Alpha / BT (Custom IQ)
non-Gaussian
filters: .05 to 1
Gaussian filters: .05 to 100
Minimum increment: .01
determines the filter characteristics of the filters used by the VSA's custom IQ demodulator.
If one of the VSA's Nyquist (cosine) or Gaussian filters is selected, the filter's characteristics must be specified. For Nyquist filters, the "alpha" of the filter must be specified; for Gaussian filters, the Bandwidth Time Product (BT Bandwidth Time Product) of the filter must be specified.
The value that is specified for is used by both the measured filter and the reference filter. For example, if a value of 0.35 is entered, the VSA sets the alpha or BT of the measured and reference filters to 0.35.
Your communications system should specify the "alpha" or "BT" for filters in the system. Set the value of to match that of your communications system.
Because the filter alpha/BT can easily be changed with a minimum amount of delay, the filters can be manually adjusted to achieve a minimum EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. (Error Vector Magnitude).
Alpha
Alpha describes the shape of a Nyquist (cosine) filter. Alpha is also called the roll-off or the excess bandwidth factor. A higher value for alpha increases the bandwidth that is used in excess of the theoretical minimum.
Modulation theory states that the minimum bandwidth needed to transmit a signal is equal to half of the symbol rate. However, to realize this system would require a perfect brick-wall filter. This filter is equivalent to an alpha of 0. This would give the best possible spectral efficiency. Unfortunately, this filter is unrealizable.
In practice, radios typically use a filter alpha of about 0.3. This means that they take up 30% more bandwidth than the theoretical minimum.
BT
BT (Bandwidth Time Product) describes the shape of a Gaussian filter. BT indicates the filter roll-off (or excess bandwidth) of the Gaussian filter.
See Also
