Optimize network for mobility with RM502Q-AE

[kuon45]

I have a RM502Q-AE modem connected to a raspberry pi.

On the modem, I have this antenna: https://www.digikey.ch/en/products/deta ... C/22108774

I am using this setup for streaming while flying a small general aviation plane.

The current modem configuration is the factory default.

I am wondering if there is anything I can do to improve the bandwidth and connection stability.

I need about 2mbit/s upload. I often get about 1.5mbit/s and it can be steady but the connection changes tower, it cuts completely for several seconds.

I am looking for advice like:

- other antenna
- modem configuration change
- linux network stack adjustment

I use modem manager, and here is the output I get:

Code: Select all

  --------------------------------
  General  |            dbus path: /org/freedesktop/ModemManager1/Modem/0
           |            device id: 8676dd622bdeb94cf751d0f6740810142d789752
  --------------------------------
  Hardware |         manufacturer: Quectel
           |                model: RM502Q-AE
           |    firmware revision: RM502QAEAAR11A04M4G
           |            supported: gsm-umts, lte
           |              current: gsm-umts, lte
           |         equipment id: 867826050911932
  --------------------------------
  System   |               device: /sys/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.2
           |              drivers: option1, cdc_ether
           |               plugin: quectel
           |         primary port: ttyUSB2
           |                ports: ttyUSB0 (qcdm), ttyUSB1 (gps), ttyUSB2 (at), ttyUSB3 (at), 
           |                       usb0 (net)
  --------------------------------
  Status   |       unlock retries: sim-pin (3), sim-puk (10), sim-pin2 (3), sim-puk2 (10)
           |                state: disabled
           |          power state: on
           |       signal quality: 0% (cached)
  --------------------------------
  Modes    |            supported: allowed: 2g, 3g, 4g; preferred: none
           |              current: allowed: 2g, 3g, 4g; preferred: none
  --------------------------------
  IP       |            supported: ipv4, ipv6, ipv4v6
  --------------------------------
  3GPP     |                 imei: 867826050911932
  --------------------------------
  3GPP EPS | ue mode of operation: csps-2
  --------------------------------
  SIM      |            dbus path: /org/freedesktop/ModemManager1/SIM/0

[Didneywhorl]

Need to know the carrier and plan you are on as well as your signal metrics when you are having the issues:

AT commands:

AT+QRSRP

AT+QSINR

AT+QENG="servingcell"

[kuon45]

At present I have about 5/1.5mbit/s bandwidth from iperf. And here are the results:

'+QENG: "servingcell","NOCONN","LTE","FDD",228,02,4FD405,39,6200,20,3,3,A488,-108,-17,-74,7,0,-,19'
'+QRSRP: -110,-108,-32768,-32768,LTE'
'+QSINR: -13,-4,-32768,-32768,LTE'

my carrier is sunrise switzerland with unlimited data plan.

[Didneywhorl]

RSRP is low, you'll want to get that under 99, preferably as close to -65 as possible. -65 RSRP means your sitting on the cell tower, pretty much.

Better antennas is going to likely be the key here, as the modem is struggling to communicate BACK to the tower, way worse than it is struggle to "Hear" the tower, and it is struggle to hear the tower at RSRP in the -110's range.

[kuon45]

Ok, I thought the antenna I picked would be a good fit. Do you have a recommendation?

[BillA]

Ok, I thought the antenna I picked would be a good fit. Do you have a recommendation?

Due to your constantly changing direction and location in a plane, I would try some high gain omni directional antennas (flat or round) stuck onto a window where you get the most amount of signal. Otherwise the plane acts as a Faraday cage with the antennas inside. It would be even better to stick the four antennas onto separate windows on opposite sides of the cabin to maximize your signal pickup. Directional or flat panel antennas would not work well due to your constantly changing direction and location. Unfortunately carriers point their antennas downwards to cover more ground level uses, therefore you're probably picking up signals bounced off the ground and buildings.
Another important settings would be to set the modem to fully automatic band selection, again due to the constantly changing towers as you're flying by.

[kuon45]

What is the configuration to check on the model to be sure it has fully automatic band selection?

I tried a few antennas I had running around, but they are all worse than the one I have (around -112 instead of -109).

The modem has 4 antenna connector, but it seems only one is used by LTE, I see signal change only when port 2 is unplugged. I found out that port 4 is GPS, so I guess port 1 and 3 depends on the band used.

[kuon45]

Do you have any antenna reference I should try? I looked at digikey, but there are so many shape and types, I am lost to what would be best.

[BillA]

What is the configuration to check on the model to be sure it has fully automatic band selection?

I tried a few antennas I had running around, but they are all worse than the one I have (around -112 instead of -109).

The modem has 4 antenna connector, but it seems only one is used by LTE, I see signal change only when port 2 is unplugged. I found out that port 4 is GPS, so I guess port 1 and 3 depends on the band used.

While it works even with two antennas with good signal, the other two diversity antennas can help in weak signal locations.
So it's important to have all 4 connected to get the best possible signal.
Instead of DigiKey look for antennas at TheWirelessHaven.com
https://store.thewirelesshaven.com/coll ... r-antennas
Specifically get 4 of these Quectel Sticker Type Omni Antennas and you can stick them with the included adhesive onto four different windows in your plane (two on the left, two on the right) which should give you the best possible signal from all sides.
https://store.thewirelesshaven.com/prod ... -type-mhf4

I don't have the specific auto-band selection commands, but you can find it in the Quectel RM502Q-AE user-manual, or some routers have a GUI settings built in.

[kuon45]

Thanks, I will try this configuration. I use digikey, because we have daily package from them at work and I can include my items in it with free shipping, but they have those quectel antennas.

For the modem configuration, I downloaded the list of AT commands, but I find nothing about manual band selection, so I guess it's always automatic. (I can restrict the allowed band, but by default all are enabled)

[BillA]

Thanks, I will try this configuration. I use digikey, because we have daily package from them at work and I can include my items in it with free shipping, but they have those quectel antennas.

For the modem configuration, I downloaded the list of AT commands, but I find nothing about manual band selection, so I guess it's always automatic. (I can restrict the allowed band, but by default all are enabled)

If you have never done any band settings, it should be on automatic by default. Some routers have built-in auto or band locking feature (ex GoldenOrb firmware).
As I mentioned earlier, due to your constantly changing location in a plane, it's best to leave it on automatic to ensure that the modem always seeks out the best bands/channels. If you were stationary then band locking may be useful.
If possible stick the antennas onto the inside of your plane's window, two in the front left&right side, and two in the rear left&right side.
That should provide good separation between the main and diversity antennas, and the best possible signal from all directions.