What LoRaWAN and "Chirp Spread Spectrum" (CSS) technology have in common?

LoRaWan is a type of network that uses "Chirp Spread Spectrum" so LoRa modulation reference to the extremely long-range data links. Let considered the following quote

"LoRa chirping — Inspired by nature.

It’s not called a chirp modulation by accident. The modulation does have a ton of similarity with bird chirps."

It is getting a lot attention recently because the Internet of Things IoT. You can find specification on SEMTECH website, here or the following link.

According this company "LoRa is the de facto wireless platform of Internet of Things (IoT). LoRa chipsets connect sensors to the Cloud and enable real-time communication of data and analytics that can be utilized?to enhance efficiency and productivity. LoRa devices enable?smart IoT?applications that solve some of the biggest challenges facing our planet: energy management, natural resource reduction, pollution control, and infrastructure efficiency".

The following figure shows why LoRaWAN is a disruptive technology in the IoT.

Let us see the main characteristic of this type of Network:

• Data communication over a long range while using very little power
• Fill the technology gap of Cellular, Wi-Fi and Bluetooth Low Energy (BLE) networks that require either high bandwidth or high power
• Ability to penetrate deep indoor environments and long range
• Flexible for rural or indoor use cases in a wide range of industries: including smart cities, homes and buildings, communities, agriculture, metering and utilities, healthcare, environment, and supply chain and logistics
• A proprietary spread-spectrum modulation technique derived from existing Chirp Spread Spectrum (CSS) technology, LoRa offers a trade-off between sensitivity and data rate, while operating in a fixed-bandwidth channel of either 125 KHz or 500 KHz (for uplink channels), and 500 KHz (for downlink channels)
• LoRa is purely a physical (PHY), or “bits” layer implementation, as defined by the OSI seven-layer Network Model. See following OSI model (page 5 on this doc).
• Air is used as a medium for transporting LoRa radio waves from an RF transmitter in an IoT device to an RF receiver in a gateway, and vice versa
• LoRa uses Chirp Spread Spectrum (CSS) technology offers a low-cost and low-power, it does not require a highly-accurate reference clock
• In a LoRaWAN timing and frequency offsets between transmitter and receiver are equivalent, greatly reducing the complexity of the receiver design
• LoRa modulation has a total of six spreading factors (SF7 to SF12). The larger the spreading factor used, the farther the signal will be able to travel and still be received without errors by the RF receiver

• LORA Frame

• "Chirp Spread Spectrum (CSS) which is method consisting of sweeping a sinusoidal signal frequency in a defined bandwidth. This is performed linearly and can be done increasingly or decreasingly which is what we call either an "upchirp" or "downchirp". Following figure shows an example of both:

• Two main types of CSS: Binary Orthogonal Keying (BOK) and Direct Modulation (DM).
• BOK system transmits the symbols ‘1’ and ‘0’ as a linear up-chirp and down-chirp, respectively and DM uses some kind of linear chirp to spread the data symbol.
• Two parameters to get a symbol period of Ts = 2 x SF x BW method used to encode a symbol a chirp is to add a starting set to the frequency sweep which is given by an offset can be determined by the following relation

• Spreading factors are orthogonal, so signals modulated with different spreading factors and transmitted on the same frequency channel at the same time do not interfere with each other.
• Chirp length is determined by the spreading factor SF.
• LoRa uses two types of packet formats for data transmission: Explicit and Implicit:

• Explicit Format: Preamble 8 symbols + PHDR (Physical Header) optional +PHYPayload (region-specific) + CRC (Error detection uplink messages).

• Implicit Format: the header is removed from the packet where the payload size and Coding Rate are fixed or known in advance.

• The following figure shows Physical layer structure of Uplink and Downlink packets that uses Explicit Mode

• The following figure shows Physical layer structure of Uplink and Downlink packets that uses Implicit Mode

• PHDR (Physical Header) is an optional element only present in the explicit mode that contains information about payload size and CRC (Cyclic Redundancy Check).
• PHDR_CRC (Header CRC) is an optional field that contains an error detecting code for correcting errors in header.

The PHDR and PHDR_CRC are encoded with the Coding Rate of 4/8.

