Abstract:
A method of detecting the power loadings at a receiver wherein estimation of power loadings based on the received reference signals is unnecessary. Channel condition is obtained for each channel and transmission power loading per channel is detected according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary. A received encoded information bit stream is then decoded according to the detected power loading per channel.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to data communication, and more particularly, to power loading in multi-channel data communication systems such as multiple-input multiple-output (MIMO) systems. 
   BACKGROUND OF THE INVENTION 
   A multiple-input-multiple-output (MIMO) communication system employs multiple transmit antennas in a transmitter and multiple receive antennas in a receiver for data transmission. A MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels, wherein each channel is a spatial sub-channel (or a transmission channel) of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. 
   MIMO increases system link robustness and spectral efficiency. To optimize spectral efficiency for MIMO system, many efforts have been made, which can be broadly classified into two categorists: open-loop approaches and closed-loop approaches. The open-loop approaches include spatial multiplexing, space-time coding and the tradeoff therebetween them. The closed-loop approaches focus on maximizing the link capacity, which results in a “water-filling” solution, and on minimizing the weighted MMSE which provides an “inverse water-filling” solution. 
   In an open-loop MIMO system, the MIMO transmitter has no prior knowledge of the channel condition (i.e., channel state information). As such, space-time coding techniques are usually implemented in the transmitter to combat fading channels. In a closed-loop system, the channel state information (CSI) can be fed back to the transmitter from the receiver, wherein some pre-processing can be performed at the transmitter in order to separate the transmitted data streams at the receiver side. 
   Such techniques are referred to as beamforming techniques, which provide better performance in desired receiver&#39;s directions and suppress the transmit power in other directions. Beamforming techniques are considered for IEEE 802.11n (high throughput WLAN) standard. Closed-loop eigen-beamforming generally provides higher system capacity compared with the closed-loop solution, assuming the transmitter knows the down-link channel. Singular vector decomposition (SVD) based eigen-beamforming decomposes the correlated MIMO channel into multiple parallel pipes. 
   When applying the closed-loop approach to MIMO-OFDM (orthogonal frequency division multiplexing), the optimal solution requires a bit loading and power loading per OFDM subcarrier. In order to simplify the complexity, commonly assigned patent applications Ser. No. 11/110,346, filed Apr. 19, 2005, entitled “Power Loading Method and Apparatus for Throughput Enhancement in MIMO systems,” and patent application Ser. No. 11/110,337, filed Apr. 19, 2005, entitled “A Method and Apparatus for Quantization and Detection of Power Loadings in MIMO Beamforming System,” incorporated herein by reference, provide adapting coding/modulation and power level across all subcarriers, fixing coding/modulation for all data streams and only adjusting the uneven power level for all OFDM symbols. In such methods, the receiver needs to know the power loadings before decoding the received signals. Therefore, the transmitter needs to acknowledge the receiver about the power loadings used at the transmitter, or the receiver needs to do automatic detection to estimate the power loading values based on the received reference signals. 
   BRIEF SUMMARY OF THE INVENTION 
   In one embodiment the present invention provides a method of automatically detecting the power loadings at the receiver wherein estimation of power loadings based on the received reference signals is unnecessary. The present invention is applicable to power loading methods which are calculated based on the channel eigenvalues. The present invention is also applicable to quantized power loading cases. 
   As such, in one implementation, the present invention provides a closed-loop signaling method over multiple channels in a telecommunication system, comprising the steps of: obtaining channel condition for each channel; detecting transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary; and decoding a received encoded information bit stream according to the detected power loading per channel. 
   The step of detecting transmission power loading further includes the steps of: determining rank-ordered channel eigenvalues based on the channel matrix; determining un-quantized power loadings; and determining quantized power loadings based on the un-quantized power loadings. 
   In another implementation the present invention provides a telecommunication system, comprising: a wireless transmitter and a receiver, wherein the transmitter transmits data streams via multiple channels over a plurality of antennas to the receiver by selecting transmission power loading per channel, and the receiver detects transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary. The receiver further obtains channel condition for each channel, detects transmission power loading per channel according to channel condition, wherein estimation of power loadings based on the received reference signals is unnecessary, and decodes a received encoded information bit stream according to the detected power loading per channel. Preferably, the receiver detects transmission power loading by further determining rank-ordered channel eigenvalues based on the channel matrix, determining un-quantized power loadings, and determining quantized power loadings based on the un-quantized power loadings. 
   These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a functional block diagram of a MIMO system implementing un-quantized uneven power loading and detection. 
       FIG. 2  shows a functional block diagram of a MIMO system implementing quantized uneven power loadings and detection according to an embodiment of the present invention. 
       FIG. 3  shows a functional block diagram of a detector for detecting power loading at a receiver according to an embodiment of the present invention. 
       FIG. 4  shows a flowchart of the steps of quantized power loading detection according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a functional block diagram of a MIMO system  300  which implements un-quantized uneven power loading and detection according to said commonly assigned patent application Ser. No. 11/110,337, filed Apr. 19, 2005, entitled: “A Method and Apparatus for Quantization and Detection of Power Loadings in MIMO Beamforming System.” The system  300  comprises a transmitter TX and a receiver RX. The transmitter TX includes an information bits generation unit  302 , a demultiplexer  304 , a power loading calculation unit  306 , a multiplication unit  308 , a V function unit  310 , and transmit antennas  312 . The receiver RX includes receiver antennas  314 , a U H  function unit  316 , a channel estimation unit  318 , a power loading detection unit  330 , an inverse power loading unit  322  and a multiplier  324 . 
   The system  300  provides adapting coding/modulation and power level across all subcarriers, fixing coding/modulation for all data streams and only adjusting the uneven power level for all OFDM symbols. The receiver needs to know the power loadings before decoding the received signals. Therefore, the transmitter needs to acknowledge the receiver about the power loadings used at the transmitter, or the receiver needs to do automatic detection to estimate the power loading values based on the received reference signals. 
   In one embodiment the present invention provides a method of automatically detecting the power loadings at the receiver wherein estimation of power loadings based on the received reference signals is unnecessary. The present invention is applicable to power loading methods which are calculated based on the channel eigenvalues. The present invention is also applicable to quantized power loading cases. 
   For channel eigenvalue-based algorithms for power loading calculation (such as a reverse water filling method described in the above mentioned commonly assigned patent applications) the receiver can estimate the power loading based on the channel eigenvalues. The channel eigenvalues are calculated from the estimated channel matrix H at the receiver. Assuming the power loading α i  at the ith channel is a function of channel eigenvalues λ i , i=1, 2, . . . , N ss . At the receiver, the power loadings can be estimated by performing the f function operations on the channel eigenvalues, as in relation (1) below:
 
