LTE OFDM Modulator

Modulate LTE resource grid and return time-domain OFDM samples

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Libraries:
Wireless HDL Toolbox / Modulation

Description

The LTE OFDM Modulator block implements an algorithm for modulating LTE resource grid samples specified by LTE standard TS 36.212 [1]. The block uses an orthogonal frequency-division multiplexing (OFDM) mechanism in its operation and converts the resource grid input samples to an equivalent time-domain signal output. OFDM is effective for communication over channels with high-frequency selectivity and is widely used in the development of the LTE downlink transmitter. The block implements a windowing feature to reduce the spectral regrowth, or adjacent channel leakage ratio (ACLR), of an OFDM signal.

The block provides an interface and architecture suitable for HDL code generation and hardware deployment.

You can select the number of downlink resource blocks (NDLRB) and choose either normal or extended cyclic prefix (CP), as described in the LTE standard. The latency from the first input sample to the first output sample depends on your selection of the NDLRB.

NDLRB	Latency
6	6268
15	6376
25	6496
50	6796
75	7096
100	7396

Examples

OFDM Modulation of LTE Resource Grid Samples

Use LTE OFDM Modulator block to modulate LTE resource grid samples to equivalent time-domain signal output.

Open Script

Modulate and Demodulate LTE Resource Grid

Modulate and demodulate LTE resource grid samples.

Open Script

Ports

Input

data — Input data

scalar

Input data, specified as a signed real or complex number. 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

valid — Indicates valid input data

scalar

Control signal that indicates if the data from the data input port is valid. When this value is 1 (true), the block captures the value on the data input port. When this value is 0 (false), the block ignores the values on the data input port.

Data Types: Boolean

NDLRB — Number of downlink resource blocks

6 | 15 | 25 | 50 | 75 | 100

Number of downlink resource blocks, specified as 6, 15, 25, 50, 75, or 100. The NDLRB must be one of these six values specified by LTE standard TS 36.212 [1]. The block samples this port at the start of each subframe and ignores any changes within a subframe.

Dependencies

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

Data Types: uint8 | uint16 | uint32 | fixdt(0,K,0), K >= 7 | single | double

cyclicPrefixType — Type of CP

scalar

Type of CP, specified as a Boolean scalar. When this value is 0 (false), the block selects normal CP. When this value is 1 (true), the block selects extended CP. The block samples this port at the start of each subframe and ignores any changes within a subframe.

Dependencies

To enable this port, set the Cyclic prefix source parameter to Input port.

Data Types: Boolean

reset — Clears internal states

scalar

Clears internal states, specified as a Boolean scalar. When this value is 1 (true), the block stops the current calculation and clears all internal states. When this value is 0 (false) and the valid input value is 1 (true), the block begins a new subframe.

Dependencies

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

Data Types: Boolean

Output

data — Output data

scalar

Output data, returned as a signed real or complex number. The data type is the same as the data type of the data input port. When you clear the Divide butterfly outputs by two parameter, the word length increases by 1 bit per stage in inverse fast fourier transform (IFFT).

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

valid — Indicates valid output data

scalar

Control signal that indicates if the data from the data output port is valid. The block sets this value to 1 (true) when the modulated samples are available on the data output port.

Data Types: Boolean

ready — Indicates block is ready

scalar

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

Data Types: Boolean

Parameters

Main

NDLRB source — Source of NDLRB

Property (default) | Input port

You can set the NDLRB by selecting a parameter value or using an input port. To enable the NDLRB parameter, select Property. To enable the NDLRB port, select Input port.

NDLRB — Number of downlink resource blocks

6 (default) | 15 | 25 | 50 | 75 | 100

Number of downlink resource blocks, specified as 6, 15, 25, 50, 75, or 100. NDLRB must be one of these six values specified by LTE standard TS 36.212 [1].

Dependencies

To enable this parameter, set the NDLRB source parameter to Property.

Cyclic prefix source — Source of cyclic prefix

Property (default) | Input port

You can set the cyclic prefix by selecting a parameter value or using an input port. To enable the Cyclic prefix type parameter, select Property. To enable the cyclicPrefixType port, select Input port.

Cyclic prefix type — Type of cyclic prefix

Normal (default) | Extended

Type of cyclic prefix, specified as Normal or Extended.

Dependencies

To enable this parameter, set the Cyclic prefix source parameter to Property.

Windowing — Spectral growth reduction

off (default) | on

Select this parameter to perform a windowing operation that reduces spectral growth and uses the NDRLB window length specified by the Window length per NDLRB parameter. Clear this parameter to disable windowing operation.

