Modulation is how information is encoded in a wireless signal. All information transmitted over a wireless signal is modulated in some way. Having more complex modulation allows for much higher bandwidth.

Two well-known modulation schemes are Amplitude Modulation (AM) and Frequency Modulation (FM), most commonly used for radio broadcasts.

AM works by adjusting the amplitude, or intensity, of the signal. FM works by adjusting the frequency of the signal.

For example, think of visible light. AM is similar to adjusting the brightness of the light to send a signal whereas FM is similar to adjusting the color of the light to send a signal.

There is also PM (Phase Modulation), which is similar to frequency modulation but adjusts the phase of a signal.

All three of these different techniques can be used to "modulate" digital data onto a radio signal.

QAM, or Quadrature Amplitude Modulation, is a high-bandwidth method of modulation frequently used for 4G and 5G cellular systems.

QAM encodes data by adjusting both the amplitude and the phase of the signal at the same time. Cell signals also sometimes use phase-based modulation techniques such as PSK (Phase Shift Keying).

Wireless ("radio frequency") transmissions are inherently "analog." Amplitude, frequency, and phase can all be any value on a continuum. However, computers work with binary digital data that's comprised of zeroes and ones.

While modulation allows any kind of data to be encoded on a signal, for cellular signal we specifically need to encode digital data on the wireless signal.

For this, a constellation is used. A constellation is essentially a grid of points; Each point has a digital value assigned to it. Cellular signals use variations in frequency and amplitude to communicate a specific coordinate on the constellation.

Using constellations with more points can achieve higher speeds.

PSK and QAM are the main forms of modulation used in cellular signals.

PSK (Phase Shift Keying) adjusts only the phase of the signal. QPSK is frequently used with cellular. QPSK (Quadrature Phase Shift Keying) has 4 possible values.

QAM (Quadrature Amplitude Modulation) adjusts both the phase and the amplitude of the signal. 16QAM, 64QAM, and 256QAM are commonly used in LTE/5G and 1024QAM is planned for use in the future. 16QAM has 16 possible values, 64QAM has 64 possible values, 256QAM has 256 possible values, etc.

Let's look at constellation diagrams for a few different modulation schemes.

Not necessarily. As you decrease the space between points it becomes more susceptible to interference and data corruption becomes more likely.

As you can see in the constellation diagrams above higher modulation schemes have much less distance between each point, for example with 16QAM each point is separated by 2/√10 on the constellation diagram whereas with 1024QAM each point is separated by 2/√682.

Alongside modulation schemes, coding rates are a critical factor in determining 4G and 5G data speeds.

Whenever a signal is transmitted wirelessly there's always a risk of the signal getting corrupted due to interference. To prevent this, complex error correction systems are used. The "coding rate" is how much of the signal is being used for data transfer and how much is redundant info for error correction. A lower coding rate helps deal with the fact that more complex constellations are more susceptible to interference.

For example, with a coding rate of 948/1024 (the highest used in LTE/NR) 92.58% of the signal is useful information while 7.42% is redundant info used for error correction.

Cellular devices assign a CQI (Channel Quality Indicator) to the signal; The CQI is used to tell the tower what modulation and coding rate the device thinks should be used.

SNR (Signal to Noise Ratio) is the main factor used to determine the CQI, however other factors are considered as well. ICACT's SNR estimates seem to line up best with what I have observed.

Here is a table of LTE/NR CQI values and what modulation/coding rate is used:

There are multiple CQI tables that can be used; Carriers can choose which CQI table to use. Each table has 15 modulation/coding rate combinations. There is a total of 6 different tables as of 3gpp release 15. When LTE was first deployed table 1 was used, which only went up to 64QAM. Currently carriers use table 2, which can go up to 256QAM, however it has fewer modulation/coding rate combos at lower modulations/coding rates. In the future table 4 is planned to be used, which goes up to 1024QAM. The table listed above is table 2.

Here are estimated SNR requirements for CQI tables 1 and 2:

The easiest way to get a better modulation/coding rate is by improving your SNR, which can be achieved using an external antenna or a signal booster.

If you're using a 4G or 5G router, gateway or modem, we've probably written up a guide for improving cell signal to that device. Check out our guides to external antennas for routers here.

If you're looking for a signal booster to improve coverage throughout your home, we've written a list of the 11 best signal boosters of 2022 here.

Here are just a few of our top picks for most people:

Need help picking the right external antenna or signal booster to improve your cell signal? Reach out to our team of signal specialists; we love solving tricky signal problems!