From VLF to THz, the past and present of the radio spectrum

Date:Jan 05, 2021

The frequency spectrum is a physical quantity existing in nature, which cannot be increased or decreased, so it is extremely precious. According to the definition of the International Telecommunication Union, the frequency range of electromagnetic waves used when humans can identify is from 3kHz to 300GHz. For ease of presentation, the frequency range from 3kHz to 300GHz is divided into VLH (very low frequency), LF (low frequency), MF (intermediate frequency), HF (high frequency), VHF (very high frequency), UHF (ultra high frequency) according to the frequency. , EHF (extreme high frequency) and THF (terahertz radiation), a total of 8 parts.

It is worth noting that, in general, the higher the frequency, the worse the penetration. The lower the frequency, the smaller the bandwidth that can be provided. There is an old saying in the communication field-"Wired resources are unlimited, while wireless resources are limited." In a specific frequency band, the achievable transmission rate is not unlimited. It is also affected by the signal-to-noise ratio and channel Restrictions on objective physical conditions such as bandwidth. Just like a car on a city road, you cannot drive as fast as you want. It is also affected by factors such as the width of the road and the number of other vehicles. In the past hundred years, the entire communications industry has continuously challenged the limits, hoping to obtain higher transmission rates under limited spectrum resources, or to further utilize higher frequency spectrum resources.

1Very low frequency VLF and low frequency LF (3KHz~300KHz)-the beginning of wireless applications

Like all physical quantity applications, humans start from low to high in the use of wireless spectrum. In the definition of the International Telecommunication Union, 3KHz~300KHz is called very low frequency and low frequency. This frequency band has extremely strong penetrating power, and the wavelength is easily tens of kilometers, so it can easily cover the entire earth, so it was originally used in aviation , Nautical navigation. Many civil aviation passenger planes and ships use the VLF frequency band for navigation and management, and there are sonar systems used by submarines in this frequency band.

For technology players, the radio waves accepted by the hot radio wave timekeeping watches in recent years are also in this frequency band. The 6 radio waves claimed by various radio time synchronization watches actually correspond to the 68.5KHz frequency of the Chinese time synchronization radio wave BPC, the Japanese time synchronization radio wave JJY 40KHz/60KHz frequency, the North American time synchronization radio wave WWVB 60KHz frequency, and the European time synchronization radio wave MSF /DCF77 60KHz, 77.5KHz frequency has a total of 6 radio wave time synchronization signal transmission stations, so it is called 6 stations radio wave. Therefore, don't think that there is no high-tech if the frequency is low, and the low frequency can be used to great advantage.

MF (300KHz~3MHz)-the lifeblood of radio stations and radio navigation

If you often listen to radio stations, you will surely find that many radio stations will say frequencies like medium wave XXX. The so-called medium wave here actually means intermediate frequency. After humans have successfully mastered and used very low frequencies and low frequencies, they discovered that radio waves can also transmit information such as sound. So the intermediate frequency became the first choice for regional radio stations. Our country stipulates that the medium wave broadcasting frequency band is 525-1605KHz with an interval of 9KHz. All medium wave radio stations must comply with this regulation.

In addition to broadcasting, medium wave is also used in many navigation systems. The process navigation system NDB used by civil aviation today is also based on this frequency range.

High frequency HF (3MHz~30MHz)-the starting point of global communication

In the field of radio broadcasting, high frequency is called short wave. Because high frequency can achieve ultra-long-distance transmission through ionospheric reflection without the need for a very high strategy at the transmitting station, for the first time in the high frequency range, humans have realized a global coverage radio station and a global communication station. It is no exaggeration to say that starting from the high frequency, human beings have the global radio communication capability for the first time.

In addition to the high frequency used by international radio stations, many military communications and confidential communications also use this frequency band. Many spy stories about radio encryption and communication during World War II were all unfolding in the high frequency range. On the other hand, ITU (International Telecommunication Union), in order to thank radio enthusiasts for their contributions, has also specially planned amateur frequencies for radio enthusiasts to use without the approval and authorization of relevant agencies.

It is worth mentioning that the familiar RFID and NFC actually work in this frequency range. Among them, NFC works at 13.56MHz, and RFID uses 27.12MHz additionally. The reason for choosing such a frequency band is not to increase the transmission distance, but to reduce the design difficulty and manufacturing cost of the receiver and transmitter.

Regarding high frequencies, there is also a story full of mystery called the weird radio station MDZhB-since the 1970s, radio enthusiasts around the world have listened to a mysterious radio station at 462.5KHz. This radio station has been sending monotonous "hums" to the outside world for nearly 40 years. One day, the harsh hums suddenly disappeared and were replaced by cold human voices. "U-V-B-7-6", a heavy Russian accent read a series of codes. After a pause, the buzzing sounded again. Around 2002, the call sign was changed to "MDZhB". To this day, you can still receive this mysterious radio station on the radio, and the debate about the specific purpose of this radio station and what content it broadcasts has never stopped in the past 40 years.


