wayne roberts / Mbed OS LoRaWAN_singlechannel_gateway

Synchronous wireless star LoRa network, central device.

Dependencies:   SX127x sx12xx_hal

Alternate to this project gateway running on raspberry pi can be used as gateway.

LoRaWAN on single radio channel

Synchronous Star Network

This project acts as central node for LoRaWAN-like network operating on single radio channel. Intended for use where network infrastructure would never exist due to cost and/or complexity of standard network. This project uses the class-B method of beacon generation to synchronize the end nodes with the gateway. OTA mode must be used. End-node will be allocated an uplink time slot upon joining. End node may transmit uplink at this assigned timeslot, if it desires to do so. This time slot is always referenced to the beacon sent by gateway.

LoRaWAN server is not necessary. All network control is implemented by this project. User can observe network activity on the mbed serial port. Downlinks can be scheduled using command on serial port.

This implementation must not be used on radio channels requiring duty-cycle transmit limiting.

alterations from LoRaWAN specification

This mode of operation uses a single datarate on single radio channel. ADR is not implemented, because datarate cannot be changed. OTA mode must be used. When join-accept is sent by gateway, it will have appended (instead of CFlist) the beacon timing answer to inform of when next beacon occurs, and two timing values: the time slot allocated to this end-node and the periodicity of the network. Periodicity means how often the end-node may again transmit. Beacon is sent for purpose of providing timing reference to end-nodes. The beacon payload may contain a broadcast command to end nodes. Time value is not sent in beacon payload. The time at which beacon is sent provides timing reference: every 128 seconds as standard.

Rx2 receive window is not implemented. Only Rx1 is used because a single radio channel is used. Rx1 delay is reduced to 100 milliseconds. Original LoRaWAN has 1000 millisecond Rx1 delay to accommodate internet latency.

LoRaWAN standard class-B beacon requires GPS timing reference. This implementation does not use GPS, instead a hardware timer peripheral generates interrupts to send beacons. Absolute accuracy is not required, only relative crystal drift between gateway and end-nodes is considered.

Timing values are always specified as 30ms per step as in LoRaWAN standard. Each beacon period has 4096 30ms steps per beacon period.

join OTA procedure

The join procedure has changed the join-accept delay to 100 milliseconds (standard is 5 seconds). This allows end-node to hunt for gateway on several channels during joining. When gateway starts, it will scan available channels for the optimal choice based on ambient noise on the channels. End node will retry join request until it finds the gateway. Gateway might change channel during operation if it deems current channel too busy.

configuration of network

End nodes must be provisioned by editing file Comissioning.h. The array motes lists every end node permitted on network. It contains appEui, devEUI and appKey just as specified in standard LoRaWAN. All provisioning is hard-coded; changing provisioning requires reprogramming gateway. When changing number of motes, N_MOTES definition must be changed in lorawan.h.

#define N_MOTES     8
extern ota_mote_t motes[N_MOTES];   /* from Comissioning.h */

configuring number of end-nodes vs transmit rate

Trade-off can be selected between number of end-nodes on network vs. how often each end-node can transmit.
In this example, where DR_13 is SF7 at 500KHz:

    #elif (LORAMAC_DEFAULT_DATARATE == DR_13)
        #define TX_SLOT_STEPPING        8  //approx 0.25 seconds
        #define PERIODICITY_SLOTS       (TX_SLOT_STEPPING * 6)
    #endif

Here, each end-node is given time of 240ms = 8 * 30ms; accommodating maximum payload length for both uplink and downlink.
6 in this code is the maximum count of end nodes using this gateway. Each end-node can transmit every 1.44 seconds, in this example.
If you wanted to change 6 to 20 end-nodes, each would be able to use network every 4.8 seconds.
Another example: If you wanted to use DR_12 = SF8, you would have approx 2.5 to 3dB more gain compared to SF7, but each end-node must be given double time, resulting in 20 nodes able to use network every 9.6 seconds at DR_12.

network capacity limitation

The number of end-nodes which can be supported is determined by number of SLOT_STEPPING's which can occur during BEACON_PERIOD. Beacon guard is implemented same as standard LoRaWAN, which is 3 seconds prior to beacon start and 2.12 seconds after beacon start, which gives 122.88 seconds for network traffic for each beacon period.

gateway configuration

spreading factor is declared at #define LORAMAC_DEFAULT_DATARATE in lorawan.h, valid rates are DR_8 to DR_13 (sf12 to sf7). In the end-node, the same definition must be configured in LoRaMac-definitions.h. This network operates at this constant datarate for all packets.
Band plan can be selected by defining USE_BAND_* in lorawan.h. 434MHz can be used on SX1276 shield. TypeABZ module and sx1272 only support 800/900MHz channels band.

end-node provisioning

Security permits only matching DevEUI / AppEui to join the network, due to unique AES root key for each end device; in this case the DevEUI must be programmed in advance into gateway. However, if the same AES root key could be used for each end device , then any number of end devices could be added at a later time, without checking DevEUI on the gateway when an end device joins the network. On the gateway, the end device join acceptance is performed in file lorawan.cpp LoRaWan::parse_receive() where MType == MTYPE_JOIN_REQ. A memcmp() is performed on both DevEUI and AppEUI.

If you wish to allow any DevEUI to join, define ANY_DEVEUI at top of lorawan.cpp . In this configuration, all end devices must use same AppEUI and AES key. N_MOTES must be defined to the maximum number of end devices expected. Test end device by commenting BoardGetUniqueId() in end node, and replace DevEui[] with 8 arbitrary bytes to verify gateway permits any DevEUI to join.

RAM usage

For gateway CPU, recommend to consider larger RAM size depending on number of end devices required. ota_motes_t has size of 123 bytes. Each end device has one of these, however if less RAM usage is required for more end-devices, the MAC command queue size may be reduced.

hardware support

The radio driver supports both SX1272 and SX1276, sx126x kit, sx126x radio-only shield, and sx128x 2.4GHz.. The selection of radio chip type is made by your choice of importing radio driver library.

Beacon generation requires low power ticker to be clocked from crystal, not internal RC oscillator.

Gateway Serial Interface

Gateway serial port operates at 115200bps.

Any received uplink will be printed with DevAddr and decrypted payload.