SX1280 Fast LoRa

 

The SX1280s LoRa modules can use far higher data rates than the UHF band SX127Xs. At the fastest settings, spreading factor 5 and bandwidth 1625khz, the effective data rate is 203kbs.

High gain antennas such as yagis are very low cost for 2.4ghz since so many are produced for use on WiFi. The yagi below cost me £4.50 delivered from a UK supplier.

 

At what distance would a pair of these yagis, one for each transceiver allow a line of sight (LOS) data link to operate at 203kbs ? I decided to find out.

I configured my custom link test software to run at the 203kbs setting, put the link test transmitter on the mast on my shed and went to the receiving location North of Cardiff, 4.4km away that I have used for the SX1280 ranging tests described in a previous blog.

 

 

With the yagi connected to the NiceRF module on my receiver, I was getting packets down to -3dBm.

So at the maximum TX power the SX1280 can provide, 12dBm, there would be 15dBm of link gain, which suggests a LOS range at that power of around 25km. Which would be impressive.

I exchanged the yagi antenna I was using for the receiver of the test packets to a simple 2dBi one, see below;

 

 

I was then receiving packets down to +5dBm, so the receiving end yagi was providing me with 8dBm of gain.

The implication was that the simple 2dBi antennas fitted either end would be on the verge of working over the 4.4km link. So I swapped the transmitting end yagi with a simple 2dBi one and tried again. Much to my surprise it worked, I received all the 12dBm packets. This  also shows how useful the decending power method of testing can be in predicting actual link performance.

So with the simplest of antennas a pair of SX1280 LoRa modules can communicate at 4.4km at 203kbs using only 12mW of transmit power, impressive.

 

Semtech SX1280 2.4ghz LoRa Ranging Tranceivers

The Semtech SX1280 LoRa devices bring long range LoRa technology to the 2.4ghz bands. The devices allow for much higher data throughput than the lower frequency LoRa devices, data transfer speeds are up to 203kbps.

The SX1280 also has a ranging mode that can measure distance by recording the time of flight of a special packet exchange between two SX1280s.

I have designed some plug in Mikrobus boards for the Ebyte and G-NiceRF SX1280 LoRa devices. The larger green PCBs are an ATMega328P based Mini Logger I also designed. It can be fitted with a single plug in Mikrobus compatible board, see the picture below;

 

 

The Mikrobus PCBs shown, the yellow ones, are breadboard friendly so will plug into a standard 0.1″ breadboard, very useful for prototyping.

A complete Arduino based transceiver fitted with an G-NiceRF SX1280 device is shown on the left of the picture above, complete with 2dBi WiFi antenna. The board was programmed in the Arduino IDE.

Semtech publish sample code for the SX1280 but its not directly suitable for the Arduino environment so I used the Semtech code as a basis for writing some Arduino library files.

First Ranging tests

I used a large open field to test the ranging, these were the results;.

Actual Distance     Indicated Distance.
0                               4.4M
50M                         57.6M
100M                       103M
150M                       148M
200M                       201M
250M                       253M

So not too bad, the results did seem to be consistent for large open areas. So short distance ranging seems OK, what about longer distances ?

4.4km Ranging test

North of the Cardiff where I live there is a ridge and the location is around 150M higher than where my shed is located in the city. With a SX1280 transceiver on top of the 6M mast attached to my shed there is a relatively clear line of sight from the ridge to the device on the mast, the distance is 4.4km.

I wrote ranging software that would first transmit a ranging request at high power (10dBm!) and report whether the ranging request completed. The transmit power was then reduced and another ranging request made.

Over the 4.4km link the ranging worked down to -14dBm. Thus if the full power was used, 10dBm, this test indicates the ranging may have potential to work at over 69km.

40km Ranging Test

There is a location North of Cardiff (altitude 264m) suitable for a 40km link test to Black Down (altitude 325m) in the Mendips, the other side of the Bristol channel.

