Pilot transmission and channel estimation for multiple transmitters

Abstract

Each transmitter is assigned a time-only pilot code, a frequency-only pilot code, or a time-frequency pilot code to use for pilot transmission. The pilot codes may be pseudo-random, orthogonal, and/or cyclic-shift codes. To obtain a channel estimate for a transmitter using a time-frequency pilot code composed of a time-only code and a frequency-only code, a receiver multiplies a set of received symbols for each symbol period with a set of code values for the frequency-only code to obtain a set of detected symbols and performs an IDFT on the set of detected symbols to obtain an initial impulse response estimate. The receiver performs code matching on multiple initial impulse response estimates derived for multiple symbol periods with the time-only code to obtain a final impulse response estimate for the desired transmitter. The receiver retains the first L channel taps and zeroes out remaining channel taps, where L is the expected channel length.

1 . A method of performing channel estimation in a wireless communication system, comprising:
 deriving a plurality of detected symbols based on a plurality of received symbols for a plurality of pilot subbands and a cyclic-shift code for a transmitter, wherein the plurality of pilot subbands is a subset of a plurality of frequency subbands for the system;   deriving an initial impulse response estimate based on the plurality of detected symbols; and   deriving a final impulse response estimate for the transmitter based on the initial impulse response estimate.   
   
   
       2 . The method of  claim 1 , wherein the deriving the plurality of detected symbols comprises
 determining a plurality of phase values based on the cyclic-shift code and the plurality of pilot subbands, and   multiplying the plurality of received symbols with the plurality of phase values to obtain the plurality of detected symbols.   
   
   
       3 . The method of  claim 1 , wherein the deriving the final impulse response estimate comprises
 setting first L channel taps in the final impulse response estimate to first L channel taps in the initial impulse response estimate, where L is an integer greater than one, and   setting remaining channel taps in the final impulse response estimate to zero.   
   
   
       4 . A method of performing channel estimation in a wireless communication system, comprising:
 deriving a plurality of sets of detected symbols based on a plurality of sets of received symbols for a plurality of symbol periods and a cyclic-shift code for a transmitter, wherein each set of received symbols is for a set of pilot subbands that is a subset of a plurality of frequency subbands for the system;   deriving at least one initial impulse response estimate based on the plurality of sets of detected symbols; and   deriving a final impulse response estimate for the transmitter based on the at least one initial impulse response estimate.   
   
   
       5 . The method of  claim 4 , wherein the deriving the plurality of sets of detected symbols comprises, for each of the plurality of symbol periods,
 determining a set of phase values based on the cyclic-shift code and the set of pilot subbands used for pilot transmission in the symbol period, and   multiplying a set of received symbols for the symbol period with the set of phase values to obtain a set of detected symbols for the symbol period.   
   
   
       6 . The method of  claim 4 , wherein the deriving the at least one initial impulse response estimate comprises
 averaging the plurality of sets of detected symbols to obtain a sequence of detected symbols, and   performing an inverse transform on the sequence of detected symbols to derive a single initial impulse response estimate.   
   
   
       7 . The method of  claim 4 , wherein the deriving the at least one initial impulse response estimate comprises
 forming a sequence of detected symbols with the plurality of sets of detected symbols, and   performing an inverse transform on the sequence of detected symbols to derive a single initial impulse response estimate.   
   
   
       8 . The method of  claim 7 , wherein the deriving the final impulse response estimate comprises
 setting first L channel taps in the final impulse response estimate to first L channel taps in the single initial impulse response estimate, where L is an integer greater than one, and   setting remaining channel taps in the final impulse response estimate to zero.   
   
   
       9 . The method of  claim 4 , wherein the deriving the at least one initial impulse response estimate comprises, for each code value among a plurality of code values for a second code for the transmitter,
 forming a sequence of detected symbols with at least one set of detected symbols for which the code value applies, and   performing an inverse transform on the sequence of detected symbols to derive an initial impulse response estimate for the code value.   
   
   
       10 . The method of  claim 9 , wherein the deriving the final impulse response estimate comprises
 performing code matching on a plurality of initial impulse response estimates, derived for the plurality of code values, with the second code.   
   
   
       11 . The method of  claim 4 , further comprising:
 retaining first L channel taps in the final impulse response estimate, where L is an integer greater than one; and   setting remaining channel taps in the final impulse response estimate to zero.   
   
   
       12 . A method of transmitting a pilot in a wireless communication system, comprising:
 generating a set of code symbols for a set of pilot subbands based on a cyclic-shift code assigned to a transmitter, wherein the set of pilot subbands is a subset of a plurality of frequency subbands for the system; and   transmitting the set of code symbols on the set of pilot subbands.   
   
   
       13 . The method of  claim 12 , wherein the generating the set of code symbols comprises, for each symbol period with pilot transmission,
 identifying the set of pilot subbands used for pilot transmission in the symbol period, wherein at least two different sets of pilot subbands are used for pilot transmission in different symbol periods,   determining a set of phase values based on the cyclic-shift code and the set of pilot subbands used for the symbol period, and   generating the set of code symbols based on the set of phase values.   
   
   
       14 . The method of  claim 12 , wherein the generating the set of code symbols comprises, for each symbol period with pilot transmission,
 generating the set of code symbols based on the cyclic-shift code and a code value for a second code assigned to the transmitter.   
   
   
       15 . An apparatus in a wireless communication system, comprising:
 a pilot processor operative to generate a set of code symbols for a set of pilot subbands based on a cyclic-shift code assigned to a transmitter, wherein the set of pilot subbands is a subset of a plurality of frequency subbands for the system; and   a transmitter unit operative to transmit the set of code symbols on the set of pilot subbands.   
   
   
       16 . The apparatus of  claim 15 , wherein the pilot processor is operative, for each symbol period with pilot transmission, to identify the set of pilot subbands used for pilot transmission in the symbol period, to determine a set of phase values based on the cyclic-shift code and the set of pilot subbands used for the symbol period, and to generate the set of code symbols based on the set of phase values, wherein at least two different sets of pilot subbands are used for pilot transmission in different symbol periods. 
   
   
       17 . The apparatus of  claim 15 , wherein the pilot processor is operative to, for each symbol period with pilot transmission, generate the set of code symbols based on the cyclic-shift code and a code value for a second code assigned to the transmitter. 
   
   
       18 . The apparatus of  claim 17 , wherein the second code is orthogonal to at least one other second code assigned to at least one other transmitter. 
   
   
       19 . The apparatus of  claim 17 , wherein the second code is pseudo-random with respect to at least one other second code assigned to at least one other transmitter. 
   
   
       20 . An apparatus in a wireless communication system, comprising:
 means for generating a set of code symbols for a set of pilot subbands based on a cyclic-shift code assigned to a transmitter, wherein the set of pilot subbands is a subset of a plurality of frequency subbands for the system; and   means for transmitting the set of code symbols on the set of pilot subbands.   
   
   
       21 . The apparatus of  claim 20 , wherein the means for generating the set of code symbols comprises, for each symbol period with pilot transmission,
 means for identifying the set of pilot subbands used for pilot transmission in the symbol period, wherein at least two different sets of pilot subbands are used for pilot transmission in different symbol periods,   means for determining a set of phase values based on the cyclic-shift code and the set of pilot subbands used for the symbol period, and   means for generating the set of code symbols based on the set of phase values.   
   
   
       22 . The apparatus of  claim 20 , wherein the means for generating the set of code symbols comprises, for each symbol period with pilot transmission,
 means for generating the set of code symbols based on the cyclic-shift code and a code value for a second code assigned to the transmitter.