LTE Transmission Modes Part 4 SRS (Sounding Reference Signal)
Malaka Dahanayake
Telecommunication Engineering Professional | Expertise in 4G/5G Radio Access Network
Prior to delving into TM7 and TM8, it would be beneficial to familiarize ourselves with the following two topics:
1) SRS - Sounding Reference Signal
2) UE Specific Reference Signal
This article will focus on exploring various aspects of SRS. Beamforming, a key component in TM7 and TM8, depends on accurate channel estimation. In LTE TDD deployments, most vendors utilize uplink SRS for this purpose.
It is highly recommended that you review the previous articles before continuing.
SRS stands for Sounding Reference Signal. Literally it says a kind of "Reference Signal".
Reference Signal for who ? Is it for UE or for eNodeB ?
It is reference signal for eNodeB to figure out the channel quality of uplink path for each subsections of frequency region.
Why we need this kind of signal ?
As you know, in LTE eNodeB often allocates only a partial section of full system bandwidth for a specific UE and at a specific time. So, it would be good to know which section across the overall bandwidth has better channel quality comparing to the other region. In this case, Network can allocate the specific frequency region which is the best for each of the UEs. (If we always must use full bandwidth, we may not need this kind of reference signal since there is no choice even when there is a better or worse spots within the bandwidth).
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How often UE transmit SRS ?
It depends on the configuration set by the signaling message (SIB2, RRC Connection Setup, RRC Connection Reconfiguration etc), but UE can transmit it every two subframes at the most and every 32 frame (320 subframe) at the least (10 bit signaling parameter srs-ConfigIndex tells UE of the periodicity of SRS transmission and the period can be 2,5,10,20,40,80,160,320 ms). Actually, there is an option in which UE does not transmit SRS at all.
SRS is transmitted at the last symbol of UL slot with full system band area, and it is transmitted by a certain interval.
What if multiple UE has the same SRS transmission cycle(interval) ? Would there be any possibility that a bunch of SRS from multiple UEs are overlapped ?
Yes, this is possible. To avoid this kind of situation, we can configure each of UE to transmit SRS in hopping mode with different hopping schedule.
If we summarize above all,
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.
?There are two methods of transmitting the SRS:
Non-frequency-hopping SRS
Frequency-hopping mode — the SRS transmission is split into a series of narrowband transmissions that will cover the whole bandwidth region of interest; this mode is the preferred method under poor channel conditions.
Frequency-hopping SRS
SRS Configuration
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.
For the following parameters, refer to 36.211 5.5.3.2 Mapping to physical resources.
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There is no flag for "srs-Hopping ON or OFF", but according to the specification , can figure "srs-Hopping ON or OFF" indirectly by combining the following two parameters.
RRC Parameters for SRS
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.
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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.
For srs-Bandwidth, refer to following tables from 36.211. Basically, these tables defines how many resource block (frequency bandwidth) is allocated for SRS transmission.
Interpretation of SRS configuration Table – Example
Visual Interpretation of SRS configuration Table – Example
Frequency domain resource selection for SRS transmission
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)
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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 , srs-Bandwidth of bw0 corresponds to 48 PRBs.
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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).
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Example 2: SRS Frequency Hopping
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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 = 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.
Time domain resource selection for SRS transmission
In the time domain, a resource is nothing but the subframe where SRS transmission must 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’.
srs-SubframeConfig
What is the Frame Structure Type mention in above tables ?
Frame structure type 1
Frame structure type 2
SRS Sub-Frame Configuration
where ns here means "slot number within a radio frame".
For example, if we chose subframe configuration 7, T_SFC become 5 and Delta_SFC become {0,1}. According to this, the SRS Status on each subframe become as follows.
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For srs-ConfigIndex, please refer to the following table from TS 36.213
Can use below link to simulate the SRS position as per the configuration
Uplink Frame Structure with SRS - Example on srs-subframeConfig?
Below example is from the test and trace log with Amari Callbox from Amarisoft.
DL NRB : 100, UL NRB : 100
SIB 2 :
soundingRS-UL-ConfigCommon setup: {
srs-BandwidthConfig bw2,
srs-SubframeConfig sc3,
ackNackSRS-SimultaneousTransmission TRUE
}
RrcSetup :
soundingRS-UL-ConfigDedicated setup: {
??srs-Bandwidth bw3,
??srs-HoppingBandwidth hbw0,
??freqDomainPosition 0,
??duration TRUE,
??srs-ConfigIndex 37,
??transmissionComb 0,
??cyclicShift cs2
},
According the RrcConfiguration above, following configurations are?applied.
36.213-Table 8.2-1:UE Specific SRS Periodicity T_SRS and Subframe Offset Configuration T_offset for trigger type 0, FDD
With this configuration SRS is being transmitted across 4 RBs every 4?radio frames and transmit 20 consecutive SRS to cover the whole bandwidth. In this example, you would notice that the pattern between SFN 56.0 and 132.0 repeats.
References :