Abstract:
A method and apparatus for efficiently transmitting channel quality information on a radio channel in a mobile communication system are provided. The reporting cycle of channel quality information is changed adaptively according to time-varying characteristics associated with the Doppler frequency of a radio channel or the variation of the channel quality information. Therefore, the channel quality information is efficiently transmitted. Also, the decrease of unnecessary frequent information transmissions reduces an uplink interference power level and power consumption in user equipment (UE), as well.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of an application entitled “Method and Apparatus for Controlling Transmission of Channel Quality Information According to Characteristics of Time-Varying Channel in a Mobile Communication System” filed in the Korean Intellectual Property Office on Feb. 26, 2004 and assigned Serial No. 2004-13141, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a mobile communication system. In particular, the present invention relates to a method and apparatus for reporting channel quality information necessary to determine modulation, coding rate, and data rate as transmission parameters.  
         [0004]     2. Description of the Related Art  
         [0005]     The recent increasing demands for data and multimedia service are not being met by existing communication systems. Hence, along with the trend, the 3 rd  Generation Partnership Project (3GPP) and 3GPP2 committees are standardizing efficient communication systems for packet data communication. High Speed Downlink Packet Access (HSDPA) has recently been standardized and implemented, wherein data can be transmitted to mobile terminals at high data rates.  
         [0006]     HSDPA provides packet transmission service very efficiently using Adaptive Modulation and Coding Scheme (AMC) and Hybrid Automatic Repeat Request (HARQ). Especially, AMC is a scheme for maximizing transmission throughput by controlling modulation, coding rate, and data rate adaptively according to a radio channel quality. To support AMC, information about the radio channel quality must be reported.  
         [0007]      FIG. 1  illustrates an AMC operation in a conventional HSDPA mobile communication system.  
         [0008]     Referring to  FIG. 1 , a User Equipment (UE)  10  measures the Signal-to-Interference power Ratio (SIR) of a Common Pilot Channel (CPICH) received from a base station  20  as a reference signal, and determines a Channel Quality Indicator (CQI) according to the measurement, to thereby maximize the whole transmission throughput. In a 3GPP system called Wideband Code Division Multiple Access (WCDMA), the CQI is sent on a High Speed-Dedicated Physical Control Channel (HS-DPCCH)  22  related to a High Speed-Dedicated Shared Channel (HS-DSCH).  
         [0009]      FIG. 2  illustrates a CQI reporting format in the WCDMA communication system. Referring to  FIG. 2 , the CQI is sent in one HS_DPCCH subframe of 2 ms. The HS-DPCCH subframe includes a HARQ Acknowledgement (ACK) in a 2560-chip time slot and the CQI in two time slots of 5120 chips. One radio frame has 5 subframes and thus it is 10 ms in duration.  
         [0010]     The number of actual bits transmitted is 20 bits. Five bits among the 20 bits represent information, and the remaining 15 bits are used for redundancy information produced from channel encoding. The 5-bit information represents 31 CQI values according to a UE category. According to the CQI, the base station selects Quadrature Phase Shift Keying (QPSK) or 16 Quadrature Amplitude Modulation (QAM) as a modulation scheme and determines an appropriate data rate, that is, an appropriate transport block size for the UE.  
         [0011]     The CQI is determined by the SIR over the entire frequency band. It is sent to the base station according to transmission parameters including a predetermined reporting cycle and time offset. Let the reporting cycle be denoted by k and the time offset be denoted by l. Then, k and l are called CQI transmission parameters and the Node B notifies the UE of k and l by higher-layer signaling.  
         [0012]      FIG. 3  is a diagram illustrating a message flow for transmitting CQI transmission parameters in a radio link (RL) setup procedure in the conventional HSDPA system. The base station is illustrated separately as a Node B  22  for actually establishing an RL with a UE  10  and a Radio Network Controller (RNC)  24  for controlling the RL connection.  
