DL - 4G Americas

Other SON-related aspects that were discussed in the framework of Rel-9 included a new OAM interface
to control home eNodeB, and studies on self-testing and self-healing functions. SON-related functionality
continues to expand through the subsequent releases of the LTE standard.
The SON specifications are built over the existing 3GPP network management architecture, reusing much
functionality that existed prior to Rel-8. These management interfaces are being defined in a generic
manner to leave room for innovation on different vendor implementations. OAM plays a key role in the
application of the SON algorithms to network deployments. Targets for the various SON functions are set
by the OAM, such as the number of handover events and failures, additional PM counts, KPIs, etc. The
OAM also controls the enabling/disabling of SON function in addition to setting trigger conditions for
optimization function and setting specific policies. Taken together, the OAM and the SON algorithms
together allow operators to cost effectively deploy LTE networks on a wide scale.
More information on the SON capabilities in 3GPP can be found in 3G Americas’ December 2009 white
paper, The Benefits of SON in LTE. 147
B.2.7 ENHANCED DOWNLINK BEAMFORMING (DUAL‐LAYER)
In LTE Rel-8, five types of multi-antenna schemes are supported on the downlink. This includes transmit
diversity, open-loop and closed-loop SM, MU-MIMO, and single layer UE-specific reference symbolbased
Beamforming. In UE-specific reference symbol-based BF (also referred to as Mode-7) the eNodeB
can semi-statically configure a UE to use the UE-specific reference signal as a phase reference for data
demodulation of a single codeword at the UE. At the eNodeB transmitter, a set of transmit weights are
computed and applied to each sub-carrier within a desired band to both the data and the corresponding
dedicated reference symbol described in 3GPP TS36.211 Section 6.10.3. The simplest way to compute
the transmit antenna weights is to first compute a covariance matrix of the channel over the band of
interest and then taking the largest eigenvector of this covariance matrix, and applying it to all the subcarriers
within the band. For TDD 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 be possible for some cases or a codebook feedback can be used.
To enhance the performance of Mode-7, dual-layer BF has been standardized in LTE Rel-9. In this new
mode (Mode-8), the presence of two layers of UE-specific reference signals enables an eNodeB
scheduler to schedule a DL transmission using Single-User MIMO (SU-MIMO) – rank-1 or 2 – or MU-
MIMO – based on covariance matrices and CQI information feedback from UEs. The estimate of the
covariance matrix at the eNodeB may be obtained using channel reciprocity from SRS in an LTE TDD
system.
The existing semi-static MU-MIMO scheme in Rel-8 uses a 4 bit codebook feedback (for 4 transmit
antennas) where the codebook is a subset of the SU-MIMO codebook. There is only 1 layer of UEspecific
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, two orthogonal streams of UE-specific reference signals (RS) are supported as shown in
Figure B. for MU-MIMO transmission in the new transmission mode. The two orthogonal streams of UEspecific
RS are CDMed, have the same overhead as Rel-8 one stream UE-specific RS, and allows for
147 The Benefits of SON in LTE, 3G Americas, December 2009.
www.4gamericas.org February 2011 Page 136