VHF Very High Frequency (30MHz-300MHz)-Meet the TV Era

From low frequency to high frequency, we have mastered the method of transmitting sound and information on a global scale. Next, of course, we must achieve two-way communication, and it is best to see the picture. So VHF VHF was developed and utilized. In this frequency band, FM broadcasting, walkie-talkies, BP pagers, cordless phones, and wireless TV have all appeared on the stage, making ordinary people feel the charm of wireless communication for the first time. The popularity of these products has also profoundly affected social development.

In addition to radio and television familiar to ordinary people, VHF is also responsible for international maritime communications, aviation navigation, and aviation ground ATC communications.

UHF UHF (300MHz~3GHz)-the main digital communication channel

GSM, WCDMA, WiFI, Bluetooth, GPS, most of the digital wireless communication technologies you know are in this range. Due to the intensive application of this frequency band, countries around the world have adopted the form of authorization to strictly regulate the use. National licenses for this frequency band are often issued in the form of mobile phone operator licenses. The LTE Band 1234567 you know is actually the code name of each frequency band in the spectrum range. Different countries have different frequency bands approved for use, so it is necessary to optimize and design for each frequency band. The full Netcom that mobile phone modems have been talking about actually refers to the support of different frequency bands and different network standards.

Why don't you need a license for WiFi at home? Because countries have also defined unlicensed frequency bands in this frequency band-as long as the power of the device does not exceed the legal specifications, the use of the 2.4GHz frequency band does not require a national license. Now you know why WiFi, Bluetooth, etc. like to get together 2.4GHz, right? What’s interesting is that microwave ovens also work at 2450MHz, so it’s also an unlicensed band device-there are instructions on radio interference in all microwave oven manuals, which roughly means don’t put microwave ovens and WIFi routers, TVs and other equipment together, otherwise There may be interference.

A large number of wireless communications work in the UHF frequency band has caused the entire frequency band to be very crowded. Therefore, in order to further increase the transmission rate, in addition to breakthroughs in modulation methods and coding, only higher frequency bands can be used. Starting from 802.11n, the 5GHz unlicensed frequency band has been used to achieve WiFi speeds above gigabit. With 802.11ac, a transmission speed of 1700Mbps and MU-MIMO function can be achieved under 5GHz, which greatly improves the transmission speed and carrying capacity of WiFI. Due to the large bandwidth of the 5GHz unlicensed frequency band, Qualcomm also proposed a licensed assisted access technology (LAA) in the evolution of 4G LTE, so that 4G and 5G networks can also use unlicensed frequency bands to further improve the transmission rate and carrying capacity.

As for the well-known 5G communication standard, in addition to 5G working in the 2.4GHz band of the original LTE network, the 28GHz mmWave millimeter wave subset is also added to ensure that the ultra-high-speed transmission rate established in the 5G era can be realized. The first announced mass production Qualcomm X50 Modem can achieve a download speed of 5Gbps in the 28GHz frequency band, which is almost 10 times that of the current LTE network!


EHF very high frequency (30~300GHz)-a new wireless journey

In the next-generation WiFi standard 802.11ad, the 60GHz frequency band is directly selected to achieve a maximum transmission rate of 7Gbps-do not think that 802.11ad is far away from us. In fact, support for the 802.11ad standard has been added to the baseband of Qualcomm Snapdragon 835 . Netgear Nighthawk AD7200, the first home router that supports the 802.11ad standard, has been on the market for a long time.

Before the advent of 802.11ad, the Wireless HDMI standard realized the wireless transmission of HDMI signals within 10 meters through the 60GHz frequency band, and UWB (Ultra Wide Band) used by the once popular technology Wireless USB was also at extremely high frequencies. Although UHF has many limitations, it is definitely another journey of wireless communication. If you want to achieve more than 10Gbps wireless communication, you must fully grasp and use EHF extremely high frequency.

THF terahertz radiation (300GHz~3THz)-the next sky

The terahertz radiation wave is 0.3~3THz (1THz=10^12Hz), connect EHF at the top and infrared ray at the bottom. Electromagnetic waves in this frequency band already have all the characteristics of light waves, so that THF can scan objects like rays. Although the image quality is not as good as X-rays, it has no radioactive effect on objects.

The THF frequency band has completely different characteristics from other frequency bands before, so that THF is used in imaging and safety. On the contrary, there are not many breakthroughs in communication. The whole body scanner used in the American airport is based on the principle of terahertz radiation.