OFDM Modulator Baseband

Modulate using orthogonal frequency division modulation

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Description

The OFDM Modulator Baseband block applies Orthogonal Frequency Division Multiplexing modulation to an incoming data signal. The output is a baseband representation of the OFDM modulated signal.

Ports

Input

In — Input signal

3-D array

Input signal, specified as a 3D vector. The block accepts one or two inputs depending on the state of the Pilot input port. The input signal dimensions are :

Pilot Input Port	Signal Input	Pilot Input
off	N data -by-N sym -by-N t	N/A
on		N pilot -by-N sym -by-N t

where

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

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

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

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

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

• N_CPTotal represents the cyclic prefix length over all the symbols. When N_CP is a scalar, N_CPTotal = N_CP × N_sym. When N_CP is a row vector, N_CPTotal = ∑ N_CP.

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

Data Types: double
Complex Number Support: Yes

Output

Out — Baseband modulated signal

3-D array

Baseband modulated signal, returned as a3D array. The datatype of the output follows the input datatype. The output signal has dimension N _CPTotal +N _FFT ×N _sym -by-N _t.

Parameters

FFT Length — Number of DFT points

positive integer

Number of DFT points, specified as a positive integer. The length of the FFT, N_FFT, must be greater than or equal to 8 and is equivalent to the number of subcarriers.

Number of guard bands — Number of subcarriers to the left and right guard bands

[6;5] (default) | 2-by-1 integer-valued vector

Number of subcarriers allocated to the left and right guard bands, specified as a 2-by-1 integer-valued vector. The number of subcarriers must fall within [0,⌊N_FFT/2⌋ − 1] where you specify the left, N_leftG, and right, N_rightG, guard bands independently in a 2-by-1 column vector.

Insert DC null — Option to insert DC null

off (default) | on

Select this parameter to insert a null on the DC subcarrier.

Pilot input port — Option to specify pilot input port

off (default) | on

Select this parameter to allow the specifying of pilot input port.

Pilot subcarrier indices — Pilot subcarrier indices

[12; 26; 40; 54] (default) | column vector

Pilot subcarrier indices, specified as a column vector. 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

$$\left[N_{\text{leftG}}+1,N_{\text{FFT}}/2\right]\cup \left[N_{\text{FFT}}/2+2,N_{\text{FFT}}-N_{\text{rightG}}\right],$$

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 N_pilot-by-1. When the pilot indices vary across symbols, the property has dimensions of N_pilot-by-N_sym. If there is only one symbol but multiple transmit antennas, the property has dimensions of N_pilot-by-1-by-N_t. If the indices vary across the number of symbols and transmit antennas, the property will have dimensions of N_pilot-by-N_sym-by-N_t. 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 — Length of cyclic prefix

16 (default) | positive scalar | positive vector

Length of cyclic prefix, specified as a positive integer. 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 N_sym, the prefix length can vary across symbols but remains the same through all antennas.

Apply raised cosine windowing between OFDM symbols — Option to apply raised cosine window between OFDM symbols

off (default) | on

Select this parameter to apply raised cosine windowing between OFDM symbols. 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 — Length of raised cosine window

1 (default) | positive scalar

Length of raised cosine window, specified as a positive scalar. This 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 — Number of OFDM symbols

1 (default) | positive scalar

Number of OFDM symbols in the time-frequency grid, specified as a positive scalar.

Number of transmit antennas — Number of transmit antennas

positive scalar

Number of transmit antennas, specified as a real positive scalar. Specify the number of transmit antennas, N_t, as a positive integer such that N_t ≤ 64.

Simulate using — Type of simulation to run

Code generation (default) | Interpreted execution

Type of simulation to run, specified as Code generation or Interpreted execution.

• Code generation –– Simulate the model by using generated C code. The first time you run a simulation, Simulink^® generates C code for the block. The C code is reused for subsequent simulations unless the model changes. This option requires additional startup time, but the speed of the subsequent simulations is faster than Interpreted execution.

• Interpreted execution –– Simulate the model by using the MATLAB^® interpreter. This option requires less startup time than the Code generation method, but the speed of subsequent simulations is slower. In this mode, you can debug the source code of the block.

