ofdmPilotConfig

R2026b

Specify pilot locations and pilot symbols for OFDM systems

Since R2026a

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Description

The ofdmPilotConfig object specifies the pilot location and pilot symbol configuration for OFDM systems. Use this object to construct pilot symbol patterns for user-defined OFDM systems and provide pilot symbol information to the ofdmChannelEstimate function.

Creation

Syntax

pilotcfg = ofdmPilotConfig
pilotcfg = ofdmPilotConfig(PropertyName=Value)
pilotcfg = ofdmPilotConfig(fftlen,numGuardBandCarriers,numSym,numTxStreams)

Description

pilotcfg = ofdmPilotConfig creates an OFDM pilot configuration object that specifies the pilot locations and the pilot symbol configuration for a MIMO-OFDM system.

example

pilotcfg = ofdmPilotConfig(PropertyName=Value) sets one or more properties using name-value arguments. For example, FFTLength=64 sets the total number of subcarriers to 64.

example

pilotcfg = ofdmPilotConfig(fftlen,numGuardBandCarriers,numSym,numTxStreams) configures the output object by setting the:

The object computes values for the StreamGroups, PilotLocations, and PilotSymbols properties based on input argument values. For this syntax, the number of transmit streams must be a nonnegative integer power of 2. For more information, see Computing Pilot Cluster Locations from FFT Length and Transmit Streams and Choosing Pilot Symbols for Pilot Locations.

example

Properties

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FFT length for OFDM symbols, specified as a positive integer scalar.

Data Types: double

Number of subcarriers allocated to the left and right guard bands, specified as 2-by-1 integer vector, [NleftG; NrightG]. Total number of subcarriers in the guard bands, sum(NumGuardBandCarriers) < FFTLength.

Data Types: double

Number of OFDM symbols in the OFDM grid, specified as a positive integer scalar.

Data Types: double

Number of transmit streams, specified as a positive integer scalar.

Data Types: double

Grouping of transmit streams, specified as a nonempty cell array of vectors. Vectors in the cell array indicate streams that transmit at the same OFDM subcarriers and using the same OFDM symbols. The property must assign each transmit stream [1:NumTransmitStreams] in exactly one of the vector groups. Each transmit stream must appear in exactly one group and you must include all streams as specified by the NumTransmitStreams property.

Example: For NumTransmitStreams = 3, setting StreamGroups = {[1 2], [3]} divides the three transmit streams into two groups. Streams 1 and 2 transmit on the same subcarriers and use the same symbols and must be orthogonal to avoid interference with each other. Stream 3 transmits in a separate group.

Data Types: double

Pilot locations for each pilot cluster in each stream group, specified as a nonempty cell array in which each cell contains a 3D array defining pilot clusters for one stream group. To specify pilot locations based on rectangular patterns, use the ofdmPilotGrid function.

  • The number of elements in PilotLocations must be equal to the number of elements in StreamGroups.

  • For each stream group, g, PilotLocations{g} is an M(g)-by-2-by-N(g) array where:

    • M(g) is the number of pilot symbols per cluster for stream group g. This value must be greater than or equal to the number of streams groups.

    • The columns dimension configures 2 columns. The first column, PilotLocations{g}(:,1,:), specifies the subcarrier indices of all the pilots and the second column, PilotLocations{g}(:,2,:), specifies OFDM symbol indices of all the pilots.

    • N(g) is the number of clusters for group g.

  • Subcarrier indices are integers in the range [(NumGuardBandCarriers(1) + 1), (FFTLength – NumGuardBandCarrier(2))].

  • Symbol indices are integers in the range [1, NumSymbols].

Example: The default setting for PilotLocations places pilot symbols in two OFDM symbols and spreads them evenly across subcarriers. 7:4:59 repeated defines two sets of pilot symbol subcarrier indices to 7,11,15,…,59, spaced by 4 subcarriers. ones(1,14) are the pilot symbol indices for the first set. All these pilot symbols are in the first OFDM symbol. 14*ones(1,14) are the symbol indices for the second set. All these pilots symbols are in the fourteenth OFDM symbol. reshape(...,1,2,28) organizes the values into a 3D array of one cluster group, two columns (subcarrier index and symbol index), and 28 pilot positions.

