1. R1-094856: SRS Enhancements for LTE-A
Agenda: 7.4.2
Document for: Discussion/Decision
3GPP RAN WG1 #59, Jeju, South Korea
November 9-13, 2009
Motorola

2. Introduction LTE Rel-10 uplink enhancements compared to Rel-8/9
Multi-antenna transmission up to four Tx antenna
Uplink spatial multiplexing of up to four layers
Frequency Non-contiguous resource allocation
Possible support for CoMP
Rel-8 SRS design features
SRS transmitted on last symbol in configured subframes
IFDMA comb SRS structure reusing DM RS base sequences
SRS BW restricted to be multiple of 4 RBs (contiguous)
Four SRS BWs can be simultaneously supported in a cell
Cyclic shift orthogonality: 8 equally spaced cyclic time shifts per comb supported (spacing = 4.17us)
SRS Frequency hopping supported; Optional SRS transmit antenna switching
Configurable UE-specific SRS BW, Starting subcarrier, comb index, CS, SRS periodicities – 2, 5, 10, 20, 40, 80, 160 or 320 ms via RRC signaling
SRS base sequence group = PUCCH base sequence group
No cyclic shift hopping for SRS
Rel-8 SRS design provides sufficient flexibility and can be straightforwardly extended to support Rel-10 multi-antenna transmissions
Propose to re-use Rel-8 SRS structure as much as possible
New features/enhancements/signaling should be considered only if they provide “significant” benefit over Rel-8 mechanisms

3. SRS Extension to support UL MIMO in LTE-A (slide 1 of 2)
Current Baseline Agreement [TR ]
Non-precoded and Antenna-specific SRS
Rel-8 principles re-used for SRS multiplexing
UL MIMO
Likely a new transmission mode is defined for SU-MIMO (2 Tx and 4 Tx UE transmit antenna configuration based on UE capability)
As in DL Rel-8/9, UE is configured in new MIMO mode via RRC signaling
SRS transmission can be configured via RRC signaling
Simultaneous SRS transmission from multiple transmit antennas when configured via RRC
Most likely when UE is suitable for UL-MIMO (i.e., precoding)
Take into account SNR condition and antenna effect (e.g., antenna gain imbalance, coupling, etc.)
Antenna-specific SRS have same SRS BW, Frequency RB position
Orthogonality via cyclic shift multiplexing and/or different comb assignments
CS, comb assignment can be explicit or implicit relative to first antenna configuration

4. SRS Extension to support UL MIMO in LTE-A (slide 2 of 2)
Similar to “Single Antenna Port Mode” defined for UE with multiple transmit antennas, such mode can be configured for SRS too
In order to save power and SRS resource if UE is not likely to support UL-MIMO/precoding (e.g., low SINR, large AGI, etc.)
Different transmit PA architectures supported [R1=093750, RAN4 LS Response]
E.g 2 Tx antenna UE:
A: 20dBm + 20dBm
B: 23dBm + 23dBm
C: 23dBm + x, where x ≤ 23dBm
the total transmit power, Pmax , defined as 23dBm regardless of the number of antennas, PAs and precoding used
Implementation issue on how PAs are used in “single-port” SRS transmission
Achieve same coverage (23dBm max Tx power) as Rel-8 single antenna UE
“Long-term” precoding: based on UE implementation
Antenna virtualization: correspond to a fixed synthesized pattern
Turn-off all but one PA
But the same implementation should be used for SRS and PUSCH, since eNB still use SRS to track power control and frequency-selectiveness of the UL channel

5. Non-contiguous RA and Carrier Aggregation
No non-contiguous SRS transmissions with a component carrier
Independent SRS transmission and configuration for each component carrier with UL Carrier Aggregation
Component carrier-specific SRS sequence

6. Conclusions
Rel-8 SRS design provides sufficient flexibility and can be straightforwardly extended to support Rel-10 multi-antenna transmissions
Propose to re-use Rel-8 SRS structure as much as possible
New features/enhancements/signaling should be considered only if they provide “significant” benefit over Rel-8 mechanisms
Simultaneous SRS transmission from multiple transmit antennas when configured via RRC
Take into account SNR condition and antenna effect (e.g., antenna gain imbalance, coupling, etc.)
Antenna-specific SRS have same SRS BW, Frequency RB position
Orthogonality via cyclic shift multiplexing and/or different comb assignments
SRS configured in Single Antenna Port Mode for Multi-transmit antenna UE
Implementation issue on how PAs are used in “single-port” SRS transmission
Long-term” precoding, Antenna virtualization, Turn-off all but one PA
Same implementation should be used for SRS and PUSCH, since eNB still use SRS to track power control and frequency-selectiveness of the UL channel
No non-contiguous SRS transmissions with a component carrier
Independent SRS transmission and configuration for each component carrier with UL Carrier Aggregation
Component carrier-specific SRS base sequence