Amateur radio ESP32 based LoRa APRSDroid KISS Bluetooth modem + LoRa APRS-IS RX/TX iGate server over WiFI + digipeater + Codec2 DV modem (in conjunction with Android frontend application)
- you need to use **APRSDroid** application (https://aprsdroid.org), connect to the modem using bluetooth, data will be re-transmitted through the LoRa radio, this is similar to APRSDroid micromodem - https://unsigned.io/micromodem/, received data will be sent back to the APRSDroid using bluetooth. By having two clients you can not only send your position, but also send and receive APRS messages
- it is also possible to use **other KISS APRS clients** over Bluetooth serial, just use `rfcomm` on Linux to setup serial over Bluetooth and put up AX25 interface with `kissattach`, then use any existing Linux APRS clients, such as `xastir`, see section below for alternative Linux usage
- **LoRa APRS iGate RX/TX server over WiFi + Digipeater**
- **RF to APRS-IS gating**, it will connect to your WiFI and will forward received APRS positions from RF LoRa into the APRS-IS network, it also reports client signal level, by appending it into the APRS comment, so you can see your signal reports in different locations (could be enabled or disabled from config). This way, it is also possible to setup portable iGate by connecting to your mobile phone's hotspot and provide power from the phone USB port by using OTA cable
- just install https://github.com/sh123/codec2_talkie on you Android phone, pair with the modem and you can communicate with each other by using digital voice [Codec2](http://www.rowetel.com/?page_id=452)
All work was done on ESP32-WROOM with custom made LoRa shield, if your ESP32 board is compatible then it should work, but there might be need to redefine pinouts to LoRa module if it differs (see further description in Software Setup section), currently pinouts are connected from LoRa to ESP32-WROOM as (SS/RST/DIO0 could be redefined in loraprs_service.h):
- for server mode fill `cfg.AprsFilter`, see http://www.aprs-is.net/javAPRSFilter.aspx for various formats, do not include `filter` directive, just space separated values
- change `cfg.LoraFreq` if you are planning to use different frequency or if planning to calibrate clients, currently it is set to **433.775MHz** as per https://vienna.iaru-r1.org/wp-content/uploads/2019/01/VIE19-C5-015-OEVSV-LORA-APRS-433-MHz.pdf
- if you are planning to experiment with different bandwidths/spread factors then modify values in `initializeConfig()`, with current parameters APRS packet time on air is around **1-2 seconds** @ 292bps/36Bps, to decode with as lower level as possible with reasonable speed (comparable to 300bps HF APRS), use https://github.com/tanupoo/lorawan_toa or https://www.rfwireless-world.com/calculators/LoRa-Data-Rate-Calculator.html to make further calculations
- use 80 MHz ESP32 frequency in Arduino SDK, it will prolong battery life when operating portable, higher CPU speed is not required, there are no CPU intensive operations
- or calibrate clients based on server frequency drift report by changing `cfg.LoraFreq`, for example, let client and server run for an 30-60 minutes and if server reports err: -1500, then set client frequency to about 1000 kHz less, e.g. instead of 433.775 set it to 433.774, this will give couple of additional dB
- alternatively automatic calibration could be done on server or client side by enabling automatic frequency correction by setting `cfg.EnableAutoFreqCorrection` to `true`, might be suitable for experiments where only one client is operating or if server broadcast messages, so clients can correct their frequency. Use it either on client side to tune all clients to server frequency if TX is enabled on iGate side or to tune server to single client for test purposes
-`cfg.EnableBeacon` set to `true` to enable periodic beacons specified in `cfg.AprsRawBeacon` with period specified in `cfg.AprsRawBeaconPeriodMinutes` into RF and APRS-IS if `cfg.EnableRfToIs` is enabled
- Make sure LoRa sync word and other LoRa parameters match
- **Client** should be interoperable with other clients, which transmit raw text aprs messages, in this case select "TNC (plaintext TNC2)" in APRSDroid protocol options
- **Server** supports only classical `AX25` frames over LoRa (as defined in http://www.aprs.org/doc/APRS101.PDF page 12). It should enable interoperability with classical Linux APRS software, such as Xastir with `kissattach`. Some LoRa ARPS implementations transfer plain text APRS messages over LoRa, as a result **Server** will not be able to process these messages and gate them to APRS-IS, also clients won't be able to decode messages gated from APRS-IS to RF by the server. Server support for plaintext APRS messages is planned.
- You can setup `AX25` network interface with `sudo kissattach /dev/rfcomm0 ax25` command, but previously need to update `/etc/ax25/axports` with new line as `ax25 CALLSIGN-10 9600 255 1 comment`, you can also specify IP address if there is a need to run TCP/IP over AX25
- LoRa library, which is in use by this project does not implement CAD, but CSMAp is utilized by this project as per KISS specification. TX path is executed only when there is no incoming data returned by `LoRa::parsePacket` and TX path is executed with probability p (CSMA persistence), configured by const `Loraprs::Service::CfgCsmaPersistence` in `loraprs_service.h`. Random value is selected between 0 and 255 and TX is executed only when it is lower than `CfgCsmaProbBoundary`.
- To decrease TX probability in case of high traffic use lower value. `Loraprs::ServiceCsmaSlotTimeMs` configures the amount of time in milliseconds to wait if transmission was not performed due to persistence, select lower value for lower TOA (time on air). It is also possible to dynamically override these parameters with KISS P 0x02 and SlotTime 0x03 command codes from the client.
- This modem could be used in conjuction with [Android Codec2 Walkie-Talkie](https://github.com/sh123/codec2_talkie), when application connects to the modem, instead of sending AX25 APRS packets it sends Codec2 speech encoded frames. This enables digital voice communicaiton between one or multiple modems.
- Select appropriate lora spread factor `cfg.LoraSf` and bandwidth `cfg.LoraBw` depending on Codec2 speech rate from 450-3200 bps. For example, if you are using 450 bps mode and 20 kHz bandwidth then set spreading factor to 6 or 7. See data rate table above.
- When using modem for voice communication `Loraprs::Service::CfgCsmaPersistence` must be set to maximum 255 value to disable CSMA, otherwise real time voice communication won't be guaranteed. Android codec2_talkie application automatically sets this parameter to 255 by using KISS P command code.
- Also, it might be useful to disable CRC check for LoRa packets with `cfg.LoraEnableCrc` parameter equal to `false`. Some broken bits in one speech frame will cause audio being scrambled, it might be better then longer gap when complete packet is dropped.
- With such low power it is very important to have antenna SWR close to 1, many rubber duck antennas are claimed to be 433MHz, but they do not resonate at that frequency at all or resonate only when attached to its native large handheld transceiver, which has enough metal inside to behave as a counterpoise, these antennas have SWR 2 or higher. Check your antenna on antenna analyzer before using, add wire counterpoise if needed or better to use dipole or halo home made antenna for that matter
- Range (20 KHz channel width and 9 spreading factor, also got similar results with 125 kHz and 12 SF), it will mostly depend on your base station's antenna elevation
- Successful decodes down to **-19.75dB** below the noise floor when using compressed APRS coordinates (smaller packets, about 50 bytes, **32 bytes without PATH, speed, altitude**), see APRSDroid discussions on compressed corrdinates support and custom branches
- Monitor your planned frequency, such as 433.775 MHz for ISM device activity, if there is strong interference from other users tune up or down it to minimize interference, it might be critical for long range
- It might be useful to add additional pass band filter or broadcast FM radio reject filter, it seem to improve sensitivity when using external base antenna
