Lora vs Xbee - Page 1
Forum section: electronics / rf-microwave

author: Deku Tree
author_rank: Contributor
timestamp: March 02, 2019, 03:19:32 am
content: I'm not an RF engineer, so please forgive my ignorance when it comes to these things. I'm currently working on getting my HAM license so I suppose that will help.I've been tasked with picking a radio system for a high-altitude balloon system. We want full duplex communication and are currently using these modules at 9600 baud sending GPS and some telemetry info back and forth between the balloon and our ground station.https://www.digi.com/products/embedded-systems/rf-modules/sub-1-ghz-modules/xbee-sxThe system works fairly well, but we have issues with signal strength at some of the altitudes we are pushing. I think most of the issue is poor antenna placement, but as I was looking around at other systems I came across LoRa modules like the SX1278/SX1276 and thought they might be a good alternative. Without buying each module and range testing, do any of you have experience with either of these systems? Which would be a good choice for this kind of application?From an innitial look at the specs, it looks like the LoRa modules have a much higher sensitivity, but are half-duplex so we would need to write some code to handle this. It also looks like the LoRa has a lower data rate, but with the amount of data we are sending this doesn't limit us much. Also the XBee modules are rated to 1W whereas the LoRa modules are around 100mW.If performance and ease-of-integration are the only metrics, what's the best choice here?

author: OwO
author_rank: Super Contributor
timestamp: March 02, 2019, 03:28:17 am
content: Also look into the AX5043, it has sensitivity on par with LoRa at the same bitrates.

author: Deku Tree
author_rank: Contributor
timestamp: March 02, 2019, 04:47:34 am
content: Looks like these might not be legal in the United States? I don't believe we can transmit on 868MHz.

author: radiolistener
author_rank: Super Contributor
timestamp: March 02, 2019, 05:33:09 am
content: you're need to take into account signal loss and antenna gain.Since your balloon system will works in the line of sight (with no use reflections), you can calculate it as free space loss.For example for 10 km and 900 MHz, signal loss will be for about -111.53 dB.transmitter power 1W = 30 dBm.It means that with isotropic antenna on both side you will get signal level on the receiver side:30 dBm - 111.53 dB = -81.53 dBmWith half wavelength dipole (antenna gain = 2.15 dBi) on both side, you will get signal level on the receiver side:30 dBm + 2.15 dBi + 2.15 dBi - 111.53 dB = -77.23 dBmBy using directional antennas you can simply increase signal level on receiver side, just replace 2.15 dBi with your antenna gain in the formula above.But directional antenna requires to be directed in the right direction, because it radiates power primary at it's main direction.Signall loss, you can calculate here:https://www.everythingrf.com/rf-calculators/free-space-path-loss-calculatorIn such way, you can estimate which power and antenna gain is required for your system depends on the balloon altitude, and frequencyQuote from: Deku Tree on March 02, 2019, 04:47:34 amhe can use 145 or 433 MHz band, signal loss will be even 10 dB smaller.But there is may be also problem with license, because it may require specific license for balloon flight transmitter.

author: georges80
author_rank: Frequent Contributor
timestamp: March 02, 2019, 05:45:14 am
content: There are 900MHz Lora modules and they are 'legal' for use in the 900MHz US ISM band.Lora has some amazing range capability - this achieved by reducing bandwidth and/or increasing the spreading factor. This reduces bit rate and increases sensitivity.The real question is what range do you require and what bit rate can you live with for your telemetry. From that you can start calculating sensitivity (Semtech has a Lora calculator you can download).Given you balloon is presumably line of sight to your base station, you should be able to reach many many kilometres of range. By many we're talking in the 50km+ range (with reduced bit rate). There's been a few multi-hundred kilometre (balloon) packet records - but of course that is not likely 100% packet reliabilityWould pay you to google for Lora Range Tests to get an idea of what folk have achieved. Getting line of sight over land in the 20km - 30km range is fairly typical with excellent packet reliability.cheers,george.

author: Deku Tree
author_rank: Contributor
timestamp: March 02, 2019, 06:02:48 am
content: radiolistener: This makes sense, and from this it seems like a dipole in the air and a relatively low gain patch/yagi could give me the range I need with either system. I'll also look into the legality, although it's nice to know that the XBees are FCC certified which makes my life a little easier.georges80: Some of those range tests online are what initially got me interested in the possibility of using LoRa for this. One of my other concerns is that the LoRa modules I'm looking at are only half-duplex, so presumably I would need to write some software to deal with two-way communication.Good tip on the LoRa calculator, I'll check it out. At this point I just need a solid link up to around 11km, so LoRa is certainly looking capable of that.

author: radiolistener
author_rank: Super Contributor
timestamp: March 02, 2019, 06:12:34 am
content: By using high gain antenna, you can achieve much better performance than by using more powerful transmitter. But the high gain antenna needs to be properly directed.Higher antenna gain = better precise of antenna direction is needed.Also high gain antenna is more complicated and more expensive.So there is need for some compromise which depends on your needs.

author: georges80
author_rank: Frequent Contributor
timestamp: March 02, 2019, 06:24:09 am
content: Quote from: Deku Tree on March 02, 2019, 06:02:48 amYeah, 11km is nothing for Lora. Even a simple 1/4 wave antenna on each end can deal with that easily. You wouldn't even need to run any of the crazy high spreading factors and/or low bandwidths.Lora - as is xbee etc are all 1/2 duplex. You can't receive while transmitting... There's a software stack that deals with bidirectional communications. Radiohead is a free radio stack that you can extract the bits you need for your purposes. It targets many radios, so you can just select the 'driver' for the SX1276 modules. If you want to source within the US you can get an RFM95 module on digikey (900MHz flavour).cheers,george.