DL MIMO efficiency enhancements for LTE
MIMO is an effective technique to improve spectral efficiency and increase overall network capacity. SRS can be utilized to improve DL MIMO performance, especially for massive MIMO in TDD. In release-16 SRS capacity and coverage are enhanced by introducing more than one SRS symbol in a UL normal subframe and introducing virtual cell ID for SRS.
More than one symbol for SRS in a UL normal
subframe
With the introduction of more than one SRS symbol
in a UL normal subframe, the SRS capacity and coverage can be increased. These
additional SRS symbol(s) are referred to as trigger type 2 SRS.
1 to 13 symbols of the first 13 symbols of a UL
normal subframe can be configured to a UE for aperiodically triggered SRS
transmission. Intra-subframe repetition, frequency hopping and antenna
switching of the additional SRS symbols can be supported. A guard period of one
SC-FDMA symbol can be configured for frequency hopping and antenna switching.
The number of repetitions can be configured from
the set {1, 2, 3, 4, 6, 7, 8, 9, 12, 13}. If a UE has more receive antennas
than transmit chains (e.g. 1T2R), the UE can be configured to transmit the
additional SRS with antenna switching. And a UE can additionally be configured
with frequency hopping for the additional SRS. The number of antennas (pairs)
to switch is:
- 2 for 1T2R, or the number of
pairs is configured as 2 for 2T4R
- 3 if the number of pairs is
configured as 3 for 2T4R
- 4 for 1T4R
Both legacy SRS and additional SRS can be
configured to the same UE, and transmission of legacy SRS and additional SRS
symbol(s) in the same subframe for the UE is supported.
For sequence generation, per-symbol group hopping
and sequence hopping are supported.
Independent open loop and close loop power control
is supported for additional SRS, and DCI formats 3/3A/3B are used for close
loop power control.
UEs not configured with SPUCCH/SPUSCH are not
expected to be triggered with additional SRS in the same subframe as
PUSCH/PUCCH in the same serving cell. UEs configured with SPUCCH/SPUSCH drop
the SRS transmission in the symbols colliding with SPUCCH/SPUSCH in the same
serving cell.
The additional SRS transmission in the symbols
colliding with PUSCH/PUCCH of another serving cell in the same TAG, the same
band and with the same CP, is dropped.
The additional SRS transmission in the symbols
colliding with PUSCH/PUCCH of another serving cell in the different TAG is
dropped if the total transmission power exceeds the PCMAX in any overlapping
portion.
Virtual cell ID
Virtual cell ID within the range from 0 to 503 can
be configured for SRS to increase SRS capacity.
The virtual cell ID can be configured to only
additional SRS symbol(s) or both legacy and additional SRS symbol(s). If
virtual cell ID is not configured, the physical cell ID is used.