Data Types: cell

Pilot symbol values for each stream group, specified as a nonempty cell array of 3D array of complex numbers.

  • The number of elements in the pilot symbols must be equal to the number of elements in the stream groups.

  • In the 3D array:

    • size(PilotLocations{g},1) — The first dimension is the number of pilot symbols per cluster. This value must match the number of pilot positions in PilotLocations{g}.

    • size(PilotSymbols{g},2) — The second dimension is the number of streams in the group. Each pilot position has only one value for each stream.

    • size(PilotSymbols{g},3) — The third dimension is the number of clusters. This value must match the number of clusters in PilotLocations{g}.

  • For each cluster n, the matrix of pilot symbols (PilotSymbols{g}(:,:,n)) must have full rank. Full rank means that pilots for all streams in that cluster must be orthogonal, so the receiver can separate them when estimating the channel.

  • PilotSymbols{g}(m,:,n) —These values are the pilot symbol values for the mth pilot position in the nth cluster for all streams in the group g.

For more information, see Choosing Pilot Symbols for Pilot Locations.

Example: The default setting for PilotSymbols assigns one stream group with 28 clusters and one pilot symbol per cluster. comm.PNSequence(SamplesPerFrame=56).step() defines a 56 bit PN sequence. pskmod(...,4,pi/4,'InputType','bit'), QPSK modulates the bits to values for 28 pilot symbols. reshape(...,1,1,28) organizes the modulated sequence into a 3D array of one pilot cluster position, one stream group, and 28 pilot positions to match the structure of PilotLocations.

Data Types: double
Complex Number Support: Yes

Output Arguments

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Pilot configuration object for an OFDM system, returned as an ofdmPilotConfig object.

Object Functions

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validateValidate contents of ofdmPilotConfig object
pilotSignalReturns pilot symbols and linear indices of pilot locations
plotPlot pilot locations in 3-D

Examples

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Open Live Script

Plot the default OFDM pilots for the ofdmPilotConfig configuration object.

Specify an OFDM pilot configuration using default values.

pilotcfg = ofdmPilotConfig
pilotcfg = 
  ofdmPilotConfig with properties:

               FFTLength: 64
    NumGuardBandCarriers: [2×1 double]
              NumSymbols: 14
      NumTransmitStreams: 1
            StreamGroups: {[1]}
          PilotLocations: {[1×2×28 double]}
            PilotSymbols: {[1×1×28 double]}

Plot the pilot configuration. The tilted 3D-style plot shows all pilot symbols in an OFDM system. Colored cubes represent individual complex pilot symbols, arranged diagonally by transmit stream. The horizontal axis is OFDM Symbols (indices 1–14), the vertical axis is Subcarrier Indices (7–59). Each cube marks a pilot location across subcarriers and OFDM symbols.

plot(pilotcfg)

Figure contains an axes object. The axes object with xlabel Subcarrier Indices, ylabel OFDM Symbols contains an object of type patch.

Open Live Script

Create an OFDM system with 64-subcarriers and two transmit streams.

fftlen = 64;               % FFT length (number of subcarriers)
numGuardCarriers = [6; 5]; % Number of guard band carriers (lower; upper)
numSym = 14;               % Number of OFDM symbols
numTxStreams = 2;          % Number of transmit streams
pilotcfg = ofdmPilotConfig(fftlen,numGuardCarriers,numSym,numTxStreams)
pilotcfg = 
  ofdmPilotConfig with properties:

               FFTLength: 64
    NumGuardBandCarriers: [2×1 double]
              NumSymbols: 14
      NumTransmitStreams: 2
            StreamGroups: {[1 2]}
          PilotLocations: {[2×2×52 double]}
            PilotSymbols: {[2×2×52 double]}

  • The StreamGroups property configures the grouping of pilot symbols. Two streams are grouped together. At each pilot location, both streams transmit pilot symbols at the same time and frequency, but with different orthogonal values.

  • The PilotLocations property configures the placement of pilot symbols. The 3D array defines [Number of pilot locations per cluster per stream group; Number of pilot symbols per cluster for each stream group; Number of clusters]. As configured, each cluster has two pilot locations, each with a (subcarrier, symbol) coordinate.

