Precoding techniques for wireless communications

Abstract

Methods, systems, and devices for wireless communications are described in which a base station may identify a null space matrix that lies within a null space of an effective channel matrix for communications between the base station and a user equipment (UE). An indication of the null space matrix may be provided to the UE, and the null space matrix used to determine modifications to a precoding matrix. The base station and UE may determine a redistribution matrix that provides a reduced variance of transmission powers for a number of transmission channels, where a product of the null space matrix and the redistribution matrix may be computed and added to the precoding matrix to generate a modified precoding matrix. The modified precoding matrix may be used to generate the communications from the base station and UE with reduced power variance across channels.

1 . (canceled) 
     
     
         2 . An apparatus for wireless communications at a user equipment (UE), comprising:
 at least one processor; and   at least one memory coupled with the at least one processor, the at least one memory storing instructions executable by the at least one processor to cause the apparatus to:
 transmit, to a network node, a power amplifier saturation indication for a plurality of power amplifiers associated with a plurality of transmit chains at the UE, wherein at least one power amplifier of the plurality of power amplifiers has a different power amplifier saturation than one or more other power amplifiers of the plurality of power amplifiers; 
 receive, from the network node, an uplink grant that includes an indication of a precoding matrix associated with a channel between the UE and the network node, wherein the precoding matrix is an unbalanced precoding matrix based at least in part on the power amplifier saturation indication; and 
 apply the precoding matrix to the plurality of transmit chains for uplink transmission of an uplink communication to the network node. 
   
     
     
         3 . The apparatus of  claim 2 , wherein the instructions are further executable by the at least one processor to cause the apparatus to:
 receive, from the network node, configuration information for transmitting the power amplifier saturation indication to the network node.   
     
     
         4 . The apparatus of  claim 3 , wherein the configuration information is received via radio resource control (RRC) signaling that configures a precoding matrix codebook. 
     
     
         5 . The apparatus of  claim 2 , wherein the instructions are further executable by the at least one processor to cause the apparatus to:
 perform an iterative algorithm to generate a redistribution matrix for use as the precoding matrix.   
     
     
         6 . The apparatus of  claim 5 , wherein the instructions to perform the iterative algorithm are executable the at least one processor to cause the apparatus to:
 normalize each row of a plurality of rows of the precoding matrix to generate a plurality of normalized rows;   adjust each row of the precoding matrix based at least in part on a difference between a corresponding row of the precoding matrix and an associated normalized row to generate a plurality of adjusted rows;   evaluate a vector norm for each row of the plurality of adjusted rows to determine if a variance of the vector norms is within a threshold variance value; and   repeat, based at least in part on the variance of the vector norms being greater than the threshold variance value, the normalizing, adjusting, and evaluating.   
     
     
         7 . The apparatus of  claim 5 , wherein the redistribution matrix provides reduced power variance across the plurality of transmit chains relative to a power variance based at least in part on the precoding matrix. 
     
     
         8 . The apparatus of  claim 5 , wherein the redistribution matrix provides a similar power amplifier backoff parameter for each of the plurality of transmit chains. 
     
     
         9 . The apparatus of  claim 2 , wherein the instructions to transmit the power amplifier saturation indication are executable by the at least one processor to cause the apparatus to:
 transmit an uplink control information (UCI) comprising the power amplifier saturation indication.   
     
     
         10 . The apparatus of  claim 2 , wherein the unbalanced precoding matrix is based at least in part on a null space matrix associated with a channel between the UE and the network node. 
     
     
         11 . A method for wireless communications at a user equipment (UE), comprising:
 transmitting, to a network node, a power amplifier saturation indication for a plurality of power amplifiers associated with a plurality of transmit chains at the UE, wherein at least one power amplifier of the plurality of power amplifiers has a different power amplifier saturation than one or more other power amplifiers of the plurality of power amplifiers;   receiving, from the network node, an uplink grant that includes an indication of a precoding matrix associated with a channel between the UE and the network node, wherein the precoding matrix is an unbalanced precoding matrix based at least in part on the power amplifier saturation indication; and   applying the precoding matrix to the plurality of transmit chains for uplink transmission of an uplink communication to the network node.   
     
     
         12 . The method of  claim 11 , further comprising:
 receiving, from the network node, configuration information for transmitting the power amplifier saturation indication to the network node.   
     
     
         13 . The method of  claim 12 , wherein the configuration information is received in radio resource control (RRC) signaling that configures a precoding matrix codebook. 
     
     
         14 . The method of  claim 11 , further comprising:
 performing an iterative algorithm to generate a redistribution matrix for use as the precoding matrix.   
     
     
         15 . The method of  claim 14 , wherein performing the iterative algorithm comprises:
 normalizing each row of a plurality of rows of the precoding matrix to generate a plurality of normalized rows;   adjusting each row of the precoding matrix based at least in part on a difference between a corresponding row of the precoding matrix and an associated normalized row to generate a plurality of adjusted rows;   evaluating a vector norm for each row of the plurality of adjusted rows to determine if a variance of the vector norms is within a threshold variance value; and   repeating, based at least in part on the variance of the vector norms being greater than the threshold variance value, the normalizing, adjusting, and evaluating.   
     
