Patent Publication Number: US-2011058506-A1

Title: Device and method for transmitting channel information in wireless communication system

Description:
TECHNICAL FIELD  
     The present invention relates to a device and method for transmitting channel information in a wireless communication system and, in particular, to a device and method for transmitting channel information obtained by quantizing the Frequency Division Duplex (FDD) downlink channel in a Multi user Multiple Input Multiple Output (MIMO) system based on an Orthogonal Frequency Division Multiple Access (OFDMA) scheme. 
     BACKGROUND ART 
     MIMO is an advanced antenna system using multiple transmission and reception antennas, and various researches have been done for improving the communication capacity of the MIMO-based communication systems. 
     The MIMO systems can be divided into two structures: open loop MIMO in which transmission is done with no specific channel information and closed loop MIMO in which transmission is done with reference to the channel information fed back from the receiver. The open loop MIMO is implemented with a complex space-time coding scheme to achieve the transmission rate close to the theoretical channel capacity and hence the decoding complexity increases system complexity increases exponentially with the increase of the number of antennas. Accordingly, in reality the closed loop MIMO, in which each transmission antenna has an independent modulation and coding scheme according to channel state, is preferable to achieve the theoretical capacity of the open loop MIMO. 
     The conventional feedback methods for transmitting the channel information from a receiver to a transmitter in closed loop MIMO are exemplarily described hereinafter. 
     The first approach is to use a code book. In this case, the receiver estimates the channel and sends an index of a codeword selected from the code book shared between the transmitter and receiver based on an appropriate metric to the transmitter. In the IEEE 802.16e standard, the receiver performs a Singular Value Decomposition (SVD) on a channel matrix H to obtain a precoding matrix V as shown in equation (1). For reference, SVD decomposes the channel matrix H into 2 unitary matrices U and V and a diagonal matrix S, where U is a unitary matrix of left eigenvectors of H, V is a unitary matrix of right eigenvectors of H, and S is a diagonal matrix of eiganvalues of H. 
       H=U S V H    (1)
 
