Path loss
Path loss is the reduction in power density of an electromagnetic wave as it propagates through space. Path loss may be due to many effects, such as free reflection, aperture-medium coupling loss, and absorption. The general formula by which received power is calculated is
Where \(\eta\) is the path loss exponent, whose value is normally in the range of 2 to 5, \(G_{t}\) is the transmitter antenna gain, and \(G_{R}\) is the receiver antenna again. In NetSim, the default value for path loss exponent \(\eta\), is 2.
\(D\) is the distance between transmitter and the receiver, measured in meters. \(D\) is assumed to be greater than \(d_{0}\), the far field reference distance. \(PL_{do}\)is the path loss at reference distance, \(d_{0}\) (\(d_{0}\) is assumed as 1m). \(PL_{do}\) depends on the protocol and is a user input available in the PHY layer of the radios. For 802.11b the default value of \(PL_{d0}\ \)is 40dB.
Example: Calculate the received power at node 2 due to node 1’s transmission. The transmit power of node1 is 100mW (20dBm), frequency is 2412 MHz, and \(G_{t}\)and \(G_{R}\) are 0. The distance between the nodes is 100m and \(\eta\) is assumed as 2
The default value for reference distance \(d_{0}\) and path loss at reference distance \(PL_{d0}\ \)are
802.11 a / b / g / n / ac / p
2.4 GHz: Default \(d_{0}\)= 1m and \(PL_{d0}\ \)= 40dB
5 GHz: Default \(d_{0}\) = 1m and \(PL_{d0\ }\)= 47 dB
802.15.4
Default \(d_{0}\) = 8m and \(PL_{d0}\ \)= 58.5dB
In LTE and 5G NR the calculations are done for each carrier for uplink and download
Default \(d_{0}\) = 1m and \(PL_{d0}\ \)= 32dB
Path loss models
Free space propagation model
The free space propagation model is used to predict received signal strength when the transmitter and receiver have a clear, unobstructed line-of-sight path between them. Satellite communication systems and microwave line-of-sight radio links typically undergo free space propagation. The free space power received by a receiver antenna which is separated from a radiating transmitter antenna by distance d, is given by the free space equation.
where \(P_{t}\)is the transmitted power, \(P_{r}\) is the received power, \(G_{t}\) is the transmitter antenna gain, \(G_{r}\) is the receiver antenna gain, d is the T-R separation distance in meters and λ is the wavelength in meters.
Log distance
The average received power logarithmically decreases with distance, whether in outdoor or indoor radio channels. The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using path loss exponent n.
Where \(\eta\) is path loss exponent. NetSim allows users to set 2.0 \(\leq \eta \leq\) 5.0
\(d_{0}\) is the reference distance, and the model is applicable only for \(d > d_{0}\)
\(d\) is the Transmitter Receiver separation distance
\(\lambda\) is the wavelength and is equal to \(\frac{c}{f}\ \)where \(c\) is the speed of light and \(f\) is the frequency
Default settings in NetSim
Hata Urban
The hata model is an empirical formulation of the graphical path loss data provided by Okumura. Hata presented the urban area propagation loss as a standard formula and supplied correction equations for applications to other situations. The standard formula for median path loss in urban areas is given by
Where,
\(L_{50}\ \)(dB) = 50th percentile (median) value of path loss
\(f_{c}\ \) = Frequency in MHz
\(h_{te}\) = Transmitter antenna height (Range 30m to 200m, default 30m)
\(h_{re}\) = Receiver antenna height (Range 1m to 10m, default 1m)
\(d\) = Separation distance in km. Since the input is in meters, it is divided by 1000 to convert to km.
\(a\ (h_{re})\) = correction factor for effective mobile antenna height which is a function of the size of coverage area.
\(a\ (hre) = \ 8.29 \times \left( \log_{10}{(1.54 \times h_{re})}\ \right)^{2}\ - \ 1.1\) in dB for \(f_{c} <\) 300 MHz
\(a\ (hre) = \ 3.2 \times \ \left( \log_{10}{(11.74 \times h_{re})\ } \right)^{2}\ - \ 4.97\) in dB for \(f_{c} \geq \ \) 300 MHz
Hata Suburban
To obtain path loss in suburban area, the standard Hata urban formula is modified as
COST231 Hata Urban and COST231 Hata Suburban
The European Co-operative for Scientific and Technical Research (EURO-COST formed COST231 working committee to develop an extended version of the Hata model COST231 proposed the following formula to extend Hata’s model. The proposed model for path loss is
Indoor office and Indoor factory
Indoor home
Where \(\eta\) = 3
Two Ray
The Two-Rays Ground Reflected Model is a radio propagation model which predicts the path losses between a transmitting antenna and a receiving antenna when they are in LOS (line of sight). Generally, the two antenna each have different height. The received signal having two components, the LOS component and the multipath component formed predominantly by a single ground reflected wave. The standard formula for Two-ray model is
Where
\(G_{t},\ G_{r}\)= Transmit and receive antenna gains
\(h_{t}\) = z coordinate of the transmitter plus transmitter antenna height
\(h_{r}\)= z coordinate of the receiver plus receiver antenna height
d = Distance between transmitter and receiver
Range Based
The propagation loss depends only on the distance (range) between transmitter and receiver. There is a single Range attribute that determines the path loss. This is not a real-world loss model but a theoretical model useful for experimentation.