Window length per NDLRB — NDRLB window length

[4, 6, 4, 6, 8, 8] (default) | row vector of NDLRB window lengths

NDRLB window length, specified as a row vector of nonnegative integers whose elements correspond to the window lengths for NDRLB 6, 15, 25, 50, 75, and 100 respectively. By default, the window lengths for NDRLB 6, 15, 25, 50, 75, and 100 are 4, 6, 4, 6, 8, and 8, respectively. The window length for each NDLRB can range from 0 to the minimum CP value.

• For normal CP, the minimum CP values for NDRLB 6, 15, 25, 50, 75, and 100 are 9, 18, 36, 72, 144, and 144, respectively.

• For extended CP, the minimum CP values for NDRLB 6, 15, 25, 50, 75, and 100 are 32, 64, 128, 256, 512, and 512, respectively.

Dependencies

To enable this parameter, select the Windowing parameter.

Enable reset input port — Reset signal

off (default) | on

Select this parameter to enable the reset port on the block icon.

Output data sample rate — Output sample rate

Use maximum output data sample rate (default) | Match output data sample rate to NDLRB

This parameter specifies the type of sample rate for the block to select for the output data.

• To provide an output data sample rate of 30.72 MHz, select Use maximum output data sample rate.

• To provide an output data sample rate based on the NDLRB parameter, select Match output data sample rate to NDLRB. The output sampling rates for NDLRB 6, 15, 25, 50, 75, and 100 are 1.92 MHz, 3.84 MHz, 7.68 MHz, 15.36 MHz, 30.72 MHz, and 30.72 MHz, respectively.

For more information, see Base Rate Controller.

IFFT Block Parameters

Divide butterfly outputs by two — Bit-width control

on (default) | off

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

Rounding Method — Rounding mode for internal fixed-point calculations

Floor (default) | Ceiling | Convergent | Nearest | Round | Zero

This parameter specifies the type of rounding mode for internal fixed-point calculations. Rounding applies to twiddle-factor multiplication and scaling operations. For more information about rounding modes, see Rounding Modes.

When the input is any integer or fixed-point data type, the IFFT algorithm uses fixed-point arithmetic for internal calculations. This parameter does not apply when the input data is of data type single or double.

Algorithms

The LTE OFDM Modulator block operation sequence is carried over using these blocks: OFDM Symbol Formation, IFFT, FFT Shift, CP Addition, Windowing, and Base Rate Controller. The OFDM Symbol Formation block maps the resource grid input to active subcarrier bins to form 2048 subcarriers. The IFFT block converts the frequency-domain signal to time-domain signal, and the FFT Shift block performs time-domain FFT shift. The CP Addition block adds CP-length samples from the end of the symbol to its prefix. The Windowing block performs windowing and overlapping of adjacent OFDM symbols of complex symbols in the resource array. The Base Rate Controller block defines the sample rate of the output data. The parameters shown in the following figure configure the behavior of the block.

OFDM Symbol Formation

An OFDM Symbol Formation subsystem calculates the number of active and inactive subcarriers and the number of CP samples. It generates a ready signal and subcarriers (active, inactive, and DC) for each OFDM symbol as per LTE standard.

The OFDM Symbol Formation subsystem calculates the number of active subcarriers based on the NDLRB value.

Number of active subcarriers = 12 x NDLRB

The number of inactive subcarriers is a difference of IFFT size and the number of active subcarriers. To save hardware resources by avoiding multiple IFFTs, the IFFT size is fixed to 2048.

Number of inactive subcarriers = 2048 — Number of active carriers

The block outputs a signal from ready port to indicate when the block is ready to accept input data. This signal depends on the valid input signal, NDLRB, CP type, and OFDM symbol number. The ready output signal is generated for one time step, and the valid input signal is checked for next time step. The block accepts the input data and the ready output signal remains high (1) until the OFDM symbol data is received. If the valid input is low (0), the ready signal extends until the valid input signal receives high (1). After receiving active subcarriers, the block sets the ready output signal to low (0) for a time period equal to the sum of the number of inactive subcarriers and the number of CP samples.

These figures show the timing diagrams of the ready output signal for NDLRB values 25 and 100, respectively.

IFFT

The IFFT block converts a frequency-domain signal to a time-domain signal. LTE supports six standard bandwidth options: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz. These bandwidth options require an FFT length of 128, 256, 512, 1024, and 2048, respectively. The block uses 2048 FFT length, which corresponds to the maximum bandwidth of LTE, that is 20-MHz. The FFT length of IFFT is configured to the highest FFT size to generate single hardware, which supports all LTE bandwidth options.

