Abstract:
An apparatus and method for closed-loop signaling over multiple channels in a telecommunication system. Channel condition for each channel is obtained, and transmission rate per channel is determined according to channel condition. The information bit streams is transmitted via the multiple channels over a plurality of transmitter antennas according to the transmission rates.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to data communication, and more particularly, to data communication in multi-channel communication system such as multiple-input multiple-output (MIMO) systems.  
       BACKGROUND OF THE INVENTION  
       [0002]     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.  
         [0003]     MIMO techniques are adopted in wireless standards, such as 3GPP, for high data rate services. In a wireless MIMO system, multiple antennas are used in both transmitter and receiver, wherein each transmit antenna can transmit a different data stream into the wireless channels whereby the overall transmission rate is increased.  
         [0004]     There are two types of MIMO systems, known as open-loop and closed-loop. 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 prevent 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.  
         [0005]     The SVD (singular value decomposition) type of beamforming technique is widely used in closed-loop MIMO systems. Using SVD, a MIMO channel can be decomposed into several independent channels for data transmission resulting in no interferences between different data streams at the receiver.  
         [0006]     Since the MIMO channels can be decomposed into independent channels with different eigenvalues, the transmission rates for each channel can be selected based on the channel eigenvalues, as described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification”, a contribution to IEEE 802.11, 11-04-889r1, November 2004 (incorporated herein by reference).  
         [0007]     The algorithm to select the transmission rates can be adapted to the channel conditions (i.e., link adaptation algorithm). However, in a beamforming system with uneven power loadings, the signal-to-noise-ratio (SNR) is also tightly related to the power loadings in all the channels, as shown in D.-S. Shiu, G. J. Fochini, M. J. Gans, and J. M. Kahn, “Fading correlation and its effect on the capacity of multi-element antenna systems”,  IEEE Trans. Communication , vol. 48, pp. 502-513, March 2000.  
         [0008]     Using the link adaptation algorithm based only on the channel eigenvalues causes significant performance degradations, especially for the beamforming systems supporting even transmission rates for all channels.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     In one embodiment the present invention provides an apparatus and method for closed-loop signaling over multiple channels in a telecommunication system. Channel condition for each channel is obtained, and transmission rate per channel is determined according to channel condition. The information bit streams is transmitted via the multiple channels over a plurality of transmitter antennas according to the transmission rates.  
         [0010]     In order to increase system capacity, a link adaptation algorithm according to the present invention is utilized in selecting channel transmission rates. According to an embodiment of the present invention, a method to determine the transmission rates for each channel in a beamforming system selects the transmission rates based on the channel conditions (i.e., link adaptation algorithm).  
         [0011]     The present invention further provides a general criterion for determining the SNR for transmission rate selections in a beamforming MIMO system. For a beamforming MIMO system with uneven power loading, the present invention provides better link adaptation quality than transmission rate selections based on channel eigenvalues. For a beamforming system supporting even transmission rates for all channels, the present invention together with uneven power loadings provides significant performance improvements over the prior art.  
         [0012]     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  
       [0013]      FIG. 1  shows a functional block diagram of a SVD MIMO system implementing Link Adaptation according to an embodiment of the present invention.  
         [0014]      FIG. 2  shows a flowchart of example steps of rate selection algorithm according to an embodiment of the present invention.  
         [0015]      FIG. 3  shows a flowchart of example steps of beamforming systems supporting even transmission rate according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]      FIG. 1  shows an example block diagram of a MIMO system  100  including beamforming, according to an embodiment of the present invention. The MIMO system  100  in  FIG. 1  includes a transmitter TX comprising a demultiplexer DeMUX  102 , a power loading unit  104  that implements power control for each transmitter antenna, a Combiner  106  and a V processing function  108 . The demultiplexer DeMUX  102  splits the incoming information bits into N ss  streams. Each data stream is multiplied in the Combiner  106  by the respective power loading P which is provided by the power loading unit  104 . The MIMO system  100  further includes a receiver RX comprising a U H  processing function  110 , a P −1  (i.e., inverse of P) function  112  and a combiner  114 . The matrix P −1  in function  112  is a N ss -by-N ss  diagonal matrix with inverse of the power loading P for each stream along the diagonal. The combiner  114  provides a multiplication operation. Further, a bit generator  116  generates information bits and an adaptation unit  118  provides pre-defined look-up table for selecting data rate based on SNR information.  
         [0017]     In the MIMO system  100  of  FIG. 1 , the receiver RX is provided with the power loading information used by the transmitter TX, via the P −1  function  112 . Using the power loading information the receiver RX can properly demodulate the received signals. In one example, the transmitter TX provides the power loading information to the receiver RX. In another example, the receiver RX estimates the power loading of the transmitter TX.  
         [0018]     The power loading unit  104  of the MIMO system  100  implements adaptive power loading for different transmit channels according to the present invention. In one embodiment, where the SNR thresholds for peak rate transmission are known, the power loading unit  104  performs channel power loading.  
         [0019]     For the MIMO system  100  having a channel H with N t  transmit antennas and N r  receiving antennas, without V processing at the transmitter TX, the received signal y can be represented as: 
 
