Why the 433-435 MHz Band is Good for LoRa (and other innovative digital modes)

By Jon Adams, N7UV

LoRa is a digital mode with technical characteristics that are well documented publicly. LoRa has extremely good performance in noisy environments, and can successfully transfer data at SNRs well below the noise floor. Here's a snap from the second-generation Semtech SX1261/2 data sheet that shows typical worst case low SNR values for all 8 spreading factor (SF) settings.

 For all SFs, the LoRa receiver is able to work at 20 dB below the noise floor (the noise is 100x the signal)! As I've said before, it's not magic, it's math and good silicon engineering design. It's because this particular FSK modulation technique is quite unusual in the RF world, and until about 20 years ago it was really difficult to use such a modulation technique without going to great costs and complexities. 

Standards are great - you have so many to pick from! 

From the early 2000s to today, I am a member of the IEEE Standards Association, and have been sometimes a voting member involved in standards processes for nowadays commodity wireless solutions like IEEE 802.11, 802.15, 802.16, and 802.20. Like many such things, a few really caught on, as Wi-Fi uses the IEEE 802.11 standard, ZigBee and other local personal area networks use standards within IEEE 802.15. IEEE 802.16 is WiMAX, which generated a burst of products back in the 2008-2015 time frame, but was outcompeted by the 3GPP (another standards organization) cellular technologies like 4G, 4G HSDPA, 4G LTE and 5G. 

Chirp spread spectrum had some strong adherents back in the early 2000s. I knew and worked with some of those engineers/scientists/strategists in the standards committees. Chirp was becoming a real thing, and implementation in low-cost silicon was beginning to be practical. It was still early, and a bit messy, and never made its way into any known IEEE standard, but IEEE and other standardization organizations are not the only place that standards are created. As an example, public safety two-way radio in the US broadly uses a technology called Project 25 (P25), and its creation was deep in the research labs at Motorola (at which I was a tiny part). Motorola pushed hard to get the industry to adopt its approach to two-way radio, and ultimately P25 became a "standard". Its success has been outstanding. 

Semtech did something similar: they had this powerful, lost cost, low energy-consuming, easy-to-use radio technology, and they built an industry alliance (The LoRa Alliance) to promote and create a LoRa-enabled ecosystem for connecting Internet of Things devices. In so doing, they created a global market for LoRa chips, and have sold more than 400 million of them since the first ones were released over a decade ago. We hams get to reap the benefits of that giant market. 

A glimpse of the 433 - 434.5 MHz band 

Here's a 30-second snap of the 432.5 to 434.5 MHz (let's call it 433/434) spectrum from a SDR receiver on a roof-mounted antenna at my QTH.  (BEWARE: My SDR receiver's antenna is about 1 meter from my 10 watt 2 meter AFSK AX.25 APRS digipeater/igate, and the same distance from my 1 watt 70 cm LoRa APRS digipeater/iGate, so every time those transmit, the SDR can't handle strong signals; those lines from screen edge to edge are artifacts of the SDR.)  

 Looking at the waterfall display, at 433.17 MHz there's the 3rd harmonic of the 144.39 MHz APRS digipeater at my QTH. 

What does the US FCC and the global marketplace say about 70 cm? 

Under US FCC Part 15.231 Periodic operation in the band 40.66-40.70 MHz and above 70 MHz, specifically the part "above 70 MHz", may be how universally available 433.05 - 434.79 MHz unlicensed devices come into play inside US jurisdiction. This spectrum chunk is considered a global unlicensed band, what many in the world call an "ISM" band. (For folks under US jurisdiction, ISM means FCC Part 18, Industrial Scientific Medical Equipment, which is different, but close enough.) Products that use the 433/434 band are ubiquitous! In the spectrum waterfall diagram above, there are periodically operated FM-modulated signals like the one at about 433.4 MHz (seems like some kind of on-off-keyed or pulse-width-keyed signal), devices like weather station wireless sensors, garage door openers, home security wireless sensors, even automotive keyfobs. (Note that in the US these items used to be concentrated down around 315 MHz, but with so many wanting to harmonize, there are fewer 315 MHz devices nowadays.) 

