IMST IC880A Supported Channels

Gateways
1

To my understanding in the IMST IC880A gateway, the Semtech SX1301 IC acts as a DSP which takes in 2 streams of IQ data from 2 SX-1257 ICs. The datasheet of SX1301 IC can says it can listen to 10 channels. 8 standard, 1 GFSK and 1 backhaul.

In the Global Config File , in addition to 8 multi_sf channels (normal channels nodes send data on) these two additional channels are also configured. Please see that part below.

 "chan_Lora_std": {
            /* Lora MAC channel, 250kHz, SF7, 868.3 MHz */
            "enable": true,
            "radio": 1,
            "if": -200000,
            "bandwidth": 250000,
            "spread_factor": 7
        },
        "chan_FSK": {
            /* FSK 50kbps channel, 868.8 MHz */
            "enable": true,
            "radio": 1,
            "if": 300000,
            "bandwidth": 125000,
"datarate": 50000

I have two questions based on this.

  1. What is the dwell time for these channels? I believe the front end SX-1257 ICs must be going through these channels as well. Is the dwell time different for these?

  2. In a normal IMST IC880A gateway, do both SX-1257 ICs keep shifting through the 10 channels all the time?

Thanks a lot for help.

2

David, reading your questions, it looks like you assume there is one receiver that is scanning the different channels and as such you are asking how long does it “dwell” on each channel. Actually there are 10 separate receivers, all are active 100% of the time. Each SX1257 also have one transmitter but only one is activated in a gateway, see config file, radio 1 tx is disabled, only radio 0 tx is enabled. Hope this helps with your understanding.

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3

Dear Tony,

Thanks for the eye opening answer.

I know for a fact that the IMST IC880A gateway has only 2 SX1257 receivers which share the load of 8 channels and receive simultaneously from all 8 LoRa channels. Did you mean to say that a single SX1257 has multiple receivers inside?

I’ve always has this problem as to how could the SX1301 DSP know which IQ data correspond to which physical channel if the SX1257 RF front end is cycling through multiple physical channels time to time(dwelling through channels). Your answer makes me think differently. Thank you. However, your answer lead me to few different questions. Perhaps you may like to help me a bit further.

To my understanding, the IQ data sent from the SX1257 RF front end to the LoRa SX1301 DSP via a serial interface, I.E bit by bit. A standard LoRa gateway ( such as the IMST IC880A) has only 2 SX1257 front ends. I paste a diagram of the SX1257 internals below.
sx125x

sxr

It seems to me from the diagram that the SX1257 can only tune to one PLL frequency at a time. I.e. It sends a single frequency quadrature clock (2 clock signals, 90 degrees offset) to the Mixer.

Now lets compute the channels in the 868MHz ISM Band. It has 8 channels 200KHz apart.
so that’s a total of more than 800KHz BW per each front end IC as the Standard gateway has to sample 8 channels.

So I am confused how these 2X SX1257 ICs manage to sample a BW of 1800KHz to capture all LoRa channels.

Please help.

4

This equals 8 * 200 = 1600. And 1600 / 2 = 800 which fits perfectly within the 925 KHz available when using 125 kHz LoRa channels.

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