What Does SNR Mean?
SNR is short for “Signal to Noise Ratio,” and can also be expressed as S/N ratio. It is the ratio of signal power to that of all other electrical signals in the area, known as the noise level. Noise is measured by the Root-Mean-Square (RMS) value of the fluctuations over time. This ratio is expressed in decibels (dB).
In a statistical sense, SNR can also be defined as what is equal to the mean divided by the standard deviation. However, for the scope of this article, we are going to focus on SNR as it relates to wireless communications.
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What is SNR? Why is SNR Important?
Providing a definition of SNR can get very technical. It is, essentially, a balance beam between your signal power and noise power. The higher the value on one end in relation to the value on the other, the greater or lower your overall SNR value will be.
In the most basic terms, SNR determines how usable your signal will be.
A higher SNR value means the signal is clearer. With a lower value, you start to introduce Gaussian noise into your network (expressed as static), and as the number becomes closer to 1, the worse the static gets. The noise interferes with the signal processing capabilities of your network, causing random noise and amplitude modulation. If the SNR value gets lower than one, the signal becomes unusable. This is called the “noise floor.”
Signals close to the noise floor can be subject to data corruption, which will result in retransmissions between the transmitter and receiver. This will degrade wireless throughput and latency as the retransmitted signals will take up airtime in the wireless environment. When SNR increases, the channel's data throughput also increases. This means that for a given signal level, an increase in noise will decrease the data throughput. The higher the noise level, the less space there is for the actual data that is being transmitted on the channel.
The noise floor varies depending on the background noise in the environment. One of the great challenges in providing cellular, radio, and WiFi is providing enough signal strength to rise above any other forms of interference. Raising the signal level will help in such situations.
What is a Good SNR Value?
Whether or not you have a good SNR value depends on what type of signal you’re working with. Generally speaking, you want as high an SNR value as possible.
Here’s a basic rundown:
- >40dB SNR = Excellent signal (5 bars). Lightning fast, always associated
- 25dB to 40dB SNR = Very good signal (3 - 4 bars). Very fast, always associated
- 15dB to 25dB SNR = Low signal (2 bars). Usually fast, always associated
- 10dB - 15dB SNR = Very low signal (1 bar). Mostly slow, usually associated
- 5dB to 10dB SNR = No signal, almost never associated, agonizingly slow
These values were provided by Wireless-Nets.
However, different networks will require different SNR levels to really function. For example, data networks don’t require as high a ratio as voice, due to its speed being less critical to its function. A data network with 20 dB of SNR will still function relatively quickly. However, a voice-over network such as cellular or Voice-Over LTE will require a higher SNR value to provide reliably clear voice and high signal quality.
How is SNR Calculated?
SNR calculations can be either simple or complex. If your SNR measurements are already in decibel form, then you can subtract the noise quantity from the desired signal: SNR = S - N. This is because when you subtract logarithms, it is the equivalent of dividing normal numbers.
The SNR is equal to the difference in the numbers. For example, you measure a radio signal with a strength of -10 dB and a noise signal of -50 dB. -10 - (-50) = 40 dB, 40 dB being the SNR. Quite a good signal-to-noise ratio!
For more complex calculations, divide the value of the desired signal by the amount of the noise. Then, take the common logarithm of the result, i.e., log (S ÷ N). After this, if the signal strength measurements are in watts (power), multiply by 20. However, if they are units of voltage, then you will multiply by 10.
For power, SNR = 20 log (S ÷ N) and for voltage, SNR = 10 log (S ÷ N). Also, the resulting calculation is the SNR in decibels. For example, your measured noise value (N) is 2 microvolts, and your signal (S) is 300 millivolts. The SNR is 10 log (.3 ÷ .000002) or approximately 62 dB. This is expressed mathematically as a fourier transform.
Got all that? It can get even more complicated when you try and factor in SINAD to the mix, but we’ll stop there, as this is already getting far more complicated than you were likely hoping. Your chances of winning drastically go down.
What Causes Low Signal to Noise Ratio?
All real measurements are disturbed by noise. The level of background noise in an environment depends on numerous factors, including:
- Electronic systems
- Power spectral density
- Phenomenon such as wind, rain, snow, temperature, humidity, gravitational attraction of the moon, etc.
- Sound and radio wave density
- Magnetic fields
The most common situations where low signal to noise ratio shows up is combination of weak input signal (or signal in your environment) combined with abnormally strong electrical currents nearby, such as transmission lines, generators, power plants, etc. Numerous smaller sources of noise in a small area, such as computers or cellular phones, can also raise the noise floor.
How to Improve Your SNR
There are two obvious ways to improve your SNR:
- Improve the signal power
- Reduce the background noise in your area
These are only somewhat in your control. For instance, if you’re right next to a major transmission line, it will be very hard to shut that down and reduce your noise floor. But if you have a massive server hub, shutting down a few unneeded servers might go a long way into making your wireless signal clearer.
For cellular signal, there is a way to increase your signal power. They’re called cell phone signal boosters, and they dramatically increase signal strength inside a building.
How We Can Help
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