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multiplexing (SM) techniques. The TXD techniques will increase diversity order which may result in
reduced channel variation and improved system coverage. These techniques include Transmit Antenna
Switching (TAS), Space-Frequency Block Coding (SFBC), Cyclic Delay Diversity (CDD) and Frequency
Shift Transmit Diversity (FSTD). SM techniques allow multiple spatial streams that are transmitted sharing
the same time-frequency resource.
In the case where the eNB sends CSI to the UE, CL-MIMO can be used to significantly increase spectral
efficiency. CL-MIMO utilizes the CSI feedback from the eNB to optimize the transmission for a specific
UE’s channel condition. As a result of this feedback, it is vulnerable to sudden channel variations. In
general, CL-MIMO has better performance than OL-MIMO in low-speed environments. SM techniques
can also be used to significantly increase the spectral efficiency of CL-MIMO. The multiple spatial streams
are separated by an appropriate receiver processing (e.g. using successive interference cancellation
[SIC]). This processing can increase peak data rates and potentially the capacity due to high SINR and
uncorrelated channels. The SM techniques can be classified into Single-Codeword (SCW) and Multiple-
Codewords (MCW) techniques. In the former case, the multiple streams come from one turbo encoder,
which can achieve remarkable diversity gain. In the latter case, when multiple streams are encoded
separately, an SIC receiver can be used to reduce the co-channel interference between the streams
significantly.
Specifically for Rel-10 the uplink enhancements are divided into three major areas:
1. Inclusion of TxD for uplink control information transmission via Physical Uplink Control Channel
(PUCCH). The Spatial Orthogonal-Resource Transmit Diversity (SORTD) mode was selected for
many PUCCH formats where the same modulation symbol from the uplink channel is transmitted
from two antenna ports, on two separate orthogonal resources.
2. SU-MIMO Physical Uplink Shared Channel (PUSCH) transmission with two transmission modes:
a single antenna port mode that is compatible with the LTE Rel-8 PUSCH transmission and a
multi-antenna port mode that offers the possibility of a two and a four antenna port transmission.
Discussions are ongoing in 3GPP regarding the refinements of PUSCH multi-antenna port
transmission such as handling rank-1 transmissions, the SRS options, the UCI multiplexing on
PUSCH as well as the precoder design for retransmissions.
3. Uplink Reference Signals (RS). The UL reference signal structure in LTE-Advanced will retain the
basic structure of that in Rel-8 LTE. Two types of reference signals were enhanced:
Demodulation Reference Signals (DM RS) and Sounding Reference Signals (SRS). The
demodulation reference signal is used by the receiver to detect transmissions. In the case of
uplink multi-antenna transmission, the precoding applied for the demodulation reference signal is
the same as the one applied for the PUSCH. Cyclic shift (CS) separation is the primary
multiplexing scheme of the demodulation reference signals. Orthogonal Cover Code (OCC)
separation is also used to separate DM RS of different virtual transmit antennas. The sounding
reference signal is used by the receiver to measure the mobile radio channel. The current
understanding is that the sounding reference signal will be non-precoded and antenna-specific
and for multiplexing of the sounding reference signals, the LTE Rel-8 principles will be reused.
7.1.3 DOWNLINK TRANSMISSION ENHANCEMENTS
In order to improve the SU-MIMO spatial efficiency of the downlink, the LTE downlink SM has been
enhanced to support up to eight layers per component carrier in LTE Rel-10. The maximum number of
codewords supported remains two.
www.4gamericas.org February 2011 Page 54