The article focuses on radio connectivity for Marvelmind Precise Indoor “GPS.” But most hints and advice are applicable and valid for the wide range of systems using radio connectivity: from remote-controlled toys to complex autonomous robotics systems for industrial applications.
First of all, check the Architecture comparison document. Note, that the modem controls beacons over radio in a star architecture, i.e. beacons can hear each other, for example, hedgehogs can know other hedgehogs’ locations in this way, but currently all beacons talk to the modem – the main controller of the system – they don’t talk to other beacons over radio.
Thus, the answer is that the radio connectivity must be provided between the modem and the beacons it controls.
Today, Marvelmind Indoor “GPS” system typically uses only one modem, which typically supports up to 250 beacons (stationary and mobile combined). If there is more than one modem, it means that there are just two Marvelmind systems deployed.
For larger networks, it is possible to deploy a Multi-Modem Architecture – see a slide “Tunnel safety and performance” in the presentation dedicated to the Multi-Modem Architecture. In the Multi-Modem Architecture, each modem controls its map of stationary beacons. Mobile beacons (hedgehogs) freely roam between the maps using handovers. Modems, which are typically built on the Super-Modem’s hardware, are sending the location data from their respective maps to a Super-Super-Modem, and they are controlled by the Super-Super-Modem. The Super-Super-Modem and modems are connected using regular IP over 3G/4G/5G or WiFi, since the microsecond-level latency/jitter are not required for this connectivity unlike in the connectivity between the modem and the beacons it controls.
RSSI or Received Signal Strength Indicator is a level of radio signal received from a transmitter. What to remember in practice:
What is too strong and what is too weak?
ISM radio and SRD radio bands are the terms used almost interchangeably. Both radio bands are dedicated for the license-free usage. License-free usage for the end-user – not for the manufacturer:
Common for both – 915MHz and 868MHz:
The 2.4GHz is very popular and has plenty of advantages:
But the main advantages of the 2.4GHz made it too popular for the manufacturers and lead to its main disadvantage:
Another limitation of the 2.4GHz band is higher loss of radio
Frequency of 915MHz is about 2 times higher than the 433MHz band:
For 2.4GHz, the frequency is about 2.5 higher than the 915MHz band and 5.5 higher than for the 433MHz band. As a result:
We created several predefined radio profiles. It is done in order to simplify the options. In practice, there are so many radio settings, that it is impossible to choose properly without a special tool. But, if for some reason you need something special – apart from the predefined profiles – let us know via email@example.com and we will create a special radio profile for you.
Standard antennas are supplied with Super-Beacons, for example. They are short – 50mm. Size affects the performance. In a typical open-space environment they provide up to ~100m radio coverage radius to the similar standard antennas on the Modem v5.1. Standard antennas are omni-directional shortened quarter-wave antennas.
Full-size antennas are larger – 80-160mm – depending on the band. They are more efficient and provide 3-6dB gain – i.e. 1.5-2 time larger range in radio. If used on both ends – beacon’s side and the modem’s side – they allow up to ~400m typical radio range in open space. Full-size antenna are omni-directional quarter-wave antennas.
Directional antennas provide even larger range or stronger radio signals and they have a typical gain of 6-16dB, i.e. can provide 2-6 larger distances. Since they are directional, special considerations must be met to provide the proper coverage. For example, it is worth placing the directional antenna in one size of the elongated warehouse and coverштп the whole warehouse from that side. As an alternative – to place a larger gain full-size omni antenna or two directional antennas back to back facing outwards in the middle of the warehouse. Combining two antennas back to back requires a splitter and additional cables. All of these devices add losses to the radio signal and increase the costs and the complexity. Thus, proper radio planning becomes challenging.
Embedded antennas are used in Mini-RX and in Mini-TX. They have a poorer radio performance as compared to the regular antennas. However, they are very small and placed directly on the board. Their size is the only advantage. The range of ~50-100m is enough for many applications and many use cases. When the radio range of the embedded antenna is not enough, an external antenna is connected to the Mini-RX, for example, in the Badge configuration.
Typically, the easiest and the least expensive way to provide the radio coverage is to improve the antennas:
Polarization is a simple thing for practical implementation: in order to provide optimal radio connectivity, transmitting and receiving antennas must have the same polarization. As simple as that.
