Abstract:
A user terminal and a method for transmitting a CQI index from the user terminal to a base station. A method for performing a CQI index conversion so as to have a constant number of bits regardless of the number of bits of a calculated CQI index includes the steps of: calculating the CQI index for a signal received from the base station; converting the calculated CQI index to an one-bit CQI index through delta-sigma modulation; and transmitting the converted one-bit CQI index to the base station.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for transmitting a CQI index from a user terminal to a base station, and more particularly relates to a method for converting a CQI index to have a constant bit number regardless of a calculated bit number of the CQI index. 
       BACKGROUND ART 
       [0002]    For a comprehensive power control concept for maximizing the performance in a general wireless communication system, there are various link adaptation methods considering quality of service (QoS). For an example, there are sub-carrier allocation, bit loading, adaptive modulation, power control, and the like. The best way to satisfy QoS for adaptive link is that a base station, which is the operational body performing adaptive link, should accurately comprehend downlink channel status. Thus a method is required for a base station for comprehending downlink channel status as accurately as possible. Feedback method is generally used for such a method, wherein a terminal, which is a main operational body capable of estimating downlink channel status, directly estimates required channel status and transmits the information to the base station. Such information is called as channel quality information (CQI). 
         [0003]    In a wireless communication system, a base station periodically receives the channel quality indicator (CQI), which indicates channel status of each individual terminal, from the multiple terminals, and the downlink transmission capability is enhanced by utilizing this (i.e. CQI) and performing adaptive modulation and coding (AMC) or hybrid auto repeat reQuest (HARQ) and the like, thus the overall system capability can be enhanced. 
         [0004]    Thus, a shorter reporting period of aforementioned CQI is better in order to promptly reflect the varying channel status of the terminals for a base station. However, in a case where a CQI channel is being used with other channel at the same time, if only the CQI channel is expanded in order to shorten the CQI reporting period, the capacity of the other channel is reduced. 
         [0005]    In an OFDM system, in order to optimize sub-carrier allocation, bit loading, adaptive modulation, power control, and the like, it is naturally required to obtain channel information for each sub-carrier. In an LTE system, for example, CQI is managed in a band type wherein many sub-carriers are grouped (sub-band CQI) in order to avoid the complexity of implementation. An LTE system transmits and receives CQI using maximum 4 bits, and the CQI is classified into 16 levels and transmitted by 4 bits. 
         [0006]    Thus, the number of bits must be increased by multiple of four in order to simultaneously transmit multiple CQI information on multiple sub-bands. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    An objective of the present invention for solving the foregoing problem is to provide a method, wherein the number of bits for transmitting the CQI information is equally maintained regardless of the number of bits of the CQI index. 
         [0008]    Another objective of the present invention for solving the foregoing problem is to provide a method for reducing overhead in CQI transmission. 
       Solution to Problem 
       [0009]    To ensure this end, a user terminal of the present invention includes: a CQI index calculation unit for calculating the CQI index for a signal received from the base station; a delta-sigma modulation unit for converting the said CQI index to an one-bit CQI index through delta-sigma modulation; and a communication unit for transmitting the converted one-bit CQI index to said base station. 
         [0010]    To ensure this end, a method of the present invention for transmitting a CQI index from a user terminal to a base station includes the steps of: calculating the CQI index for a signal received from the base station; converting the said CQI index to an one-bit CQI index through delta-sigma modulation; and transmitting the converted one-bit CQI index to said base station. 
       Advantageous Effects of Invention 
       [0011]    A method for transmitting CQI index of the present invention modulates CQI index using a delta-sigma modulation unit, therefore even the number of bits representing the CQI index increases, a user terminal transmits a CQI index to the base station using equal number of bits. Thus by sending CQI index using one bit, there is an effect of reducing the overhead in transmitting CQI. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  illustrates a mobile communication system according to an exemplary embodiment of the present invention; 
           [0013]      FIG. 2  illustrates an operation performed in a user terminal according to an exemplary embodiment of the present invention; 
           [0014]      FIG. 3  illustrates a configuration of a delta-sigma modulation unit according to an exemplary embodiment of the present invention; and 
           [0015]      FIG. 4  is a block diagram illustrating a configuration of a user terminal according to an exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF SYMBOLS 
       [0016]      
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 100: base station 
                 110: user terminal 
               
