In many occasion some of my Sigfox prototypes has been in movement with difficulties to emit ; many source can cause this but basically from discussions with specialists speed is not a close friend of Sigfox technology.
As a quick test is always better than a long theorical discussion, tonight, I took my car and made some tests.
This post details the test of Sigfox running in a car at different speed.
For the test I’m using one of my prototypes with a good antenna from connit, placed in my car in front of the windshield ; inside the car.
I took two road : one for test up-to 90km/h and another for test at 110 and 130km.
For the first road, I’ve test by stopping every 200 meters that the coverage is good and equivalent all along the road. The road is quite flat in a perfect state ( no pothole ) ; not fully strait but with long strait lines.
The first road where I made the test for speed 0Km – 15 km/h – 30km/h – 50km/h – 70km/h – 90 km/h
Messages are captured by 9-12 base stations on this road
Rssi goes up to -90 dB on any location the RSSI signal for at least one basestation is better than -110dB
The second road is a highway, the first part in Orange is a 110Km limited area and the blue part is a 130 km limited area. I was not able to check the static result of a sigfox transmission as I can’t stop on these road. But they have a really good access to the mountain around where the antenna should be.
Messages has been captured from 3 to 6 antenna with RSSI up-to -100dB.
For the different speed, each time I send 2×10 frames : 1 for 12 bytes, 1 of 6 bytes ; every 20 seconds. The results are based on 10 frames. Sometime I had to get them in two separate measure due to the road not long enough. Frame are received or not received by sigfox
Received 12B LOSS Received 6B LOSS frame frame 15km/h 11/11 0% 10/11 9% 30km/h 10/10 0% 10/10 0% 50km/h 11/11 0% 11/11 0% 70km/h 10/11 9% 11/11 0% 90km/h 11/11 0% 11/11 0% 110km/h 13/14 9% 14/14 0% 130km/h 14/14 0% 14/14 0%
Basically, there is no real message loss related to the speed of the vehicle. But, now if we are looking more in details in the transmission, the impact is larger : every message is sent 3 times at different frequencies ; so if we look at repetition the result is different :
Frame Frame Avg Frame LOSS sent Received received per msg 0km/h 54 54 3 0% 15km/h 63 53 2.65 16% 30km/h 60 50 2.63 17% 50km/h 66 48 2.28 27% 70km/h 66 51 2.42 23% 90km/h 66 48 2.28 27% 110km/h 93 55 1.83 41% 130km/h 87 49 1.75 44%
We can notice that speed directly impacts the performance even for a small speed ; loss is growing with speed growth. It is not possible to conclude to a large impact of going over 90Km/h from this test as the road has been different. When average frame received per message is decreasing it means that chance to loose it is higher. In the test case many antennas where listen to me with good signal so impact of speed when reception is not as good as here can be dramatic for performance.
If you want to take a look in details to the data, you can get them from this file : export-device-messages
This level of loss in repetition is not a strong issue when the number of antennas around the emitter is large but when you are in an area covered with only 1 or 2 antennas it starts to be a problem. For sure this is related to 2 important factors : density of the antenna and quality of the emitter radio, this ensuring to touch more antennas.
In the above example we see that on GREEN area where we have highway but city environment where we can eventually find more antenna, we have a lot of GPS point. But in the area in RED and ORANGE area the fix density is really lower due to communication loss.
To understand the problem we are most of the time referring to Doppler effect : by moving in a certain direction the message frequency is impacted by the vehicle speed like in this picture :
In fact, when we apply the Doppler formula, with car speed
F' = ( light speed / (light speed light + vehicle speed) * F
We have a ratio of 0,99988% the frequency shift is really small and should not impact the communication like we seen and antennas perpendicular to the vehicle should not be concerned by this effect.
This ratio means a frequency shift about 104kHz at 868MHz, regarding sigfox communication bandwidth per channel (100Hz) it means a shift of 1000 channels and in 50% of the case it will be out of the 200KHz global Sigfox bandwidth. In fact this will become a real issue if the speed is changing during the transmission : in this case the message will move from a channel to another and the Sigfox decoding system will have big issues to follow it. In the case of a car driving on a highway this is not changing so fast, the main impacting situation is when the car is taking a large turn as it change the Doppler direction. So the antenna in front or back to the car at 130km/h may have difficulties to capture the message (about 50%) but the perpendicular antenna may have no problems. (more antenna you have, larger is the chance to get the message)
So we have to find the reason in another direction : when a signal is transmitted, it takes multiple path to reach is destination, depending on the area topography. By making multiple path the signal is making different time to reach the destination. Longer is the distance to the antenna and larger are the possible path.
Longer path also means :
- Different reception power
- Time shift at reception
The problem when the vehicle is moving, as the transmission is during a long time is that the path with higher radio signal to reach the antennas will change during the transmission and this will create shifts in the communication. Regarding the Sigfox modulation based on BPSK, a shift can be interpreted as a kind of phase change echo or it can create a phase change where you did not had one (all the Green Circles can correspond to a phase changed due to a bit change or due to a path changed (longueur path)
More you have transmission, more you have reception point (antennas) and more you have a chance to get a stable path where this phenomena does not appear. Lower is the speed and safer is the path also.