Vertically polarized transmitting antennas work the best with vertically polarized receiving antennas and vice versa. The same for horizontal polarization.
Vertically placed antennas, for example, Standard antennas or Full-size antennas, will have the vertical polarization. Placed horizontally, they will have horizontal polarization.
We recommend to use the vertical polarization. It is simply more implementable and practical in real-life cases:
Standard and Full-size antennas are formally speaking omni-directional. But, in reality they are omni-directional in the horizontal plane, if they are placed vertically. In the vertical plane they have a theoretical gain dip to zero. In practice, the gain in the direction where antenna points is not zero, but can be significantly smaller than to the direction perpendicular to the antenna.
It is very difficult to talk about the radiation pattern of the embedded antennas. They may be considered to be omni-directional, but with lower gain and efficiency.
Directional antennas clearly have the direction(s) with higher gain.
There could be multiple sources of interference:
Typical potential suspects:
Antennas are a mechanical element and they can be pretty easily broken, because antennas stick out from the beacons/modems. Make sure they are not mechanically hit or bent where they are not bendable.
We are using reverse SMA connectors in the majority of our equipment. They are considered non-standard connectors, because they are reversed. It is done in order to pass, for example, the FCC (USA) radio certification that requires that antennas are not easily interchangeable, i.e the antennas having non-standard mating connectors as well.
It is very easy to wrongly connect SMA female and reverse SMA male. The connectors will be screwed together well, but there will be no electrical contact. For the external user everything will look like poor radio connectivity.
Antennas must be 1-2 wavelengths from metal parallel to antennas. For the Marvelmind system, it is 50-100cm.
At the same time, antennas/beacons can be placed on the metal easily, if the antennas are perpendicular to the metal. It is possible, because there will be a mirrored virtual antenna in the metal. If parallel and close, it would kill the radio signal. If perpendicular, it will affect mildly or may even improve in some cases.
Proximity to other materials than metal can be affecting as well. For example, concrete, wood, etc. The impact depends on the conductivity of the materials, its permittivity and permeability.
In case of doubts, the recommendation is very simple: keep at least 50-100cm from metal, wires, walls. If not possible, make antennas perpendicular to those wires, metal, walls. Remember to keep the antennas vertical and parallel to antennas of other radio devices in the Marvelmind network.
Example of a very bad antenna placement: antenna is just a couple cm from a metal. The radio performance of the antenna would be completely ruined the radio range suffers most likely significantly
A better placement, but not perfect: antenna is bent and from vertical polarization it becomes horizontal. But it is perpendicular to the metal now and its performance won’t be killed. The horizontal polarization is potentially reducing the performance, but it is a forced and reasonable compromise with the results most likely much better than that in the example above. The antenna position is also not perfect because it is too close to the grey electric splitter. The splitter must be 50-100cm away or behind the antenna to reduce the impact on the antenna
Antenna is placed OK: it is vertical; relatively far from the metal or perpendicular to metal. What shall be improved? – to move right more – farther from the parallel metal part on the left
Obviously, radio cannot go through metal walls. Thus, if you need to track something inside a huge oil or water tank or even inside a van, you must place the beacons and the modem inside the tank as well. See the example:
In some cases, the modem may still be outside, but not because the radio goes through the walls, because it can leak inside via holes or slots. Yes, with losses, but the signal (RSSI) can be still sufficiently strong, and the system may operate without problems. So, you can always test and check.
Already discussed in detail above. Keep antennas with the same polarization – the easiest: vertical and parallel. Otherwise, it is possible to significantly reduce the radio range.
Since Marvelmind Indoor “GPS” systems are typically used indoors, there is always a multi-path radio propagation. The result is that the radio signals may randomly sum up from different directions and be 2-3 times stronger in a particular point; but they can subtract and become 10-100 times lower.
Moreover, the multi-path propagation is not a static process. Something huge and made of metal may be moving dozens of meters away, for example, a forklift or a crane, but the RSSI will be impacted at the antenna point, which is next to you.
Often, if you have difficulties with RSSI, it is worth trying to move the antenna/beacon/modem 10-30cm around the point of the current location. You may see a significant change in the RSSI. But remember, that everything affects the performance. You can’t just tune it and be sure it will stay, since your own body/arms affect the performance. Thus, often it is a try-and-error work until the best radio performance (strongest RSSI and no packet loss) is achieved.