LoRa

Stands for “Long Range”.

To be used in long-lived battery-powered devices scenarios.

Semi-proprietary.

• Parts of the protocol are well documented, others not.

• They own the radio part (but sub-licensing is on the way).

• You can install your own gateways.

LoRa usually means two different things:

• LoRa: a physical layer that uses Chirp Spread Spectrum (CSS) modulation.

• LoRaWAN: a MAC layer protocol.

Stack

Physical layer

Developed by Semtech.

Low-range, low-power and low-throughput.

Operates on 433-, 868- (EU) or 915 (US) MHz bands.

Payload from 2 to 255 octets (2Kb).

• Depends on configuration parameters.

Datarate: up to 50Kbps.

In Europe, 8 channels with a bandwidth of 0.3MHz are used.

WAN

MAC mechanism for controlling communications between end devices and LoRaWAN gateways. For all devices, it manages:

• Communication frequencies.

• Data rate.

• Power.

Open Standard developed by the LoRa Alliance.

Network

Star of stars topology.

Devices transmit data asynchronously.

• Data is received by multiple gateways.

• Each gateway forwards received data to a centralized network server, using a backhaul link (Ethernet or cellular).

• The network server:

  • Filters duplicate packets

    • Packet with the strongest signal gets decoded

  • Realizes security checks.

  • Manages the network

Architecture

Physical Layer

Modulation.

• (changing a signal, the carrier, in a way that allows it to contain information to be transmitted).

LoRa uses a proprietary Spread-Spectrum modulation technique: Chirp Spread Spectrum (CSS).

(A chirp is a signal in which frequency raises or lowers with time)

• Tries to increase range by:

  • Sending information with more power (within regulated values - <14dBm or 25mW).

  • Or by lowering the data rate.

• Increases link budget.

• Increases immunity to in-band interference.

This, along with Forward Error Correction techniques, contribute to extend the range and robustness of radio communication links.

• Compared to FSK.

Has different Spread Factors (SF7 to SF12).

• Spread factors can set the modulation rate and tune the distance.

• They indicate how fast or slow is the chirp (how many chirps you get per second) → how much data you can encode per second.

  • The higher the SF, the lower the datarate.

  • Each SF is 2x slower than the one before.

  • The slower you send your data, the farther you can send it.

    • The interface has more time to decode and sensititviy is increased.

This helps on scaling the network.

• Closer nodes receive data much faster.

• Air is ”cleared” for other nodes to transmit.

• By adding more gateways, devices get nearer to them, applying the above.

For a 125kHz bw (configurable by design).

The Bandwidth (kHz), Spreading Factor and Coding Rate are design variables that allow a system to optimize the trade-off between.

• Occupied bandwidth.

• Data rate.

• Link budget.

• Interference immunity.

By using software, it is possible to combine these values to define a transmission mode.

Bandwidth.

• Show how wide is going to be the transmission signal.

• 3 options: 125 kHz, 250 kHz or 500 kHz.

• Greater reach: 125 kHz.

• Greater transmission speed: 500 kHz.

• Less bandwidth = more airtime = more sensitivity = more battery consumed.

Coding Rate.

• 4 options: 4/5, 4/6, 4/7 and 4/8.

• Meaning:

  • Every 4 useful bytes are going to be encoded by 5, 6, 7 or 8 transmission bits.

• Smaller coding rate: 4/8.

• Lower coding rate = more airtime.

Spreading Factor

• Number of chips per symbol used in data treatment before the transmission signal.

• 7 options: 6, 7, 8, 9, 10, 11 and 12.

• Greater Spreading Factor = Greater Range = more air time.

WAN

Components

End-Device.

• Devices (low-power) that communicate with the LoRa Gateway.

• They are not associated to a particular gateway.

• They are, however, associated to a Network Server.

Gateway.

• Intermediate devices that relay packets between end-devices and a network server.

• Linked to the Network server via a higher bandwidth backhaul network.

• They add information about the quality of reception, when forwarding a packet from an end-device to a network server.

• They are transparent to the end-devices.

• There are multiple gateways in a network.

• Multiple gateways can receive the same packet transmitted from the same end-device.

Network Server.

• Decodes and de-duplicates packets sent from devices.

• Generates packets to be sent towards devices.

• Choses the appropriate gateway to send packets to a specific end-device.

Device Classes

End-devices classes

Class A – bi-directional.

Lowest power consumption.

Devices schedule uplink transmissions according to their requirements, with a small variation before transmission.

Each uplink transmission is followed by two short downlink receive windows.

• Downlink transmissions at any other time have to wait until the next uplink transmission.

• Less flexibility for downlink.

Class B – bi-directional with scheduled receive slots

Devices open more receive windows at scheduled times.

There is a synchronized beacon from the gateway to the network server, indicating when the device is listening.

Class C – bi-directional with maximal receive slots

Greatest power consumption.

Almost continuous receiving windows.

• Server can initiate transmission almost anytime.

End-Device Duty Cycle

Besides transmission frequency, duty cycle regulations apply.

Delay between successive frames sent by a device.

1% limitation for end-devices.

• Device has to wait 100x the time it took for it to send the message, in order to be able to send again in the same channel.

Gateways: 10%.

Payload

MAC Commands.

• Allows the network to customize end-device parameters.

Checks.

• Link status (this can be send by the end-device itself).

• Device battery.

• Device margin (SNR).

Settings.

• Datarate.

• TX power.

• TX and RX channels.

• RX timing.

• Repetition.

• Duty cycle.

• Dwell time.

End-Device Connection to a network.

• Also known as Activation.

This process provides the end-device with:

• End-device address (DevAddr): An identifier composed by the network identifier (7bit) and by the end-device’s network address (25bit).

• App identifier (AppEUI): Unique identification of the end-device owner.

• Network Session Key (NwkSKey): A key used by both the network server and end-device to verify and ensure message integrity.

• App Session Key (AppSKey): A key used by both the network server and end-device to encrypt the payload of received messages.

Note on security:

• LoRaWAN protocol security is based on 802.15.4.

  • AES-128.

To activate the device, there are two procedures:

• Over-the-Air Activation (OTAA).

  • Join-Request and Join-Response messages are exchanged in each new session, allowing the end-devices to obtain the network and application session keys.

• Activation By Personalization (ABP).

  • The devices have both keys already stored internally.

Adaptive Data Rate.

• The network tells the node at which data rate it can send data.

  • Manages the SF for each end-device.

• The aim is to:

  • Optimize for fastest data rate versus range.

  • Maximize battery life.

  • Maximize network capacity.

Typically, there is no node-to-node direct communication.

• LoRaWAN allows this by having 2 gateways and a network server in between the nodes.

However, most end-device vendors also include (for testing, mostly) a raw form of LoRa.

• Allows peer-to-peer communication between nodes.

• Contains only the link layer protocol.

• Only allows a very small number of nodes in a topology.

  • There is no packet management.