# fixed.complexQuantizationNoiseStandardDeviation

Estimate standard deviation of quantization noise of complex-valued signal

## Syntax

``noiseStandardDeviation = fixed.complexQuantizationNoiseStandardDeviation(precisionBits)``

## Description

example

````noiseStandardDeviation = fixed.complexQuantizationNoiseStandardDeviation(precisionBits)` returns an estimate of the quantization noise standard deviation of a complex-valued signal with a quantization level q=2-precisionBits, where `precisionBits` is the required number of bits of precision.```

## Examples

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Quantizing a complex signal to $p$ bits of precision can be modeled as a linear system that adds normally distributed noise with a standard deviation of ${ϛ}_{\text{noise}}=\frac{{2}^{-p}}{\sqrt{6}}$ [1,2].

Compute the theoretical quantization noise standard deviation with $p$ bits of precision using the `fixed.complexQuantizationNoiseStandardDeviation` function.

```p = 14; theoreticalQuantizationNoiseStandardDeviation = fixed.complexQuantizationNoiseStandardDeviation(p);```

The returned value is ${ϛ}_{\text{noise}}=\frac{{2}^{-p}}{\sqrt{6}}$.

Create a complex signal with $n$ samples.

```rng('default'); n = 1e6; x = complex(rand(1,n),rand(1,n));```

Quantize the signal with $p$ bits of precision.

```wordLength = 16; x_quantized = quantizenumeric(x,1,wordLength,p);```

Compute the quantization noise by taking the difference between the quantized signal and the original signal.

`quantizationNoise = x_quantized - x;`

Compute the measured quantization noise standard deviation.

`measuredQuantizationNoiseStandardDeviation = std(quantizationNoise)`
```measuredQuantizationNoiseStandardDeviation = 2.4902e-05 ```

Compare the actual quantization noise standard deviation to the theoretical and see that they are close for large values of $n$.

`theoreticalQuantizationNoiseStandardDeviation`
```theoreticalQuantizationNoiseStandardDeviation = 2.4917e-05 ```

References

1. Bernard Widrow. “A Study of Rough Amplitude Quantization by Means of Nyquist Sampling Theory”. In: IRE Transactions on Circuit Theory 3.4 (Dec. 1956), pp. 266–276.

2. Bernard Widrow and István Kollár. Quantization Noise – Roundoff Error in Digital Computation, Signal Processing, Control, and Communications. Cambridge, UK: Cambridge University Press, 2008.

## Input Arguments

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Required number of bits of precision, specified as a positive integer-valued scalar.

Data Types: `double`

## Output Arguments

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Noise standard deviation, returned as a scalar.

## Tips

`fixed.complexQuantizationNoiseStandardDeviation` is used in these functions.

## Algorithms

The variance of a complex-valued error sequence e(k) with quantization level q=2-precisionBits  is

`${\sigma }_{q}^{2}=\frac{2}{q}{\int }_{-q/2}^{q/2}{e}^{2}de=\frac{{q}^{2}}{6}=\frac{{2}^{-2precisionBits}}{6}.$`

The standard deviation of a real error sequence e(k) is

`${\sigma }_{q}=\frac{{2}^{-precisionBits}}{\sqrt{6}}.$`

 Widrow, Bernard. "A Study of Rough Amplitude Quantization by Means of Nyquist Sampling Theory." IRE Transactions on Circuit Theory 3, no. 4 (December 1956): 266-276.

 Widrow, Bernard, and Kollár, István. Quantization Noise – Roundoff Error in Digital Computation, Signal Processing, Control, and Communications. Cambridge, UK: Cambridge University Press, 2008.