The Divide butterfly outputs by two parameter controls if 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 IFFT in the same amplitude range as its input. When the Divide butterfly outputs by two parameter is cleared, the block avoids overflow by increasing the word length by 1 bit after each butterfly multiplication.

FFT Shift

Conventionally, transceivers perform an FFT shift in the frequency domain. However, this method requires memory and introduces latency related to the size of the FFT. Instead, a transceiver can execute the same operation in the time domain using the frequency shifting property of Fourier transforms. Shifting a function in one domain corresponds to a multiplication by a complex exponential function in the other domain. To reduce hardware resources and latency, this block performs the FFT shift by multiplying the time-domain samples by a complex exponential function.

These equations describe an FFT shift. The equation for an N-point FFT is

$$X(k)=F[x(n)]={\displaystyle \sum _{n=0}^{N-1}x(n)e^{-\frac{j2\pi nk}{N}}}$$

For an FFT shift of N/2 carriers in either direction, substitute $$k=k-\frac{N}{2}$$, resulting in

$$X(k-\frac{N}{2})={\displaystyle \sum _{n=0}^{N-1}x(n)e^{-\frac{j2\pi n(k-\frac{N}{2})}{N}}}$$

This equation simplifies to

$$X(k-\frac{N}{2})={\displaystyle \sum _{n=0}^{N-1}{e}^{j\pi n}x(n)e^{-\frac{j2\pi nk}{N}}}$$

Since $${\displaystyle \sum _{n=0}^{N-1}x(n)e^{-\frac{j2\pi nk}{N}}}$$ is equivalent to $$F[x(n)]$$, and $$e^{j\pi }=-1$$, this equation simplifies to

$$X(k-\frac{N}{2})=F[{{\displaystyle (-1)}}^{n}x(n)]$$

The final equation shows that an FFT shift in the time domain simplifies to multiplication by (-1)ⁿ. Therefore, the block implements the FFT shift by multiplying the time-domain samples by either +1 or –1.

CP Addition

Cyclic prefix addition is a process of adding the last samples of an OFDM symbol as a prefix to each OFDM symbol. CP addition for an OFDM symbol with Nfft samples and CP samples N_CP is shown in this figure.

The LTE OFDM Modulator block uses FFT size of 2048 for all NDLRB resources to avoid multiple IFFTs. The block uses CP values corresponding to NDLRB 100.

The Cyclic prefix type parameter controls whether the block expects a normal or an extended CP. The block requires the input to maintain a sample rate of 30.72 MHz. It assumes that each symbol is 2048 samples plus the cyclic prefix size associated with the rate. When using a normal CP, the prefix of the first symbol in each slot has 160 samples, while each subsequent symbol contains a prefix of 144 samples. When using an extended CP, all symbols contain 512 samples. For more information about the cyclic prefix length (in samples) of each OFDM symbol in a subframe, see the lteOFDMModulate (LTE Toolbox) function.

Windowing

Windowing reduces the spectral regrowth, or adjacent channel leakage ratio (ACLR), of an OFDM signal. The feature is optional. To enable windowing, select the Windowing parameter.

For more information about windowing, see the lteOFDMModulate (LTE Toolbox) function.

Base Rate Controller

This block generates the output data at a sample rate of 30.72 MHz by using the maximum output data sample rate or output data sample rate with the specified NDLRB.

This figure shows the output data when you set the Output data sample rate parameter to Use maximum output data sample rate.

This figure shows the output data when you set the Output data sample rate parameter to Match output data sample rate to NDLRB parameter. For NDLRB value 25, the output sample rate is 7.68 MHz, and the block returns valid output data at every fourth cycle.

Performance

The performance of the synthesized HDL code varies with your target and synthesis options. The input data type used for generating HDL code is fixdt(1,16,14).

This table shows the resource and performance data synthesis results when using the block with default configuration. The generated HDL targeted to Xilinx^® Zynq^® XC7Z045I-FFG900-2L FPGA. The design achieves a clock frequency of 247 MHz.

Resource	Number Used
LUTs	8050
Registers	9682
DSPs	22
Block RAM	22
F7 Muxes	0
F8 Muxes	0
RAMB36/FIFO	12
RAMB18	20

References

[1] 3GPP TS 36.212. "Multiplexing and channel coding." 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA). URL: https://www.3gpp.org.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using Simulink® Coder™.

This block supports C/C++ code generation for Simulink^® accelerator and rapid accelerator modes and for DPI component generation.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Restrictions

You cannot generate HDL for this block inside a Resettable Synchronous Subsystem (HDL Coder).

Version History

Introduced in R2019a