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Home > White Papers > LoRa Range Survey


 Surveying the Operating Range of LoRa1 Wireless in Glasgow CBD

Published: Fri 08 April 2016 | by Andrew Wixted

LoRa is a low-power wireless technology operating in unlicensed ISM bands. It is one of several competing technologies aimed at capturing a segment of the IoT market for cost sensitive, low data rate IoT applications. To assess the viability of the technology a survey was carried out across the Glasgow CBD using different base station locations.

The survey map linked below contains the results relating to one base station.


Introduction

LoRa, which stands for 'Long Range Wireless', is a Chirp Spread Spectrum (CSS) technology capable of operating in various ISM bands in different jurisdictions. Bands include 169, 433, 490, 868 and 915MHz. While limited in power to the limits of the particular ISM band the LoRa devices can communicate over tens of kilometres in line-of-sight situations and shorter but still considerable distances in built up areas. Data speeds up to 300kbps are possible but speed is a trade-off for distance. Compared to a WiFi wireless G (802.11g), the transmit power is approximately one eighth (200mW EIRP2,3 vs 25mW EIRP), the data rate is only a fraction of WiFi's rate but the LoRa wireless range opens many opportunities for IoT applications.

LoRa chips can be combined with various software layers and servers to create LoRaWAN4, a Low Power Wide Area Network (LPWAN) with data rates of 0.3 kbps to 50 kbps. A full LoRaWAN comprises end devices, gateways, network servers, network controllers and application servers. A LoRa end point could be received by multiple gateways with the network server managing any multiple reception. The network controller can monitor the network via the network server and adjust end point modulation and power levels to reduce power usage. LoRaWAN also defines the encryption protocols allowing end points and application servers to communicate over a LoRa network managed by a third party. Go here for a LoRaWAN '101' (2.8Mbyte PDF) from the LoRa-Alliance

Since the critical and differentiating component of this network is the physical layer, comprising the LoRa wireless chips and associated hardware, the ability of these devices to function in the intended environment needed to be validated and their performance evaluated. One phase of this evaluation was a survey of performance across a Central Business District with multiple gateways (base stations).


Glasgow CBD LoRa Survey Design

A survey was designed where a roaming station monitored beacons (physical layer packets) from a fixed base station. Different base station locations were used to assist in ascertaining the appropriate number and location of base stations. The two units utilised identical transceiver components (Semtech SX1272) at each end giving a network that was RF symmetrical. The transceivers were operated in the 868MHz ISM band using a bandwidth (BW) of 125kHz, a spreading factor (SF) of 12 and a coding rate (CR) of 4/5. This gave an RF link budget of approximately 154dB. One SX1272 was managed by an Arduino and transmitted a beacon with a packet sequence number. The other end used an SX1272 connected to a Raspberry Pi with a 3G/GPS mobile modem used for GPS assisted location. To provide user feedback the Raspberry Pi transmitted the current location and location of the most recent successfully received LoRa packet to a web server. The user could receive feedback by viewing their location on a map. This feedback meant that the edges of the reception area or black spots could be explored. The LoRa receiver unit was carried in a backpack and walked at street level through the Glasgow CBD. The Raspberry Pi was programmed using Python.

During the survey the GPS location, the received signal strength indicator (RSSI) of each packet, the mobile network RSSI and the contents of each packet were logged. Independent of this, the ambient RF signal strengths for bands between 1MHz and 2.7GHz were recorded using a portable Spectrum Analyser. This survey is reported elsewhere on this white paper and the data can be viewed on the map linked above.

LoRa Survey results Example

Complete LoRa Survey System with data and feedback paths. Assisted GPS used Google Location Services to obtain satellite data. When satellite lock was obtained an SMS was sent to the user. Location and LoRa RSSI data was fed to a web server for user feedback.

The above process was repeated for different base station locations and height and for internal building reception but only the results for the Glasgow Caledonian University (GCU) base station and external reception are presented here.


Glasgow CBD LoRa Survey Results

Data logged in the surveys was downloaded from the survey device and loaded into a database. This database could then be interrogated to extract data relating to particular days, locations, base stations, signal strength or other factors of interest. The resultant data was also used for plotting markers on Open Street Maps with the marker colour used to indicate the LoRa RSSI. On the full map clicking the markers shows the RSSI at that point.

LoRa Survey Results Example
Example of LoRa survey Results (click here for full map then select 'LoRa')

From the mapped data:

  • In one direction reception reached 2.2km from the base station with the possibility of going further.
  • In another direction the reception reached 1.57 km after which reception was lost.
  • In streets with tall buildings running orthogonal to the transmission path, reception was reduced.
  • Bad reception on some streets tended to be in the mid-block area.
  • Comparing the street map data with topographical maps indicated that terrain played a strong role in LoRa wireless packet reception.
    • Hills blocked the transmission path
    • A depression such as a highway underpass could reduce reception
    • An open area had widespread reception

Glasgow CBD LoRa Survey Application

The application of the survey results falls into the 'common sense' arena. Taking into account the grid layout of the streets and the location of hills, the location of base stations should ensure that base stations can transmit along the key streets and also ensure that coverage into both sides of a hill are provided. Using a base at GCU leaves an area in the south west of the survey area with poor reception (behind Blythswood Hill). A roof mounted antenna on a commercial building west of the motorway provides RF down into this area and along these key commercial streets. Similarly, on the eastern side and below Queen Street Station when the land falls away very quickly, this area was covered with an antenna mounted at Strathclyde University. The area west of the motorway is covered by the previously mentioned base and black spots not covered by the combination of these antennas will be covered by a future antenna located as Glasgow University.

Gateways use a receiver with a slightly better sensitivity (-142dBm) than the devices used in the survey (-137dBM) improving the overall link budget in the end-node-to-gateway direction by 8dB over the values used in the survey (sensitivity used was -134dBm). This will give an overall improved coverage and reliability.


Conclusion

Overall the survey process and related testing gave a strong level of confidence that the technology could perform and would be a valuable tool in the IoT toolbox. A variety of research into LoRa sensor applications, low power operation and energy harvesting for battery-less nodes is continuing. Development of gateway and network intelligence is also ongoing.

[ Link To Maps ] (select 'LoRa RSSI')

Dr Andrew Wixted,
Glasgow Caledonian University

This work funded by Stream Technologies and Innovate UK as a Knowledge Transfer Partnership with Glasgow Caledonian University


[1] LoRa is a registered trademark of Semtech Corporation.

[2] EIRP: Effective (or Equivalent) Isotropic Radiated Power. The equivalent isotropic radiated power to get the same power level. 'Isotropic' is a theoretical 'point source' of radiation. EIRP is a function of both the transmitter power and the gain of the antenna, for example: a 14dB gain sector antenna coupled with a 125W (51dBm) transmitter has an EIRP of 65 dBm (equivalent to a 3200W point source).

[3] WiFi maximum power varies from jurisdiction to jurisdiction and sometimes within a jurisdiction. WiFi devices are sometimes designed for the market's 'lowest common denominator' so 100mW power limited devices are common even though some jurisdictions allow 1W (30dBm) transmitters.

[4] LoRaWAN is a trademark of the LoRa Alliance.