Introduction

This example shows how to generate sounding reference signals (SRS), as defined in TS 38.211 Section 6.4.1.4, including SRS configuration, symbol and indices generation, OFDM resource grid mapping, and SRS waveform generation.

Full-band transmissions: Generate an SRS spanning all the available bandwidth.

Frequency-hopping transmissions: Generate periodic and aperiodic SRS transmissions with different frequency-hopping patterns.

Multi-user transmissions: Generate orthogonal SRS using time, frequency, and cyclic shifts.

Sounding reference signals are uplink physical signals employed by user equipment (UE) for uplink channel sounding, including channel quality estimation and synchronization. Unlike demodulation reference signals (DM-RS), SRS are not associated to any physical uplink channels and they support uplink channel-dependent scheduling and link adaptation. SRS assist in:

Codebook-based closed-loop spatial multiplexing.

Control uplink transmit timing.

Reciprocity-based downlink precoding in multi-user MIMO setups.

Quasi co-location of physical channels and reference signals.

SRS Frequency Positioning

Example of onfiguring a 15 MHz bandwidth carrier with 15 kHz subcarrier spacing (SCS).

carrier = nrCarrierConfig;
carrier.NSizeGrid = 79;             % Bandwidth in RB
carrier.SubcarrierSpacing = 15;

The bandwidth configuration parametersCSRSandBSRScontrol the bandwidth allocated to the SRS, which normally increases withCSRSand decreases withBSRS.

srs = nrSRSConfig;
srs.CSRS = 10;  % Bandwidth configuration C_SRS (0...63)
srs.BSRS =  1;  % Bandwidth configuration B_SRS (0...3)

srs.FrequencyStart = 30; % Frequency position of the SRS in carrier in RB (0...271)
srs.NRRC = 0;            % Frequency domain position in blocks of 4 PRB (0...67)

This figure displays a single-slot OFDM resource grid containing an SRS. ForCSRS = 10andBSRS = 1, the frequency position of the SRS(F0)shifts byNRBTwhenNRRCis in the range (3...5) and by2NRBTwhenNRRCis in the range (6...8). The SRS returns to the initial position (FrequencyStart) whenNRRCis 9.NRBTis the number of resource blocks (RBs) allocated to the SRS per transmission.

This figure illustrates the concepts introduced above forCSRS = 10andBSRS = 1.

NRRCis an additional frequency offset specified as a number of 4 RBs and it corresponds to the higher layer parameterfreqDomainPosition(see TS 38.331 Section 6.3.2 SRS-Config [2]). For values ofNRRCbetweenk⋅(NRBT/4)and(k+1)⋅(NRBT/4)−1, the frequency position of the SRS is shifted byk⋅NRBT, wherekis an integer. TS 38.211 Section 6.4.1.4 refers toNRBTasmSRS.bwithb=BSRS. For more information, see theNRBPerTransmissionproperty of thenrSRSConfigconfiguration object.

This equation determines the origin of the SRS in frequency:

NSBdenotes the number of SRS subbands (frequency bands of sizeNRBT) where the SRS can be positioned through the parameterNRRC. To calculateNSB, you can use the SRS bandwidth configuration table (see TS 38.211 Table 6.4.1.4.3-1).

The first column contains possible values of the parameterCSRS. ForCSRS = 10andBSRS = 1, the number of unique SRS subbands isNSB=N0?NBSRS=3, whereN0=1andN1=3.

% Calculate and display the number of SRS subbands configurable by NRRC
NSBTable = hSRSNumberOfSubbandsOrHoppingPatterns(srs);

% Number of SRS subbands (NRRC < 9):  3

% Calculate the frequency origin of the first SRS symbol
f0 = hSRSFrequencyOrigin(srs);

% The frequency origin of the SRS is F0 = 30 RB.

When intra-slot frequency hopping is enabled, the calculation ofFrequencyOriginis only valid for the first SRS symbol in the slot.

Frequency-Hopping Configuration

Example of configuring a 15 MHz bandwidth carrier with 15 kHz SCS.

carrier = nrCarrierConfig;
carrier.NSizeGrid = 79;
carrier.SubcarrierSpacing = 15

Create a four-symbol SRS located at the end of the slot. Select the repetition factor to indicate the number of equal consecutive SRS transmissions (OFDM symbols) in a slot. For frequency-hopping configurations,Repetitionmust be lower than the number of SRS symbols.

srs = nrSRSConfig;
srs.NumSRSSymbols = 4;
srs.Repetition = 1;
srs.SymbolStart = 10; % Time-domain position of the SRS in the slot. (8...13) for normal cyclic prefix (CP) and (6...11) for extended CP