         [0013]     Referring to  FIG. 3 , upon receipt of an RL SETUP REQUEST message for the UE  10  from the RNC  24  in step  32 , the Node B  22  transmits to the RNC  24  an RL SETUP RESPONSE message including k and l in step  34 . In step  36 , the RNC  24  transmits a RADIO BEARER REQUEST message including k and l to the UE  10 . The UE  10  transmits a RADIO BEARER SETUP COMPLETE message to the RNC  24 , thereby completing the setup of the RL in step  38 .  
         [0014]      FIG. 4  illustrates a CQI transmission on the HS-DPCCH in the HSDPA system. In the illustrated case, three UEs transmit CQIs to one Node B.  
         [0015]     Referring to  FIG. 4 , UE  1  transmits a CQI in a first, third, fifth and seventh time slots with k=2 and l=0. UE  2  transmits a CQI in the first and fifth time slots with k=4 and l=0. UE  3  transmits a CQI in the third and seventh time slots with k=4 and l=2.  
         [0016]     In the conventional HSDPA system, the Node B usually determines k depending on whether the LE is in a handover situation. The UE then reports a CQI at a time when (5×CFN+[(nx256 chips+ix2560 chips)/7680 chips]) mod k is 0 and i mod  3  is 0. Here, n is a timing offset and i is a slot count. Because one frame comprises 15 time slots, i ranges from 0 to 14. Connection Frame Number (CFN) is a frame count and incremented by 1 each time i reaches 14. [(nx256 chips+ix2560 chips)/7680 chips] increments by 1 each time i increments by 3 and by 5 each time i increments by 15. Considering that one frame has 15 slots in the WCDMA system, the CFN is incremented by 1 at the end of each frame. Consequently, the CQI reporting is performed in at once (multiple of 3) th  and (multiple of k) th  slots. Therefore, the CQI is sent on the uplink every 3k slots, that is, every k subframes. The CQI is repeated as many times as N_cqi_transmit. The repetition factor is also indicated to the UE by higher-layer signaling.  
         [0017]     As described above, the time to report the CQI on the uplink is determined by k. However, the conventional system gives no consideration to channel condition in determining k. In practice, each UE moves at a different speed with a different Doppler frequency. Therefore, it is not efficient to report the CQI at the same cycle in each UE. A slow-moving UE can transmit the CQI within a coherence time even at a long reporting cycle, whereas a fast-moving UE needs a shorter reporting cycle.  
         [0018]     The CQI reporting cycle must be determined efficiently for the following reasons.  
         [0019]     1. Interference always exists between uplink signals from UEs. Since uplink transmission on the HS-DPCCH is performed in a Discontinuous Transmission (DTX) mode, as infrequent data transmission as possible reduces interference power, which is favorable to accurate reception in the Node B. Therefore, it is efficient to set a long reporting cycle in terms of reception in the Node B.  
         [0020]     2. Continuous uplink transmission amounts to great power consumption in a UE. The increase of power consumption rapidly decreases the life of the battery. Therefore, a long reporting cycle is efficient in terms of power consumption in the UE.  
         [0021]     3. The Node B makes a resource map based on CQIs received from a plurality of UEs and allocates appropriate resources to them through scheduling. The CQI information must be reliable for appropriate resource allocation, which is equivalent to minimization of CQI transmission delay. Since the delay minimization requires frequency CQUI reporting, it is efficient to set a short reporting cycle in terms of resource management in the Node B.  
       SUMMARY OF THE INVENTION  
       [0022]     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 a method and apparatus for determining a Channel Quality Indicator (CQI) reporting cycle efficient for both a Node B and a user equipment (UE) in a high-speed mobile communication system.  
         [0023]     Another object of the present invention is to provide a method and apparatus for controlling the cycle of reporting a CQI from a UE to a Node B according to channel conditions.  
         [0024]     The above objects are achieved by providing a method and apparatus for efficiently transmitting channel quality information on a radio channel in a mobile communication system.  
         [0025]     According to one aspect of the present invention, in an apparatus and method of reporting channel quality information in a mobile communication system, mobility information of a mobile station is received from the mobile station, a reporting cycle of channel quality information is determined based on the mobility information, and the channel quality information is acquired from the mobile station at the reporting cycle.  