At the Cardiff end there was a SX1280 ranging receiver fixed to a pole and another for a basic link test. The view from here towards the Mendips is below;

At the Mendips end I had a matching set of devices, one as the ranging test instigator/transmitter and another for receiving basic link test packets. The results were logged to a micro SD card for later analysis. Basic 2dBi antennas were used at both ends.

At the Mendips end the ranging request transmitter would emit a long beep at the start of the test sequence, send a ranging request at 10dBm, then 9dBm, 8dBm etc. If the ranging request was successful then the would be a short beep. Thus to work out the power level when the ranging stopped working you only had to listen for the long beep and then count the short beeps.

The LoRa settings for the ranging were SF10 and bandwidth 406khz, the longest range settings that the SX1280 allows for ranging mode.

Ranging calculations

The path used for the ‘4.4km Ranging test’ mentioned above was used to test and calibrate the ranging function. The ground distance was 4.42km as measured on Google maps. The average ranging result from the SX1280 was 24515. This gave a conversion factor to metres of 0.1803.

The average ranging result for the Cardiff to Mendips link was 225982. Applying the 0.1803 conversion factor gives a distance of 40.745km. The distance taken from Google Maps was 40.650km. So the SX1280 ranging produced a measurement of +0.2% over actual. Not bad.

Summary – Distance Potential of the SX1280 ranging and point to point.

The ranging requests were received down to 4dBm over the 40km link. This would imply a potential range of 80km LOS at 10dBm. Even modestly improved antennas at both ends (see antenna comparisons above) could double this range.

The longest range settings for point to point mode, SF12 and bandwidth 203khz should have a link advantage of 8dBm over the ranging settings described above, SF10 and bandwidth 406khz. Converting this 8dBm link advantage over the ranging settings, suggests a potential range for point to point LoRa in the 200km region.

SX1280 Program Code

Semtech publish source code in C++ and a development kit for the SX1280 based on ARM\MBED.   I have adapted the code to work under the Arduino IDE, its been tested on ATMega328P and ATMega1284P. There is still some work to do on the ranging part of this code, when it it ready it is my intention to publish it as an Arduino library. 

All the parts used in these tests were self funded, no complimentary parts were used.

GPS Performance Comparisons

I have carried out a performance review of the common available GPSs that you might use for a LoRa tracker.

In most cases you will run the GPS in hot fix mode, and unfortunately most of the common GPS breakout boards that you get on eBay etc, are not suited for this in a power miser tracker that needs to survive on batteries for a month or longer. The reasons are explained in the article.

There is data of the performance of 12 common GPSs, signal strength (antenna effectiveness), real world power consumption and hot fix performance over an extended period. There are notes on methods of reducing tracker power consumption.

Choose the wrong GPS and your tracker, powered by a 2800mAhr battery, might only last 14 days. Choose the GPS wisely and the same battery can last 200 days.

The report is at the link below;

GPS Performance Comparisons

GPS backup issues 3 – Whats the alternative ?

So if we don’t use a a super capacitor or a small lithium rechargeable, then what is the alternative ?

We could of course use a non-recharegeable lithium battery such as a CR1216 but in the holders these have much the same issue as the super capacitor, they are physically large, see picture.

GPS and Battery

In addition even a battery as physically big as the CR1216 only has a capacity of circa 25mAhr, so that would only keep the GPS (@15uA backup current) going for 70 days. A tracker based on this setup would need it’s GPS battery replaced around every two months.

The answer to the problem is quite simple really. With the small lithium rechargeable or supercapacitor options, then the backup power for the GPS is actually coming from the projects main battery, so why not use the main battery as the backup power source for the GPS ?

All we need to do is take a 3.3V supply from the project and feed this voltage via a diode to the GPS Vbackup supply. We then need to ensure that the 3.3V supply is present when the project (using the GPS) is in sleep mode. This is not difficult even if the project is using something like a TPL5110 to switch off \on the battery supply to the project. A low drop out regulator such as the MCP1711 has a quiescent current of 0.6uA, so that can be used to go between the projects battery and the GPS Vbackup supply without having a significant impact on total GPS backup current consumption.

