But, after tx completed, we set the Rx1 windower timer and Rx2 window timer simultaneously. So, if end-device detected the preamble during the Rx1 window and start to receive data, but the data packet duration over 1s. Then, after 1s from the beginning of RECEIVE_DELAY1, the receiving status will be abort and enter the Rx2 window timer event. So, even if end-device has been detected the preamble during Rx1 window, but as the data packet duration over 1s, and the RECEIVE_DELAY2 is 1s later after RECEIVE_DELAY1. Thus, end-device can't receive data packet correctly during the first receive window.
So, if the end-device wants to receive data completely during Rx1 window, the time on air should not be exceed 1s? That's right?
But if time on air not exceed 3s, it's will be ok in the Rx2 window, that's right?
"it means that if detected a preamble during Rx1 window or Rx2 window, the max receiving duration is 3000ms"
Yes, this is correct. In the LoRaWAN stack, the packet time will never exceed 3 seconds in TX or Rx.
On the node side, this is ensured by the function "ValidatePayloadLength (...)" which verify the amount of data to be transmitted compared to the data rate.
If the frame is longer than the predefined length, the LORAMAC_STATUS_LENGTH_ERROR status will be returned.
On the GW side, the same kind of mechanism is used so that the time on air of any given packet over the network will never attain the 3 seconds.
"So, if the end-device wants to receive data completely during Rx1 window, the time on air should not be exceed 1s"
No, the limitation is at 3 seconds.
After a Tx, the timer event "OnRxWindow1TimerEvent( )" will occur precisely after 1 second and the radio will go into Rx mode.
After 2 seconds, the timer event "OnRxWindow2TimerEvent( )" will occur and will lead to the configuration of the radio for the second reception window in the function "RxWindowSetup()".
However, before changing the radio, the code checks the radio status with the call to Radio.GetStatus( )
At this stage, or the radio is in "RF_IDLE" state and the code will configure the radio and go into Rx mode.
Or the radio is in another state (meaning the radio is already busy) and the code will simply drop the command until the next irq from the radio.
I would be glad to know who are these experts who "proved" LoRaWAN is impossible :-) There is no magic at play here, only technology and maths!
Lets go back to basics:
Each GW has at least one sx1301, the sx1301 is a huge digital baseband chip which emulates 49 LoRa demodulator and 1 FSK demodulator, all running in parallel.
The sx1301 has 8 channels and it is continuously scanning for the presence of preambles of any datarate on any of these 8 channels. Due to the orthogonalities of LoRa signals between them, the sx1301 can demodulate 8 LoRa packets at different datarate (DR0, DR1, ...DR7), at the same time, and on the same channel (providing you have a sufficient link margin between each packet). If you multiplex these parameters over time and the 8 channels, the number of nodes per GW can become staggering, even more as the network densify and the time on air of each packet decrease (due to ADR).
Everybody wants a definite answer as to how many nodes a GW can support but the answer is 4 dimensional:
- rssi/SNR of the received packets (simultaneous reception on the same channel)
- time on air of the packets (equivalent to datarate, the longer the packet, the longer one demodulator of the GW is used)
- frequency of the packets (two packets with the same datarate and the same RSSI/SNR will collide unless they are on 2 different frequencies).
- number of times per day a node will send a packet (taking resources another node could take)
The answer is thus incredibly complex to calculate because these 4 parameters all have an impact one on another. If you add the legislations which are different for each region (Duty Cycle, LBT,... ), the calcul becomes even more difficult to compute. So in the end you will be disappointed, there is no definite answer and even simulations can go from figures to figures depending on the input parameters. You can of course set your input parameters for your specific case and you will have a certain number but this number won't necessarily be representative of a real life deployment.
PS: to add some fun into the equation, LoRaWAN is all about country wide network deployment. At this level, the geographical distribution of the GWs over the territories also have an impact on the number of nodes the network can support :-)
Why SX1278 switch from SLEEP mode to STANDBY mode need to be configured STANDBY twice to succeed.
SX1278LoRaSetOpMode( RFLR_OPMODE_SLEEP );
SX1278LoRaSetOpMode( RFLR_OPMODE_STANDBY );
SX1278LoRaSetOpMode( RFLR_OPMODE_STANDBY );
If configured STANDBY Mode only once, and then read back the RegOpMode register value, still the SLEEP mode.
Is there a limit to the use of the RegOpMode register?
You don't need to configure Standby twice. The command is not immediate and the radio needs a few us to go from Sleep to Standby (please, refer to the datasheet for exact timing). If you add a small delay between the time you set the radio in Standby and the time you read the register, you will see the radio behave as expected.