FCC Part 15.240 Operation in the band 433.5-434.5 MHz allows use radio frequency systems that "... identify the contents of commercial shipping containers. Operations must be limited to commercial and industrial areas such as ports, rail terminals and warehouses" The phrase "... such as..." is a soft definition, and suggests this usage may be found wherever shipping containers are handled, which in the US, may be pretty much anywhere.  

There's a lot of claimants to this little sliver of spectrum. It's a bit of a noisy place. A technology like LoRa is right at home in the midst of these interfering signals. Back to the waterfall display above, centered on 433.775 MHz are 6 LoRa packets; the bottom 4 are transmitted from my own station, and the two at the top of the display are digipeats of one of my tracker's signals. By looking at the digipeated signals, which by the way are about -70 dBm RSSI, the waveform is tightly contained and spur-free. 

Some hams note that there are amateur TV operators in the 70 cm band. That's true! Many of these stations are using old NTSC technology (both black and white and in color). A standard NTSC signal takes up about 6 MHz of spectrum. According to the "ATV Handbook - an Introduction to Amateur TV" by KH6HTV, there are at most three channels available in the 70 cm band for amateur TV: 420-426, 426-432 MHz and 438-444 MHz. The following snap is from page 9 of 44 from his handbook. Take-away is that he points out that there are good reasons for hams to avoid ATV in the 432-438 MHz band, quite correctly he notes weak signal ops around 432 MHz and earth<>satellite operations between 435 and 438 MHz. Many have moved on to the 23 cm (1240 - 1300 MHz) where it's much more quiet, and there's plenty of spectrum to experiment with. 

Band plans! 

Band plans are a complicated thing, and everyone has their own, it seems. Southern California hams have a 30-year-old 70 cm band plan, Northern California's is confounded by the restriction on 70 cm operations around Beale AFB in northern California (note that there are similar restrictions around many other governmental and military facilities in the US), Here in Arizona there's an 3-decade-old 70 cm band plan which doesn't align with California, Colorado's got its own thing going, etc. Most 70 cm band plans are often analog-voice-repeater-focused, decades old, and seemingly obsolete. ARRL's own 70 cm band plan is not in touch with local band plans, since band plans are often influenced by local activities. Yet new technologies available to hams have moved forward unimpeded - D-Star, Allstar, NXDN, DMR, P25, POCSAG, Fusion, C4FM, wide-band modes, narrow-band modes, digital modes like LoRa, all of which have become common. 

LoRa is fortunate that it can operate well with interferers nearby or in-band, as the above minimum SNR values suggest. It is a resilient digital communications method! The majority of US hams running 70 cm LoRa are using a 125 kHz bandwidth with a baud rate that, depending on settings, has a range from 31 baud to 13.9 kbaud. (We've done some experiments here in Phoenix running LoRa at 62.5 kHz, but haven't deployed any network infrastructure for it yet.) It plays well with narrow-band receivers since the power levels are comparatively low, the transmissions are short and quick, and the rate of frequency change is so fast that narrow-band radio squelch circuits don't have time to detect it. The maximum baud rate for any LoRa settings is only ~15 kbaud, much less than the 56 kbaud FCC limitation (which is kind of antique, if you ask me - we should change that!) 

LoRa IN SPACE! 

There are now LoRa-equipped space craft, operating earth<>space in the 70 cm 435-438 MHz sub-band. TinyGS is a software app that allows anyone to stand up a satellite ground station and contribute to the downlink reception for dozens of satellites. According to the TinyGS.com website, there are now over 2200 ground stations participating. That's pretty cool, being about to collect downlink data from spacecraft with a simple LoRa radio! And since most US-based terrestrial 70 cm LoRa systems are operating on 433.775 MHz, there's no chance of interference to earth<>space operations. 

In conclusion 

While 433.775 MHz is a good home in the low-power device weeds where LoRa's modulation/demodulation strength allows it to manage well with varied and unavoidable interferers, it'd be a great idea for future band planners to carve out enough spectrum in order for hams to experiment with LoRa to its maximum ability. A non-exclusive 433 - 435 MHz for all digital data modes (LoRa, FSK, GFSK, GMSK, PSK, QPSK, QAM, etc.) could work well. 

Get yourself a 70 cm LoRa radio and get connected! 

--Cheers and 73 - Jon N7UV