Whilst the solution to providing a long term GPS backup current is simple to implement, very few GPS breakout boards provide access to the GPS Vbackup pin, so you cannot feed in an external backup supply. For this reason I designed my own GPS breakout boards for the Quectel L70 and L80 GPS. These are low cost high performing GPSs with substantially lower power consumption than most other GPS modules, see a detailed report here;

 GPS performance comparisons

 

L70 and L80 Breakout

The L80 GPS has a slightly worse power performance than the L70, but it has its own ceramic patch antenna and easy to include in PCB designs and solder in place, I like it. The L70 has a balloon mode, for use up to 80km.

GPS backup issues 2 – Super Capacitor ?

If a small lithium battery is not suitable;  what about a super capacitor that can at least be charged fairly quickly so it might accrue enough charge during a short hot fix power on to last for several hours ?

There is a web site which is an online super capacitor charge and discharge calculator;

Super Capacitor Calculator

First lets see what size of super capacitor we need. Assume a maximum backup time of one day, a fully charged voltage of 3.0V and a minimum volts for GPS backup of 1.5V, a  GPSs backup current of 15uA which is  typical for a Ublox GPS for instance. The calculator suggests a capacitor of 1Farad would last a couple of hours longer than one day.

If the hot fix gap was 10 minutes and the hot fix takes 5 seconds with a GPS backup current of 15uA, then assuming no charge\discharge efficiency losses the charge current needs to be 600/5 times the backup current or 1.8mA. That’s manageable.

So if we are to use a 1F super capacitor how big would it be ?

Checking on the Farnell web site a 1Farad 3.6V super capacitor would be around 20mm, perhaps a bit more, in diameter and 5mm high. That’s really quite big, see picture for how big the 5.5V version 1F capacitor is. The GPS shown is one of the larger 25mm ceramic patch antenna types.

 

GPS and SuperCapacitor

There are some small super capacitors around the same size as the Seiko MS621, but these super capacitors are of very limited capacity and would only provide a couple of hours backup (based on practical tests).

So if a super capacitor is too big to be practical and we have allready dismissed small lithium rechargeables, then what is the alternative ?

GPS backup issues 1

A modern GPS will normally get a fix from cold in 30 – 60 seconds when it has a good view of the sky. Once it has downloaded enough information on the GPS satellites the main GPS power can be turned off and as long as a voltage is supplied to the GPS backup pin it will retain the satellite information in GPS memory. When the GPS power is then turned back on the GPS can acquire an updated fix in as little as 2 – 5 seconds, this is called ‘hot fixing’.

In backup mode the GPS may consume only 5uA to 20uA, so clearly there are significant power savings to be had if we only want intermittent fixes from the GPS. A GPS that only needs to run for a few seconds to get an updated fix every 10 minutes or longer will use a lot less power that an GPS that is left running all the time.

A very common way to supply the backup supply to a GPS is to use a small Lithium battery, typically a Seiko MS621. However this is not without problems, the MS621 has a capacity of only 5.5mAhr and a max charge current of 100uA. So when first used with the GPS you need to leave it powered for 55 hours, just over two days, to fully charge the battery which is not very convenient. In addition you need to consider the cycle time. A typical GPS will consume 15uA in backup mode, so the MS621 will only last 15 days or so when fully charged. If the tracker is to use hot fixing for longer than 15 days then the backup battery needs to be charged, or it will just run out and hot fixing will fail.

If we are taking a GPS fix every 10 minutes and the hot fix takes 5 seconds, there is only 100uA x 5 seconds = 500uA seconds going into the battery. But in 10 minutes there is 10min x 60seconds x 15uA = 9000uA seconds going out. Thus the battery is draining 18 times faster than it is charging and will inevitably discharge quite quickly.

Although fitting an MS621 battery is common with GPS modules, its not suitable as a backup method if the GPS is to be used for a couple of weeks or more. A different backup strategy is needed.

Stuart Robinson

January 2019