This blog post describes the physical range characteristics of wireless networks – in particular, those of LoRaWAN technology. The presented information supports the planning process and the evaluation of use cases of LoRaWAN.
Part 1 of the article explains the factors and their correlations that influence the range of radio signals. In Part 2 “Range and coverage of LoRaWAN in real-life scenarios” several examples in urban areas will be presented and analyzed. It will be published soon.
In radio technology, there are essentially three characteristics that can be used to characterize a radio network:
- the speed of data transmission, and
- power consumption.
It is difficult to consider all three criteria with the same emphasis, since physical laws set clear limits in this case: For example, LoRaWAN can transmit data over long distances and requires relatively little energy, but has only a low data rate.
WiFi and Bluetooth, for example, achieve very high data rates, but the energy consumption is comparatively high and their range is very small. All smartphone users are well aware of this thirst for energy. The base stations of the large telecommunications providers offer high data rates and relatively high ranges, but a large amount of energy must be available for this. Therefore the power supply is always a very important planning factor for such installations.
In general, an optimization to a maximum of 2 of the above-mentioned criteria is possible. – Therefore, the decision has to be made which properties are given priority
The Link budget indicates the quality of a radio transmission channel.
Using a simple model, the link budget can be calculated by adding the transmit power (transmitter power, Tx), Receiver sensitivity (Receiver Power, Rx), Antenna gain, and Free Space Path Loss (FSPL).
In the further process, the link budget of LoRaWAN is calculated.
Radio Signal Path Loss
The path loss indicates how much energy is lost in free space at a distance between Tx and Rx. The further the distance between Tx and Rx, the lower the level of energy. The path loss is usually represented as follows:
FSPL = (44πd/λ)2 = (44πdf/c)2 (1)
The factors explained:
FSPL = Free Space Path Loss
d = distance between Tx and Rx in meters
f = Frequency in Hertz
There is also a widely used logarithmic calculation formula for free space path loss:
FSPL (dB) = 20log10(d) + 20log10(f) – 147.55 (2)
A doubled distance (d) means a path loss of 6dB (in free space).
On the receiver side (Rx), the sensitivity of the receiver is the value that affects the link budget. The so-called Rx sensitivity describes the minimum possible reception power and tolerance for thermal noise and is calculated as follows:
Rx-Sensitivity = -174 + 10log10(BW) + NF + SNR (3)
means to do so:
BW = bandwidth in Hz,
NF = noise factor in dB,
SNR = the signal to noise ratio indicates how much stronger the signal has to be compared to the noise
The Rx sensitivity of LoRaWAN is higher – and therefore better – than e.g. Wifi.
Equation (4) shows the extreme case of path loss without including antenna gain and other types of free space attenuation:
link budget = max. Rx Sensitivity (dB) – Max. Tx power (dB) (4)
An example of the calculation of a LoRaWAN link budget:
Tx power = 14 dBm
BW = 125KHz = 10log10(125000) = 51
NF = 6 dB (the gateways in LoRaWAN networks have lower NF values)
SNR = -20 (for SF=12)
These numbers entered in formula (3) give an Rx sensitivity of -137 dBm
Rx sensitivity = – 174 + 51 + 6 – 20 = -137 dBm</block ratio>
The link budget can then be calculated as follows using expression (4):
Link budget = -137dB – 14dB = -151dB
The values given result in a LoRaWAN Link budget of 151 dB.
With the link budget of 150 dB specific for LoRaWAN, a distance of up to 800 km can be covered under optimal conditions (pure free space loss). The current LoRaWAN world record is at a range of 702 km.
Under real conditions, these ideal values cannot be achieved. This depends on several influencing factors.
Factor 1: Free Space Path Loss
By doubling the distance, the free space path loss for LoRaWAN increases by 6 dB, so that the pass loss of the radio signal is subject to a logarithmic function (see equation (1)).
In addition to the loss of energy as a function of distance, factors such as reflection and refraction of radio waves on objects can lead to overlapping of radio waves, which can also have a negative effect on the range. (Note: These connections are very well explained in the video “LoRa crash course” by Thomas Telkamp starting at position 15:41).