Could you confirm that the CRC errors disappear when the modems are separated by a longer distance? This could be related to saturation of the receiver, although we have an internal AGC which takes care of saturation. It may fail for extremely large coupled powers, exceeding the max ratings of the chip. Also, do you see a difference when the "LowDataRateOptimize" is set?
Do you have LoRa IQ Waveform library of narrower than 62.5kHz?
Lower BW are made by simply playing the waveform files at a lower sampling rate. The OSR is set to 4, so play them at 500 kHz for LoRaBW=125 kHz, 250 kHz for LoRaBW=61.25 kHz, 125 kHz for LoRaBW=30.6 kHz, etc...
Thank you very much and best regards,
Nominal is 8 (overall 12 with the built in 4), but the devices also works with 6 at nominal performance. Below starts to be risky
What would a realistic figure be for received signal strength? I would expect:
RSS = RSSI if SNR > 0, otherwise it is RSSI + SNR
Is this correct?
For signals with an RSSI > 100dBm, the SNR seems to top in the range of 7-14dB.
What limits the SNR to this range, as I would expect to continue to increase with RSSI?
The calculation you are proposing is mostly right, and is the simplification we actually propose in our drivers and documentation. It may be made better when SNR is close to 0 (energies sum up), but this approxximation is good enough.
Concerning the SNR, by implementation choice is tails off when it becomes quite positive. You just have to consider that SNR is "suffucient" when over 5dB, and then just rely on RSSI.
Actually, in your calculation, a better description would be "packet power" instead of RSSI, which is effectively what you are trying to quantify.
My setting for SX1278 :
BW= 7.8 K
TXpower = 20 dB
Payload= 2 Byte
Preamble = 8 symbols
receive time= 15 second
Antenna = tow Ygi (7.5 dBi) made myself
My questions :
1- If use from CAD mode i can get more distance?
2- What your offer for receive packets? CAD mode or normal receive mode such as RX continuous - RX single ?
3- for get the long distance i designed yagi antenna (Gain = 7.5 dBi) and get 14 km (LOS) but today i see 30 dBi Omani antenna in Alibaba store i can't believe this, 30 dBi is very very high Gain, i share this antenna link here please tell me your idea about this.
4- what is your offer for preamble lengths for get more distance ?
5- My LoRa module use from 26 MHz TCXO, in the page 81 of sx1278 data sheet say if use from TCXO, bit TcxoInputOn of register RegTcxo should be set to 1. i don't set this bit of register but module worked fine. this is Normal ??! why ?
—- My Ygi Antenna picture (made myself)---
Let's first state the obvious, which is that this type of extreme modulation may give good results in certain propagation conditions, but is also more challenging in terms of reference frequency stability, vibration, channel coherence time vs. symbols time.... in other words unless you have static objects, no vibrations, good TCXOs, I would steer away of the 18 bps mode you are experimenting. Oh, I forgot fast fading issues and moving objects which may create packet loss as your symbol time exceeds 500ms. Now onto your questions:
1. Can is fast to detect LoRa Preamble, but the tradeoff is 3dB in sensitivity. So the answer is no
2. It depends on your power consumption tradeoffs. RxSingle to detect Preamble is good, for an infinite power receiver I'd go for Rx Continuous
3. Yagi is good. 14 km LOS with SF12 and 7.8 k is disappointing. I think you are terrain-limited in your experiment. I recommend a propagation simulation in your location first, we've seen customers reporting 100s of km in these conditions.
4. No impact of preamble length on distance as loong as your preamble gets detected (8 symbols)
5. The TCXO mode will save you power, but the chip will work ireespective of this setting. 26 MHz ????? That is not so good, all LoRa is validated at 32 MHz reference, I strongly recommend sticking to 32 MHz
I am new to LoRa and I would like to learn some more information on how the nodes change their channel. As I have understand LoRa GWs listen (at the same time) at least to 8 different channel frequencies (it depends on GW manufacture). My question is when and how the LoRa node changes it's transmitting channel?
The node can choose its transmission frequency. In case of a LoRaWAN protocol it is a random selection between all available frequencies, see LoRaWAN specification 1.0.2 p6 "The end-device changes channel in a pseudo-random fashion for every transmission. The resulting frequency diversity makes the system more robust to interferences."
Also the aim of having a gateway with 8 channels is to be able to listen to several nodes at the same time.
In the LinkADRReq command, can the network disable the 3 default frequencies with ChMask?
No, the 3 default frequency are part of the specifications and cannot be changed by a MAC command.