LTE Sounding reference signal

36.211 §5.5.3 Uplink SRS

Uplink SRS is a reference signal sent by the UE, used by the eNb to evaluate the channel quality of uplink path and uplink timing transmission
This is a simplified view of the SRS

SRS
Sounding reference signal
PRB
Physical Resource Block
RE
Resource Element
SF
Subframe
UE
User Equipment

Graphical view of SRS in one uplink radio frame (10 ms)

The sounding reference signal shall be transmitted in the last symbol of the uplink subframe
FDD-LTE, Cell bandwidth : 6 PRBs, Cyclic prefix : Normal, SRS subframe config : 2
SRS bandwidth config : changed from 0 to 7, SRS bandwidth : 0

Uplink radio frame

Sounding reference signal subframe configuration, FDD

SRS subframe
config
Configuration period
TSFC (subframes)
Transmission offset
ΔSFC (subframes)
0 1 {0}
1 2 {0}
2 2 {1}
3 5 {0}
4 5 {1}
5 5 {2}
6 5 {3}
7 5 {0,1}
8 5 {2,3}
9 10 {0}
10 10 {1}
11 10 {2}
12 10 {3}
13 10 {0,1,2,3,4,6,8}
14 10 {0,1,2,3,4,5,6,8}
15 reserved reserved

SRS subframe config 2
Slot ns 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
SRS subframes 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1

SRS subframes are the subframes satisfying the criteria : [ns/2] mod Tsfc ϵ ΔSFC

Length of the sounding reference signal sequence Msc,bRS = mSRS,b * NscRB / 2 = 4 * 12/2 = 48
K0 = ( NRBUL / 2 - mSRS,0 / 2 ) * NscRB + kTC = (6/2 - 4/2) * 12 + 0 = 12
The frequency-domain starting position K0 = 12 + 2 * 4 * 6 * 0 = 12

mSRS,b and Nb, b = 0,1,2,3, values for the uplink bandwidth of 6 ≤ NRBUL ≤ 40
SRS bandwidth configuration
CSRS
SRS bandwidth
BSRS = 0
SRS bandwidth
BSRS = 1
SRS bandwidth
BSRS = 2
SRS bandwidth
BSRS = 3
mSRS,0 N0 mSRS,1 N1 mSRS,2 N2 mSRS,3 N3
0 36 1 12 3 4 3 4 1
1 32 1 16 2 8 2 4 2
2 24 1 4 6 4 1 4 1
3 20 1 4 5 4 1 4 1
4 16 1 4 4 4 1 4 1
5 12 1 4 3 4 1 4 1
6 8 1 4 2 4 1 4 1
7 4 1 4 1 4 1 4 1

mSRS,b and Nb, b = 0,1,2,3, values for the uplink bandwidth of 40 < NRBUL ≤ 60
SRS bandwidth configuration
CSRS
SRS bandwidth
BSRS = 0
SRS bandwidth
BSRS = 1
SRS bandwidth
BSRS = 2
SRS bandwidth
BSRS = 3
mSRS,0 N0 mSRS,1 N1 mSRS,2 N2 mSRS,3 N3
0 48 1 24 2 12 2 4 3
1 48 1 16 3 8 2 4 2
2 40 1 20 2 4 5 4 1
3 36 1 12 3 4 3 4 1
4 32 1 16 2 8 2 4 2
5 24 1 4 6 4 1 4 1
6 20 1 4 5 4 1 4 1
7 16 1 4 4 4 1 4 1

mSRS,b and Nb, b = 0,1,2,3, values for the uplink bandwidth of 60 < NRBUL ≤ 80
SRS bandwidth configuration
CSRS
SRS bandwidth
BSRS = 0
SRS bandwidth
BSRS = 1
SRS bandwidth
BSRS = 2
SRS bandwidth
BSRS = 3
mSRS,0 N0 mSRS,1 N1 mSRS,2 N2 mSRS,3 N3
0 72 1 24 3 12 2 4 3
1 64 1 32 2 16 2 4 4
2 60 1 20 3 4 5 4 1
3 48 1 24 2 12 2 4 3
4 48 1 16 3 8 2 4 2
5 40 1 20 2 4 5 4 1
6 36 1 12 3 4 3 4 1
7 32 1 16 2 8 2 4 2

mSRS,b and Nb, b = 0,1,2,3, values for the uplink bandwidth of 80 < NRBUL ≤ 110
SRS bandwidth configuration
CSRS
SRS bandwidth
BSRS = 0
SRS bandwidth
BSRS = 1
SRS bandwidth
BSRS = 2
SRS bandwidth
BSRS = 3
mSRS,0 N0 mSRS,1 N1 mSRS,2 N2 mSRS,3 N3
0 96 1 48 2 24 2 4 6
1 96 1 32 3 16 2 4 4
2 80 1 40 2 20 2 4 5
3 72 1 24 3 12 2 4 3
4 64 1 32 2 16 2 4 4
5 60 1 20 3 4 5 4 1
6 48 1 24 2 12 2 4 3
7 48 1 16 3 8 2 4 2

eNb broadcasts RRC (36.331) SystemInformationBlockType2

  SoundingRS-UL-ConfigCommon ::= CHOICE {
  release NULL,
  setup SEQUENCE {
    srs-BandwidthConfig ENUMERATED {bw0, bw1, bw2, bw3, bw4, bw5, bw6, bw7},
    srs-SubframeConfig ENUMERATED {sc0, sc1, sc2, sc3, sc4, sc5, sc6, sc7, sc8, sc9, sc10, sc11, sc12, sc13, sc14, sc15},
    ackNackSRS-SimultaneousTransmission BOOLEAN,
    srs-MaxUpPts ENUMERATED {true} OPTIONAL -- Cond TDD
    }
  }

eNb sends RRC (36.331) RRCConnectionSetup to the UE

radioResourceConfigDedicated RadioResourceConfigDedicated
  physicalConfigDedicated PhysicalConfigDedicated OPTIONAL
    SoundingRS-UL-ConfigDedicated ::= CHOICE{
      release NULL,
      setup SEQUENCE {
        srs-Bandwidth ENUMERATED {bw0, bw1, bw2, bw3},
        srs-HoppingBandwidth ENUMERATED {hbw0, hbw1, hbw2, hbw3},
        freqDomainPosition INTEGER (0..23),
        duration BOOLEAN,
        srs-ConfigIndex INTEGER (0..1023),
        transmissionComb INTEGER (0..1),
        cyclicShift ENUMERATED {cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7}
        }
      }

UE specific SRS periodicity and subframe offset configuration for trigger type 0, FDD (36.213 §8.2)

SRS configuration index
Isrs
SRS periodicity
Tsrs (ms)
SRS subframe offset
Toffset
0 – 1 2 Isrs
2 – 6 5 Isrs - 2
7 – 16 10 Isrs - 7
17 – 36 20 Isrs - 17
37 – 76 40 Isrs - 37
77 – 156 80 Isrs - 77
157 – 316 120 Isrs - 157
317 – 636 320 Isrs - 317
637 – 1023 reserved reserved

Graphical view of SRS in one SFN period (1024 frames of 10ms)

ISRS index = 14

SRS period

Type 0 triggered SRS transmission instances in a given serving cell for TDD serving cell with TSRS > 2 and for FDD serving cell are the subframes satisfying :
(10 - nf + kSRS - Toffset) mod TSRS = 0
The SRS transmission instances for TDD serving cell with TSRS = 2 are the subframes satisfying :
(kSRS - Toffset) mod 5 = 0