OFDM Demodulator

Demodulate time-domain OFDM samples and return subcarriers for custom communication protocols

  • Library:
  • Wireless HDL Toolbox / Modulation

Description

The OFDM Demodulator block demodulates time-domain orthogonal frequency division multiplexing (OFDM) samples and outputs subcarriers based on the OFDM parameters. The block supports 5G, LTE [1], Wireless Local Area Network (WLAN 802.11a/b/g/n/ac) [2], WiMAX (802.16 m and e), digital video broadcast (DVB), and digital audio broadcast (DAB) standards.

The block accepts input data along with a valid control signal and these OFDM parameters: FFT length, CP length, and the number of right and left guard subcarriers. The block outputs demodulated data along with valid and ready controls signals. The block enables the ready output port only when these OFDM parameters are provided to the block through input ports. The block samples the corresponding OFDM parameters only when the ready port is 1 (high) and the first valid port of each OFDM symbol is 1 (high).

The block supports scalar and vector inputs. You can use a vector input to increase the data throughput and achieve a giga-sample-per-second (GSPS) throughput. The block provides an interface and architecture suitable for HDL code generation and hardware deployment.

Ports

Input

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Input data, specified as a scalar or column vector of real or complex values. The vector size must be a power of 2, in the range from 1 to 64, and less than or equal to the FFT length.

double and single data types are supported for simulation, but not for HDL code generation.

Data Types: single | double | int8 | int16 | int32 | signed fixed point
Complex Number Support: Yes

Indicates valid input data, specified as a Boolean scalar.

This port is a control signal that indicates when the sample from the data input port is valid. When this value is 1, the block captures the values on the data input port. When this value is 0, the block ignores the values on the data input port.

Data Types: Boolean

Length of the FFT, specified as a real scalar. The FFT length must be power of 2 and in the range from 8 to 65,536. This value must be less than or equal to the Maximum FFT length parameter value.

To support the minimum FFT length of 8, the FFTLen data type must be fixdt(0,k,0), where k is greater than or equal to 4.

Dependencies

To enable this port, set the OFDM parameters source parameter to Input port.

Data Types: single | double | uint8 | uint16 | uint32 | unsigned fixed point

Length of the cyclic prefix, specified as a real scalar. CPLen port values must be in the range from 0 to FFTLen.

To support the minimum FFT length of 8, the CPLen data type must be fixdt(0,k,0), where k is greater than or equal to 4.

Dependencies

To enable this port, set the OFDM parameters source parameter to Input port.

Data Types: single | double | uint8 | uint16 | uint32 | unsigned fixed point

Number of left guard carriers of OFDM symbol, specified as a scalar. numLgSc port values must be in the range from 0 to (FFTLen/2) – 1.

To support the minimum FFT length of 8, the numLgSc data type must be fixdt(0,k,0), where k is greater than or equal to 2.

Dependencies

To enable this port, set the OFDM parameters source parameter to Input port.

Data Types: single | double | uint8 | uint16 | uint32 | unsigned fixed point

Number of right guard carriers of OFDM symbol, specified as a scalar. numRgSc port values must be in the range from 0 to (FFTLen/2) – 1.

To support the minimum FFT length of 8, the numRgSc data type must be fixdt(0,k,0), where k is greater than or equal to 2.

Dependencies

To enable this port, set the OFDM parameters source parameter to Input port.

Data Types: single | double | uint8 | uint16 | uint32 | unsigned fixed point

Clear internal states, specified as a Boolean scalar. When this value is 1 (true), the block stops the current calculation and clears all internal states.

Dependencies

To enable this port, select the Enable reset input port parameter.

Data Types: Boolean

Output

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Demodulated output data, returned as a complex-valued scalar or column vector. Output data type is dependent on the data type of the input data port.

  • When you set the OFDM parameters source parameter to Property and clear the Divide butterfly outputs by two parameter, the output word length increases by log2(FFT length) bits.

  • When you set the OFDM parameters source parameter to Input port and clear the Divide butterfly outputs by two parameter, the output increases by log2(Maximum FFT length) bits.

To avoid overflow, select the Divide butterfly outputs by two parameter.

