What is 5G cellular ?

The term 5G just means 5th generation digital cellular technology.
To recap, 1G was CDMA (1xRTT) voice and data, 2G was the EVDO CDMA data upgrade, 3G is HSPA (or WCDMA) voice and data, and 4G is LTE data and VoLTE (voice over LTE).

5G is a shift in frequency use though, as opposed to a new transmission protocol. There are a number of frequency bands used for the current HSPA and LTE signals. 600 Mhz, 700 MHz, 850 MHz, 1900 MHz, and 1700/2100 MHz. Any technology can be used in any band. In fact, LTE is deployed in every band, but HSPA is only in the 850 & 1900 bands.

One issue though, and this is where 5G comes in, is with data throughput (speed) and loading (congestion). The LTE data speed in the lower bands (600, 700, 850) max's out under 20 meg download (in best conditions), and in the upper bands (1900, 1700/2100) max's out at about 100 meg download (in best conditions). I say best conditions, because numerous other factors come into play as well. Such as signal strength and signal to noise ratio. The weaker the signal or greater the noise floor, the slower the speed.

The whole purpose of 5G is to use un-used high-frequency spectrum where download speed bandwidth can be maximized and loading minimized. Initially, the manufacturers bragged about how they could achieve 1000 meg (gigabit) download speeds, but are now finding that 200 meg is a more realistic maximum.

The spectrum

To understand how 5G will work, lets recap the electromagnetic spectrum.
Sound waves, radio waves, microwaves, visible light, infrared, and ultraviolet are all the same type of non-ionizing wave (more on that later). The only difference is the frequency of the wave, its cycles per second, measured in Hertz (Hz). The higher the frequency, the more cycles per second, and inversely, the shorter the wavelength.
Here are some examples . . .
20 - 20,000 Hz
sound (to us)
540 - 1700 KHz
AM broadcast radio
26 - 28 MHz
CB radio
88 - 108 MHz
FM broadcast radio
136 - 174 MHz
VHF mobile radio
450 - 470 MHz
UHF mobile radio
600 - 2100 MHz
the above mentioned cellular bands
2.412 - 2.484 GHz
2.45 GHz
microwave oven
5 - 6 GHz Wi-Fi
25 GHz
5G cellular
300 GHz - 430 THz
infrared light     (also expressed as 1 mm to 700 nm)
430 - 790 THz
visible light     (also expressed as 700 nm to 400 nm)
790 THz - 30 PHz
ultraviolet light     (also expressed as 400 nm to 10 nm)

Hz (hertz)
KHz (kilohertz, 1000)
MHz (megahertz, 1,000,000)
GHz (gigahertz, 1000 megahertz)
THz (tetrahertz, 1000 gigahertz)
PHz (petrahertz, 1000 tetrahertz)
mm (millimeter)
nm (nanometer)

Ionizing & Non-ionizing waves

Above 30 PHz are the X-rays, gamma rays, etc. These are also what's know as ionizing waves because their wavelength is so short, shorter than the size of an atom actually, that they can actually knock electrons out of an atom, making the atom more positive, hence ionized. These are the waves that can disrupt cell structure and cause DNA damage leading to mutations and cancers.

Everything below roughly 30 PHz are non-ionizing waves. They do not cause cell damage. They pass right through us harmlessly.

Microwave ovens work by agitating the atoms in the food, which causes them to bounce around more, which, due to atomic friction causes heat dissipation. There is no "radiation".

millimeter waves and 5G cellular

5G will use millimeter wave frequencies around 25 GHz.
Because of the very short wavelength, these waves do not travel very far in free air (in comparison to lower frequencies at the same power). They are also very susceptible to anything in the air. Cloud, fog/mist, rain, snow, wood, even your hand will all degrade or block the signal. In best case testing, these signals only travel 300-500 feet. Also, because of the very high frequency, they are very directional. Essentially, the transmitting and receiving antennas have to be lined up perfectly. One thing the manufacturers of the equipment are finding is that they'll need an antenna on every side of the phone to track the signal from the tower. And the tower will have to employ some kind of beaming/tracking to maintain a direct path to the phone.

final thoughts

Due to the nature/physics of the 25 GHz millimeter waves, it is impossible to achieve wide-ranging cellular coverage with them like we enjoy today with the existing systems. This technology will only be deployed in key small areas were return on investment can be maximized by the maximum number of users. Simple economics says there is no way the carriers will spend hundreds of thousands of dollars to deploy a tower/radio on each streetlight to get signal to 2-3 houses (which can barely penetrate the walls anyway) with no guarantee of financial return. With a range of only a few hundred feet and the signal being degraded by pretty much anything in it's path, its ubiquitous use could not and never be achieved.