Receivers at or within Range meters see a 0 dB pathloss. Hence received power equals transmit power. Receivers beyond Range see a 1000 dB pathloss. Hence received power will be close to -1000 dBm i.e., zero in linear units.
Figure-1: This figure explains a typical range based pathloss model. In this example, Node A and B are in transmission range with each other which is denoted by a blue circle where the pathloss is 0dB i.e, successful transmission. And Node C is beyond the range i.e, pathloss is 1000dB all packets are errored and dropped. In NetSim, user has to specify the range of the node in meters in the links.
This is a link-level property and would apply to all devices connected to that point-to-multipoint or multi-point-to-multipoint link. Thus, users can have AP1 and associated Wireless nodes set to Range based pathloss, and AP2 and its wireless nodes set to a different pathloss model.
This pathloss is applied not just for transmissions but for Carrier sensing and Interference calculations also. For example, as shown in Figure-2, consider a scenario with A transmitting to B and C transmitting to D. Let B be within range of A, D be within range of C and B also be within range of C. In this case, if there is a simultaneous transmission from A and C, the transmission from A will fail at B due to interference of the C to D transmission.
Figure-2: Transmission failure due to interference from neighbouring transmissions. In this case A > B transmission fails due to interfering signal from C, when C transmits to D. This occurs because B is within range of C.
Similarly let us consider another example as shown in Figure-3. where B is within the range of A, D is within the range of C. Further, A and C are within range of one another. In this case if there is a transmission from A, C detects the medium to be busy and vice versa which leads to carrier sense blocking at transmitters.
Figure-3: Carrier sense (CS) blocking due to neighbouring transmissions. If A transmits, then C is CS blocked and if C transmits then A is CS blocked.
Given below in Table-1 are the lower bound, upper bound, and default values for the Range (m) parameter in the GUI for different protocols.
Network |
Range (m) |
||
|---|---|---|---|
Min |
Max |
Default |
|
Wi-Fi (Internetworks, MANET, VANET), TDMA, Pure Aloha, Slotted Aloha, Cognitive Radio |
1 |
1000000 |
50 |
WSN, IoT |
1 |
1000000 |
20 |
UWAN |
100 |
1000000 |
10000 |
Table-1: Min, Max, and Default values of Range(m) for Range Based pathloss model
Pathloss Matrix File
With this option users can define the pathloss for the wireless links. The name of the trace file generated should be PathlossMatrix<Wireless Link Id>.txt and it should be per the NetSim Pathloss Matrix File format.
The propagation loss is fixed for each pair of nodes and does not depend on their actual positions. This model should be useful for synthetic tests. By default the propagation loss is assumed to be symmetric. The value of pathloss for each pair of nodes is read from a file.
The name of the trace file generated should be kept as PathlossMetrics<Wireless Link Id>.txt and it should be in the NetSim Pathloss Metrics File format. The NetSim Pathloss Matrix File format is as follows
Step 1: Open node (Wireless_Link) properties -> select pathloss model as PATHLOSS_MATRIX_FILE and click on Configure Pathloss metrics.
Step 2: Inside the text file and write the code in format shown below
# Commented lines
# Empty lines will be ignored
# Format for writing this file is
# SNR value must be in increasing order
# time(sec),tx,rx,loss(dB)
time <Time_in_Secs>, <Tx_Node_ID>, <Rx_Node_ID>, <Pathloss(dB)>
Default value of pathloss exponent
The default value of path loss exponent for all path loss models in NetSim are as shown below Table-2.
Path loss model |
Path loss exponent (default) |
|---|---|
Free space |
2 |
Log distance |
2 |
COST231 Urban |
-- |
COST231 Hata Suburban |
-- |
Hata Urban |
-- |
Hata Suburban |
-- |
Indoor Office |
2.6 |
Indoor Factory |
2.1 |
Indoor Home |
3 |
Table-2: Default value of path loss exponent for path loss models