Data Types: single | double | int8 | int16 | int32 | signed fixed point
Complex Number Support: Yes

Indicates valid input data, returned as a Boolean scalar.

This port is a control signal that indicates when the data output port is valid. The block sets this value to 1 when the data samples are available on the data output port. When you select the Remove DC subcarrier parameter, this value is set to 0 at the center of the output samples to exclude the DC carrier.

Data Types: Boolean

Control signal that indicates when the block is ready for new input data. When this value is 1, the block accepts input data in the next time step. When this value is 0, the block ignores input data in the next time step.

Dependencies

To enable this port, set the OFDM parameters source parameter to Input port.

Data Types: Boolean

Parameters

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Main

You can set OFDM parameters with an input port or by selecting a value for the parameter.

Select Property to enable the FFT length, Cyclic prefix length, Number of left guard subcarriers, and Number of right guard subcarriers parameters.

Select Input port to enable the FFTLen, CPLen, numLgSc, numRgSc input ports and the MaxFFTLength parameter.

Specify the maximum length of the FFT.

Dependencies

To enable this parameter, set the OFDM parameters source parameter to Input port.

Specify the FFT length. When you set the OFDM parameters source parameter to Property, the block uses this FFT length value as the maximum FFT length.

Dependencies

To enable this parameter, set the OFDM parameters source parameter to Property.

Specify the length of the cyclic prefix.

Dependencies

To enable this parameter, set the OFDM parameters source parameter to Property.

Specify the number of left guard subcarriers.

Dependencies

To enable this parameter, set the OFDM parameters source parameter to Property.

Specify the number of right guard subcarriers.

Dependencies

To enable this parameter, set the OFDM parameters source parameter to Property.

Select this parameter to enable the CP Fraction parameter on the block mask.

Cyclic prefix fraction, specified as a value from 0 to 1, inclusive. This parameter specifies the percentage of CP samples that the block removes from the start of the OFDM symbol. The block shifts the remaining CP samples to the end of the OFDM symbol.

When this parameter is 0.55, the block removes 55% of the CP from the beginning of the symbol, and shifts 45% to the end of the symbol. When you set this parameter to 1, the block removes 100% of the CP from the start of the OFDM symbol, and does not shift any samples to the end.

Dependencies

To enable this parameter, select the Enable CP Fraction parameter.

When you select this parameter, the block excludes the DC subcarrier in the output by setting the output valid signal to 0 for the center of the output subcarriers.

Select this parameter to enable the reset input port.

FFT Parameters

This parameter controls the scaling option of the FFT HDL Optimized block inside the OFDM Demodulator block.

When you select this parameter, the FFT implements an overall 1/N scale factor by dividing the output of each butterfly multiplication by two. This adjustment keeps the output of the FFT in the same amplitude range as its input. If you clear this parameter, the block avoids overflow by increasing the word length by one bit after each butterfly multiplication.

This parameter specifies the type of rounding mode for internal fixed-point calculations. For more information about rounding modes, see Rounding Modes (DSP System Toolbox). When the input is any integer data type or fixed-point data type, the FFT algorithm uses fixed-point arithmetic for internal calculations. This parameter does not apply when the input is of data type single or double. Rounding applies to twiddle-factor multiplication and scaling operations.

Algorithms

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The OFDM Demodulator block operation sequence is implemented using these blocks: Ready Generator, Cyclic Prefix Remover, Sample Repeater, FFT Shifter, FFT, Down Sampler, and Subcarrier Selector. The parameters shown in this figure configure the behavior of the block.

References

[1] 3GPP TS 36.211 version 14.2.0 Release 14. "Physical channels and modulation." LTE - Evolved Universal Terrestrial Radio Access (E-UTRA).

[2] "Wireless LAN Medium Access Control (MAC) and Physical layer (PHY) Specifications." IEEE Std 802.11 – 2012.

[3] Stefania Sesia, Issam Toufik, and Matthew baker. LTE - THE UMTS Long Term Evolution from theory to practice.

[4] Erik Dahlman, Stefan Parkvall, and Johan Skold. 4G - LTE/LTE - Advanced for Mobile broadband Second edition.

Extended Capabilities

Introduced in R2019b