToAs of the signals from the MS and the RS. The time information obtained by the MS timer includes the processing time of the MS, and the time information obtained by the RS timer includes the processing time of the RS includes the processing time of the RS and a time span between reception of the signal from the BS and reception of a signal from the MS.  
      In step  509 , the BS estimates the distance between the BS, the MS and the RS using the propagation delays by Equation (7).  
      The BS then controls the transmission power of a signal to be sent according to the estimated distances in step  511 . For example, the power is controlled referring to a quantitatively made-up power scheduling table like Table 1 below.  
      Table 1 is an example of a power scheduling table by which to perform a quantitative power control proportional to distance.  
                                       TABLE 1                                   Power Level   R S→D     R D→R     R R→S     R S→D : R D→R : R R→S                                                              Within cell   Class A   Default   Default   Default           radius   Class B   Class A + X   Class A + Y   Class A + Z           Class C   Class B + X   Class B + Y   Class B + Z           Class D   Class C + X   Class C + Y   Class C + Z           .   .   .   .           .   .   .   .           .   .   .   .           Class N   Class N   Class N   Class N       Outside cell       Starts from   Starts from   Starts from       radius       Default for   Default for   Default for               Class N or   Class N or   Class N or               higher class   higher class   higher class                  
 
      Referring to Table 1, transmission power is controlled to a predetermined power level according to the distance information. The distance-based power levels listed in Table 1 may be continuously changed by a channel estimation algorithm when communication paths are established.  
      Then the BS ends the process.  
      As described above, the transmission power of signals to be sent to the RS and the MS is controlled by estimating the distances between the BS, the RS, and the MS. The BS notifies the RS and the MS of the estimated distances so that the RS and the MS can control transmission power between them.  
       FIG. 6  is a flowchart illustrating a power control operation in the RS in the multi-hop relay broadband wireless communication system according to the present invention.  
      Referring to  FIG. 6 , the RS monitors reception of a signal from the BS in step  601  and upon receipt of the signal, it activates the RS timer in step  603 .  
      In step  605 , the RS monitors reception of a signal from the MS. Upon receipt of the signal from the MS, the RS measures the ToA of the received signal using the RS timer in step  607 . The ToA covers the processing time Δ x  of the MS, the propagation delay T D→R  from the MS to the RS, and the difference a between the propagation delay from the BS to the MS and the propagation delay from the BS to the RS.  
      In step  609 , the RS forwards the signal received from the BS to the MS and forwards the signal received from the MS to the BS.  
      The RS monitors reception of estimated distances between the BS, the MS and the RS from the BS in step  611 . Upon receipt of the estimated distances, the RS sends a signal to the BS or the MS at a power level controlled according to the estimated distances in step  613 . The RS then ends the process of the present invention.  
       FIG. 7  is a flowchart illustrating a power control operation in the MS in the multi-hop relay broadband wireless communication system according to the present invention.  
      Referring to  FIG. 7 , the MS monitors reception of a signal from the BS in step  701 . Upon receipt of the signal from the BS, the MS activates the MS timer in step  703 .  
      In step  705 , the MS sends a control signal including the processing time of the MS to the BS and the RS.  
      The MS then monitors reception of estimated distances between the BS, the MS and the RS from the BS in step  707 .  
      Upon receipt of the estimated distances, the MS sends a signal to the RS and the BS at a power level controlled according to the estimated distances in step  709 . Then the MS ends the process.  
       FIG. 8  illustrates a signal flow through signal links in the multi-hop relay broadband wireless communication system according to the present invention.  
      Referring to  FIG. 8 , when two RSs  805  and  807  exist, the distances between a BS  801 , an MS  803 , and the RSs  805  and  807  are estimated using the propagation delays between them, as illustrated in  FIG. 2 .  
       FIG. 9  illustrates a signal flow through signal links in the multi-hop relay broadband wireless communication system according to the present invention. The distances between the BS, the MS and the RS are estimated in the same manner as in  FIG. 8 .  
      Referring to  FIG. 9 , the distances between RSs  905 ,  911  and  913  are estimated. When it receives a signal from a BS  901 , the RS  905  (RS  1 ) activates its timer and estimates the distances between the RSs  905 ,  911  and  913  in the same manner as in  FIG. 2 .  
      For power control, the distances between the BS, the MS and the RS may be estimated in an early stage of communications or periodically estimated taking into account of the mobility of the RS or the MS.  
      In the above embodiments, the propagation delays are calculated by signaling among the BS, the MS and the RS and the distances are estimated using the propagation delays. Also, the distances can be calculated using Global Positioning System (GPS) without time synchronization.  
      In accordance with the present invention as described above, the distances between the BS, the MS and the RS are estimated using the propagation delays and power control is carried out according to the distances in the multi-hop relay broadband wireless communication system. Therefore, interference is reduced between systems and the power consumption of the RS or the MS is efficiently decreased.  
      While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.