LTE Innovation Mining: Down Link MIMO Essential Patent (2)

Patent application publication US20080279301, entitled Multiple antennas transmit diversity scheme, is a candidate for LTE DL MIMO essential patent.

BACKGROUND

The scheme of diversity is widely used to combat the effect of fast fading by providing a receiver with multiple faded replicas of the same information-bearing signal. The schemes of diversity in general fall into the following categories: space, angle, polarization, field, frequency, time and multipath diversity. Space diversity can be achieved by using multiple transmit or receive antennas. The spatial separation between the multiple antennas is chosen so that the diversity branches, i.e., the signals transmitted from the multiple antennas, experience fading with little or no correlation. Transmit diversity, which is one type of space diversity, uses multiple transmission antennas to provide the receiver with multiple uncorrelated replicas of the same signal. Transmission diversity schemes can further be divided into open loop transmit diversity and closed-loop transmission diversity schemes. In the open loop transmit diversity approach no feedback is required from the receiver.

In one type of closed loop transmit diversity, a receiver knows an arrangement of transmission antennas, computes a phase and amplitude adjustment that should be applied at the transmitter antennas in order to maximize a power of the signal received at the receiver. In another arrangement of closed loop transmit diversity referred to as selection transmit diversity (STD), the receiver provides feedback information to the transmitter regarding which antenna(s) to be used for transmission.

An example of open-loop transmission diversity scheme is the Alamouti 2.times.1 space-time diversity scheme. The Alamouti 2.times.1 space-time diversity scheme contemplates transmitting a Alamouti 2.times.2 block code using two transmission antennas using either two time slots (i.e., Space Time Block Code (STBC) transmit diversity) or two frequency subcarriers (i.e., Space Frequency Block Code (SFBC) transmit diversity). One limitation of Alamouti 2.times.1 space-time diversity scheme is that this scheme can only be applied to two transmission antennas. In order to transmit data using four transmission antennas, a Frequency Switched Transmit Diversity (FSTD) or a Time Switched Transmit Diversity (TSTD) is combined with block codes.

PRIOR ART

In order to transmit data using four transmission antennas, a Frequency Switched Transmit Diversity (FSTD) or a Time Switched Transmit Diversity (TSTD) is combined with block codes.

The problem with combined SFBC+FSTD scheme and STBC+TSTD schemes is that only a fraction of the total transmission antennas and hence power amplifier capability is used for transmission in a given frequency or time resource. This is indicated by `0` elements in the SFBC+FSTD and STBC+TSTD matrix. When the transmit power on the non-zero elements in the matrix is increased, bursty interference is generated to the neighboring cells degrading system performance. Generally, bursty interference manifests itself when certain phases of a frequency hopping pattern incur more interference than other phases.

In the downlink reference signals mapping for four transmission antennas in the 3GPP LTE, the density on antenna ports 2 and 3 is half the density on antenna ports 0 and 1. This leads to weaker channel estimates on antenna ports 2 and 3 relative to channel estimates on antenna ports 0 and in conventional SFBC+FSTD scheme.

INVENTION

Embodiments of this invention propose to exchange the second and the third row of the conventional SFBC-FSTD matrix, thus resulting in new SFBC. By this operation, symbols S.sub.1 and S.sub.2 are transmitted over antennas ports 0 and 2 while symbols S.sub.3 and S.sub.4 are transmitted over antenna ports 1 and 3. This is useful for evening out pilot-density disparity inherent in the reference signal structure of the LTE system.

CLAIM

21. A method for data transmission, the method comprising the steps of:modulating data to be transmitted into four modulated symbols;generating an output matrix, the output matrix being established by: [ y ( 0 ) ( 4 ) y ( 1 ) ( 4 ) y ( 2 ) ( 4 ) y ( 3 ) ( 4 ) y ( 0 ) ( 4 + 1 ) y ( 1 ) ( 4 + 1 ) y ( 2 ) ( 4 + 1 ) y ( 3 ) ( 4 + 1 ) y ( 0 ) ( 4 + 2 ) y ( 1 ) ( 4 + 2 ) y ( 2 ) ( 4 + 2 ) y ( 3 ) ( 4 + 2 ) y ( 0 ) ( 4 + 3 ) y ( 1 ) ( 4 + 3 ) y ( 2 ) ( 4 + 3 ) y ( 3 ) ( 4 + 3 ) ] = [ 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 0 - 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 1 0 0 0 - j 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 0 - 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 j 0 0 0 0 0 0 0 0 0 0 1 0 0 0 - j 0 ] [ Re ( S 1 ( ) ) Re ( S 2 ( ) ) Re ( S 3 ( ) ) Re ( S 4 ( ) ) Im ( S 1 ( ) ) Im ( S 2 ( ) ) Im ( S 3 ( ) ) Im ( S 4 ( ) ) ] where S.sub.1(i), S.sub.2(i), S.sub.3(i) and S.sub.4(i) are the four modulated symbols transmitted on the subcarriers 4i, 4i+1, 4i+2 and 4i+3; andtransmitting the symbols in the output matrix via four antennas on four frequency subcarriers, with y.sup.(n)(m) in the output matrix being transmitted via an (n+1)-th antenna and an (m+1)-th subcarrier:

DL MIMO specification in 3GPP TS36.211, V890, Section 6.3.4.3 for precoding for transmit diversity


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