  • The PilotSymbols property configures the actual pilot values. The 3D array defines [Number of pilot symbols per cluster; Number of streams per group; Number of clusters]. As configured, each cluster has a 2-by-2 matrix of complex pilot values. Each column corresponds to a stream, and each row to a pilot in the cluster. These are chosen so that, in each cluster, the pilots sent by the two streams are orthogonal.

A 3D plot shows colored squares representing the placement of pilot symbols for the MIMO-OFDM system.

  • Subcarrier Indices: Represents the subcarriers frequency bins used in the MIMO-OFDM system. Each tick is a subcarrier index.

  • OFDM Symbols: Represents the time dimension where each tick is an OFDM symbol index from 1 to 14.

  • Transmit Streams: Represents the streams transmitted out the antenna. There are two streams, labeled 1 and 2.

  • Each cube marks a pilot symbol location in time–frequency, with two parallel bands indicating the two transmit streams. Each pilot symbol is a known complex value transmitted at a specific subcarrier frequency, OFDM symbol time, and transmit stream spatial layer.

  • Each color corresponds to a cluster of pilots. The receiver uses the pilot symbol clusters of same color cubes together for channel estimation.

  • The cubes are arranged in a regular, repeating pattern across subcarriers and OFDM symbols. For each transmit stream (1 and 2), the cubes are stacked at the same (subcarrier, symbol) positions, but at different heights (streams).

Plot cardinal views to show the OFDM grid allocations.

plot(pilotcfg)
view([90 -90]) % symbol-subcarrier view

Figure contains an axes object. The axes object with xlabel Subcarrier Indices, ylabel OFDM Symbols contains an object of type patch.

plot(pilotcfg)
view([90 0]) % symbol-stream view

Figure contains an axes object. The axes object with xlabel Subcarrier Indices, ylabel OFDM Symbols contains an object of type patch.

plot(pilotcfg)
view([0 0]) % subcarrier-stream view

Figure contains an axes object. The axes object with xlabel Subcarrier Indices, ylabel OFDM Symbols contains an object of type patch.

Open Live Script

Create the pilot configuration object.

pilotcfg = ofdmPilotConfig( ...
    'FFTLength', 64, ...
    'NumGuardBandCarriers', [6; 5], ...
    'NumSymbols', 14, ...
    'NumTransmitStreams', 4, ...
    'StreamGroups', {[1 2], [3 4]} );

Create pilot locations for each stream group. For the first group, create 2-by-2 pilot matrices for each of nine clusters. For the second group, create 2-by-1 pilot matrices for each of four clusters.

pilotLocations1 = ofdmPilotGrid([1 2], [1 2], [7 31 57], [1 7 13]);
pilotLocations2 = ofdmPilotGrid([1 2], 1, [19 44], [4 10]);

Check the number of clusters for each group.

numClusters1 = size(pilotLocations1, 3)
numClusters1 = 
9

numClusters2 = size(pilotLocations2, 3)
numClusters2 = 
4

Create pilot symbols for each group (must match number of clusters).

pilotSymbols1 = randn(size(pilotLocations1, 1), ...
    numel(pilotcfg.StreamGroups{1}), ...
    numClusters1);
pilotSymbols2 = randn(size(pilotLocations2, 1), ...
    numel(pilotcfg.StreamGroups{2}), ...
    numClusters2);

Assign pilots to the configuration object, pilotcfg.

pilotcfg.PilotLocations = {pilotLocations1, pilotLocations2};
pilotcfg.PilotSymbols   = {pilotSymbols1, pilotSymbols2};

Validate and plot. The validate function displays a return message for invalid configurations only. For valid pilot configurations no message is displayed.

validate(pilotcfg);
plot(pilotcfg);

Figure contains an axes object. The axes object with xlabel Subcarrier Indices, ylabel OFDM Symbols contains an object of type patch.

Algorithms

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References

[1] Van de Beek, J.-J., O. Edfors, M. Sandell, S. K. Wilson, and P. O. Borjesson. “On Channel Estimation in OFDM Systems." Vehicular Technology Conference, IEEE 45th, Volume 2, IEEE, 1995.

Extended Capabilities

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Version History

Introduced in R2026a

See Also

Functions