         [0026]     According to another aspect of the present invention, in an apparatus and method of reporting channel quality information in a mobile communication system, a variation in channel quality information received from a mobile station is estimated, a reporting cycle of the channel quality information is determined based on the variation, and the channel quality information is acquired from the mobile station at the reporting cycle. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     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:  
         [0028]      FIG. 1  illustrates an Adaptive Modulation and Coding Scheme (AMC) operation in a conventional mobile communication system;  
         [0029]      FIG. 2  illustrates a Channel Quality Indicator (CQI) reporting format in a conventional Wideband Code Division Multiple Access (WCDMA) communication system;  
         [0030]      FIG. 3  is a diagram illustrating a message flow for transmitting CQI transmission parameters in a radio link (RL) setup procedure in the conventional mobile communication system;  
         [0031]      FIG. 4  illustrates a CQI transmission on the High Speed-Dedicated Physical Control Channel (HS-DPCCH) in the conventional mobile communication system;  
         [0032]      FIG. 5  is a block diagram illustrating the configuration of a system for determining a CQI according to an embodiment of the present invention;  
         [0033]      FIG. 6  is a flowchart illustrating the operation of a Node B according to the embodiment of the present invention;  
         [0034]      FIG. 7  is a block diagram illustrating the configuration of a system for determining a CQI according to another embodiment of the present invention;  
         [0035]      FIG. 8  is a block diagram of a CQI variance measurer according to the second embodiment of the present invention;  
         [0036]      FIG. 9  is a flowchart illustrating the operation of the Node B according to the second embodiment of the present invention; and  
         [0037]      FIG. 10  illustrates an example of CQI transmission on the HS-DPCCH according to the embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]     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.  
         [0039]     The embodiments of the present invention are intended to control a Channel Quality Indicator (CQI) reporting cycle appropriate for both a Node B and a user equipment (UE). According to channel conditions, a long reporting cycle does not matter for some UEs, and a short reporting cycle is required for other UEs. Therefore, the CQI reporting cycle must be determined such that there are no problems with scheduling on behalf of the Node B, interference power is reduced, and battery consumption in UEs is reduced.  
         [0040]     Two embodiments regarding CQI reporting cycle determination are provided herein. In one of the embodiments, a UE transmits information concerning its movements and a Node B determines a CQI reporting cycle according to the UE&#39;s movements. In the other embodiment, the Node B monitors the change in a CQI and determines the CQI reporting cycle adaptively according to the CQI change. Common to both the embodiments, once the CQI reporting cycle is determined, a time offset is determined so as to minimize uplink interference.  
       First Embodiment  
       [0041]     The UE estimates its Doppler frequency and speed as its mobility information and reports the mobility information to the Node B. The Node B then determines an appropriate CQI reporting cycle and time offset according to the mobility information.  
         [0042]      FIG. 5  is a block diagram illustrating the configuration of a system for determining a CQI according to an embodiment of the present invention.  
         [0043]     Referring to  FIG. 5 , a UE  110  roughly estimates its mobility information, quantizes it, and transmits the quantized value on the uplink. To do so, the UE  110  has a Doppler estimator  112  for estimating Doppler information from a signal received from a Node B  120 , for example a Common Pilot Channel (CPICH), and a Doppler quantizer  114  for quantizing the Doppler estimation value at an appropriate quantization interval. The quantized value is transmitted to the Node B  120  on a High Speed-Dedicated Physical Control Channel (HS-DPCCH).  
         [0044]     A reporting cycle determinator  122  in the Node B  120  determines a CQI reporting cycle k for the UE  110  based on the quantized value and determines a time offset l according to k. Final determination of the reporting cycle will be described later in more detail.  
         [0045]     In an embodiment of the present invention, the Node B  120  can report k and l to a Radio Network Controller (RNC)  130  by a radio link (RL) SETUP RESPONSE message. The RNC  130  then notifies a CQI determinator  116  of the UE  110  of k and l by a RADIO BEARER REQUEST message. The CQI determinator  116  transmits a CQI on a HS-DPCCH at a time determined by k and l.  
