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Raised Cosine Receive Filter - Filter input signal, possibly downsampling, using raised cosine FIR filter

Library

Comm Filters

Description

The Raised Cosine Receive Filter block filters the input signal using a normal raised cosine FIR filter or a square root raised cosine FIR filter. It also downsamples the filtered signal if you set the Output mode parameter to Downsampling. The FIR Decimation block implements this functionality. The Raised Cosine Receive Filter block's icon shows the filter's impulse response.

Characteristics of the Filter

Characteristics of the raised cosine filter are the same as in the Raised Cosine Transmit Filter block, except that the length of the filter's input response has a slightly different expression: 2 * N * Group delay + 1, where N is the value of the Input samples per symbol parameter (not the Upsampling factor parameter, as in the case of the Raised Cosine Transmit Filter block).

If the Filter gain parameter is chosen to be User-specified, then the passband gain of the filter is:

for a normal filter.
for a square root filter.

Downsampling the Filtered Signal

To have the block downsample the filtered signal, set the Output mode parameter to Downsampling. By default, downsampling is on. If L is the Downsampling factor parameter value, then the block retains 1/L of the samples, choosing them as follows:

If the Sample offset parameter is zero, then the block selects the samples of the filtered signal indexed by 1, L+1, 2*L+1, 3*L+1, etc.
If the Sample offset parameter is a positive integer less than L, then the block initially discards that number of samples from the filtered signal and downsamples the remaining data as in the previous case.

To preserve the entire filtered signal and avoid downsampling, set Output mode to None. This setting is appropriate, for example, when the output from the filter block forms the input to a timing phase recovery block such as Squaring Timing Recovery. The timing phase recovery block performs the downsampling in that case.

Input and Output Signals

The input signal must be a scalar value or a frame-based column vector. For information about the data types each block port supports, see the Supported Data Type table on this page. You can select either Maintain input frame size or Maintain input frame rate for the Framing parameter.

If you set Output mode to None, then the input and output signals share the same sampling mode, sample time, and vector length.

If you set Output mode to Downsampling and Downsampling factor is L, then L and the input sampling mode determine characteristics of the output signal:

If the input is sample-based, then the output is sample-based and the output sample time is 1/L times the input sample time.
If the input is frame-based, then the output is a frame-based vector whose length is 1/L times the length of the input vector. The output frame period equals the input frame period.

Exporting Filter Coefficients to the MATLAB Workspace

To examine or manipulate the coefficients of the filter that this block designs, select Export filter coefficients to workspace. Then set the Coefficient variable name parameter to the name of a variable that you want the block to create in the MATLAB workspace. Running the simulation causes the block to create the variable, overwriting any previous contents in case the variable already exists.

Latency

For information pertaining to the latency of the block, see details in FIR Decimation.

Dialog Box

Filter type: The type of raised cosine filter: Square root or Normal.
Input samples per symbol: An integer greater than 1 representing the number of samples per symbol in the input signal.
Group delay: A positive integer that represents the number of symbol periods between the start of the filter response and its peak.
Rolloff factor: The rolloff factor for the filter, a real number between 0 and 1.
Framing: Selects the framing method. The framing method choices are: Maintain input frame size or Maintain input frame rate .
Output mode: Determines whether or not the block downsamples the signal after filtering. Choices are Downsampling and None.
Downsampling factor: The factor by which the block downsamples the signal after filtering. This field appears only if Output mode is set to Downsampling.
Sample offset: The number of filtered samples the block discards before downsampling. This field appears only if Output mode is set to Downsampling.
Filter gain: Determines how the block scales the filter coefficients. Choices are Normalized and User-specified.
Linear amplitude filter gain: A positive scalar used to scale the filter coefficients. This field appears only if Filter gain is set to User-specified.
Export filter coefficients to workspace: If you check this box, then the block creates a variable in the MATLAB workspace that contains the filter coefficients.
Coefficient variable name: The name of the variable to create in the MATLAB workspace. This field appears only if Export filter coefficients to workspace is selected.
Visualize filter with FVTool: If you click this button, then MATLAB launches the Filter Visualization Tool, fvtool, to analyze the raised cosine filter whenever you apply any changes to the block's parameters. If you launch fvtool for the filter, and subsequently change parameters in the mask, fvtool will not update. You will need to launch a new fvtool in order to see the new filter characteristics. Also note that if you have launched fvtool, then it will remain open even after the model is closed.

