LTE: Sounding Reference Signal Procedure

The Sounding Reference Signal (SRS) is a reference signal transmitted by the UE in the uplink direction which is used by the eNodeB to estimate the uplink channel quality over a wider bandwidth. The eNodeB may use this information for uplink frequency selective scheduling.

The eNodeB can also use SRS for uplink timing estimation as part of timing alignment procedure, particularly in situations like there are no PUSCH/PUCCH transmissions occurring in the uplink for a long time in which case, the eNodeB relies on SRS for uplink timing estimation.

SRS doesn’t need to be transmitted in the same physical resource blocks where PUSCH is transmitted as SRS may stretch over a larger frequency range.

There are 3 types of SRS transmissions defined in LTE. From release-8 onwards ‘Single SRS’ transmission and ‘Periodic SRS’ transmissions are supported. In release-10, ‘Aperiodic SRS’ transmission is introduced.  We will be discussing each of these types in detail.

SRS Configuration

‘Single SRS’ and ‘Periodic SRS’ transmissions are called ‘trigger type 0’ SRS transmissions which are configured by RRC signalling. ‘Aperiodic SRS’ transmission is called as ‘trigger type 1’ SRS transmission which is configured by RRC but triggered by DCI.

The eNodeB configures the UE with UE specific SRS configuration as shown below. UE specific SRS configuration provides the UE with time domain (subframes) as well as frequency domain resources.

UE specific SRS configuration for ‘trigger type 0’ (Periodic or Single)

UE specific SRS configuration for ‘trigger type 1’ (Aperiodic)

In addition to the UE specific SRS configuration, cell specific SRS configuration defines the subframes that can contain SRS transmissions as well as the set of SRS bandwidths available in the cell. In order to prevent SRS transmissions in the PUCCH regions of the cell, several SRS bandwidth configurations (srs-SubframeConfig) are defined.

Single and Periodic SRS transmissions

The parameter duration in the UE specific SRS configuration informs the UE whether single or periodic SRS transmission to be used.

Single SRS transmission is very simple one. After receiving RRC Connection Reconfiguration message with UE specific SRS configuration and parameter duration set to FALSE, the UE transmits SRS only once which is called ‘Single’ SRS transmission.

If the parameter duration is set to be TRUE, then the UE transmits Periodic SRS indefinitely until disabled.

srs-ConfigIndex defines SRS periodicity and an offset. The periodicity ranges from 2 ms to 320 ms.

srs-Bandwidth parameter defines the bandwidth that needs to be used while transmitting SRS in a subframe.

srs-HoppingBandwidth is defined for the purpose of frequency hopping of SRS. If frequency hopping of the SRS is enabled, then srs-HoppingBandwidth is smaller than srs-Bandwidth. SRS hopping procedure will also be discussed in detail.

freqDomainPosition defines the starting  position of the SRS in the frequency domain

cyclicShift can vary from 1 to 8 which generates up to 8 different SRSs which are orthogonal to each other. The eNodeB can configure SRS for up to 8 UEs in the same subframe and frequency resources but to use different cyclic shift. The cyclic shift multiplexed signals need to have the same bandwidth to maintain the orthogonally.

transmissionComb: Actually, SRS is transmitted in every alternate (every even or every odd) subcarrier in the assigned SRS bandwidth. transmissionComb takes values 0 or 1 which informs whether to transmit SRS in every even or odd subcarrier in the assigned SRS bandwidth. By doing this the eNodeB can multiplex two UEs with same cyclicShift, frequency and time resources but different transmissionComb (0 or 1).

Aperiodic SRS transmissions

Aperiodic SRS transmissions are defined from Release-10 onwards. Aperiodic SRS transmission, as the name implies, is single shot SRS transmission based a trigger.

Aperiodic SRS is configured by RRC but triggered by ‘SRS request’ flag in PDCCH DCI Formats 0/4/1A (for FDD and TDD) and DCI Formats 2B/2C for TDD alone.

Before triggering Aperiodic SRS using DCI Format 0, a single set of parameters srs-ConfigApDCI-Format0 need to be configured by RRC. Similarly, Aperiodic SRS using DCI formats 1A/2B/2C, a single common set of parameters srs-ConfigApDCI-Format1a2b2c should be configured by RRC.

