| Communications Blockset™ | |
Comm Filters
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 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.
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.
The underlying implementation varies as follows:
If Sample offset is set to zero
offset or Framing is set to Maintain
input frame size, the block uses FIR
Decimation under the mask. This polyphase decimation improves
the filter efficiency in terms of simulation speed and quality of
code generation.
If Framing is set to Maintain input frame rate and Sample offset is set to a nonzero number, the block uses Digital filter under the mask when
The input signal must be a scalar value or a frame-based column vector. The block supports double, single, and fixed-point data types. 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.
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.
The type of raised cosine filter: Square root or Normal.
An integer greater than 1 representing the number of samples per symbol in the input signal.
A positive integer that represents the number of symbol periods between the start of the filter response and its peak.
The rolloff factor for the filter, a real number between 0 and 1.
Selects the framing method. The framing method choices are: Maintain input frame size or Maintain input frame rate .
Determines whether or not the block downsamples the signal after filtering. Choices are Downsampling and None.
The factor by which the block downsamples the signal after filtering. This field appears only if Output mode is set to Downsampling.
The number of filtered samples the block discards before downsampling. This field appears only if Output mode is set to Downsampling.
Determines how the block scales the filter coefficients. Choices are Normalized and User-specified.
A positive scalar used to scale the filter coefficients. This field appears only if Filter gain is set to User-specified.
If you check this box, then the block creates a variable in the MATLAB workspace that contains the filter coefficients.
The name of the variable to create in the MATLAB workspace. This field appears only if Export filter coefficients to workspace is selected.
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.
Select the rounding mode for fixed-point operations. The filter coefficients do not obey this parameter; they always round to Nearest.
Select the overflow mode for fixed-point operations. The filter coefficients do not obey this parameter; they are always saturated.
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.
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.
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.
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.
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.
Gaussian Filter, rcosine, rcosflt
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