α i =ƒ(λ 1 , λ 2 , . . . , λ N     ss   )  (1)
 
   This method is also applicable to quantized power loading cases (one such quantized power loading case is described in the above mentioned commonly assigned patent applications). 
   In the reverse water filling method described in the above mentioned commonly assigned patent applications, the un-quantized power loadings α i  are calculated according to relation (2) below: 
   
     
       
         
           
             
               
                 
                   α 
                   i 
                 
                 = 
                 
                   
                     P 
                     total 
                   
                   
                     
                       λ 
                       i 
                     
                     ⁢ 
                     
                       
                         ∑ 
                         
                           j 
                           = 
                           1 
                         
                         
                           N 
                           ss 
                         
                       
                       ⁢ 
                       
                         1 
                         
                           λ 
                           j 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   where N ss  is the number of spatial data streams and P total  is a fixed number representing total power. The quantized power loadings P i  are calculated according to relation (3) below:
 
P i =Q[α i ]  (3)
 
   wherein Q(·) is a quantization function. 
   According to an embodiment of the present invention, the process for detecting P i  at the receiver includes the steps of:
         1. Calculate rank-ordered channel eigenvalues λ i , i=1, 2, . . . , N ss , from the estimated channel matrix H based on sent packets, where λ are the eigenvalues of the matrix HH* and the (·)* is the Hermitian operation.   2. Calculate α i  based on relation (2) above.   3. Calculate P i , which are the nearest quantized values of α i . Here the same quantizer is used as in the transmitter.       

