Antenna Port Mapping for Demodulation Reference Signals

Abstract

A unified, rank independent mapping between antenna ports and group/code pairs. Each antenna port is uniquely associated with one code division multiplexing (CDM) group and one orthogonal cover code (OCC). The mapping between antenna ports and group/code pairs is chosen such that, for a given antenna port, the CDM group and OCC will be the same for every transmission rank.

What is claimed is: 
     
         1 . A method, implemented by a base station, for transmitting demodulation reference signals to a user terminal, the method comprising:
 determining a selected transmission rank from a plurality of transmission ranks for a downlink transmission to the user terminal;   determining or more antenna ports for the downlink transmission based on a predetermined mapping between a set of antenna ports and a set of group and code pairs, wherein each group and code pair comprises a code division multiplexing group and orthogonal cover code;   wherein the mapping between antenna ports in the set of antenna ports and the group and code pairs in the set of group and code pairs is such each antenna port in the set of antenna ports is associated a same group and code pair in the set of group and code pairs regardless of selected transmission rank; and   transmitting the demodulation reference signals over the antenna ports corresponding to the selected transmission rank.   
     
     
         2 . The method of  claim 1 , wherein the predetermined mapping is further characterized in that, within a given code division multiplexing group, antenna ports associated with a low transmission rank in the plurality of transmission ranks are a subset of antenna ports associated with a higher transmission rank in the plurality of transmission ranks. 
     
     
         3 . The method of  claim 1 :
 wherein the orthogonal cover codes comprise length-4 cover codes; and   wherein the mapping of the antenna ports to the group and code pairs is such that, for selected antenna ports, the length-4 orthogonal cover codes comprise two length-2 cover codes for channel estimation.   
     
     
         4 . The method of  claim 2 , further comprising sending a control signal to the user terminal to indicate whether channel estimation should be performed using length-2 or length-4 orthogonal cover codes for selected antenna ports. 
     
     
         5 . A method, implemented by a user terminal, for receiving demodulation reference signals transmitted by a base station, the method comprising:
 determining a selected transmission rank from a plurality of transmission ranks for a downlink transmission to the user terminal;   determining or more antenna ports for the downlink transmission based on a predetermined mapping between a set of antenna ports and a set of group and code pairs, wherein each group and code pair comprises a code division multiplexing group and orthogonal cover code;   wherein the mapping between antenna ports in the set of antenna ports and the group and code pairs in the set of group and code pairs is such each antenna port in the set of antenna ports is associated a same group and code pair in the set of group and code pairs regardless of selected transmission rank; and   receiving the demodulation reference signals over the antenna ports corresponding to the selected transmission rank.   
     
     
         6 . The method of  claim 5 , wherein the predetermined mapping is further characterized in that, within a given code division multiplexing group, antenna ports associated with a low transmission rank are a subset of antenna ports associated with a higher transmission rank. 
     
     
         7 . The method of  claim 5 :
 wherein the orthogonal cover codes comprise length-4 cover codes; and   wherein the mapping of the antenna ports to the group and code pairs is such that, for selected antenna ports, the length-4 orthogonal cover codes comprise two length-2 cover codes for channel estimation.   
     
     
         8 . The method of  claim 7 , further receiving a control signal from the base station and performing channel estimation using either length-2 or length-4 orthogonal cover codes for the selected antenna ports depending on the control signal. 
     
     
         9 . A base station, comprising:
 a plurality of transmit antennas;   a transmitter operatively connected to the transmit antennas, the transmitter including a transmit signal processor and transmit controller;   the transmitter configured to:
 determine a selected transmission rank from a plurality of transmission ranks for a downlink transmission to a user terminal; 
 determine or more antenna ports for the downlink transmission based on a predetermined mapping between a set of antenna ports and a set of group and code pairs, wherein each group and code pair comprises a code division multiplexing group and orthogonal cover code; 
 wherein the mapping between antenna ports in the set of antenna ports and the group and code pairs in the set of group and code pairs is such each antenna port in the set of antenna ports is associated a same group and code pair in the set of group and code pairs regardless of selected transmission rank; and 
 transmit demodulation reference signals over the antenna ports corresponding to the selected transmission rank. 
   
     
     
         10 . The base station of  claim 9 , wherein the predetermined mapping is further characterized in that, within a given code division multiplexing group, antenna ports associated with a low transmission rank will be a subset of antenna ports associated with a higher transmission rank. 
     
     
         11 . The base station of  claim 9 :
 wherein the orthogonal cover codes comprise length-4 cover codes; and   wherein the mapping of antenna ports to the group and code pairs is such that, for selected antenna ports, the length-4 orthogonal cover codes comprise two length-2 cover codes for channel estimation.   
     
     
         12 . The base station of  claim 11 , wherein the base station is further configured to send a control signal to a user terminal to indicate whether channel estimation should be performed using length-2 or length-4 orthogonal cover codes for the selected antenna ports. 
     
     
         13 . A user terminal, comprising:
 a plurality of receive antennas;   a receiver operatively connected to the receive antennas, the receiver including a receive signal processor and receive controller;   wherein the receiver is configured to:
 determine a selected transmission rank from a plurality of transmission ranks for a downlink transmission to a user terminal; 
 determine or more antenna ports for the downlink transmission based on a predetermined mapping between a set of antenna ports and a set of group and code pairs, wherein each group and code pair comprises a code division multiplexing group and orthogonal cover code; 
 wherein the mapping between antenna ports in the set of antenna ports and the group and code pairs in the set of group and code pairs is such each antenna port in the set of antenna ports is associated a same group and code pair in the set of group and code pairs regardless of selected transmission rank; and 
 receive demodulation reference signals over the antenna ports corresponding to the selected transmission rank. 
   
     
     
         14 . The user terminal of  claim 13 , wherein the predetermined mapping is further characterized in that, within a given code division multiplexing group, antenna ports associated with a low transmission rank are a subset of antenna ports associated with a higher transmission rank. 
     
     
         15 . The user terminal of  claim 13 :
 wherein the orthogonal cover codes comprise length-4 cover codes; and   wherein the mapping of the antenna ports to the group and code pairs is such that, for selected antenna ports, the length-4 orthogonal cover codes comprise two length-2 cover codes for channel estimation.   
     
     
         16 . The user terminal of  claim 15 , wherein the receiver is further configured to receive a control signal from the base station and perform channel estimation using either length-2 or length-4 orthogonal cover codes for the selected antenna ports depending on the control signal.