     The receiver compares the precoding matrix V with respective codewords and feeds backs an index of the corresponding codeword to the transmitter. Here, the codeword index consists of 3 or 6 feedback bits per subcarrier. 
     This approach is advantageous with the small number of feedback bits, however, performing SVD on the channel matrix and repeated codeword comparison results in computational overload of the receiver. 
     The second approach is to use a sounding signal. This approach can be considered under the assumption of the reciprocity of downlink and uplink channels. The receiver sends a sounding signal to the transmitter through a sounding channel and the transmitter estimates the uplink channel based on the sounding signal and then obtains downlink channel information. This method is appropriated for the Time Division Duplex (TDD) system in which the uplink and downlink channels are reciprocal in the same frequency band but not for the Frequency Division Duplex (FDD) system in which different frequency bands are used for the uplink and downlink channels. 
     Meanwhile, IEEE 802.16m system, as the next generation communication system, support FDD as well as TDD, whereby there is a need to develop an efficient channel information feedback method for supporting the FDD operational mode that is capable of reducing the computation complexity of the receiver without degradation of the system performance in the OFDMA-based multi user MIMO system. 
     DISCLOSURE 
     Technical Problem 
     Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a channel information transmission device and method that is capable of supporting FDD mode in a multi user MIMO system. 
     It is another object of the present invention to provide a channel information transmission device and method that is capable of feeding back the quantized channel information to the transmitter. 
     It is still another object of the present invention to provide a device and method that is capable of feeding back only the differential information indicating difference between the current channel information and the previous channel information, thereby reducing feedback data amount and improving system performance. 
     Technical Solution  
     According to one aspect of the present invention, there is provided a device for transmitting channel information of complex channels established between a transmitter and a receiver in a wireless communication system, comprising: a channel estimator for estimating a channel by decomposing a channel matrix element on a complex channel into a real part and an imaginary part: a channel quantizer for outputting quantization values by quantizing the real part and the imaginary part based on predetermined boundary values, respectively; a channel information generator for generating channel information corresponding to the quantization values of the real part and the imaginary part; and a channel information transmitter for transmitting the channel information. 
     According to another aspect of the present invention, there is provided a method for transmitting channel information of a complex channel established between a transmitter and a receiver in a wireless communication system, comprising: estimating a channel by decomposing a channel matrix element of the complex channel into a real part and an imaginary part; quantizing the real part and imaginary part of the estimated channel based on predetermined boundary values; generating channel information corresponding to the quantization results of the real part and imaginary part; and transmitting channel information. 
     According to further another aspect of the present invention, there is provided a device for transmitting channel information in a wireless communication system supporting Multi user Multiple Input Multiple Output (MU MIMO) based on Frequency Division Duplex (FDD), comprising: a channel estimator for estimating a channel from an FDD downlink signal: a channel information generator for generating channel information by quantizing a element of a channel matrix; and a channel information transmitter for feeding back the channel information. 
     According to still further another aspect of the present invention, there is provided a method for transmitting channel information in a wireless communication system supporting Multi user Multiple Input Multiple Output (MU MIMO) based on Frequency Division Duplex (FDD), comprising: estimating a channel from an FDD downlink signal; generating channel information by quantizing an element of the channel; and feeding back the channel information. 
     Advantageous Effects 
     The channel information transmission device and method feeds back the channel information generated by quantizing a real part and an imaginary part of a channel matrix element in a multi user MIMO system, thereby reducing computation amount of the receiver and being able to implement effective feedback scheme supporting FDD. 
     The channel information transmission device and method calculates optimized boundary values using Gaussian distribution and quantizes the channel matrix elements based on the boundary values, thereby reducing feedback data without compromising system performance. 
     The channel information transmission device and method feeds back only the differential information indicating difference between the current channel information and the previous channel information, thereby reducing feedback data amount and improving system performance. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a multi user MIMO which is applicable to the present invention; 
         FIG. 2  is a block diagram illustrating a configuration of a channel information transmission device according to a first embodiment of the present invention 
         FIG. 3  is a graph illustrating how to quantize a real part and an imaginary part of complex channel matrix element with 2 bits respectively using Gaussian distribution. 
         FIG. 4  shows graphs illustrating BER vs. SNR performances in a single user environment and a multi user environment according to the variation of a number of quantization hits; 
         FIG. 5  is a block diagram illustrating a configuration of a channel information transmission device according to a second embodiment of the present invention; 
         FIG. 6  is a graph illustrating sum capacity vs. SNR performance when quantized with 8 bits in rectangular coordinate and polar coordinate schemes; 
         FIG. 7  is a flowchart illustrating a channel information transmission method according to a first embodiment of the present invention; 
         FIG. 8  is a flowchart illustrating a channel information transmission method according to a second embodiment of the present invention; and 
         FIG. 9  is a flowchart illustrating a procedure for calculating boundary values for quantization using Gaussian distribution. 
     
    
    
     MODE FOR INVENTION 
     Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. 
       FIG. 1  is a schematic diagram illustrating a MIMO system to adopt a channel information transmission method according to an exemplary embodiment of the present invention. In a MIMO system, the transmitter and receiver communicate with each other through multiple communication channels established between their multiple transmit and receive antennas.  FIG. 1  shows the exemplary 4×2 MIMO system including a transmitter with 4 transmit antennas (TxAnt 1 , TxAnt 2 , TxAnt 3 , and TxAnt 4 ) and a receiver with 2 receive antennas (RxAnt 1  and RxAnt 2 ) establishing 8 communication channels (h 11 , h 12 , h 13 , h 14 , h 21 , h 22 , h 23 , and h 24 ). 
       FIG. 2  is a block diagram illustrating a configuration of a channel information transmission device according to a first embodiment of the present invention. 
     As shown in  FIG. 2 , the channel information transmission device  100  includes a channel estimator  110 , a boundary value calculator  120 , a channel quantizer  130 , a channel information generator  140 , and a channel information transmitter  150 . 
     The channel estimator  110  estimates channels by decomposing each element of the channel matrix of complex channels between the transmitter and the receiver into a real part and an imaginary part. The channel estimation is carried out by sampling the real part and imaginary part of each channel periodically, and the estimated channel value is output to the quantizer  130 . 
     The boundary value calculator  120  calculates the boundary values as reference for quantizing the channel values and outputs the boundary values to the channel quantizer  130 . The boundary value calculator  120  calculates and/or stores N−1 boundary values for N quantization levels and outputs the boundary values to channel quantizer  130 . How to calculate the boundary values is described with reference to  FIG. 3  hereinafter. 
     In an embodiment of the present invention, the boundary values for quantizing the elements of the channel matrix in the real and imaginary parts are calculated using a probability density function (pdf) and/or a cumulative distribution function (cdf) under the assumption of Gaussian distribution of the complex channel matrix. 
     In more detail, when the probability variable is denoted by x, the provability density function by ƒ x (x), the boundary value by a, the quantization value by {circumflex over (x)}, the quantization distortion by D, and the quantization level by N, the quantization distortion can be expressed as equation (2). 
     