     FIG. 2  shows a functional block diagram of a MIMO system  400  which implements quantized uneven power loadings and detection according to an embodiment of the present invention. The system  400  comprises a transmitter TX and a receiver RX. The transmitter TX includes an information bits generation unit  402 , a demultiplexer  404 , a power loading quantization unit  406 , a power loading calculation unit  407 , a multiplication unit  408 , a V function unit  410 , and transmit antennas  412 . The receiver RX includes receiver antennas  414 , a U H  function unit  416 , a channel estimation unit  418 , a power loading detection unit  420  (detailed by example in detector  100  of  FIG. 3 , described below), a power loading quantization unit  421 , an inverse power loading unit  422  and a multiplier  424 . 
     FIG. 3  shows a block diagram of an embodiment of a power loading detector  100  for detecting P i  at the receiver RX according to an embodiment of the present invention. The detector  100  includes a channel estimation unit  102 , an SVD unit  104 , an eigenvalue calculation unit  106 , a power loading calculation unit  108  and a quantizer  110 . The channel estimation unit  102  estimates the channel H, the SVD unit  104  determines H=UDV H  where U and V are unitary matrices and D is a diagonal matrix with elements equal to the square-root of eigenvalues of the matrix HH*, where (·)* is the Hermitian operation, the eigenvalue calculation unit  106  determines eigenvalues λ i =D ii   2 , the power loading calculation unit  106  determines the un-quantized power loading α i  from relation (2) above, and the quantizer  110  determines the quantized power loading P i  according to relation (3) above. 
   An alternative method for implementation of the power loading detector is described in the following to reduce the complexity in determining the un-quantized power loading and determining the quantized power loading, including the steps of:
         1. Calculate and rank order channel eigenvalues λ i , i=1, 2, . . . , N ss , in descending order from the estimated channel matrix H, where λ are the eigenvalues of the matrix HH* and the (·)* is the Hermitian operation.   2. For i=1, calculate c i =λ 2i−1 /λ 2i .       

   3. Find the quantized set P=(P 1 , P 2 , . . . , P Nss ), from a set of power-loading sets pre-defined and fixed by the transmitter, with the smallest |c i -d i | where d i =P Nss-2(i−1) /P Nss-2i+1 .
         4. Repeat steps 2 and 3 with i=i+1, if multiple sets of P in step 3 exist and i&lt;Nss.       

   For example, in a case where N ss  =2 and 2-bit quantization, then the pre-defined power loading set P={(P 1 , P 2 ): 0.4, 1.6), (0.7, 1.3)} for quantized power loading values, which are under the fixed power constraint. If the transmitter chooses (P 1 , P 2 ) =(0.4, 1.6) as the power loading pair, and the estimated channel has eigenvalues (3.6, 1.2) at the receiver. From step 2, c 1 =3.6/1.2 =3. Therefore, the selection is (P 1 , P 2 )=(0.4, 1.6), because its d 1 =(P 2 /P 1 )=4 is near c 1 =3, rather than the other pair with d 1 =(1.3/0.7)=1.86. Since only one set of power loading pair exists, no repetitions of step 2 and 3 are necessary. 
     FIG. 4  shows a flowchart of an implementation of the abovementioned alternative method of quantized power loading detection, according to the present invention, including the steps of: 
   Step  200 : Start, 
   Step  202 : Calculate and rank order channel eigenvalues λ i , in descending order, for i=1, . . . , N ss , 
   Step  204 : set i=1, 
   Step  206 : Compute 
   
     
       
         
           
             
               c 
               i 
             
             = 
             
               
                 λ 
                 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     i 
                   
                   - 
                   1 
                 
               
               
                 λ 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   i 
                 
               
             
           
           , 
         
       
     
   
   Step  208 : Find all P=(P 1 , . . . , P NSS ) with smallest |c i -d i |, from all pre-defined quantized power loading sets, where 
   
     
       
         
           
             
               d 
               i 
             
             = 
             
               
                 P 
                 
                   
                     N 
                     ss 
                   
                   - 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     i 
                   
                   + 
                   2 
                 
               
               
                 P 
                 
                   
                     N 
                     ss 
                   
                   - 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     i 
                   
                   + 
                   1 
                 
               
             
           
           , 
         
       
     
   
   Step  210 : Determine if multiple sets of P&#39;s exist and i&lt;Nss, 
   Step  214 : If so, i =i +1, and proceed back to step  206 . 
   Step  212 : Otherwise, if multiple sets of P&#39;s still exist, randomly choose one, 
   Step  216 : The process is completed. 
   Because according to the present invention there is no need to transmit the reference signal for power loading detection, system complexity is reduced. Further, the above methods can be implemented at the transmitter for quantized power loading selections. 
   The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.