Downlink control information (DCI) can trigger aperiodic SRS transmissions using the higher layer parameterresourceType(see TS 38.331 Section 6.3.2 SRS-Config). As the frequency hopping pattern is reset for aperiodic SRS resource types at each slot, selectperiodicorsemi-persistentSRS resource types to enable inter-slot frequency hopping oraperiodicto disable it. You can configure the slot-wise periodicity and offset of the SRS transmissions by using the propertySRSPeriod. Foraperiodicresource types, the parameterSRSPeriodcontrols the periodicity and offset of DCI signals triggering aperiodic SRS transmissions.

srs.ResourceType =  "periodic";
srs.SRSPeriod = [2 0];          % Periodicity in slots (1,2,4,5,8,10,...)
srs.SRSPeriod(2) = 0;           % Offset in slots must be smaller than the periodicity

To select a hopping configuration and observe the changes to the OFDM resource grid.

srs.CSRS = 19; % Bandwidth configuration C_SRS (0...63)
srs.BSRS = 2;  % Bandwidth configuration B_SRS (0...3)
srs.BHop = 0;  % Frequency hopping configuration (0...3). Set BHop >= BSRS to disable frequency hopping
srs.NRRC = 14; % Frequency domain position in blocks of 4 PRB (0...67)

The bandwidth over which the SRS symbols hop increases with the value ofCSRSand decreases withBHop(untilBHop = BSRSdisables hopping). IncreasingBSRSreduces the allocated bandwidth per OFDM symbol (propertyNRBPerTransmission) and can reduce the intra-slot frequency hopping as well. To disable frequency hopping, setBHop >= BSRS. For non-hopping configurations, the roles ofCSRSandBSRSare analogous, as the allocated bandwidth increases withCSRSand decreases withBSRS.

You can use the bandwidth configuration table (TS 38.211 Table 6.4.1.4.3-1) to calculate the number of different frequency-hopping patterns configurable by the SRS parameterNRRCasNFHP=NBHop+1?NBSRS. The frequency-hopping patterns repeat whenNRRC>NFHP⋅NRBT/4.

Set the bandwidth configurations parameters asCSRS = 20, BSRS = 2, andBHop = 1. The conditionBHop < BSRSdoes not guarantee frequency hopping. However, the opposite is true:BHop >= BSRSalways disables frequency hopping. You can determine when an SRS bandwidth configuration (CSRS,BSRS,BHop) produces frequency hopping using theNbparameter from the bandwidth configuration table and evaluating the conditionNFHP=NBHop+1?NBSRS>1.

Multi-User Configurations

To configure multiple SRS transmissions suitable for multi-user setups, use time-domain, frequency-domain, and sequence-domain parameterscan be adopted to create sets of orthogonal (interference-free) SRS transmissions.

Time-Domain Orthogonal SRS

You can create time-domain orthogonal SRS transmissions in multiple ways, for example:

Configure different slot periodicity and offset for different SRS by using the propertySRSPeriod.

Configure different symbol-wise time domain allocations for different SRS by using the propertySymbolStart.

Configure a 10 MHz bandwidth carrier with 15 kHz SCS.

carrier = nrCarrierConfig;
carrier.NSizeGrid = 52;
carrier.SubcarrierSpacing = 15;

To specify the slot periodicity and offset to create orthogonal SRS transmissions.

First SRS configuration:

srs = nrSRSConfig;
srs(1).SRSPeriod = [1 0];
srs(1).SRSPeriod(1) = 4; % Slot periodicity and offset 
srs(1).SRSPeriod(2) = 0; % Slot offset of first SRS

Second SRS configuration:

srs(2) = srs(1); % Create a copy of the configured SRS
srs(2).SRSPeriod(1) = 2; % Periodicity in slots
srs(2).SRSPeriod(2) = 1; % Offset in slots

This figure displays an OFDM resource grid containing the configured SRS transmissions.

To specify the number of SRS symbols and location in the slot to create orthogonal SRS transmissions.

First SRS configuration:

srs = nrSRSConfig;
srs(1).NumSRSSymbols = 1;  % Number of SRS symbols in a slot (1,2,4)
srs(1).SymbolStart = 12; % Time-domain position of the SRS in the slot. (8...13) for normal cyclic prefix (CP) and (6...11) for extended CP

Second SRS configuration:

srs(2) = srs(1); % Create a copy of the configured SRS
srs(2).NumSRSSymbols = 2;   
srs(2).SymbolStart = 9;

This figure displays the OFDM resource grid containing the SRS transmissions.

Frequency-Domain Orthogonal SRS

You can create frequency-domain orthogonal SRS transmissions in multiple ways:

Configure different comb offsets for different SRS by using the propertyKBarTC.

Configure different frequency-hopping patterns usingCSRS,BSRS,BHopandNRRC.

Configure a 10 MHz bandwidth carrier with 15 kHz SCS.

carrier = nrCarrierConfig;
carrier.NSizeGrid = 52;
carrier.SubcarrierSpacing = 15;

Specify the comb numbers and offsets to create orthogonal SRS transmissions.

