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# OFDM Modulator Baseband

Modulate using orthogonal frequency division modulation

## Library

OFDM, in Digital Baseband sublibrary of Modulation

## Description

The OFDM Modulator Baseband block applies OFDM modulation to an incoming data signal. The block accepts one or two inputs depending on the state of the Pilot input port.

### Signal Dimensions

Pilot Input PortSignal InputPilot InputSignal Output
falseNdata-by-Nsym-by-NtN/ANCPTotal+NFFT×Nsym-by-Nt
trueNpilot-by-Nsym-by-Nt

where

• Ndata represents the number of data subcarriers. For further information on how Ndata is determined, see the `comm.OFDMModulator.info` reference page.

• Nsym represents the number of symbols determined by Number of OFDM symbols.

• Nt represents the number of transmit antennas determined by Number of transmit antennas.

• Npilot represents the number of pilot symbols determined by the first dimension size in the Pilot subcarrier indices array.

• NCP represents the cyclic prefix length as determined by Cyclic prefix length.

• NCPTotal represents the cyclic prefix length over all the symbols. When NCP is a scalar, NCPTotal = NCP × Nsym. When NCP is a row vector, NCPTotal = ∑ NCP.

• NFFT represents the number of subcarriers as determined by FFT length.

## Parameters

FFT Length

Specify the FFT length, which is equivalent to the number of subcarriers. The length of the FFT, NFFT, must be greater than or equal to 8.

Number of guard bands

Assign the number of subcarriers to the left and right guard bands. The input is a 2-by-1 vector. The number of subcarriers must fall within [0,NFFT/2 − 1].

Insert DC null

Select to insert a null on the DC subcarrier.

Pilot input port

Select to allow the specifying of pilot subcarrier indices.

Pilot subcarrier indices

Specify the pilot subcarrier indices. This field is available only when the Pilot input port check box is selected. You can assign the indices to the same or different subcarriers for each symbol. Similarly, the pilot carrier indices can differ across multiple transmit antennas. Depending on the desired level of control for index assignments, the dimensions of the indices array vary. Valid pilot indices fall in the range

`$\left[{\text{N}}_{\text{leftG}}+1,\text{\hspace{0.17em}}{\text{N}}_{\text{FFT}}/2\right]\cup \left[{\text{N}}_{\text{FFT}}/2+2,\text{\hspace{0.17em}}{\text{N}}_{\text{FFT}}-{\text{N}}_{\text{rightG}}\right],$`

where the index value cannot exceed the number of subcarriers. When the pilot indices are the same for every symbol and transmit antenna, the property has dimensions Npilot-by-1. When the pilot indices vary across symbols, the property has dimensions of Npilot-by-Nsym. If there is only one symbol but multiple transmit antennas, the property has dimensions of Npilot-by-1-by-Nt. If the indices vary across the number of symbols and transmit antennas, the property will have dimensions of Npilot-by-Nsym-by-Nt. If the number of transmit antennas is greater than one, ensure that the indices per symbol are mutually distinct across antennas to minimize interference. The default value is ```[12; 26; 40; 54]```.

Cyclic prefix length

Specify the length of the cyclic prefix. If you specify a scalar, the prefix length is the same for all symbols through all antennas. If you specify a row vector of length Nsym, the prefix length can vary across symbols but remains the same through all antennas.

Apply raised cosine windowing between OFDM symbols

Select to apply raised cosine windowing. Windowing is the process in which the OFDM symbol is multiplied by a raised cosine window before transmission to reduce the power of out-of-band subcarriers, which serves to reduce spectral regrowth.

Window length

Set the length of the raised cosine window. The field is available only when Apply raised cosine windowing between OFDM symbols is selected. Use positive integers having a maximum value no greater than the minimum cyclic prefix length. For example, in a configuration in which there are four symbols with cyclic prefix lengths of ```[12 16 14 18]```, the window length cannot exceed 12.

Number of OFDM symbols

Specify the number of OFDM symbols in the time-frequency grid.

Number of transmit antennas

Specify the number of transmit antennas, Nt, as a positive integer such that Nt ≤ 64.

Simulate using

Select the simulation type from these choices:

• `Code generation`

• `Interpreted execution`

## Algorithms

This block implements the algorithm, inputs, and outputs described in the `OFDM Modulator` System object reference page. The object properties correspond to the block parameters.

## Supported Data Types

PortSupported Data Types

Input

• Double-precision floating point

Pilot (optional)

• Double-precision floating point

Output

• Double-precision floating point

## Pair Block

OFDM Demodulator Baseband

## References

[1] Dahlman, E., S. Parkvall, and J. Skold. 4G LTE/LTE-Advanced for Mobile Broadband.London: Elsevier Ltd., 2011.

[2] Andrews, J. G., A. Ghosh, and R. Muhamed. Fundamentals of WiMAX.Upper Saddle River, NJ: Prentice Hall, 2007.