               
                   
                 400: communication unit 
                 402: CQI index calculation unit 
               
               
                   
                 404: control unit 
                 406: delta-sigma modulation unit 
               
               
                   
                   
               
             
          
         
       
     
       DESCRIPTION OF EMBODIMENTS 
       [0017]    Foregoing and additional features of the present invention will be more apparent through the exemplary embodiments being described with reference to the accompanying drawings. Hereinafter it will be described in detail for a person of ordinary skill in the art can easily understand and implement through these exemplary embodiments of the present invention 
         [0018]      FIG. 1  illustrates a mobile communication system according to an exemplary embodiment of the present invention. Hereinafter the element of a mobile communication system according to an exemplary embodiment of the present invention will be investigated in detail. 
         [0019]    According to  FIG. 1 , the mobile communication system includes a user terminal and a base station. Of course it is apparent that another element may further be included except the foregoing elements. For an example, the mobile communication system may further include a base station control unit. 
         [0020]    As described above, an LTE system calculates 4 bit CQI having 16 levels. A CQI index has values from 0 to 15; BPSK is used as a modulation method for a zero CQI index, and it is modulated with a high channel coding rate. In addition, as a CQI index increases modulation is performed with a high modulation order and a low channel coding rate. In other words, if the CQI index is 15, 64 QAM is used and the most high channel coding rate is used in signal processing. 
         [0021]    The user terminal  110  receives and processes a signal from the base station  100 . According to the present invention, the user terminal  110  calculates CQI using the signal-to-noise ratio and the channel gain information of the signal received from the base station  100 . In addition, the user terminal  110  verifies whether an error is detected in the signal received from the base station  100 . If no error is detected in the received signal, the user terminal  110  creates an ACK signal; if an error is detected, an NACK signal is created. The user terminal  110  transmits the created ACK signal or NACK signal to the base station  100 . 
         [0022]    Separately from this, the user terminal  110  creates a CQI index for the received signal, and transmits the created CQI index to the base station  100 . According to the present invention, the user terminal  110  sets up the ACK signal or the NACK signal transmission period differently from a CQI index transmission period. For an example, while the user terminal  110  transmits the CQI index to the base station with an interval of a first transmission period, the ACK or the NACK signal is transmitted to the base station  100  with an interval of a second transmission period. In other words, the user terminal  110  can transmit the ACK signal or the NACK signal to the base station  100  at the time when it does not transmit the CQI index; the CQI index can be transmitted at the time when it does not transmit the ACK signal or the NACK signal. Of course the user terminal  110  may transmit an ACK signal or an NACK signal with a CQI index at the same time. 
         [0023]    The base station  100  determines a modulation method for the signal to be transmitted to the user terminal  110  using the CQI index received from the user terminal  110 . In other words, the base station  100  performs modulation with a high modulation order and a low channel coding rate when the received CQI from the user terminal  110  is good; and the modulation is performed with a low modulation order and a high channel coding rate when the received CQI is poor. 
         [0024]    In addition, according to the present invention, the base station  100  estimates the channel state of the present base station  100  and the user terminal  110  using the HARQ signal received from the user terminal  110  at the time when the CQI index is not received from the user terminal  110 . The HARQ signal includes the above described ACK signal or the NACK signal. In other words, the base station  100  replaces HARQ signal with the CQI index. 
         [0025]    In relation with this, when an ACK signal is received from the user terminal  110 , the base station  100  presumes that the current channel state is good, and maintains previous CQI index. On the other hand, when an NACK signal is received from the user terminal  110 , the base station  100  presumes that the current channel state is poor, and adjusts the previous CQI index. In other words, as explained here in detail, the LTE system creates 4 bit CQI having 16 levels. The CQI index has values from 0 to 15. When an NACK signal is received from the user terminal  110 , the base station  100  presumes that the current channel state is poor, and deducts one from the previous CQI index. In other words, if existing index is 10, the base station  100  changes present index to 9 when an NACK signal is received from the user terminal  110 , and modulates the signal that is to be transmitted to the user terminal  110  in accordance with the changed index. 