         [0046]     It can be further contemplated as another embodiment of the present invention that the Node B  120  manages k and l without reporting them to the RNC  130  and requests the UE  110  to report the CQI at a corresponding time point. The CQI determinator  116  of the UE  110  transmits the CQI on the HS-DPCCH in response to the request. In this case, a High Speed-Dedicated Shared Channel (HS-DSCH) scheduling delay can be minimized, obviating the need for additional downlink signaling.  
         [0047]     An Adaptive Modulation and Coding Scheme (AMC) controller and scheduler  124  in the Node B  120  schedules data transmission for all UEs and performs an AMC function based on CQIs from the UEs including the UE  110 .  
         [0048]     To transmit Doppler information to the Node B  120  on the uplink, the Doppler estimator  112  of the UE  110  estimates the Doppler frequency or speed of the UE  110  using a signal received from the Node B  120 . An algorithm for the estimation is known and the estimation is made using the covariance function of a channel by way of an example.  
         [0049]     As known, the power characteristics of a channel can determined using the CPICH. The power auto-covariance function of the channel is expressed as 
 
 Cov   c   [i ]( n )= E[C ( n ) C ( n+i )]− E[C ( n )] 2   (1) 
 
 where Cov c [i](n) is the power auto-covariance function of an i th  slot, C[n] is an n th  sample of a channel power response, and E[ ] is energy. 
 
         [0050]     The UE  110  calculates the value of i, that is, i 0  at which the covariance function of Eq. (1) has a maximum value. The UE  110  then estimates a Doppler frequency W Doppler  by substituting i 0  into  
               ω   Doppler     =     3.8317       T   b     ⁢     i   0                 (   2   )             
 
         [0051]     In general, a carrier frequency in which communications are conducted has already been set and the Doppler frequency can be expressed in terms of a UE&#39;s speed. Also, the relationship between coherence time and speed can be acquired through actual measurement. Thus, the coherence time of the channel of the UE is quantized by means of a mapping table such as Table 1 below. The coherence time refers to the time over which channel characteristics are relatively coherent.  
                                                   TABLE 1                           Order of   0000   0001   0010   0011   0100   0101   0110   0111   1000       output       Speed   1.0   1.5   3   6   12   15   30   60   120       (Km/hr)       Coherence   200   160   80   40   20   16   8   4   2       time       (ms)                  
 
         [0052]     The UE  110  transmits the value quantized by Table 1 to the reporting cycle determinator  122 .  
         [0053]     The quantized value is delivered on the HS-DPCCH. Specifically, the UE  110  transmits a 4-bit quantized coherence time value instead of a 5-bit CQI in the first HS-DPCCH frame.  
         [0054]     The reporting cycle determinator  122  determines the speed of the UE  110  from the quantized coherence time referring to the same mapping table as used in the UE  110 , Table 1.  
         [0055]      FIG. 6  is a flowchart illustrating the operation of a Node B according to the embodiment of the present invention. When the UE establishes a new RL, the procedure illustrated in  FIG. 6  is performed.  
         [0056]     Referring to  FIG. 6 , the Node B determines whether an RL setup has been requested from the RNC or UE in step  200 . Upon request for the RL setup, the Node B determines k using feedback information from the UE, that is, a coherence time value in step  202 .  
         [0057]     For example, if the Node B receives “0101” on the HS=DPCCH from the UE, it determines k to be 8 considering that 2 ms is taken to transmit one CQI, because “0101” represents a coherence time of 16 ms in Table 1.  
         [0058]     In step  204 , a proper l value that minimizes a maximum overlap between the UE and other UEs is determined based on k. Specifically, the Node B selects a proper l that minimizes the maximum overlap between CQI time slots for the UE and CQI time slots for other UEs, while changing l from 0 to k−1. It is possible since k and l values of other UEs which have already established RL are known.  