       
         
           
             
               
                 
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     From equation (6), it is known that the quantization value {circumflex over (x)} i , is a function of the boundary value a i  and also is a conditional mean of x over the range (a i−1 , a i ). 
     The a i  and {circumflex over (x)} i  are finally obtained by calculating a i  and {circumflex over (x)} i  iteratively until the significant digit does not change. In this manner, the boundary values and quantization values are determined according to the quantization bits (quantization level) as shown in table 1. In an exemplary embodiment the number of significant digits is 4 below decimal point. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 boundary and quantization values according to number 
               
               
                 of quantization bits 
               
            
           
           
               
               
               
            
               
                 Number of 
                   
                   
               
               
                 bits 
                 Boundary value 
                 Quantization value 
               
               
                   
               
               
                 1 
                 0 
                 ±0.5642 
               
               
                 2 
                 0, ±0.6941 
                 ±0.3202, ±1.0677 
               
               
                 3 
                 0, ±0.3540, ±0.7425, ±1.2360 
                 ±0.1733, ±0.5346, 
               
               
                 4 
                 0, ±0.1826, ±0.3694, ±0.5654, 
                 ±0.9504, ±1.5217 
               
               
                   
                 ±0.7771, ±1.0161, ±1.3039, ±1.6978 
                 ±0.0908, ±0.2744, 
               
               
                   
                   
                 ±0.4644, ±0.6664, 
               
               
                   
                   
                 ±0.8881, ±1.1441, 
               
               
                   
                   
                 ±1.4630, ±1.9325 
               
               
                   
               
            
           
         
       
     
     The channel quantizer  130  quantizes the estimated channel values output by the channel estimator  110  with reference to the boundary values provided by the boundary value calculator  120 . As aforementioned, the quantization is performed on each element of the channel matrix in the real and imaginary parts and the quantized results are output to the channel information generator  140 . 
     The channel information generator  140  generates the channel information by encoding the quantized values output by the channel quantizer  130 . Here, the channel information for each element of the channel matrix is generated with the respective real part and the imaginary part encoded into the number of bits corresponding to the quantization level. 
     For instance, when the quantization level (number of quantization bits) is 4, the channel information generated by the channel information generator  140  and the quantized channel values corresponding to the channel information can be listed as shown in table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 channel information and quantized channel values 
               
               
                 with 4 quantization levels 
               
            
           
           
               
               
               
            
               
                   
                 Channel info. 
                 Quantized channel value 
               
               
                   
                 (4 bits) 
                 (representative value) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 0000 
                 −1.9325 
               
               
                   
                 0001 
                 −1.4630 
               
               
                   
                 0010 
                 −1.1441 
               
               
                   
                 0011 
                 −0.8881 
               
               
                   
                 0100 
                 −0.6664 
               
               
                   
                 0101 
                 −0.4644 
               
               
                   
                 0110 
                 −0.2744 
               
               
                   
                 0111 
                 −0.0908 
               
               
                   
                 1000 
                 0.0908 
               
               
                   
                 1001 
                 0.2744 
               
               
                   
                 1010 
                 0.4644 
               
               
                   
                 1011 
                 0.6664 
               
               
                   
                 1100 
                 0.8881 
               
               
                   
                 1101 
                 1.1441 
               
               
                   
                 1110 
                 1.4630 
               
               
                   
                 1111 
                 1.9325 
               
               
                   
                   
               
            
           
         
       