         [0026]      FIG. 2  illustrates an operation performed in a user terminal according to an exemplary embodiment of the present invention. Hereinafter, using  FIG. 2 , the operation performed in a user terminal according to an exemplary embodiment of the present invention will be investigated in detail. 
         [0027]    In step S 200 , the user terminal extracts channel state information. As described above, the user terminal calculates CQI index using the signal received from the base station. The procedures for CQI index calculation is same as explained here in detail. 
         [0028]    In step S 202 , the user terminal performs delta-sigma modulation on the calculated CQI index using the delta-sigma modulation unit. The CQI index is converted into one bit using delta-sigma modulation. Delta-sigma modulation will be explained later. 
         [0029]    In step S 204 , the user terminal transmits the CQI index, which has gone through delta-sigma modulation, to the base station. 
         [0030]      FIG. 3  is a drawing of a sigma-delta analog-digital converter according to an exemplary embodiment of the present invention. According to  FIG. 3 , the sigma-delta analog-digital converter  300  includes a sigma-delta modulator  310  and an accumulator  320 . 
         [0031]    Said modulator  310 , which operates in response to a clock signal CLK, modulates input signal into sigma-delta modulated signal, and outputs the modulated digital output signal. To ensure this end, though it is not shown here, said modulator  310  may include a loop filter, a quantizer for digital signal conversion (not shown here), a DAC, and an adder/subtractor which adds input signal and feedback signal. 
         [0032]    Said accumulator  320  receives said digital output signal for accumulation, and outputs an accumulated output value. At this time, said accumulator  320  may accumulate said digital output signal by digital integration thereof, and said accumulated output value may be a digital output value. 
         [0033]    Said accumulator  320  may include a first accumulator (1, 2, . . . , N), and if said accumulator  320  includes N number of first accumulators (1, 2, . . . , N), said modulator  11  may also include N number of modulators (or Nth order modulator). 
         [0034]      FIG. 4  is a block diagram illustrating a configuration of a user terminal according to an exemplary embodiment of the present invention. Hereinafter, using  FIG. 4 , the elements of a user terminal according to an exemplary embodiment of the present invention will be investigated in detail. 
         [0035]    According to  FIG. 4 , the user terminal includes a communication unit, a control unit, and a delta-sigma modulation unit. Of course it is apparent that other elements may be included in the base station. 
         [0036]    The communication unit  400  transmits a signal to the base station, or receives a signal transmitted from the base station. 
         [0037]    The CQI index calculation unit  402  calculates CQI index for the signal received from the base station according to the control instruction of the control unit  404 . The procedures for CQI index calculation is same as explained here in detail. 
         [0038]    The control unit  404  controls each element comprising the user terminal. For an example, the control unit  404  calculates CQI index for a signal received from the base station by controlling the CQI index calculation unit  402 , and performs delta-sigma modulation on the calculated CQI index by controlling the delta-sigma modulation unit  406 . 
         [0039]    Delta-sigma modulation unit  406  performs delta-sigma modulation on the calculated CQI index by the CQI index calculation unit  402 . As explained here in detail, the CQI index has one bit value through the delta-sigma modulation process. The CQI index which has been modulated by the delta-sigma modulation unit  406  is transmitted to the base station through the communication unit as explained here in detail. 
         [0040]    Thus, according to the present invention, the user terminal transmits CQI index to the base station using equal number of bits even the number of bits representing the CQI index increases by modulating the CQI index using the delta-sigma modulation unit. 
         [0041]    Although the present invention is described with reference to one exemplary embodiment as illustrated in the drawings, it is merely exemplary and it will be understood for the person of ordinary skill in the art that various variations and equivalent other exemplary embodiments are possible from the foregoing disclosure.