First SRS configuration:

srs = nrSRSConfig;
srs(1).KTC    = 2; % Comb number (2,4). It indicates the allocation of the SRS every KTC subcarriers
srs(1).KBarTC = 1; % Comb offset (0...KTC-1)

Second SRS configuration:

srs(2) = srs(1); % Create a copy of the configured SRS
srs(2).KTC    = 4;
srs(2).KBarTC = 0;

This figure displays the OFDM resource grid containing the SRS transmissions.

Create frequency-domain orthogonal SRS configurations using different frequency-hopping patterns.

srs = nrSRSConfig;
srs.NumSRSSymbols = 4;  % Number of SRS symbols in a slot
srs.SymbolStart =8; % Allocate SRS in OFDM symbols 10:13

Select the desired bandwidth configuration parameters, resource type and repetition factor.

srs.CSRS = 10;  % Bandwidth configuration C_SRS (0...63)
srs.BSRS =  2;  % Bandwidth configuration B_SRS (0...3)
srs.BHop =  0;  % Frequency hopping configuration (0...3). Set BHop >= BSRS to disable frequency hopping
srs.ResourceType = "periodic"; % ('periodic','semi-persistent','aperiodic')
srs.Repetition   = 1; % Number of equal consecutive SRS transmissions (1,2,4). It must be lower than NumSRSSymbols

This example calculates the number of orthogonal SRS sequences that can be configured byNRRCand creates frequency non-overlapping SRS configurations. The figure displays the OFDM resource grid containing the SRS transmissions.

Note that the SRS transmissions are never overlapping regardless of theRepetitionfactor andResourceType(aperiodicdisables inter-slot frequency hopping). Disable frequency hopping by settingBHop >= BSRSand observe that the different SRS are still allocated to different subbands.

Cyclic-Shift Orthogonal SRS

This section generates multiple SRS allocated to the same time and frequency resources (OFDM symbols and subcarriers) but different time-domain cyclic shifts. Due to the properties of Zadoff-Chu sequences, this configuration produces orthogonal SRS. To demonstrate the orthogonality among the configured SRS, this section performs CP-OFDM modulation and calculates the cross-correlation of the time-domain waveforms.

Matlab Codes,

% Configure a 10 MHz bandwidth carrier with 15 kHz SCS.
carrier = nrCarrierConfig;
carrier.NSizeGrid = 52;
carrier.SubcarrierSpacing = 15;

% Create a full-band SRS
srs = nrSRSConfig;
srs.CSRS = hSRSBandwidthConfiguration(srs,carrier.NSizeGrid);

% All SRS share the same physical resources, but they are configured with different cyclic shifts.
for i = 1:8
    srs(i) = srs(1);
    srs(i).CyclicShift = i-1;
end

% Create a resource grid containing SRS
numSlots = 1; % Numer of slots to generate
for ich = length(srs):-1:1
    slotGrid{ich} = hSRSGrid(carrier,srs(ich),numSlots);
end

% Get OFDM modulation related information
ue.NRB = carrier.NSizeGrid;
ue.SubcarrierSpacing = carrier.SubcarrierSpacing;
ue.CyclicPrefix = carrier.CyclicPrefix;
ofdmInfo = hOFDMInfo(ue);

% OFDM modulation
nsrs = length(srs); % Numer of SRS waveforms
numSamples = numSlots*ofdmInfo.SamplesPerSubframe/ofdmInfo.SlotsPerSubframe;
txWaveform = zeros(numSamples,nsrs);
for i = 1:nsrs
    txWaveform(:,i) = hOFDMModulate(ue,slotGrid{i});
end

% Cross correlation of SRS waveforms generated with different cyclic shifts
C = txWaveform'*txWaveform;
srsCorr = C./diag(C);

This figure shows the time-domain cross-correlation of the SRS waveforms with different cyclic shifts.

The low correlations among SRS waveforms generated using different time-domain cyclic shifts show their orthogonality.

Conclusion

The relationship between SRS configuration parameters and their effects on both the resource grid and SRS waveform properties:

The bandwidth allocated to the SRS generally increases withCSRSand decreases withBSRS. For frequency-hopping configurations, the instantaneous bandwidth and hopping bandwidth decrease withBSRSandBHop, respectively.

The frequency position of the SRS depends on theFrequencyStartandNRRCparameters.NRRCallows to select different SRS subbands and frequency-hopping patterns.

BHop < BSRSgenerally produces frequency hopping, but it is not guaranteed. However,BHop >= BSRSis sufficient to disable frequency hopping.

Inter-slot frequency hopping is active only forperiodicandsemi-persistentresource types.

Time-, frequency-, and sequence-related parameters can be used to create orthogonal SRS transmissions.

Reference

　 1. 38.211

　　2. 38.331

　　3. MathWorks

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