# Wave Walker DSP

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Calculate Signal to Noise Ratio (SNR) in Python Simulation
July 1, 2024

#### Introduction

This blog performs a simple math derivation for SNR and then verifies the results with a Python simulation.

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#### Calculating SNR Mathematically

An example received signal x[n] is the addition of the signal s[n] and the noise w[n],

(1)

The SNR of the received signal x[n] is the ratio of the power for the signal to the power of the noise ,

(2)

where

(3)

(4)

The expectation operator operates as an average or mean. It is assumed that x[n] is N samples long beginning at time n=0, therefore the average signal power (3) can be written according to

(5)

Similarly, the average power of the noise (4) is

(6)

The SNR of x[n] (2) is therefore [Lyons2011, p.875]

(7)

which can be written in decibels as

(8)

#### Calculating SNR with Python

Python is used to generate two signals, a BPSK signal and real Gaussian noise. The BPSK signal is generated by the following Python code:

 import numpy as np numSamples = 4096 symbolMap = np.array([-1, 1]) mapIndex = np.random.randint(0,len(symbolMap),numSamples) symbols = symbolMap[mapIndex]

The average power of the symbols (5) is calculated and printed by:

 Ps = np.mean(np.abs(symbolMap)**2) print('signal power = ' + str(np.round(Ps,2)))

which results in:

 signal power = 1.0

The real Gaussian noise is generated by:

 noise = np.sqrt(0.01)*np.random.normal(0,1,numSamples)

The average noise power (6) is calculated and printed by:

 Pn = np.mean(np.abs(noise)**2) print('noise power = ' + str(np.round(Pn,2)))

which results in:

 noise power = 0.01

The SNR is the ratio of the signal power to the noise power, which is calculated and printed by:

 SNRdB = 10*np.log10(Ps/Pn) print('SNR (dB) = ' + str(np.round(SNRdB)))

which results in:

 SNR (dB) = 20.0

Figure 1 is an example of s[n], w[n] and x[n] in the time domain as generated by the example Python code.

#### Conclusion

This blog provided a simple mathematical backing for how to compute SNR mathematically and then provided Python code for how to generate an example BPSK and then compute it’s SNR.

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