How to correctly calculate the path loss of 2.4ghz frequency module

the large-scale model is used to estimate the average performance of long-distance transmission. Large-scale models depend on distance and important environmental characteristics that have little to do with frequency. With the shortening of the distance, the model will completely collapse, but it is very useful for determining the working range of wireless systems and roughly planning the network capacity. The small-scale (fading) model describes one-to-one signal changes. This kind of model mainly involves multipath effect (phase cancellation). Path attenuation is considered to be constant, but it mainly depends on frequency and bandwidth.

however, the initial emphasis is usually on small-scale models in which the signal changes rapidly in a short distance or time. If the estimated received power is large enough (usually related to the receiver sensitivity, but also related to the communication protocol used), this link can be used to send data. The amount by which the received power exceeds the receiver sensitivity is called link margin.

link margin or fading margin is defined as the power (margin) beyond the receiver sensitivity level required to ensure a reliable wireless link between the transmitter and the receiver. Under ideal conditions (the antenna is precisely aligned, there is no multipath or reflection, and there is no loss), the necessary link margin is dB. The exact fading margin required depends on the reliability of the link, but according to experience, it is best to always maintain a fading margin of 22dB to 28dB. If the fading margin is not less than 15dB in good weather conditions, the RF system can be fully guaranteed to continue to operate effectively in bad conditions (caused by weather, sunlight and radio frequency interference).

the path loss between the receiving antenna and the transmitting antenna is usually recorded in a dimensionless form by normalizing the relationship between distance and wavelength. However, sometimes it is more convenient to consider the loss caused by distance and wavelength separately. In this case, it is particularly important to pay attention to the units used, because different units are selected, and the offset constants involved are different.

for example, evaluate the feasibility of a 1km link (range) with two RF nodes (Node 1 and Node 2), where the nodes use MRF24J4MB module and the output power is 2dBm. Node 1 is connected to an omnidirectional PCB antenna with a gain of 1dBi, and node 2 is also connected to a similar PCB antenna with a gain of 1dBi. The transmitting power of node 1 is 1mW (or 2dBm) and the sensitivity is -12dBm. The transmission power of node 2 is 1mW (or 2dBm), and the sensitivity is similar to that of node 1. The length of the cable is very short, and the loss at both ends is about 1dB. After that, all the gains are added and all the losses of the link from node 1 to node 2 are subtracted (only the free space loss of the 1km link path is considered).

since -6dB is greater than the minimum receiving sensitivity of node 2 (-12dBm), the signal level is just enough for node 2 to communicate with node 1. At this time, the margin is 42 dB (12 dB ~ 6 dB), which can realize effective transmission in good weather conditions, but it may not be enough to realize reliable communication in bad weather conditions.

because the path loss on the round trip path is the same, the signal level received at node 1 is -6dB. The receiving sensitivity of node 1 is -12dBm, so the fading margin is 42 dB (12 dB ~ 6 dB). In addition, there is loss (fading) caused by the environment [within the line of sight (LoS)], which will further reduce the signal level by 2dB, which meets the communication requirements without any additional gain.

now, we replace node 2 with MRF24J4MA module with a gain (output power) of dB. Because the receiving sensitivity of node 1 is -95dBm, the fading margin is 35dBm(95dB~6dB). In addition, there is a loss (fading) caused by the environment [within the line of sight (LoS)], which will further reduce the signal level by 2dB, and the communication at this time only has an additional gain of 15dB to 2dB.