Tuesday, February 3, 2015

Real-life performance of the LoRa radio

Numbers put in datasheet is one thing, but experimenting the performance in real environment gives you the confidence. According to our experiments, LoRa really does outperform other competing RF technologies.

Today I performed some RF link length and performance testing in customers premises in production facilities. The purpose was to detect the limits of different frequencies and radio technologies commonly used in Wireless Sensor Networks, which means low power and low cost. Testing was limited to license-free IMS bands with transmit power within legal limits. Protocol issues were not covered in this experiment since the purpose was to set theoretical limits of any radio.

National Instruments Finland kindly supported us by supplying necessary wireless measurement instruments: NI PXI FlexRIO SDR (Software Defined Radio) which acted as a base station, and NI USRP (Universal Software Radio Peripheral) which was used as mobile station together with laptop running LabVIEW.

RF testing ongoing: measurement man at the left,
USRP in the middle, and PXI FlexRIO on the right.
The test environment consist of 160m long warehouse with interconnected manufacturing and office spaces. All in all rather complex building with many concrete walls, number of metal plated walls, metal fire doors, and plenty of metal structures inside. Mobile phones didn't work in many spots and Wifi had very limited coverage. Definitely difficult environment for any radio, just like real industrial deployment sites tends to be.

Typical narrowband tranceiver have receiving sensitivity of -110dBm, thus we decided to map the border where typical radio can not detect signal from noise. Test results of 434MHz, 868MHz and 2.4 GHz didn't surprised much. 2.4 GHz achieved 80m link length with direct line of sights. It couldn't penetrate through the first concrete wall.

434 and 868 MHz had very similar performance to each other. Almost the whole building complex was covered with recognizable signal. Only at the other end of the building in a completely metal covered hall with tiny windows in doors the signal was completely lost.  434 MHz benefits from open doors, even if they are not within the direct line in between transmitter and receivers. This indicates the lower frequency reflects from structures better and can bounce within the labyrinth, whereas the higher frequency tends to travel more straightforward though obstacles. When fire doors were closed, there was no significant difference in performance.

Then we compared the results of narrowband RF with spread spectrum LoRa radio. We changed from NI equipment to actual LoRa hardware, as generating the spread spectrum signal with software radio would be way too complex and effort consuming exercise for the purpose of this test. ARM mbed-enabled SX1276 LoRa development board from Semtech and FRDM-KL25Z MCU development board from Freescale was used instead.

LoRa RF test equipment for 434 and 868 MHz with battery pack.
In this experiment, we did not only measured carrier signal strength, but two-way packet communication over LoRaMac was used. Other radio unit was located in the same spot with the original base station. With this setup, we couldn't identify the limits of the connectivity, as we got packets through in all over the building, even in most difficult locations.

We went event to a bomb shelter in the basement of the office wing, with heavy steel-reinforced concrete walls and blast doors. There was no problems with signal when doors were closed. We event put the radio into a all-metal locker in the bomb shelter and locked the door. Still the signal passed happily, obviously through the small ventilation holes of the locker, and through all the concrete walls up to ground and into next wing of the building.

Really incredible result!

At last we moved the base station into most distant location within the building at the end of the warehouse hall. Now we managed to create challenging enough conditions for the radio. To block the signal, several concrete walls, metal fire doors and dozen or so metallic warehouse shelves were needed. Performance of 434MHz and 868MHz Lora was similar to each other, just like recognized in narrowband Sub-GHz tests.

The conclusion: We did not quite reach the promised 150-160 dB link budget advertised by LoRa datasheets and marketing material. However, we did got 145 dB link budget in a real and challenging environment. It really does outperform any competing technologies, not only in paper but in practice as well, with actual two-way data link established.

The test was performed with two LoRa end-nodes. I'm really waiting for a possibility to repeat the experiment with real LoRa-gateway having a LoRa concentrator chip in place. LoRa Alliance was publish at the beginning of January 2015, thus it is still pretty new technology.

4 comments:

  1. Let me clarify my point on LabVIEW and touch ready...
    I mean that the concept of direct manipulation in LabVIEW, of picking blocks and placing them and then wiring them together is more amenable to "touch-based development" than using a touch-based text editor to write code. I am optimistic, that for many use cases, developing code on a pure touch interface with LabVIEW will be preferable to mouse and keyboard



    labview programming

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  3. Kindly Specify RF Parameters Such as Spreading Factor, Coding Error rate, Bandwidth and output power that you have used in you experiment, because i don't have great experience with Semtech SX1276 while testing it with following configuration: SF=12, BW=125KHz, CR=4/5 and output power +2dBm at 868MHz Frequency band..

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  4. Nice article .. Can you point to some links on the external antenna that you have used in this testing. Any pointers on where i can buy these antennas will also help. Thanks -Ajay

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