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rcosdesign

Raised cosine FIR pulse-shaping filter design

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Syntax

b = rcosdesign(beta,span,sps)
b = rcosdesign(beta,span,sps,shape)

Description

b = rcosdesign(beta,span,sps) returns the coefficients b that correspond to a square-root raised cosine FIR filter with rolloff factor specified by beta. The filter is truncated to span symbols, and each symbol period contains sps samples. The order of the filter, sps*span, must be even. The filter energy is 1.

example

b = rcosdesign(beta,span,sps,shape) returns a square-root raised cosine filter when you set shape to "sqrt" and a normal raised cosine FIR filter when you set shape to "normal".

example

Examples

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Open Live Script

Specify a rolloff factor of 0.25. Truncate the filter to 6 symbols and represent each symbol with 4 samples. Verify that "sqrt" is the default value of the shape parameter.

h = rcosdesign(0.25,6,4);
mx = max(abs(h-rcosdesign(0.25,6,4,"sqrt")))
mx = 
0

impz(h)

Figure contains an axes object. The axes object with title Impulse Response, xlabel n (samples), ylabel Amplitude contains an object of type stem.

Open Live Script

Compare a normal raised cosine filter with a square-root cosine filter. An ideal (infinite-length) normal raised cosine pulse-shaping filter is equivalent to two ideal square-root raised cosine filters in cascade. Thus, the impulse response of an FIR normal filter should resemble that of a square-root filter convolved with itself.

Create a normal raised cosine filter with rolloff 0.25. Specify that this filter span 4 symbols with 3 samples per symbol.

rf = 0.25;
span = 4;
sps = 3;

h1 = rcosdesign(rf,span,sps,"normal");
impz(h1)

Figure contains an axes object. The axes object with title Impulse Response, xlabel n (samples), ylabel Amplitude contains an object of type stem.

The normal filter has zero crossings at integer multiples of sps. It thus satisfies Nyquist's criterion for zero intersymbol interference. The square-root filter, however, does not:

h2 = rcosdesign(rf,span,sps,"sqrt");
impz(h2)

Figure contains an axes object. The axes object with title Impulse Response, xlabel n (samples), ylabel Amplitude contains an object of type stem.

Convolve the square-root filter with itself. Truncate the impulse response outward from the maximum so it has the same length as h1. Normalize the response using the maximum. Compare the convolved square-root filter to the normal filter.

h3 = conv(h2,h2,"same");

stem(0:span*sps,[h1/max(abs(h1));h3/max(abs(h3))]',"filled")
xlabel("Samples")
ylabel("Normalized Amplitude")
legend("h1","h2 * h2")

Figure contains an axes object. The axes object with xlabel Samples, ylabel Normalized Amplitude contains 2 objects of type stem. These objects represent h1, h2 * h2.

The convolved response does not coincide with the normal filter because of its finite length. Increase span to obtain closer agreement between the responses and better compliance with the Nyquist criterion.

Open Live Script

This example shows how to pass a signal through a square-root, raised cosine filter.

Specify the filter parameters.

rolloff = 0.25;     % Rolloff factor
span = 6;           % Filter span in symbols
sps = 4;            % Samples per symbol

Generate the square-root, raised cosine filter coefficients.

b = rcosdesign(rolloff, span, sps);

Create a vector of bipolar data. 

d = 2*randi([0 1], 100, 1) - 1;

Upsample and filter the data for pulse shaping.

x = upfirdn(d, b, sps);

Add noise.

r = x + randn(size(x))*0.01;

Filter and downsample the received signal for matched filtering. 

y = upfirdn(r, b, 1, sps);

For information on how to use square-root, raised cosine filters to interpolate and decimate signals, see Interpolate and Decimate Using RRC Filter (Communications Toolbox).

Input Arguments

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Rolloff factor, specified as a real nonnegative scalar not greater than 1. The rolloff factor determines the excess bandwidth of the filter. Zero rolloff corresponds to a brick-wall filter and unit rolloff to a pure raised cosine.

Data Types: double | single

Number of symbols, specified as a positive integer scalar.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Number of samples per symbol (oversampling factor), specified as a positive integer scalar.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Shape of the raised cosine window, specified as either "normal" or "sqrt".

Output Arguments

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Raised cosine filter coefficients, returned as a row vector.

Data Types: double

Tips

  • If you have a license for Communications Toolbox™ software, you can perform multirate raised cosine filtering with streaming behavior. To do so, use the System object™ filters, comm.RaisedCosineTransmitFilter and comm.RaisedCosineReceiveFilter.

References

[1] Tranter, William H., K. Sam Shanmugan, Theodore S. Rappaport, and Kurt L. Kosbar. Principles of Communication Systems Simulation with Wireless Applications. Upper Saddle River, NJ: Prentice Hall, 2004.

Extended Capabilities

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

Version History

Introduced in R2013b

See Also

Topics