         [0059]     The Node B again determines the maximum CQI transmission overlap between the UE and other UEs according to k and l in step  206 . The overlap can be defined as the number of other UEs that transmit CQIs in time slots set for the UE to transmit a CQI according to k and l. If the maximum overlap exceeds a predetermined threshold th in step  208 , the Node B increases k by one level in step  210  and returns to step  204 . Available k values are preset: 0, 2, 4, 8, 10, 20, 40, 80, 100. Therefore, if k is set to 8 in step  202 , k is increased to 10 in step  210 .  
         [0060]     Once k and l have been determined in the above procedure, the Node B generates an RL SETUP RESPONSE message including k and l in step  212  and transmits the RL SETUP RESPONSE message to the RNC in step  214 . The RNC then notifies the UE of k and l and the UE reports a CQI to the Node B in time slots determined by k and l.  
         [0061]     While k and l are indicated to the UE in the illustrated case of  FIG. 6 , it can be further contemplated as another embodiment of the present invention that the Node B requests the UE to report the CQI in time slots by the determined k and l. The CQI report request is sent to the UE in a Channelization Code Set (CCS) field in an High Speed-Shared Control Channel (HS-SCCH). The UE transmits the CQI on the HS-DPCCH immediately after receiving the CQI report request from the Node B. The CCS field is used to indicate the number and type of spreading codes.  
       Second Embodiment  
       [0062]     The Node B determines k and l according to the variation of a CQI, while continuously receiving the CQI from the UE.  
         [0063]      FIG. 7  is a block diagram illustrating the configuration of a system for determining a CQI according to another embodiment of the present invention. Referring to  FIG. 7 , a UE  310  periodically reports a CQI on a HS-DPCCH to a Node B  320 . The Node B  320  estimates the variance or standard deviation of the CQI and determines k and l according to the CQI variance or standard deviation.  
         [0064]     A CQI determinator  312  in the UE  310  transmits the CQI on the HS-DPCCH according to initial k and l values to a RNC  330 . A CQI covariance measurer  322  in the Node B  320  calculates the variance or standard deviation of CQI values accumulated for a predetermined time period and roughly estimates the variation of the CQI over time. The CQI covariance measurer  322  determines a CQI reporting cycle based on the measurement using a predetermined mapping function.  
         [0065]      FIG. 8  is a block diagram of the CQI variance measurer according to the second embodiment of the present invention. Referring to  FIG. 8 , the CQI variance measurer  322  includes an input filter  322   a , a mean square average calculator  322   b , and a mapper  322   c . To compute a square root, the mean square average calculator  322   b  is replaced by a standard variance generator.  
         [0066]     The input filter  322   a  receives CQI values from the UE, CQI 1 , CQI 2 , . . . , CQI N . It is configured to be a low pass filter such as a moving average (MA) filter or a median filter in order to detect the variation of the CQI even if there is little change in the channel condition. The mean square average calculator  322   b  obtains a CQI standard deviation σ CQI  by squaring outputs of the filter  322   a , v 1 , v 2 , . . . , v N , summing the squares, and computing the average of the sum.  
         [0067]     The mapper  322   c  maps σ CQI  to k. For example, the mapper  322   c  determines k by  
               k   ⁡     (   σ   )       =     {           0   ,           σ   ≥     η   0                 2   ,             η   1     ≤   σ   &lt;     η   0                 4   ,             η   2     ≤   σ   &lt;     η   1               …                         100   ,           σ   &lt;     η   6                       (   3   )             
 
 where η 0  to η 6 , the number of η 0  to η 6 , and available k values are preset depending on external factors such as system configuration and radio environment. The mapper  322   c  also determines a proper l according to k. 
 
         [0068]     In an embodiment of the present invention, the Node B  120  can report k and l to the RNC  330  by an RL SETUP RESPONSE message. The RNC  330  then notifies a CQI determinator  312  of the UE  310  of k and l by a RADIO BEARER REQUEST message. The CQI determinator  312  transmits a CQI at a time determined by k and l.  
