Radio space transmission loss attenuation calculation (reprint)

Wireless Transmission path analysis is an important step in wireless transmission network design, through the analysis of the transmission path to facilitate the network designers to choose the appropriate type of antenna (direction, polarization, gain and other indicators) according to the margin size of the wireless link, install the antenna height, select the appropriate feed cable and length. The following is a brief introduction to the calculation of free space loss in wireless transmission path analysis, the calculation of the signal reception strength, and the calculation of the margin of the link system in several main aspects.

1. Calculation of free space loss

Free space loss refers to the decline of electromagnetic waves in the transmission path, the formula is as follows:

Lbf=32.5+20lgf+20lgd

lbf= free Space Loss (DB)

D= distance (miles)

f= frequency (MHz)

2400mhz:lbf=100+20lgd

5800mhz:lbf=108+20lgd

The above formula is a calculation formula for the ideal condition at a temperature of 25 degrees, 1 atmospheres.

The following table lists typical free-space loss values

Distance (miles)	Path loss @2.4ghz (DB)
1 2 3 4 5 6 7 8 9 10	-100 -106 -110 -112 -114 -116 -117 -118 -119 -120
15 20 25 30 35 40 45 50	-124 -126 -128 -130 -131 -132 -133 -134

By finding the above table and calculating by formula we can get the theoretical decline of electromagnetic wave transmission from the transmitting station to the receiving terminal.

2. Calculation of signal reception strength:

The signal reception intensity refers to the intensity of the wireless signal received by the receiving station device.

rss=pt+gr+gt-lc-lbf

Rss= Receive signal strength

Pt= Transmitting Power

gr= receive antenna gain

gt= transmitter antenna gain

lc= loss of cable and cable head

lbf= Free Space Loss

For example, if the transmitter station and the station two site distance 25Km, the device transmitting power 20dBm, the transmitter antenna gain is 17dBi, the receiving antenna gain is 24dBi, cable and cable head loss 3dBi. The received signal strength rss=20+17+24-3-128=-70db

3. Calculation of the link system Headroom SFM (Syetem Fade margin)

The link system headroom refers to the number of wealthy DB values that are actually received by the receiving station device compared to the minimum receive threshold (device reception sensitivity) allowed by the receiving station device.

Sfm= Rss-rs

Rss= Receive signal strength

rs= device Receive sensitivity (DB)

In the above example, if the device receives a sensitivity of -81DBI, the link system headroom is:

Sfm=rss-rs

=-70-(-) 81=11db

The above example is that the use of this transmission power of 20dBm, receive sensitivity of -81DBI 2400MHz Wireless Bridge, in the addition of 17dBi gain transmitter antenna and 24dBi gain receiving antenna, cable and cable head loss only 3dBi loss, There is also a 11dB link system headroom after transmission of the 25KM.

The system headroom is an important index to measure the availability and stability of the wireless link. Because the wireless channel is a parameter change channel, interference and noise signal can affect the useful signal at any time, sometimes the intensity of interference and noise signal can be achieved 20~30db, so in the construction of Wireless Link, must retain a certain amount of system headroom. Although the system headroom is larger than the 5dB system can work, it is generally recommended that the system headroom is greater than 15-20db.

Radio space transmission loss attenuation calculation (reprint)