The right path — A look at path loss calculations for modules using the 2.4 GHz band
It is important therefore to evaluate the range and performance of wireless transmission to create models for estimating the path loss for short range modules in indoor and outdoor environments to give designers an initial estimate on a wireless communications system’s performance. The performance parameters include range, path loss, receiver sensitivity, bit error rate (BER) and packet error rate (PER), which are critical in any communications system.
To do this, consider three modules with varied specifications related to power and type of antenna – Microchip’s MRF24J40MA, MRF24J40MB and MRF24J40MC. The MRF24J40MA is a certified 2.4 GHz IEEE 802.15.4 radio transceiver module with integrated PCB antenna and is suitable for wireless sensor networks, home automation, building automation and consumer applications. The MRF24J40MB is similar but better suited to longer range applications such as automatic meter reading. The MRF24J40MC has an external antenna (shown in Figure 1) and also suits longer range applications. All three connect to microcontrollers through a four-wired SPI interface and have various regulatory and modularly certified on board.
Figure 1: MRF24J40MC modules with daughter board and external antenna.
Path loss models
Large-scale models predict behaviour averaged over distances. The large-scale model is a function of distance and significant environmental features that are roughly frequency independent. This model exorbitantly breaks down as the distance decreases but is useful for modelling the range of a radio system and rough capacity planning. Small-scale (fading) models describe signal variability on a scale of one to one. They have dominating multi-path effects (phase cancellation). The path attenuation is considered constant but is mostly dependent on the frequency and bandwidth.
However, usually the initial focus is on small scale modelling with rapid change in the signal over a short distance or length of time. If the estimated received power is sufficiently large (typically relative to the receiver sensitivity), which may be dependent on the communications protocol in use, the link becomes useful for sending data. The amount by which the received power exceeds receiver sensitivity is called the link margin.
The link or fade margin is defined as the power (margin) required above the receiver sensitivity level to ensure a reliable radio link between the transmitter and receiver. In favourable conditions (antennas are perfectly aligned, no multi-path or reflections exist, and there are no losses), the necessary link margin would be 0 dB. The exact fade margin required depends on the desired reliability of the link, but a good rule of thumb is to maintain 22 to 28 dB of fade margin at any time. Having a fade margin of not less than 15 dB in good weather conditions provides a high degree of assurance that the RF system continues to operate effectively in harsh conditions due to weather, solar and RF interference.
The path loss due to propagation between the reception and transmission antennas is normally written in dimensionless form by normalising the distance to the wavelength. However, it is sometimes convenient to consider the loss due to distance and wavelength separately. In this case, it is important to track the units being used, since each choice involves a differing constant offset.
As an example, estimate the feasibility of a 1 km link (range) with RF nodes one and two of MRF24J40MB modules with 20 dBm output power. Node one is connected to an omnidirectional PCB antenna with 1 dBi gain, while node two is also connected to a similar PCB antenna with 1 dBi gain. The transmitting power of node one is 100 mW (or 20 dBm) and its sensitivity is -102 dBm. The transmitting power of node two is 100 mW (or 20 dBm) with a similar sensitivity as node one. The cables are short and are approximated with a loss of 1 dB on each side. Then add all the gains and subtract all the losses from the node one to node two link considering only the free space loss for a path loss of a 1 km link.
Since -60 dB is greater than the minimum receive sensitivity of node two (-102 dBm), the signal level is just enough for node two to communicate with node one. There is a 42 dB margin (102 dB – 60 dB), which is suitable for good transmission under good weather conditions, but may not be enough to protect against harsh weather conditions.
The path loss is the same on the return path. Therefore, the received signal level on the node one side is -60 dB. Since the receive sensitivity of node one is -102 dBm, this leaves a fade margin of 42 dB (102 dB – 60 dB). Additionally, there are losses due to environment (fading) even at LoS (line of sight) and could further reduce by 20 dB, which is within the requirement for communications without any additional gain.
Now let’s substitute node two with an MRF24J40MA module with 0 dB gain (output power). Since the receive sensitivity of node one is -95 dBm, this leaves a fade margin of 35 dBm (95 dB – 60 dB). Additionally, there are losses due to environment (fading) even at LoS and can further reduce by 20 dB, which communicates only with some additional gain of 15 to 20 dB.
Fresnel Zone
The Fresnel Zone is the area around the visual LoS that radio waves spread out after they leave the antenna, as shown in Figure 2. It is good have the LoS to maintain strength, especially for 2.4 GHz wireless systems. This is because the 2.4 GHz waves are absorbed by water. The rule of thumb is that 60% of the Fresnel Zone must be clear of obstacles. Typically, 20% Fresnel Zone blockage introduces little signal loss to the link, and beyond 40% blockage the signal loss becomes significant.
Figure 2: Fresnel Zone.
It is important to enumerate the extent to which the Fresnel Zone can be blocked. Typically, 20 to 40% Fresnel Zone obstruction introduces little to no interference into the communications link. It is better to have an inaccuracy up to more than 20% blockage of the Fresnel Zone.
The propagation losses for indoors can be significantly higher in buildings because of obstructions such as walls and ceilings. This occurs because of a combination of attenuation by walls and ceilings, and blockage due to equipment, furniture and human intervention.
Trees attenuate around 8 to 18 dB of loss per tree in the direct path. This attenuation depends on the size, shape and type of tree. A dry wood wall on both sides can result in about 6 dB loss per wall. Comparatively older buildings may have greater internal losses than new buildings due to materials and LoS issues. Concrete walls account to 10 to 15 dB depending on the size and shape of the construction. Floors in buildings account for 12 to 27 dB of loss. Concrete and steel floors attenuate more than wooden floors. Mirrored walls have very high loss because the reflective coating is conductive.
The Fresnel Zone is sometimes a good indication of an indoor environment range measurement. Generally, the LoS propagation is valid only for about the first 3 m. Beyond 3 m, the indoor propagation losses can go up to 30 dB per 30 m in dense office environments. Conservatively, it overstates the path loss in most cases. Actual propagation losses may vary significantly depending on the building construction, structure and layout.
Some of the possible reasons for propagation losses through the Fresnel Zone are collisions with other transmitters, weak error vector magnitude (EVM) from the transmitter generally in the range of 20 to 24% rms, and reflections from moving objects or people.
Figure 3 shows the received signal strength information (RSSI) in an LoS environment.
Figure 3: Location and distance in an LoS environment.
Conclusion
Take care when choosing the path loss model for predicting the RF system performance. Serious errors can occur by using the free space path loss (FSPL) model for most cases except few restricted cases. A more realistic model to use for urban environments is the ITU indoor propagation model.
For urban environments, the use of 10 to 12 dB is a good rule of thumb for predicting the required increase in the link budget to double the transmission distance. Receiver sensitivity is the first variable in a system that must be taken care of and optimised to increase the transmission distance. Other variables in any wireless system also affect distance but must be changed by a greater percentage to equal the effects presented by changing the receiver sensitivity.
Fading due to multi-path can result in a signal attenuation of more than 30 to 40 dB, and it is highly recommended that sufficient link margin is factored into the link budget to overcome this loss while designing a wireless system.
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