Communications Blockset™

Raised Cosine Transmit Filter

Upsample and filter input signal using raised cosine FIR filter

Library

Comm Filters

Description

The Raised Cosine Transmit Filter block upsamples and filters the input signal using a normal raised cosine FIR filter or a square root raised cosine FIR filter. The block's icon shows the filter's impulse response.

Characteristics of the Filter

The Filter type parameter determines which type of filter the block uses; choices are Normal and Square root.

The impulse response of a normal raised cosine filter with rolloff factor R and symbol period T is

The impulse response of a square root raised cosine filter with rolloff factor R is

The impulse response of a square root raised cosine filter convolved with itself is approximately equal to the impulse response of a normal raised cosine filter.

The Group delay parameter is the number of symbol periods between the start of the filter's response and the peak of the filter's response. The group delay and the upsampling factor, N, determine the length of the filter's impulse response, which is 2 * N * Group delay + 1.

The Rolloff factor parameter is the filter's rolloff factor. It must be a real number between 0 and 1. The rolloff factor determines the excess bandwidth of the filter. For example, a rolloff factor of .5 means that the bandwidth of the filter is 1.5 times the input sampling frequency.

The Filter gain parameter indicates how the block normalizes the filter coefficients. If you choose Normalized, then the block uses an automatic scaling:

If Filter type is Normal, then the block normalizes the filter coefficients so that the peak coefficient equals 1.
If Filter type is Square root, then the block normalizes the filter coefficients so that the convolution of the filter with itself produces a normal raised cosine filter whose peak coefficient equals 1.

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.

Input and Output Signals

The input signal must be a scalar or a frame-based column vector. double, single, and fixed-point data types are supported. Set the Input sampling mode parameter according to whether the input is sample-based or frame-based.

The input sampling mode and N, the value of the Upsampling factor parameter, determine characteristics of the output signal:

If the input is a sample-based scalar, then the output is a sample-based scalar and the output sample time is N times the input sample time.
If the input is frame-based, then the output is a frame-based vector whose length is N 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.

Dialog Box

Filter type: The type of raised cosine filter: Square root or Normal.
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.
Input sampling mode: The type of input signal: Frame-based or Sample-based.
Upsampling factor: An integer greater than 1 representing the number of samples per symbol in the filtered output signal.
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 filter coefficients do not obey this parameter; they always round to Nearest.

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 Filter Structure Diagrams in Signal Processing Blockset™ Reference Guide for illustrations depicting the use of the coefficient data types in this block:

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.

Pair Block

Raised Cosine Receive Filter