     
         16 . The method of  claim 14 , wherein the redistribution matrix provides reduced power variance across the plurality of transmit chains relative to a power variance based at least in part on the precoding matrix. 
     
     
         17 . The method of  claim 11 , wherein transmitting the power amplifier saturation indication comprises:
 transmitting an uplink control information (UCI) comprising the power amplifier saturation indication.   
     
     
         18 . An apparatus for wireless communications at a user equipment (UE), comprising:
 means for transmitting, to a network node, a power amplifier saturation indication for a plurality of power amplifiers associated with a plurality of transmit chains at the UE, wherein at least one power amplifier of the plurality of power amplifiers has a different power amplifier saturation than one or more other power amplifiers of the plurality of power amplifiers;   means for receiving, from the network node, an uplink grant that includes an indication of a precoding matrix associated with a channel between the UE and the network node, wherein the precoding matrix is an unbalanced precoding matrix based at least in part on the power amplifier saturation indication; and   means for applying the precoding matrix to the plurality of transmit chains for uplink transmission of an uplink communication to the network node.   
     
     
         19 . The apparatus of  claim 18 , further comprising:
 means for receiving, from the network node, configuration information for transmitting the power amplifier saturation indication to the network node.   
     
     
         20 . The apparatus of  claim 19 , wherein the configuration information is received in radio resource control (RRC) signaling that configures a precoding matrix codebook. 
     
     
         21 . The apparatus of  claim 18 , further comprising:
 means for performing an iterative algorithm to generate a redistribution matrix for use as the precoding matrix.   
     
     
         22 . The apparatus of  claim 21 , further comprising:
 means for normalizing each row of a plurality of rows of the precoding matrix to generate a plurality of normalized rows;   means for adjusting each row of the precoding matrix based at least in part on a difference between a corresponding row of the precoding matrix and an associated normalized row to generate a plurality of adjusted rows;   means for evaluating a vector norm for each row of the plurality of adjusted rows to determine if a variance of the vector norms is within a threshold variance value; and   means for repeating, based at least in part on the variance of the vector norms being greater than the threshold variance value, the normalizing, adjusting, and evaluating.   
     
     
         23 . The apparatus of  claim 21 , wherein the redistribution matrix provides reduced power variance across the plurality of transmit chains relative to a power variance based at least in part on the precoding matrix. 
     
     
         24 . The apparatus of  claim 18 , further comprising:
 means for transmitting an uplink control information (UCI) comprising the power amplifier saturation indication.   
     
     
         25 . A non-transitory computer-readable medium storing code for wireless communication, the code comprising instructions executable by at least one processor to cause an apparatus to:
 transmit, to a network node, a power amplifier saturation indication for a plurality of power amplifiers associated with a plurality of transmit chains at a user equipment (UE), wherein at least one power amplifier of the plurality of power amplifiers has a different power amplifier saturation than one or more other power amplifiers of the plurality of power amplifiers;   receive, from the network node, an uplink grant that includes an indication of a precoding matrix associated with a channel between the UE and the network node, wherein the precoding matrix is an unbalanced precoding matrix based at least in part on the power amplifier saturation indication; and   apply the precoding matrix to the plurality of transmit chains for uplink transmission of an uplink communication to the network node.   
     
     
         26 . The non-transitory computer-readable medium of  claim 25 , wherein the instructions are further executable by the at least one processor to cause the apparatus to:
 receive, from the network node, configuration information for transmitting the power amplifier saturation indication to the network node.   
     
     
         27 . The non-transitory computer-readable medium of  claim 26 , wherein the configuration information is received via radio resource control (RRC) signaling that configures a precoding matrix codebook. 
     
     
         28 . The non-transitory computer-readable medium of  claim 25 , wherein the instructions are further executable by the at least one processor to cause the apparatus to:
 perform an iterative algorithm to generate a redistribution matrix for use as the precoding matrix.   
     
     
         29 . The non-transitory computer-readable medium of  claim 28 , wherein the instructions to perform the iterative algorithm are further executable by the at least one processor to cause the apparatus to:
 normalize each row of a plurality of rows of the precoding matrix to generate a plurality of normalized rows;   adjust each row of the precoding matrix based at least in part on a difference between a corresponding row of the precoding matrix and an associated normalized row to generate a plurality of adjusted rows;   evaluate a vector norm for each row of the plurality of adjusted rows to determine if a variance of the vector norms is within a threshold variance value; and   repeat, based at least in part on the variance of the vector norms being greater than the threshold variance value, the normalizing, adjusting, and evaluating.   
     
     
         30 . The non-transitory computer-readable medium of  claim 28 , wherein the redistribution matrix provides reduced power variance across the plurality of transmit chains relative to a power variance based at least in part on the precoding matrix. 
     
     
         31 . The non-transitory computer-readable medium of  claim 25 , wherein the instructions to transmit the power amplifier saturation indication are further executable by at least one processor to cause the apparatus to:
 transmit an uplink control information (UCI) comprising the power amplifier saturation indication.