METHOD FOR OPTIMIZED REFERENCE SIGNAL DOWNLINK TRANSMISSION IN A WIRELESS COMMUNICATION SYSTEM
A method and system optimizes the transmission of a downlink reference signal (DLRS) in a wireless communication system that uses orthogonal division multiple access (OFDMA) for the downlink. Each Node-B (base station) is capable of transmitting the DLRS reference symbols in different subframes of the OFDM radio frame and changing both the number and location of the subframes in response to changing network conditions. The network conditions include the number of terminals being served by the Node-B and multiple access interference (MAI) from adjacent Node-Bs.
1 . A method for transmitting a reference signal in a cellular network that uses orthogonal frequency division multiplexing (OFDM) and includes (a) at least one base station having four antennas (4-TX) in a multiple-input multiple-output (MIMO) system and capable of downlink transmission of reference symbols representing said four antennas, and (b) a plurality of mobile transmit/receive terminals, each terminal capable of uplink transmission identifying itself as a 4-TX MIMO capable terminal; wherein the reference symbols are transmitted in OFDM resource blocks, a resource block comprising a plurality of OFDM subcarriers and a plurality of OFDM symbols in time slots of a radio frame, a radio frame comprising a plurality of subframes, the method comprising:
transmitting in the downlink the reference symbols in at least one subframe per radio frame;
receiving at the base station terminal identification information transmitted in the uplink from said terminals; and
in response to the number of terminals transmitting said terminal identification information, changing the number of subframes per radio frame in which the reference symbols are transmitted in the downlink.
2 . The method of claim 1 wherein changing the number of subframes comprises selecting one pattern from a set of predefined unique patterns, each pattern being predefined locations of radio subframes for the reference symbols.
3 . The method of claim 2 wherein the set of predefined unique patterns comprises a first pattern with a first subframe location and a second pattern with said first subframe location and a second subframe location.
4 . The method of claim 3 wherein the set of predefined unique patterns further comprises a third pattern with said first and second subframe locations and a third subframe location.
5 . The method of claim 2 wherein said at least one base station is associated with said set of predefined unique patterns and has a unique cell group identification (ID), and further comprising transmitting said cell group ID, whereby a terminal may detect the set of predefined patterns associated with said at least one base station.
6 . The method of claim 1 wherein the base station is a first base station and the network includes a second base station transmitting a downlink that interferes with the downlink from the first base station, the method further comprising:
determining, from the reference symbols received at a terminal, a value representing a signal-to-interference ratio (SIR);
transmitting the SIR value in the uplink from said terminal; and
in response to the received SIR value, changing the subframe to a different subframe in which to transmit the reference symbols in the downlink.