         [0069]     It can be further contemplated as another embodiment of the present invention that the CQI variance measurer  322  of the Node B  320  directly requests the CQI determinator  312  of the UE  310  to report the CQI at a corresponding time point. The CQI determinator transmits the CQI in response to the request.  
         [0070]     An AMC controller and scheduler  324  of the Node B  320  schedules data transmission for all UEs based on CQIs received from the UEs including the UE  310  and performs an AMC function.  
         [0071]      FIG. 9  is a flowchart illustrating the operation of the Node B according to the second embodiment of the present invention.  
         [0072]     Referring to  FIG. 9 , the Node B determines whether an RL setup has been requested from the RNC or UE in step  402 . Upon request for the RL setup, the Node B sets k to a small value such as 0, 2, or 4 in step  404 . The Node B also sets l according to k. Upon receipt of CQIs from the UE according to k and l, the Node B stores the CQIs in step  406 . The Node B determines whether the number of CQI reception occurrences is equal to or larger than N in step  408 . If the number of CQI reception occurrences is less than N, the Node B returns to step  406 . If the number of CQI reception occurrences is equal to or lager than N, the Node B goes to step  410 .  
         [0073]     In step  410 , the Node B calculates the variance or standard deviation of the stored CQI values and determines k according to the variance or standard deviation using Eq. (3). The Node B determines a proper l value that minimizes a maximum overlap between the UE and other UEs based on k in step  412 . Specifically, the Node B selects a proper l that minimizes the maximum overlap between CQI time slots for the UE and CQI time slots for other UEs, while changing l from 0 to k−1, which is made possible since k and l values of other UEs which have established RLs are known.  
         [0074]     The Node B again determines the maximum CQI transmission overlap between the UE and other UEs according to k and l in step  414 . The overlap can be defined as the number of other UEs that transmit CQIs in time slots set for the UE to transmit a CQI according to k and l. If the maximum overlap exceeds a predetermined threshold th in step  416 , the Node B increases k by one level in step  418  and returns to step  412 . Available k values are preset: 0, 2, 4, 8, 10, 20, 40, 80, 100. Therefore, if k is set to 8 in step  410 , k is increased to 10 in step  418 .  
         [0075]     Once k and l have been determined in the above procedure, the Node B generates an RL SETUP RESPONSE message including k and l in step  420  and transmits the RL SETUP RESPONSE message to the RNC in step  422 . The RNC then notifies the UE of k and l and the UE reports a CQI to the Node B in time slots determined by k and l.  
         [0076]     While k and l are indicated to the UE in the illustrated case of  FIG. 9 , it can be further contemplated as another embodiment of the present invention that the Node B requests the UE to report the CQI in time slots determined by k and l. The UE transmits the CQI on the HS-DPCCH immediately after receiving the CQI report request from the Node B.  
         [0077]      FIG. 10  illustrates an example of CQI transmission on the HS-DPCCH according to the embodiments of the present invention. UE  1  and UE  2  are moving fast, whereas UE  3  is moving slow in the illustrated case.  
         [0078]     Referring to  FIG. 10 , UE  1  transmits a CQI in first, third, fifth and seventh time slots with k=2 and l=0. UE  2  transmits a CQI in second, sixth, tenth, and fourteenth time slots with k=4 and l=1. UE  3  transmits a CQI in fourth and fourteenth time slots with k=10 and l=2.  
         [0079]     UE  3 , which is moving slow, reports the CQI at a longer interval than UE  1  and UE  2 , while UE  1  and UE  2  report their CQIs more frequently. This is because a radio channel environment can be fast changed for a fast-moving UE and the Node B needs to sense the change fast. Furthermore, transmissions of the CQIs from UEs are distributed by l to avoid simultaneous CQI transmissions in the same time slot as much as possible. Thus, power interference caused by overlapped CQI transmissions among UEs can be minimized.  
         [0080]     In accordance with the present invention as described above, system performance is increased, power for CQI reporting in UEs is saved, and power interference is minimized in an HSDPA communication system where channel quality information is reported for implementation of AMC.  
         [0081]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it should 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.