 y=HPx+n   (1) 
 
         [0020]     where x is the N t ×1 transmitted signal vector, P is a diagonal matrix with loading power α i  along the diagonal, and n is the additive noise in the channel.  
         [0021]     The channel H comprises a N r ×N t  matrix wherein each element h ij  of the matrix represents the channel response from j th  transmit antenna to i th  receiving antenna. By applying SVD to H, H can be expressed as: 
 
H=UDV H   (2) 
 
         [0022]     wherein U and V are unitary matrices (i.e., U is a N r ×N t  matrix, and V H  is a N t ×N t  matrix), and D is a N t ×N t  a diagonal matrix with the elements equal to the square-root of eigenvalues of the matrix (HH H ), where (•) H  is the Hermitian operation.  
         [0023]     For simplicity of explanation of the example embodiments of the present invention herein, it is assumed that N ss =N t . Hence, in the following description, the matrix dimensions are related to N t , not N ss . As those skilled in the art will recognize, the present invention applies to the generalized case where N t &gt;=N ss .  
         [0024]     As shown in  FIG. 1 , with V processing at the transmitter TX, relation (1) becomes: 
 
 y=HVPx+n   (3) 
 
         [0025]     And with U H  processing at the receiver RX, the received signal after processing X p  can be expressed as: 
 
 X   p   =U   H   y=DPx+U   H   n   (4) 
 
         [0026]     wherein the transmitted data x can be completely separated after this operation since D and P are diagonal matrices.  
         [0027]     The eigenvalues in every channel play important roles in determining the signal-to-noise ratios (SNR), which is commonly used for transmission rate selections. Since the MIMO channels can be decomposed into independent channels with different eigenvalues, the transmission rates for each channel can be selected based on the channel eigenvalues, as described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification”, a contribution to IEEE 802.11, 11-04-889r1, November 2004.  
         [0028]     The algorithm to select the transmission rates can be adapted to the channel conditions (link adaptation algorithm). However, in a beamforming system with uneven power loadings, the SNR is also tightly related to the power loadings in all the channels, as shown in the reference D.-S. Shiu, G. J. Fochini, M. J. Gans, and J. M. Kahn, “Fading correlation and its effect on the capacity of multi-element antenna systems”,  IEEE Trans. Communication , vol. 48, pp. 502-513, March 2000. It can be shown that in said reference, the capacity for a beamforming system can be expressed as the sum of multiple AWGN (additive white Gaussian noise) channels by:  
             C   =       ∑     i   =   1       N   t       ⁢     log   ⁡     (     1   +         λ   i     ⁢     p   i   2         N   0         )                 (   5   )             
 
         [0029]     where λ i  and p i   2  are the eigenvalue and transmitted power corresponding to the decomposed channels, respectively, and N 0  is the noise power.  
         [0030]     From relation (5) above, it is observed that the transmitted power plays an important role in determining the system capacity, since other parameters, λ i  and N 0 , are related to channel conditions and cannot be controlled. In fact, the signal to noise ratio for each channel is linearly proportional to the product of power loadings and channel eigenvalues. From relation (4) and (5), the SNR for each channel, SNR i , can be expressed as:  
               SNR   i     =         λ   i     ⁢     P   i         N   0               (   6   )             
 
