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What is LoRa? Insight From An Early Alliance Member

First off, this video is the best LoRa intro I have found (caveat: it is very technical):

LoRa is a unique (and awesome) modulation format that can be generated by Semtech LoRa IOT parts, including the SX1272 and SX1276 transceiver chips. This modulation format is best described as a “frequency modulated (FM) chirp.” The core IP that enables LoRa is the ability to generate a stable chirp using a frac-N phase lock loop (PLL). Here can read the core LoRa patent here. Other modulation formats include frequency shift keying (FSK), phase shift keying (PSK), etc. It is important to remember when asking “What is LoRa,” that LoRa itself does not describe system functionality above the physical (RF medium) layer.

The LoRa IoT Protocol

a physical layer (PHY- OSI Layer 1) wireless component

Semtech acquired the LoRa wireless technology through its acquisition of Cycleo SAS of Grenoble, France for $5M in 2012. A bargain!

» Want to find the best wireless technology for your M2M application?

When processing a LoRa message, additional processing gain is achieved due to the modem’s ability to filter on the constant ramp chirp signal. This is how high sensitivity is achieved. In order to achieve “lock” to the LoRa signal, a long “constant chirp” preamble is transmitted. See Figure 1. This is really the power of LoRa, that an inexpensive chip with a cheap crystal, can achieve very high sensitivity.

Figure 1. Semtech LoRa preamble

This preamble can be set as a variable number of “symbols,” which are just the number of chirps. As you might imagine, there is no selectivity between a preamble from one LoRa transmitter vs. another. If there is a constant chirp at the right frequency and at the right chirp rate, a LoRa demodulator will listen to it, whether its from the intended system or not.

Managing a LoRa receiver system to be agile in the face of not just regular power interference, but also LoRa interference, is very important, and is a key part of Symphony Link.

Once a LoRa modem has “locked” on to the preamble signal, the end of the preamble is signaled by the “reverse chirp” that is seen in Figure 1.

Then the data transmission begins, which has a series of “symbols” that function much like M-ARY FSK symbols, but instead happen on a chirp. See Figure 2.

Figure 2. LoRa Data Modulation.

Another powerful feature of LoRa is the ability to demodulate several “orthogonal” or simultaneous signals at the same frequency, assuming they have different chirp rates. In the datasheet, LoRa chirp rates are called “spreading factors,” with higher spreading factors denoting slower chirps. This function is supported by the SX1301 chip from Semtech.

All Link Labs gateway systems have the ability to decode many simultaneous LoRa chirps simultaneously, which allows very large networks to be built.

Building such a LoRa network or system requires a tremendous amount of development. Going from LoRa to a functioning wireless system, is analogous to going from having a BPSK radio chip to having a WiFi network. The OSI layer 2 and above functions of large networks that include gateways, repeaters, addressing, adaptive data rates, message retries, message acknowledgements, and high capacity OFDM downlink signals are the function of systems like LoRaWAN and Symphony Link.

» Learn more about Link Labs’ Symphony Link system.

LoRa Alliance

There has been a movement to standardize the MAC features for LoRa called the LoRa Alliance, of which Link Labs was an early member. We’ll do a future post outlining the specific ways Symphony Link is different from LoRaWAN. Symphony Link is specifically focused on ITU region 2 (915 MHz ISM Band). LoRaWAN is ideally suited for ITU region 1, where ETSI transmitter duty cycles restrictions greatly limit the role the base station can play in the network.

Some details on LoRaWAN, the LoRa Alliance protocol for LoRa:

  • LoRaWAN is a server-side implementation of a multiple access protocol designed to minimize collisions with a large number of endpoints. It requires a server application to run the MAC functions over a network connection.
  • LoRaWAN network architecture is typically laid out in a star-of-stars topology in which gateways are a transparent bridge relaying messages between end-devices and a central network server in the backend.
  • Customer logic is built into the network server
  • It is designed primarily for uplink-only applications with many endpoints, or applications where only a few downlink messages are required (limited either by application or by number of endpoints)
  • Gateways within the same network require synchronization
  • Communication between end-devices and gateways is spread out on different frequency channels and data rates. The selection of the data rate is a trade-off between communication range and message duration.
  • Different data rates do not interfere with each other and create a set of "virtual" channels increasing the capacity of the gateway.
  • The LoRaWAN network server is manages the data rate and RF output for each end-device individually by means of an adaptive data rate (ADR) scheme that is typically updated once every 24 hours
  • Multiple layers of security/encryption (EUI64 on network level and application level and EUI128 device specific key)
  • AES CCM (128-bit) for encryption and authentication
  • Works within the confines of the ETSI 1% and 10% duty cycle on transmission time in the 868 bands
  • Draft revision of class B for downlink nodes that can poll for a beacon every 1s to 128s (Engineering prototypes available now using LMiC from IBM)  Beacon period is 128s (2^n) where n is 0 to 7
  • Antenna diversity because all gateway's listen to the same uplink channels

LoRa, Semtech, LoRaWAN are trademarks of Semtech.

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Written by Brian Ray

Brian is the Founder and CTO of Link Labs. As the chief technical innovator and leader of the company, Brian has led the creation and deployment of a new type of ultra long-range, low-power wireless networking which is transforming the Internet of Things and M2M space.

Before starting Link Labs, Brian led a team at the Johns Hopkins University Applied Physics Lab that solved communications and geolocation problems for the national intelligence community. He was also the VP of Engineering at the network security company, Lookingglass, and served for eight years as a submarine officer in the U.S. Navy. He graduated from the U.S. Naval Academy and received his Master’s Degree from Oxford University.

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