     
     That is, although the channel information is provided as the quantized channel values, the channel information is preferably provided as the indices representing the quantized channel values in the form of a table or base mode codebook as shown in exemplary table 2. 
       FIG. 4  shows graphs illustrating BER vs. SNR performances in a single user environment and a multi user environment according to the variation of a number of quantization bits. 
     The graph (a) of  FIG. 4  shows the BER (Bit Error Rate) vs. SNR (Signal to Noise Ratio) performance in the single user and multi user environments when the real part and imaginary part of each element of the channel matrix are quantized with 3 bits respectively. As shown in the graph (a), a crossover occurs in the SNR range between 15 and 20 such that the single user environment&#39; s BER becomes lower than the multi user environment&#39;s BER. 
     The graph (b) of  FIG. 4  shows the BER vs. SNR performance in the single user and multi user environments when the real part and imaginary part of each element of the channel matrix are quantized with 4 bits respectively. Since no crossover occurs across the entire SNR range in the case, it is preferable to set the number of quantization bits to 4 (total 8 hits per channel) or more. 
     Finally, the channel information transmitter  150  transmits the channel information to the transmitter through a feedback channel. 
       FIG. 5  is a block diagram illustrating a configuration of a channel information transmission device according to a second embodiment of the present invention. In the second embodiment, the receiver transmits differential information indicating difference from the previous channel information rather than transmitting the entire channel information. This is preferable for transmitting the short-term channel information with which the variation of channel status is relatively low. 
     As shown in  FIG. 5 , the channel information transmission device  200  according to the second embodiment of the present invention includes a channel estimator  210 , a boundary value calculator  220 , a channel quantizer  230 , a channel information generator  240 , a differential information generator  250 , and a channel information transmitter  260 . Since the channel estimator  210 , the boundary value calculator  220 , the channel quantizer  230 , the channel information generator  240  are substantially identical with those of the channel information transmission device  100  of the first embodiment in structure and function, only the differential information generator  250  and the channel information transmitter  260  are described in detail hereinafter. 
     The differential information generator  250  compares the current channel information output by the channel information generator  240  with the previous channel information and generates the differential information based on the difference between the current and previous channel information. For this purpose, the differential information generator  250  can be provided with a buffer for temporarily storing the previous channel information provided by the channel information generator  240  so as to generate the differential information by comparing the current channel information output by the channel information generator  240  and the previous channel information stored in the buffer. 
     In case of 8-bit channel information including respective 4 real part quantization bits and 4 imaginary part quantization bits, the initial 8-bit channel information has no previous channel information. Accordingly, the differential information generator  250  generates the initial 8-bit channel information as the differential information (in this case, the buffer is set to 0) and stores the initial 8-hit channel information into the buffer. Once the initial 8-bit channel information is stored in the buffer, the differential information generator  250  compares the current channel information with the previous channel information and generates the differential information of 1 or 2 bits (2 to 4 bits per channel). 
     Here, the differential channel information can be implemented for leveling up or down (e.g. the differential channel information bit can be set to ‘1’ for leveling up and ‘0’ for leveling down) or in the form of an index indicating the channel information listed in a differential mode codebook . 
       FIG. 6  is a graph illustrating sum capacity vs. SNR performance when quantized with 8 bits in rectangular coordinate and polar coordinate schemes. In  FIG. 6 , the rectangular coordinate scheme shows the best performance when the real part and the imaginary part are quantized with 4 bits respectively, and the polar coordinate scheme shows its performance similar to that of the rectangular coordinate scheme when each element of the channel matrix is quantized with 5-bit phase and 3-hit magnitude. Since the calculation amount for obtaining the real part and the imaginary part of the channel matrix element is less than that for obtaining the phase and magnitude, the rectangular coordinate scheme-based method as adopted to the present invention is more practical in reality. 
     The channel information transmitter  260  transmits the differential information to the transmitter through a feedback channel. As aforementioned, the entire channel information, e.g. 8-bit channel information, is transmitted in an initial feedback stage, and then the 2 to 4-hit differential information is transmitted. 
     The channel information transmission method according to an exemplary embodiment of the present invention is described hereinafter with reference to  FIGS. 7 and 8 . 
     Detailed descriptions of procedures and operations of the channel information transmission method that have been described already in association with the channel information transmission device may be omitted. 
       FIG. 7  is a flowchart illustrating a channel information transmission method according to a first embodiment of the present invention. 