         [0031]     where we assume the total transmitted power is fixed, i.e.,  
                 ∑     i   =   1       N   t       ⁢     p   i   2       =     P   total             (   7   )             
 
         [0032]     Under the assumption that, before the power scaling operation, the power for each data stream, P data , is identity, the power loading α i , can be shown by:  
               α   i   2     =         p   i   2       P   data       =     p   i   2               (   8   )             
 
         [0033]     Therefore, the criterion for transmission rate selection should be determined by the product of power loading and channel eigenvalue, since the SNR for ith channel can be expressed as:  
               SNR   i     =         λ   i     ⁢     α   i         N   0               (   9   )             
 
         [0034]     The procedure of rate selection according to the present invention includes the steps of: 
        Step 1: Calculate the product (λ i α i ), representing the adjusted signal power, according to channel conditions.     Step 2: Calculate the corresponding SNR for each channel based on relation (9).     Step 3: From a pre-defined table select the transmission rate R i  based on the calculated SNR in step 2. The pre-defined table is based on the measurements and system testing results, defining the required SNRs to support certain transmission rates.     Step 4: Repeat steps 1-3 for all the channels.        
 
         [0039]      FIG. 2  shows a flowchart of an example implementation of the above steps of rate selection procedure according to the present invention. The example implementation includes the steps of: 
        Initiate index i=1 (step  200 );     Calculate (α i λ i ) based on the channel conditions (step  202 );     Compute SNR i  according to relation (9) above (step  204 );     Find transmission rate R i  corresponding to SNR i  (step  206 );     Increment index i by one (step  208 );     Determine if transmission rate for all channels been calculated: i&gt;N t ? (step  210 );     If not, then proceed to step  202  to determine transmission rate for remaining channels, otherwise terminate the process.          
         [0047]     The selection of R i  is a direct mapping from a pre-defined table. Based on the measurements and system testing results, this table defines the required SNRs to support certain transmission rates. Once the SNR is estimated, the corresponding transmission rate from the table may be selected.  
         [0048]     In general, the transmission rate is changed by changing the modulation scheme and coding rate for the transmitted data. In case of beamforming systems supporting even transmission rate for all the channels, the rate selection procedure can include the steps of: (i) finding the transmission rate in each channel R i  from steps 1-4 above, and (ii) select final rate R=minimum of R i .  FIG. 3  shows a flowchart of the steps of an example implementation of the case of even transmission rates, including the steps of: 
        Initiate index i=1 (step  300 );     Calculate (α i ×λ i ) based on the channel conditions (step  302 );     Compute SNR i  according to relation (9) (step  304 );     Find transmission rate Ri corresponding to SNR i  (step  306 );     Increment index i by one (step  308 );     Determine if transmission rate for all channels been calculated: i.e., i&gt;N t ? (step  210 );     If not, then proceed to step  302  to determine transmission rate for remaining channels, otherwise R=minimum of R i  (step  312 ).        
 
         [0056]     In another embodiment, the link adaptation/rate selection may be implemented at the receiver RX ( FIG. 1 ). In such a case, the receiver RX uses the above algorithms to perform transmission rate selection and then feedback to the transmitter TX through the uplink signaling channels. Based on the recommended rate sent by the receiver RX, the transmitter TX makes final decisions on the rate selection.  
         [0057]     The present invention provides a general criterion in determining the SNR for transmission rate selections in a beamforming system. For a beamforming system with uneven power loading, the present invention provides better link adaptation quality than the algorithm based on channel eigenvalues. For a beamforming system supporting even transmission rates for all channels, the present invention together with uneven power loadings has significant performance improvements over the prior art systems.  
         [0058]     As those skilled in the art recognize, the embodiments described herein are examples of generalized case of N t &gt;N ss  where in that case, x is N ss ×1, P is N ss ×N ss , V is N t ×N ss , U H  is N ss ×N r , etc., according to the present invention.  
         [0059]     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.