UMTS: Alive and Well - 4G Americas

6.2.7 ENHANCED DOWNLINK BEAMFORMING (DUAL-LAYER)In LTE Rel-8, five types of multi-antenna schemes are supported on the downlink. This includestransmit diversity, open-loop and closed-loop SM, MU-MIMO, and single layer UE-specificreference symbol-based Beamforming. In UE-specific reference symbol-based BF (also referredto as Mode-7) the eNodeB can semi-statically configure a UE to use the UE specific referencesignal as a phase reference for data demodulation of a single codeword at the UE. At the eNodeBtransmitter, a set of transmit weights are computed and applied to each sub-carrier within adesired band to both the data and the corresponding dedicated reference symbol described inTS36.211 Section 6.10.3. The simplest way to compute the transmit antenna weights is to firstcompute a covariance matrix of the channel over the band of interest and then taking the largesteigenvector of this covariance matrix, and applying it to all the sub-carriers within the band. ForTDD transmission, the covariance matrix can be computed from Sounding Reference Signal(SRS) due to reciprocity while for FDD the translation of UL covariance to DL covariance may bepossible for some cases or a codebook feedback can be used.To enhance the performance of Mode-7, dual-layer BF is currently being standardized in LTERel-9. In this new mode, the eNodeB scheduler may choose to schedule a DL transmission usingSingle-User MIMO (SU-MIMO) – rank-1 or 2 – or MU-MIMO – rank-1 – based on covariancematrices and CQI information feedback from UEs. The estimate of the covariance matrix at theeNodeB may be obtained using channel reciprocity and/or covariance feedback or PMI feedback.The existing semi-static MU-MIMO scheme in Rel-8 uses a 4 bit codebook feedback (for 4transmit antennas) where the codebook is a subset of the SU-MIMO codebook. There is only 1layer of UE-specific reference signals and the UE cannot suppress the cross-talk due to MU-MIMO. The performance of standalone Rel-8 MU-MIMO is inferior to Rel-8 SU-MIMO (Mode-4)or UE Specific RS based BF (Mode-7).In LTE Rel-9, 2 streams of UE specific reference signals (RS) are supported as shown in Figure6.7 for MU-MIMO transmission in the new transmission mode. The two streams of UE-specificRS are CDMed, have the same overhead as Rel-8 one stream UE specific RS, and allows forcross-talk suppression. The downlink control signaling does not indicate the presence of coscheduledUEs.Figure 6.7. UE Specific Reference Signal Structure per Resource Block (RB).www.3GAmericas.org February 2010 Page 68