Rounding mode

Select the rounding mode for fixed-point operations. The block uses the Rounding mode when the result of a fixed-point calculation does not map exactly to a number representable by the data type and scaling storing the result. The filter coefficients do not obey this parameter; they always round to Nearest. For more information, see Rounding Modes in the Signal Processing Blockset User's Guide or Rounding Mode: Simplest in the Simulink Fixed Point documentation.

Overflow mode

Select the overflow mode for fixed-point operations. The filter coefficients do not obey this parameter; they are always saturated.

Coefficients

Choose how you specify the word length and the fraction length of the filter coefficients (numerator and/or denominator).

See the Coefficients section of the FIR Decimation help page and Filter Structure Diagrams in Signal Processing Blockset Reference Guide for illustrations depicting the use of the coefficient data types in this block:

See the Coefficients subsection of the Digital Filter help page for descriptions of parameter settings.

When you select Same word length as input, the word length of the filter coefficients match that of the input to the block. In this mode, the fraction length of the coefficients is automatically set to the binary-point only scaling that provides you with the best precision possible given the value and word length of the coefficients.
When you select Specify word length, you are able to enter the word length of the coefficients, in bits. In this mode, the fraction length of the coefficients is automatically set to the binary-point only scaling that provides you with the best precision possible given the value and word length of the coefficients.
When you select Binary point scaling, you are able to enter the word length and the fraction length of the coefficients, in bits. If applicable, you are able to enter separate fraction lengths for the numerator and denominator coefficients.
When you select Slope and bias scaling, you are able to enter the word length, in bits, and the slope of the coefficients. If applicable, you are able to enter separate slopes for the numerator and denominator coefficients. This block requires power-of-two slope and a bias of zero.
The filter coefficients do not obey the Rounding mode and the Overflow mode parameters; they are always saturated and rounded to Nearest.

Product output

Use this parameter to specify how you would like to designate the product output word and fraction lengths. See Filter Structure Diagrams and Multiplication Data Types in Signal Processing Blockset Reference Guide for illustrations depicting the use of the product output data type in this block:

When you select Same as input, these characteristics match those of the input to the block.
When you select Binary point scaling, you are able to enter the word length and the fraction length of the product output, in bits.
When you select Slope and bias scaling, you are able to enter the word length, in bits, and the slope of the product output. This block requires power-of-two slope and a bias of zero.

Accumulator

Use this parameter to specify how you would like to designate the accumulator word and fraction lengths. See Filter Structure Diagrams and Multiplication Data Types for illustrations depicting the use of the accumulator data type in this block:

When you select Same as input, these characteristics match those of the input to the block.
When you select Same as product output, these characteristics match those of the product output.
When you select Binary point scaling, you are able to enter the word length and the fraction length of the accumulator, in bits.
When you select Slope and bias scaling, you are able to enter the word length, in bits, and the slope of the accumulator. This block requires power-of-two slope and a bias of zero.

Output

Choose how you specify the output word length and fraction length:

When you select Same as input, these characteristics match those of the input to the block.
When you select Same as accumulator, these characteristics match those of the accumulator.
When you select Binary point scaling, you are able to enter the word length and the fraction length of the output, in bits.
When you select Slope and bias scaling, you are able to enter the word length, in bits, and the slope of the output. This block requires power-of-two slope and a bias of zero.

Lock scaling against changes by the autoscaling tool

Select this parameter to prevent any fixed-point scaling you specify in this block mask from being overridden by the autoscaling tool in the Fixed-Point Tool.

Supported Data Type

Port	Supported Data Types
In	Double-precision floating point Single-precision floating point Signed fixed-point
Out	Double-precision floating point Single-precision floating point Signed fixed-point

Port

Supported Data Types

Double-precision floating point
Single-precision floating point
Signed fixed-point

Out

Double-precision floating point
Single-precision floating point
Signed fixed-point

Pair Block

Raised Cosine Transmit Filter