For triggering Aperiodic SRS using DCI Format 4, three sets of SRS parameters, srs-ConfigApDCI-Format4, are to be configured by RRC.

For ‘Aperiodic SRS’ trigger using DCI Formats 0/1A/2B/2C, 1-bit ‘SRS request’ field is used whereas DCI Format 4 carries 2-bit ‘SRS request’ field to indicate which of the three configured parameters set to be used.

The frequency domain behavior of Aperiodic SRS is same as Periodic SRS.

A UE configured for Aperiodic SRS transmission upon detection of a positive SRS request in subframe #n shall commence SRS transmission in the first subframe satisfying subframe #n+k, k ≥ 4 and based on the Aperiodic SRS time domain configuration.

SRS transmission in detail

The SRS configurations for different types of SRS transmissions are already discussed. We will now look at the contents of SRS, its mapping to physical resources both in time and frequency.

SRS uses same sequence as uplink Demodulation Reference Signals (DMRS). Since the cyclic shift versions of the Zadoff-Chu sequence are orthogonal, several UEs (up to 8) can transmit using different cyclic shifts on the same physical radio resource.

In the configured SRS bandwidth, the SRS will be mapped every alternate subcarrier (comb-like pattern), on the other hand, since the srs-Bandwidth is always multiple of 4 RBs, SRS sequences are always a multiple of 24 RBs.

SRS is always transmitted in the last OFDM symbol in a subframe which is based on srs-ConfigIndex.

Frequency domain resource selection for SRS transmission

There are two types of SRS, wide band SRS and narrow band SRS.

Wide band SRS doesn’t necessarily over the entire system bandwidth but on the entire bandwidth of interest, whereas narrow band SRS allows the UE to do frequency hopping between transmissions.

Wide band SRS is more beneficial from the resource utilization point of view, as the UE can sound in the entire bandwidth of interest using single SRS transmission. However, the UE at the cell edge may not have sufficient power to sound over a wide bandwidth in which case, the eNodeB might configure the UE to use frequency hopping for SRS.

In the frequency domain, SRS is transmitted in srs-Bandwidth which is multiple of 4RBs. Tables 5.5.3.2-1 to 5.5.3.2-4 in 36.311 defines 4 srs-Bandwidths based on 1 of 8 srs-BandwidthConfigs which is Cell specific bandwidth configuration. One such table is shown below.

Let us consider the following example in FDD to understand how SRS is spread in the frequency domain in terms of PRBs. Let the System Bandwidth = 10MHz (50 PRBs), srs-BandwidthConfig (From SIB2) = bw0, freqDomainPosition = 0, transmissionComb = 1.

Since the system Bandwidth is 50 PRBs, there are a total of 600 subcarriers (0…599)

Example 1: Wide band SRS (no SRS Hopping)

Consider srs-Bandwidth = bw0 and srs-HoppingBandwidth = hbw0.

Since srs-Bandwidth is equal to srs-HoppingBandwidth, frequency hopping is not enabled. From the Table 5.5.3.2-2 (presented above), srs-Bandwidth of bw0 corresponds to 48 PRBs.

In the subframe where SRS is transmitted, starting from subcarrier number 13, every alternate subcarrier (13, 15, 17… 585, 587) is used for SRS transmission.

The eNodeB can allocate same time and RBs for another UE by setting transmissionComb = 0 (all other parameters are same). This implies that second UE sends SRS on subcarriers (12, 14, 16… 584, 586).

Example 2: SRS Frequency Hopping

If frequency hopping of the SRS is enabled, then srs-HoppingBandwidth is smaller than srs-Bandwidth. Let us consider srs-Bandwidth = bw3 and srs-HoppingBandwidth = hbw0 ⇨ SRS bandwidth = 4 PRBs and SRS Hopping Bandwidth = 48 PRBs.

Consider two UEs, UE1 and UE2. Let transmissionComb = 0 for both of the UEs and freqDomainPosition = 0 for UE1 and freqDomainPosition = 2 for UE2.

Since srs-Bandwidth is set to 3, both of the UEs use 4 RBs in every subframe for SRS transmission. It can be seen that UE1 is transmitting SRS over the entire bandwidth of interest (SRS Hopping Bandwidth = 48 PRBs) but not in single shot. Similar behavior holds good for UE2 as well.