• PHYPayload contains the complete frame generated by the MAC layer. The maximum payload size varies by DR (Data Rate) and is region-specific.
• CRC is an optional field that contains an error detecting code for correcting errors in the payload of uplink messages.
• Preamble is used to synchronize the receiver with the transmitter. It MUST consist of 8 symbols for all regions as mentioned in the LoRaWAN Regional Parameters document. However, the radio transmitter will add another 4.25 symbols resulting in a final preamble length of 8 + 4.25 = 12.25 symbols.
• PHDR (Physical Header) is an optional element only present in the explicit mode that contains information about payload size and CRC (Cyclic Redundancy Check).
• PHDR_CRC (Header CRC) is an optional field that contains an error detecting code for correcting errors in header.

The PHDR and PHDR_CRC are encoded with the Coding Rate of 4/8.

• PHYPayload contains the complete frame generated by the MAC layer. The maximum payload size varies by DR (Data Rate) and is region-specific.
• CRC is an optional field that contains an error detecting code for correcting errors in the payload of uplink messages.

Chirp Spread Spectrum

Chirp modulation is type of modulation employs sinusoidal waveforms whose instantaneous frequency increases or decreases linearly over time. These waveforms are commonly referred to as linear chirps or simply chirps. Hence the rate at which their frequency changes is called the chirp rate. There are quite abundant info available, we provide few links:

• In a video explains how Chirp Spread Spectrum technology works

"The Spreading Factor (SF) is linked to the mechanism that adapts the emission power and transmission speed to the network conditions around the device. The SF enables better radio resource management and optimisation of energy consumption. Speed and radio performance vary according to the spreading factor used. The biggest factor (SF12) gives the longest signal range (highest sensitivity of the technology), and so potentially the best. This also helps with improved geolocation of the device by the network, because the signal emitted will reach the most gateways."

Following table shows the Spreading Factor and correlation with bits per second of data transmitter for 125KHz, The data speed depend on the frequency and it change accordingly: per instance for SF7: 5.5Kbps (125Khz), 10.9Kbs (250kHz) and 21.9Khz (500Khz). More info about frequency Country or region dependent here.

Spread Factor SF: Data bit per seconds bps

SF12: 293 bps SF11: 537 bps SF10: 977 bps

SF9: 1.8 kbps SFP8: 3.1 kbps SF7: 5.5 kbps

Table: Spreading Factors

Here are some conclusions of LORA:

• SF8 takes exact twice the time of SF7 and SF9 takes exact twice time of SF8.
• Symbol Rate (RS), Bandwidth (BW) and Spreading Factor (SF) relation: RS = BW / (2^SF)
• Higher the Spreading Factor -> Higher the over-the-air time.
• Lower The Spreading Factor -> Higher Data Rate
• First 8 up-chirp symbols are preamble symbols used to detect LoRa chirps, next 2 down-chirp symbols are synchronisation symbols used for timing synchronisation followed by the 5 modulated symbols (payload). The jump in the frequency represents the modulated symbol.

LoRaWAN Architecture

They are deployed in a star-of-stars topology.

A typical LoRaWAN network consists of the following elements.

Figure: A typical LoRaWAN network architecture

Receiver Sensitivity?

Higher spreading factors provide higher receiver sensitivity. Usually, LoRa uses higher spreading factors when the signal is weak.

The following table shows how spreading factors impact the receiver sensitivity.

Spreading factor Receiver sensitivity for bandwidth fixed at 125 kHz

• SF7: -123 dBm
• SF8: -126 dBm
• SF9: -129 dBm
• SF10: -132 dBm
• SF11: -134.5 dBm
• SF12: -137 dBm

Chirp Spread Spectrum (CSS) technology explained:

The following figure give a better description between the Bandwidth and Frequency in per unit:

Visit The Things Network for a very extensive info about this type of network:

Terminology:

• Bit Error Rate (BER)
• Symbol Rate (RS)
• Long Range (LoRa)
• Internet of Things (IoT)
• Chirp Spread Spectrum (CSS)
• Signal to Noise Ratio (SNR)
• Low Power Wide Area Network (LPWAN)

What LoRaWAN and "Chirp Spread Spectrum" (CSS) technology have in common? Part II