24 GHz, why can't we all just get along?

Updated: Aug 27





I recently had the opportunity to work with ITDRC Information Technology Disaster Resource Center. This particular project happens to be the Dixie fire in Northern California. The Dixie Fire is the largest single fire in California state history having burned 725.000 acres at the time of this writing. It is currently only 37% contained. According to CalFire these are the resources currently assigned to the Dixie Fire.


CalFire resources assigned to the Dixie Fire as of 23-AUG-21

This particular project was to establish a point to point data link from an existing tower site to a designated relief area for fire crew and evacuees. The ITDRC crew have quite a few solutions at their disposal. At this location they attempted to install both 5 and 24 GHz links. Yes that is 24 GHz, not 2.4.


As mentioned one end of the link was at an existing tower site so it wouldn't be a stretch to find 5 GHz saturated and unavailable for a new link. What was somewhat surprising is that 24 GHz was also unusable. In this blog I will take a deeper look into the band and some of the hardware that is capable of using it to figure out what was going on.



The 24 GHz Spectrum Band


A small segment of the 24 GHz band is dedicated for unlicensed use. This portion of the SHF (Super High Frequency) microwave radio spectrum has been allocated internationally to amateur radio (24.00 - 24.25 GHz) and amateur satellite (24.00 - 24.05 GHz) use. This band is also known as the 1.2-centimeter band.


The FCC ID website shows 815 devices authorized to use this frequency. Most are various types of sensors and radar devices, not likely to interfere with a point-to-point link based on proximity. However, for the point-to-point radio manufacturers, it is the wild west, as we will see when we look at channelization.


For point-to-point radio use, 250 MHz of frequency is available. Unlike the 2.4, 5, and 6 GHz bands, there is no regulated channelization in the 24 GHz band. This is a good news / bad news situation. Good news, no DFS, no regulations concerning incumbents, the frequency is free to use as needed. Bad news, this means that almost anything goes. One radio manufacturer may start their channel right on 24.00 GHz, and another may start it on 24.07. One radio may use 20 MHz channels, another may use 100.


This chart is intended to illustrate the relationship between channel width and number of available channels in the 24 GHz band. The chart demonstrates a 2 MHz guard, this is not regulated, and radio manufacturers are free to use different values.


Channel widths plus a 2 MHz guard at the beginning and end of the band, and between channels

Since there aren't any regulations regarding channelization in this band, where the channels start, how wide they are, inter-channel and intra-band separation, etc. All of that is up to the radio manufacturer. You begin to see why contention & interference can be a problem where non-similar manufactured links are collocated.


As always, know your spectrum. What is in use, and what is available. Most 24 GHz radios will have a built in spec-an to find the clear air.



Spectrum Management


Most enterprise or carrier-grade manufacturers employ some type of frequency or time based spectrum management methodology to provide the greatest spectrum reuse possible.


Some manufacturers time slice the spectrum between radios. These radios are given precise transmit allocation windows using highly accurate GPS-based timing (TDD Time Division Duplexing). Any other manufacturer's radios in the area cannot participate in this allocation and may transmit (TX) or receive (RX) while another radio uses the same frequency, causing interference.


FDD (Frequency Division Duplexing) is also used to increase throughput on a link. This means that each radio in the link pair (or multiple radio units) transmits and receives on different frequencies. This allows the radios to TX and RX simultaneously. Rather than the more typical arrangement where a 'conversation' take place on a single frequency, where one side will transmit while the other receives. Throughput can be doubled if each radio can TX at the same time. This method requires more bandwidth as two separate channels are needed per link pair.

There are many other methods manufacturers use to allow single-channel reuse or multiple channel cooperation. While each can be extremely useful when designed into a particular manufacturers links, they are generally not compatible between manufacturers. Even if two manufacturers use the same method, they aren't coordinated between each other and will cause destructive interference.


Excerpts of datasheets from three 24 GHz capable radios currently on the market


Ubiquiti AirFiber AF-24HD


The Ubiquiti AirFiber AF-24HD uses two 100 MHz channels by default. This leaves only a single 20 MHz channel available for use by any other radio in the area.

For Ubiquity to place multiple radios in the same space, they use what they call HDD (Hybrid Division Duplexing). This is a combination of FDD and TDD. Ubiquiti documentation says that this technology allows the installation of up to eight radios on a single tower.

Since all Ubiquiti AirFiber radios use the same two 100 MHz channels, operating more than one unit on a tower is a pretty good trick. In reality, anything more than one of these AirFibers at a location, using basically the entire 24 GHz band, would be bad news for anyone else in the area trying to use 24 GHz.


Mimosa B24


As you can see by the Operating Frequency Range, Mimosa plays well with the adjacent bands by leaving guard space at both the upper and lower ends. They also use the familiar 802.11 channel widths. Allowing for many more options with regard to operating well with others who aren't able to participate in their spectrum management scheme.


The B24 uses Auto-TDMA (Time Division Multiple Access), and coordinates radios using GPS timing. Mimosa is also capable of allowing eight collocated radios. With proper attention to channel widths the Mimosa link pair could be colocated with other brands that also allow smaller channel widths.



Cambium Networks PTP820S


Cambium offers a licensed radio in the 24 GHz band. It is unclear from online documentation why a license is required for a radio using an unlicensed band. Of the radios listed, Cambium offers the most channel width options.

Their 24 GHz radios come in two distinct models, listed as high and low. High operates on 24.170-24.150 and Low in 24.00-24.080. There is probably a good reason to split the frequency the way they do, but I have always been a fan of being able to order one model and allow an onsite decision as to which frequencies to use.

Cambium uses AFR (Advanced Frequency Reuse) and ACM (Adaptive Coding and Modulation)

AFR is an interference mitigation technique where each link samples the signal of the other to eliminate self-interference on the same channel. This reduces the interlink interference between two co-located links on the same tower where the same frequency pair is used on both links.

ACM Adaptive Modulation means dynamically varying the modulation in an errorless manner to maximize the throughput depending on current propagation conditions. ACM uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission.


Summary:

24 GHz is a good option when and where it is available. For an inexpensive, unlicensed, up to two mile shot, you can expect about 1 Gbps of aggregate TCP throughput.


We have demonstrated here that while the 24 GHz band is available for unlicensed (Cambium?) use for point-to-point radio transmission, the manufacturers in this space are free to use options that can be incompatible with each other in their implementation. Similar vendor hardware works well together with frequency and time slicing implementations, but dissimilar manufacturers don't play well together.





ITDRC


"America's premier team of volunteer technology professionals

Connecting Communities in Crisis".


We harness the collective resources of the technology community to provide no cost Information, Communications, and Technology (ICT) solutions that connect survivors and responders in crisis.


ITDRC is a great organization to be a part of. You have valuable abilities and knowledge, put them to use where they are needed most. Please take a look at the ITDRC website to see if there is anywhere you fit in. Whether it is active deployment, remote support, hardware and monetary donations. There are many ways to be a part of the solution.


Big thanks to Jeff Jones and Eric Thompson from Mimosa / Airspan for their support on this project.


Abbreviations used:

ACM - Adaptive Coding and Modulation (Link Adaptation)

AFR - Advanced Frequency Reuse

FDD - Frequency Division Duplexing

FDMA - Frequency Division Multiple Access

ITDRC - Information Technology Disaster Resource Center

RX - Receive

TDD - Time Division Duplexing

TDMA - Time Division Multiple Access

TX - Transmit



@WirelessKahuna