When exploring options for enhancing cellular signal, you may come across the terms “dB” and “dBm” used in reference to the power of signal amplifier systems. Both terms are utilized to indicate the strength of cellular signals. But what is the significance behind these terms?

Here’s a closer look at decibels (dB) and decibel milliwatts (dBm) and how they measure cellular signal strength.

What Is a dB?

The abbreviation “dB” stands for decibels, which quantifies the increase or decrease in signal.

Decibels are useful in measuring the level of signal strength generated by an amplifier system and predicting how much signal will be produced upon installation. The signal strength generated by an amplifier can be determined by adding the amount of gain (expressed in dB) from the amplifier to the signal strength of a location.

The resulting value will indicate the indoor cellular strength attainable after installing an amplifier. Please see below.

What is dBm?

The acronym “dBm” stands for decibel milliwatts, which measures the power output of an antenna or amplifier, or the intensity of signal present at a location.

This measurement is relative to milliwatts and is commonly utilized to gauge the strength of cell phone signals at a particular site.

A strong signal is one of around -70dBm or higher

A weak signal is around -100dBm or lower

Note that dBm and dB are measures of entirely different things. There’s an often repeated ‘formula’ that dBm = dB+30, so 0dB is 30dBm. This is incredibly oversimplified and incorrect.

It should be noted that dB is a measure of relative power and cannot be converted into watts. You can use dB as a tool to measure the difference between two dBm figures. The two dBm that are being compared can be converted into watts, but dB cannot be.

Understanding the meaning of the two measurements and what they are used for is essential if you wish to understand how cellular signal boosters work.

Higher Means Stronger: The Power of Logarithmic Scales

It’s easy to underestimate how strong a signal is when you’re looking at a decibel reading.

Both dB and dBm are logarithmic, not linear. This means the way most of us think about magnitude doesn’t apply.

An increase from -80 dB to -77dB may not sound like a lot since it’s an increase of just 2.4% on a linear scale, but it’s a much bigger difference on the dB scale.

Logarithmic scales are used to make it easier to represent very large numbers on a convenient scale. In the case of dB, each increase of three dB means that the signal strength has doubled, as shown in this chart below

Signal Strength and Your Cell Phone

Most cell phone users judge the strength of their phone’s signal according to the number of bars they see. But unlike a dB measurement of signal strength, these bars don’t tell you much.

Unfortunately, there’s no fixed standard for what ‘four bars of signal’ means among mobile phone manufacturers. One person could have their phone showing one bar on the signal strength reading and still be able to make calls and receive text messages. Another person could see two bars and have their SMS messages stuck on “sending” for several minutes.

Instead, users can get an accurate idea of the strength of a cellular signal by unlocking the diagnostic tools found on their phones. This guide to putting your phone into test mode works for most iOS and Android devices, allowing you to access tools that show the signal’s strength in dB, which is something that integrators can work with more easily.

As was previously mentioned, the dB scale is logarithmic, so an increase of 3dB means the power has doubled. An increase of 6dB is enough to approximately double the signal strength of a cell tower or antenna.

3G vs 4G Signal Strengths

For the most part, desirable signal strengths for 3G and 4G are similar, but they are not identical. This is because 3G and 4G operate on different wavelengths.

Signal strength of -70 dBm is considered excellent for 3G, whereas a reading of -90 dBm would be considered excellent for 4G.

For 3G service, -110 dBm is considered a ‘dead zone,’ but represents merely a ‘poor signal’ for 4G.

Recommended signal strengths for 5G will be different again. Unfortunately, while the maximum theoretical speeds for 5G are higher, the technology is more susceptible to interference and signal degradation, so some people may find they experience very poor 5G signals while in office buildings or built-up areas.

What is the Required Signal Strength?

In general, your signal strength needs to be around -100 dBm to hold a conversation over a cellular connection.

Signal strength of -100 dBm is also required to transfer data over 4G while maintaining decent speeds. The connection is unlikely to cut out entirely if the strength deteriorates, but speeds would noticeably slow down at -105 dBm.

The ‘dead zone’ for 4G is -120 dBm, but the connection would have to deteriorate to unusable speeds before the signal strength gets that low.

Signal boosters can help improve connection stability and speeds. It’s important to choose the right booster for your property type, however. Commercial retail spaces, offices, and other large venues may benefit from a powerful booster designed to cover the size of the property, and the booster would need to be installed appropriately for maximum coverage.

In some very large spaces that see high foot traffic, it may be necessary to install multiple boosters, each with a different donor antenna, so that all users of the building can have a good experience when using their mobile devices.

The Key Differences at a Glance

As you’ve seen, both dB and dBm are a way of quantifying power and intensity, so let’s focus on the differences:

dB quantifies the ratio between two values, whereas dBm expresses the absolute power level.

dBm is an absolute unit, whereas dB is a dimensionless unit.

dBm is always relative to 1mW, while dB is expressed in watts and can be relative to other powers.

Boosting the Signal

A cell phone signal booster can solve the issue of poor cell signal in a building, provided that there is some signal for the booster to pick up.

By positioning an antenna in an area with some signal, the signal can be transported to a booster unit that amplifies it, thus making it available to areas of the building that would otherwise have no signal reception.

These signal boosters work with all major networks without any carrier-specific restrictions and do not require an internet connection to function. Multiple users and devices can share the booster without encountering any problems.