sdr.raised_cosine(alpha: float, span: int, sps: int) → ndarray

Returns a raised cosine (RC) pulse shape.

Parameters:
alpha: float

The excess bandwidth \(0 \le \alpha \le 1\) of the filter.

span: int

The length of the filter in symbols. The length of the filter is span * sps + 1 samples. The filter order span * sps must be even.

sps: int

The number of samples per symbol.

Returns:

The raised cosine pulse shape with unit energy.

References

  • Michael Rice, Digital Communications: A Discrete Time Approach, Appendix A.

Examples

The excess bandwidth \(\alpha\) controls bandwidth of the filter. A smaller \(\alpha\) results in a narrower bandwidth at the expense of higher sidelobes.

In [1]: h_0p1 = sdr.raised_cosine(0.1, 8, 10); \
   ...: h_0p5 = sdr.raised_cosine(0.5, 8, 10); \
   ...: h_0p9 = sdr.raised_cosine(0.9, 8, 10)
   ...: 

In [2]: plt.figure(figsize=(8, 4)); \
   ...: sdr.plot.impulse_response(h_0p1, label=r"$\alpha = 0.1$"); \
   ...: sdr.plot.impulse_response(h_0p5, label=r"$\alpha = 0.5$"); \
   ...: sdr.plot.impulse_response(h_0p9, label=r"$\alpha = 0.9$")
   ...: 

In [3]: plt.figure(figsize=(8, 4)); \
   ...: sdr.plot.magnitude_response(h_0p1, label=r"$\alpha = 0.1$"); \
   ...: sdr.plot.magnitude_response(h_0p5, label=r"$\alpha = 0.5$"); \
   ...: sdr.plot.magnitude_response(h_0p9, label=r"$\alpha = 0.9$")
   ...:
../../_images/sdr_raised_cosine_1.png ../../_images/sdr_raised_cosine_2.png

The span of the filter affects the stopband attenuation. A longer span results in greater stopband attenuation and lower sidelobes.

In [4]: h_4 = sdr.raised_cosine(0.1, 4, 10); \
   ...: h_8 = sdr.raised_cosine(0.1, 8, 10); \
   ...: h_16 = sdr.raised_cosine(0.1, 16, 10)
   ...: 

In [5]: plt.figure(figsize=(8, 4)); \
   ...: sdr.plot.impulse_response(h_4, label="span = 4"); \
   ...: sdr.plot.impulse_response(h_8, label="span = 8"); \
   ...: sdr.plot.impulse_response(h_16, label="span = 16")
   ...: 

In [6]: plt.figure(figsize=(8, 4)); \
   ...: sdr.plot.magnitude_response(h_4, label="span = 4"); \
   ...: sdr.plot.magnitude_response(h_8, label="span = 8"); \
   ...: sdr.plot.magnitude_response(h_16, label="span = 16")
   ...:
../../_images/sdr_raised_cosine_3.png ../../_images/sdr_raised_cosine_4.png

See the Pulse shapes example.