One of the supposedly simple things that plagues me in communications theory is the idea of noise. We are all very comfortable talking about noise. We refer to noise, the noise floor and interference all the time quite glibly. Yet I must admit, I have always felt like I never truly understood the topic.
In this post I want to take a deeper look at noise: its definition, where it comes from, its characteristics and how to measure it in modern communications systems.
What is Noise?
Looking this up online, I found a wonderful definition (thanks Google!) that made some sense:
noise (noun) technical
Irregular fluctuations that accompany a transmitted electrical signal but are not part of it and tend to obscure it.
Where Does Noise Occur?
Noise exists in all forms of modern electronic and optical communications systems. Noise occurs as a result of the way electrons and photons behave in different physical mediums. The electrons that power our electronic devices and the photons that traverse fiber-optic communication systems exhibit different random behaviors as they . We define each of these phenomena as a different source of noise. In some situations a certain source of noise may be dominant due to a single physical phenomenon. In other situations noise could be dominated by another factor, or a mix of factors!
Noise vs Interference
I feel it is important to clearly differentiate between noise and interference as the terms are used separately in communications systems. For example consider Signal to Noise Ratio (SNR) vs Signal to Interference Noise Ratio (SINR). If you have ever wondered about the difference between the two terms, you may find some illumination below.
I have looked around for a good definition of the difference between noise and interference that I can quote, and I have found nothing that satisfies my need for a generalized but precise definition. I will therefore go ahead and say the following:
Interference is typically a deterministic signal (or sum of deterministic signals) that is transmitted on a specific set of frequencies that disrupts a communication signal on the same frequency. A good example of this would be multiple competing radios transmitting messages simultaneously on the same frequency in the same location. Another example of a source of interference would be a wide-band signal jammer designed to disrupt wireless communications. Interference typically comes from specific, external sources (i.e other transmitting devices) and only exists on specific frequencies. It can also be temporary, like intermittent interference caused by the duty cycle of a certain signal or transmitter.
Noise is the result of random processes that cause fluctuations in electronic signals and is produced by the physical operation of electronic/optical components and circuits. Noise can be modeled as a random process with a certain probability density function. A good example of this would be thermal noise or shot noise present in electronic equipment. Noise typically comes from inside radio/electronic equipment and you cannot move away from it and you cannot turn it off! Some forms of noise like thermal noise are completely inescapable, and cannot be reduced or removed.
Colors of Noise
One way of classifying noise signals is to look at the signal’s power as a function of frequency (called the power spectrum of the signal). Noise signals are assigned colors based on the power level as frequency increases.
Consider a noise signal that has constant power with respect to frequency. This means that the noise signal has an equal amount of power between 0 – 10 Hz, 10 – 20Hz, 100 – 110Hz, and 2010 – 2020Hz and so on. If you drew a graph of the power against frequency, the average power would be a flat line. This is termed white noise. Most forms of electronic noise can be modeled as white noise as they maintain a roughly constant power level through out the device’s band of operation.
Other Colors of Noise
Other colors of noise are defined by how the noise power changes as a function of frequency. For instance, pink noise loses power at a constant rate of 10dB / Decade of frequency. Pink Noise is actually constant on a logarithmic scale, i.e there is the same amount of power in 40-60Hz as there is between 400-600Hz and 4000-6000Hz.
Brown noise decreases faster, at a rate of -20dB/Decade. Blue noise on the other hand INCREASES with frequency at +10dB/Decade.
If you want to read about the other colors of noise and which physical phenomena they are found in, read the wikipedia article and check out their references. If you want to play with a noise generator and hear what different noise colors sound like, check out the web based noise generator at White Noise & Co.