There can be a lot of combinations considered. In a single subframe, the eNodeB can configure all 48 PRBs to UE1 with transmissionComb = 0, and configure a couple of UEs with 4 PRBs but using transmissionComb = 1. Similarly, other combinations of various SRS bandwidths of 4, 12, 24, and 48 resource blocks can be considered

One can try several combinations using different parameters for calculating SRS resources (frequency domain starting position) using the tool given at the end of this post.

Time domain resource selection for SRS transmission

In the time domain, a resource is nothing but the subframe where SRS transmission has to happen.

Based on srs-SubframeConfig in SIB2, the UE first derives cell specific SRS subframe. These subframe (s) are common to all the UEs in the cell.

Different UEs are configured with different UE specific SRS configuration, based on which each UE derives UE specific SRS subframe.

The UE transmits SRS only if the ‘UE specific SRS subframe’ coincides with ‘Cell specific SRS subframe’.

Example: Let us consider srs-SubframeConfig = sc8 and srs-ConfigIndex = 0.

From Table 5.5.3.3-1 in 36.211, subframes 2, 3, 7, and 8 are cell-specific subframes. From srs-ConfigIndex, UE specific subframes are 0, 2, 4, 6, and 8. So the UE transmits SRS in subframes 2 and 8.

When SRS is being transmitted by a UE in a subframe, it may overlap in frequency with PUSCH being transmitted by another UE. Due to this reason, none of the UEs in the cell transmits PUSCH in the last OFDM symbol of a cell specific SRS subframe. Since all UEs are aware of cell specific SRS configuration, they can take care of not transmitting PUSCH in the last OFDM symbol of the cell specific SRS subframe.

A UE does not transmit SRS whenever SRS and CQI transmissions happen to coincide in the same subframe.

A UE shall not transmit SRS whenever SRS transmission and PUCCH transmission carrying HARQ-ACK and/or Scheduling Request happen to coincide in the same subframe if the parameter ackNackSRS-SimultaneousTransmission in SIB2 is set to FALSE.

A UE shall transmit SRS whenever SRS transmission and PUCCH transmission carrying HARQ-ACK and/or Scheduling Request using shortened PUCCH format happens to coincide in the same subframe if the parameter ackNackSRS-SimultaneousTransmission is TRUE. In this case, the UE shall transmit shortened PUCCH format where the HARQ-ACK or the SR symbol corresponding to the SRS location is punctured.

The UE shall use shortened PUCCH format in a cell specific SRS subframe even if the UE does not transmit SRS in that subframe.

In case both periodic and aperiodic SRS transmissions would occur in the same subframe in the same serving cell, the UE shall only transmit the Aperiodic SRS.

A UE shall not transmit SRS whenever SRS and a PUSCH transmission corresponding to a RAR Grant or a retransmission of the same TB as part of the contention based RA procedure coincide in the same subframe.

SRS transmission in TDD

In TDD, SRS can be transmitted in uplink as well as in special subframes (UpPTS).

Based on the special subframe configuration (Table 4.2-1 from 36.211), the UpPTS length varies (one or two OFDM symbols).

When one SC-FDMA symbol exists in UpPTS, it can be used for SRS transmission.  

When two SC-FDMA symbols exist in UpPTS, both can be used for SRS transmission and both can be assigned to the same UE.

In UpPTS, whenever SRS transmission instance overlaps with the PRACH region for preamble format 4, the UE shall not transmit SRS

The following ‘SRS Calculator’ can be used to calculate SRS transmission resources (in time as well as frequency domain) for FDD. TDD support will be added very soon.

Reference: 3GPP TS 36.211, 36.213, and 36.331

SRS Calculator

srs-BandwidthConfig       srs-Bandwidth      srs-HoppingBandwidth           Cell Bandwidth
0 1 2 3 4 5 6 7                 0 1 2 3                 0 1 2 3                      1.4 3 5 10 15 20


srs-SubframeConfig       srs-ConfigIndex       freqDomainPosition           transmissionComb
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15                                                      0 1


                         

SRS Occasions/Resources

SRS Occasions/Resources will be displayed here