Factor 2: Path Loss Due to Structural Elements
The pass loss caused by structures, i.e. the absorption of radio signals when penetrating different obstacles such as buildings, influences the reception of transmitted signals and can shorten the range considerably. For example, glass reduces the signal by just 2dB. This affects the range much less than a thick concrete wall of 30 cm. The table below lists various materials and their typical effects on radio signals.
|Material||Path loss (dB)|
|Reinforced concrete (89mm)||27|
Factor 3: Fresnel Zone
To effectively cover long ranges and achieve a good link budget, it is also important to establish a direct line of sight between sender and receiver as often as possible. In radio transmission, specific spatial areas between the line of sight are referred to as Fresnel zones. If there are objects in these zones, they can have a negative influence on the wave propagation, although there is generally visual contact between the transmitting and receiving antennas. For each object located in the Fresnel zone, the signal level is lowered and the range is reduced (see figure below).
Omnidirectional antennas are normally used in LoRaWAN networks. This causes emitted energy to spread on the horizontal plane where the network nodes and gateways are located. In Europe, the power limit of an ISM band is defined as 14 dBm for a frequency of 868 MHz. In addition, the maximum antenna gain is limited to 2.15 dBi.
Factor 4: Spreading Factor
A LoRaWAN network uses Spreading Factors (SF) to specifically set the data transfer rate relative to the range. In LoRaWAN networks, SF7 to SF12 are used. Due to their Chirp Spread Spectrum modulation (CCS) as well as various phase-shifted frequencies used for chirps, insensitive to interference, multipath propagation, and fading. Chirps are used to encode data in LoRaWAN networks on the Tx side, while inverse chirps are used on the Rx side for signal decoding. The above SFs indicate how many chirps are used per second, and define bit rates, per symbol radiated power, and achievable range.
SF9, for example, is 4 times slower than SF7 in terms of bit rates. The scalability of LoRaWAN is achieved by SFs. The slower the bit rate, the higher the energy per data set and the higher the range. LoRaWAN supports an automatic adaptation of the SF factors depending on the network configuration, so-called Adaptive Data Rate (ADR).
- The link budget specifies the maximum range of a LoRaWAN network.
- The free space path loss affects the range. Doubling the distance increases the path loss by 6 dB.
- Reflections and refractions of the radio waves at obstacles and on the ground influence the signal level and range. In the LoRaWAN network, one side of the radio link is usually located near the ground.
- Obstacles inside the first Fresnel zone influence the signal level on the Rx side and shorten the range.
- SF values and thus the range of a transmitter depending on the conditions of transmission. LoRaWAN allows automatic network management via ADR and thus regulates the ranges of the transmitters.
- Rx sensitivity depends on the signal-to-noise ratio (SNR), noise factor (NF) and bandwidth (BW).
Strategies for optimizing the range of LoRaWAN
The following aspects should be considered in order to improve the range in a network with LoRaWAN technology:
- Gateway location: Provide optical visibility between the Tx and Rx antennas. Increase the height of the antennas to achieve optical visibility between them. The use of outdoor antennas is always better than indoors.
- Antenna selection: Classic pole antennas concentrate energy in the horizontal plane. Avoid obstacles in the immediate vicinity of the antenna. In addition, these should always be mounted on a post rather than on the side of the building. The range should improve if the antenna is carefully selected and the antenna polarization and the maximum defined antenna gain are optimally adjusted to each other.
- Use high-quality connectors (N-connectors) and cables (LMR 400 or equivalent, with a loss of less than 1.5 dB per 100 m). In order to reduce the loss in the connection material, the length of the connection between the station and the antennas must also be kept as short as possible.
- Co-localization: When installing in the vicinity of other radio systems, try to avoid strong interference, for example from surrounding GSM or UMTS stations. Please refer to the manufacturer’s operating instructions.
- In general, it should be mentioned briefly that the installation of a LoRaWAN gateway should ensure sufficient surge and lightning protection.