     Referring to  FIG. 7 , a receiver decomposes each element of the channel matrix of complex channels established with a transmitter into a real part and an imaginary part and estimates the channel with the real part and the imaginary part of the channel at step S 710 . Next, the receiver quantizes the channel values of the real part and the imaginary part of the channel matrix element with reference to the boundary values at step S 720 . The boundary values are preferably obtained using Gaussian distribution. 
     Next, the receiver generates channel information composed of a predetermined number of bits by encoding the quantized real part value and imaginary value at step S 730 . Finally, the receiver transmits the channel information to the transmitter at step S 740 . 
       FIG. 8  is a flowchart illustrating a channel information transmission method according to a second embodiment of the present invention. In this embodiment, differential information is transmitted as the channel information. 
     Referring to  FIG. 8 , the receiver decomposes each element of the channel matrix into a real part and an imaginary part and estimates the channel with the real part and the imaginary part of the channel at step S 810 . Next, the receiver quantizes channel values of the real part and the imaginary part of the channel matrix element with reference to the boundary values at step S 820 . Next, the receiver generates channel information composed of a predetermined number of bits by encoding the quantized real part value and imaginary value at step S 830 . 
     Sequentially, the receiver compares the current channel information with the previous channel information to output differential information at step S 840 . In this embodiment, the initial channel information is regarded as the differential information. Finally, the receiver transmits the differential information to the transmitter at step S 850 . At the initial feedback stage the initial channel information is transmitted, and differential information is transmitted from the next feedback stage. 
       FIG. 9  is a flowchart illustrating steps of quantization procedure of the channel information transmission methods of  FIGS. 7 and 8 . This procedure can be explained with reference to the description about the boundary value generators  120  and  220 . 
     The receiver first sets quantization values corresponding to the number of quantization levels at step S 910 . When using 4 quantization levels as shown in  FIG. 3 , 3 boundary values a 1 , a 2 , and a 3  are set. Next, the receiver calculates the quantization values {circumflex over (x)} 1 , {circumflex over (x)} 2 , {circumflex over (x)} 3 , and {circumflex over (x)} 4  that minimize the quantization distortion D with reference to the boundary values (see equations (5) and (6)) at step S 920 . Next, the receiver calculates the boundary values a 1 , a 2 , and a 3  with reference to the quantization values {circumflex over (x)} 1 , {circumflex over (x)} 2 , {circumflex over (x)} 3 , and {circumflex over (x)} 4  (see equations (3) and (4)) at step S 930 . Next, the receiver compares the calculated boundary values with the previously calculated boundary values at step S 940 . It is preferred that the identification test is performed to the significant digits. When the current boundary values and the previous boundary values are not identical with each other, the receiver repeats steps S 920  and S 930  to calculate the quantization values and boundary values. The steps S 910  to S 940  can be rearranged such that the receiver determines the quantization values first and then calculate the final boundary values through iterative boundary values and quantization values calculations. When the current boundary values and the previous boundary values are identical with each other at step S 940 , the receiver performs the quantization process with the finally calculated boundary vales at step S 950 . 
     For reference, the channel information transmission device and method according to the present invention exemplify quantization scheme in which the real part and the imaginary part of each channel element are encoded with 4 bits respectively. As described above, the channel information transmission device and method according to the present invention can reduce the number of bits of channel information without compromising the system performance by quantizing the real part and the imaginary part of each channel element with 8 bits (4 hits for real part and 4 bits for imaginary part). 
     In case of 4×2 MIMO system as exemplarily shown in  FIG. 1 , the receiver transmits 32-bit channel information for 4 channels through each transmit antenna, and it is observed that the feedback performance of this system substantially equals to that of the codebook-based channel information feedback in which 17-bit channel information it transmitted. In consideration of the computation complexity (SVD and repeated codeword comparison) of the codebook based feedback in the receiver, the channel information transmission method according to the present invention is more efficient than the codebook based channel information transmission method. 
     Particularly in a Collaborative MIMO system which is implemented with a base station having multiple antenna and multiple mobile terminals each having a single antenna, since each mobile terminal uses a single antenna, the channel information transmission method according to the present invention can dramatically reduce the feedback information of the mobile terminal relatively to the case that a mobile terminal uses multiple antenna. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may he made therein without departing from the spirit and scope of the invention. Therefore, the spirit and scope of the present invention must be defined not by described embodiments thereof but by the appended claims and equivalents of the appended claims.