Block Characteristics

Data Types	double
Multidimensional Signals	yes
Variable-Size Signals	no

More About

Orthogonal Frequency Division Modulation

Orthogonal frequency division modulation (OFDM) divides a high-rate transmit data stream into N lower-rate streams, each of which has a symbol duration larger than the channel delay spread and, therefore, mitigates intersymbol interference (ISI). The individual substreams are sent over N parallel subchannels which are orthogonal to each other. Through the use of an inverse fast Fourier transform (IFFT), OFDM can be transmitted using a single radio. The output is a baseband representation of the modulated signal:

$$v(t)={\displaystyle \sum _{k=0}^{N-1}{X}_k{e}^{j2\pi k\Delta ft}},0\le t\le T,$$

where {X_k} are data symbols, N is the number of subcarriers, and T is the OFDM symbol time. The subcarrier spacing of Δf = 1/T makes the symbols orthogonal over each symbol period. This orthogonality is expressed as:

$$\frac{1}{T}{\displaystyle {\int }_0^T{\left(e^{j2\pi m\Delta ft}\right)}^{*}\left(e^{j2\pi n\Delta ft}\right)}dt=\frac{1}{T}{\displaystyle {\int }_0^T{e}^{j2\pi (m-n)\Delta ft}}dt=0\text{for}m\ne n.$$

The data symbols, X_k, are usually complex and can be from any modulation alphabet such as QPSK, 16-QAM, or 64-QAM.

This figure shows an OFDM modulator consisting of a bank of N complex modulators, where each modulator corresponds to one OFDM subcarrier.

Guard Bands and Intervals

OFDM has three types of subcarriers: data, pilot, and null. Data subcarriers are used for transmitting data, and pilot subcarriers are used for channel estimation. Null subcarriers, which provide a DC null and provide buffers between OFDM resource blocks, are not transmitted. These buffers are referred to as guard bands whose purpose is to prevent intersymbol interference. The allocation of nulls and guard bands vary depending on the applicable standard such as 802.11n differs from LTE.

Analogous to the concept of guard bands, the OFDM also supports guard intervals, which to provide temporal separation between OFDM symbols so that the signal does not lose orthogonality due to time-dispersive channels. As long as the guard interval is longer than the delay spread, symbols do not interfere with other symbols. Guard intervals are created by using cyclic prefixes in which the last part of an OFDM symbol is copied and inserted as the first part of the OFDM symbol. The benefit of cyclic prefix insertion is maintained as long as the span of the time dispersion does not exceed the duration of the cyclic prefix. Using a cyclic prefix increases overhead.

Raised Cosine Windowing

While the cyclic prefix creates a guard period in time domain to preserve orthogonality, an OFDM symbol rarely begins with the same amplitude and phase exhibited at the end of the prior OFDM symbol causing spectral regrowth and therefore, spreading of signal bandwidth due to intermodulation distortion. To limit this spectral regrowth, it is desired to create a smooth transition between the last sample of a symbol and the first sample of the next symbol. This can be done by using a cyclic suffix and raised cosine windowing.

To create the cyclic suffix, the first N_WIN samples of a given symbol are appended to the end of that symbol. However, in order to comply with the 802.11g standard, for example, the length of a symbol cannot be arbitrarily lengthened. Instead, the cyclic suffix must overlap in time and is effectively summed with the cyclic prefix of the following symbol. This overlapped segment is where windowing is applied. Two windows are applied, one of which is the mathematical inverse of the other. The first raised cosine window is applied to the cyclic suffix of symbol k and decreases from 1 to 0 over its duration. The second raised cosine window is applied to the cyclic prefix of symbol k+1 and increases from 0 to 1 over its duration. This process provides a smooth transition from one symbol to the next.

The raised cosine window, w(t), in the time domain can be expressed as:

$$w(t)=\{\begin{array}{l}1,0\le \left|t\right|<\frac{T-T_W}{2}\\ \frac{1}{2}\left\{1+\cos \left[\frac{\pi }{T_W}\left(\left|t\right|-\frac{T-T_W}{2}\right)\right]\right\},\frac{T-T_W}{2}\le \left|t\right|\le \frac{T+T_W}{2}\\ 0,\text{otherwise}\end{array}$$

where:

• T is the OFDM symbol duration including the guard interval.

• T_W is the duration of the window.

Adjust the length of the cyclic suffix via the window length setting property, with suffix lengths set between 1 and the minimum cyclic prefix length. While windowing improves spectral regrowth, it does so at the expense of multipath fading immunity. This occurs because redundancy in the guard band is reduced because the guard band sample values are compromised by the smoothing.

The following figures display the application of raised cosine windowing.

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.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.