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LoRa 2.4 GHz

The ability to achieve high communication ranges both for outdoor and indoor applications is often of essential interest. In Europe, such applications are often realized in the license-free sub-gigahertz frequency bands at 433 MHz or 868 MHz. The reason for this is that applications that are operating on low frequencies are subject to less propagation losses compared to applications that are operating on higher frequencies. That means that working on lower frequencies enables to achieve higher communication ranges when the transmit power is kept the same.

However, the advantage of higher range is faced by two disadvantages of those sub-gigahertz frequency bands. First, a duty cycle that has to be fulfilled and second the fact that the sub-gigahertz frequency bands are not harmonized worldwide. Especially the latter can lead to enormous costs for the necessary radio approval outside of Europe.

Especially applications in the field of home automation, such as light control, heating, air conditioning, and automatic meter reading (AMR) are currently often developed in the sub-gigahertz frequency range due to the high range requirement. The limitation by a duty cycle is normally accepted as the application by itself rarely transmits data. But sometimes a higher transmission rate is desirable, especially when thinking about some sensor applications. From a worldwide regulatory point of view the sub-gigahertz frequency range looks like a rag rug, while the 2.4 GHz band is globally harmonized and therefore offers a better alternative. In addition, an application within the 2.4 GHz frequency band is not subject to any duty cycle restrictions.

Well known wireless technologies that work within the 2.4 GHz frequency band are WLAN, Bluetooth® or Zigbee. But without any additional power amplifiers those technologies can only achieve communication ranges of a few tens meters. Higher range requirements result directly in complex and expensive radio systems with power amplifiers and higher power consumption. Those radio systems are probable not suitable for battery-driven devices.

This balancing act between long range, battery-driven operation and the use of a worldwide harmonized frequency band is only achieved by the iM282A. It is able to achieve communication ranges of more than 10 km (LoS) within the 2.4 GHz frequency band. The iM282A provides a powerful Cortex M3 controller and the new SX1280 radio module from Semtech®. In addition to the well-known LoRa® modulation, the SX1280 offers also a conventional (G) FSK / MSK modem as well as a FLRC (Fast Long Range Communication) modem. Thus, different raw data rates from 476 bps up to 2.0 Mbps can be realized depending on the range to be achieved, see [1]. With a sensitivity of down to -130 dBm and an integrated 12 dBm power amplifier, an extraordinary link budget of up to 142 dB can be achieved. Furthermore, the 20 x 25 mm sized module consumes less than 40 mA in transmit mode, less than 10 mA in receive mode and below 1µA in sleep mode and is therefore ideal for battery-driven applications.

With the iM282A, the successful LoRa® technology from Semtech is now also available for the 2.4 GHz frequency band. The LoRa® modulation is already known since 2013 from the Semtech chip family SX127x for the sub-gigahertz frequency bands. At that time, IMST GmbH presented the world’s first LoRa® radio module iM880A at the “Innosecure” trade fair. Based on this technology, the LoRaWAN® standard has evolved over time. The LoRaWAN® standard is specified for the sub-gigahertz frequency bands and is constantly being developed by the LoRa Alliance®, which consists of several hundred well-known companies worldwide.

An essential feature of LoRa® modulation is the decoupling of bandwidth and bit rate. By means of a spreading factor for the band spread, the ratio between bandwidth and bit rate can be adjusted. Similar to its sub-gigahertz relatives SX127x, the SX1280 supports for LoRa® modulation the spreading factors (SF) from 5 to 12. There are also four adjustable bandwidths 203/406/812/1625 kHz available that enables up to 24 different radio settings for LoRa® modulation. Beside the LoRa® modem the SX1280 also includes a so called FLRC modem. The FLRC modem uses coherent GSMK demodulation in combination with Forward Error Correction (FEC) and interleaving techniques. This should result in an 8 to 10 dB better link budget compared to conventional FSK modulation. A rough rule of thumb is: doubling the distance increases the path attenuation by 6 dB. With conventional (G) FSK modulation the iM282A offers a third integrated modem.

The following figure shows the multitude of possible radio settings for the three modems LoRa®, FLRC and FSK. The achievable raw data values in bit per second (bps) are plotted against the achievable sensitivities in dBm.

iM282A Raw Data Rate and Sensitivity for different operation modes
iM282A sensitivity for the LoRa modulation for different bandwidth (BW) and spreading factors at a 1% PER

With the LoRa® modulation the best sensitivity and therefore the highest range can be achieved. This is done by scaling down the used bandwidth and scaling up the used spreading factor. Both increase the so-called “sensitivity” of the receiver. On the other hand the usable data rate is reduced. The sensitivity of a receiver refers to the lower threshold of the input power at which a tolerable reception with a low Packet Error Rate (PER) is still possible. The following figure shows the sensitivity for the LoRa® modulation for different bandwidth (BW) and spreading factors at a 1% PER (Packet Error Rate).

The sensitivity of many short-range radio systems (Bluetooth, Zigbee, WiFi) within the 2.4 GHz frequency band is in the range of around -90 dBm to -110 dBm. If it is possible to increase the sensitivity, higher ranges can be achieved. The iM282A achieves a sensitivity value of -130 dBm, which is significantly better than the above-mentioned technologies.

In fairness, it must be said that the highest sensitivity is achieved at a raw data rate of only 476 bps. However, it is essential that the iM282A provides a low-cost radio module that can be used for battery operation and which is able to transmit reliable data over several kilometres within the 2.4 GHz frequency band.

With the iM282A Starter Kit several range tests have been conducted. Some results are listed below.

Test 1 between the cities Moers and Voerde with a Line of Sight distance of 12.13 km

iM282A Range Test
RF PowerBandwidth [kHz]Spreading FactorEffective Data Rate [kb/s]Link[dB]Number of PacketsPER [%]
RF PowerBandwidth [kHz]Spreading FactorEffective Data Rate [kb/s]LinkBudget [dB]Number of PacketsPER [%]
+8dBm200120.4761381001 %
+8dBm200120.4761381003 %

Test 2 between the cities Moers and Duisburg with a Line of Sight distance of 5.74 km

RF PowerBandwidth [kHz]Spreading FactorEffective Data Rate [kb/s]Link [dB]Number of PacketsPER [%]
RF PowerBandwidth [kHz]Spreading FactorEffective Data Rate [kb/s]LinkBudget [dB]Number of PacketsPER [%]
+8dBm200120.4761381000 %
+8dBm200101.591321000 %
+8dBm400103.17 1301000 %
+8dBm20085.081361000 %
+8dBm20078.881231001 %
+8dBm800820.301231002 %
iM282A Range Test 6km

With the SK-iM282A IMST GmbH offers a plug & play solutions to explore the features and capabilities of iM282A. It consists of two radio modules iM282A assembled on so called Demo-boards, two antennas, batteries and two USB cables. Furthermore the SK-iM282A includes an easy-to-use Studio-Software (PC GUI) that allows to configure all physical parameters. Also it includes several applications like RemoteControl, PacketSniffer, DataLinkService and RadioLinkTest for easy evaluation. The latter application RadioLinkTest provides the possibility to conduct range tests with different RF setting easily.