Radio Mobile - RF propagation simulation software
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Linkbudgets

 

Radio planning starts with a linkbudget. Every individual parameter of the system definition that is applied to the unit is derived from the linkbudget. Also the planning goals are held by the linkbudget.

Because of this a simple explanantion of a linkbudget is given.

For this explanation I use a recitation that I held for the local amateurradio group of the VERON in Maastricht. The explanantion about Linkbudgets is also used in a recitation about radioplanning. The values are taken from a 70cm repeater near Maastricht: PI2MST.

The linkbudget limits the distance that a radio signal can travel at wich the required service availabilty is reached.

To be able to calculate the value and visualise the result in Radio Mobile we need more detailed information. Genarally there is a situtation where a central transmitter is having contact with a mobile or a hand held radio. But linkbudgets are also used in poit to point links. In this example we use a base station and a hend held portable.

specifications of the repeater.


  • Phone repeater at 430.075 MHz
  • TX: ~ 10 Watt = 40 dBm
  • RX: ~ 0,15 µV = -124 dBm
    at 12 dB Sinad
  • TX attenuation 3 dB
  • RX attenuation 3 dB
  • Antenna gain (dipole) 2,2 dBi
    (0 dBd)

Specifications of the handheld radio

  • THD7
  • TX: ~ 5 Watt = 37 dBm
  • RX: ~ 0,18 µV = -122 dBm
    at 12 dB Sinad
  • Antenna gain – 4 dB
  • Body loss 6 dB

Up- and downlink

With a radiolink it is important that both stations can reach each other. In that cas there is a link balance. In the cas ttha on can hear the other but cannot reach him there is a lin un-balance. This happens when one of teh stations has 1000 Watt and the other just 5 Watt.

This table shows the linkbalans calculation for the described case:

Downlink     Formula
TX power 40 dBm a
Tx attenuation -3 dB b
Antenna gain 0 dBd c
Erp power 37 dBd d=a+b+c
       
RX sensitivity -122 dBm e
RX antennagain -4 dB f
RX Bodyloss -6 dB g
minimum RX level -112 dBm h=e-f-g
       
MAPL * 149 dB i=d-h
       
Uplink     Formula
TX power 36 dBm a
TX Bodyloss -4 dB b
Antenna gain -6 dBd c
Erp power 26 dBd d=a+b+c
       
RX sensitivity -124 dBm e
RX antennagain 0 dB f
TX Attenuation -3 dB g
minimum RX level -121 dBm h=e+f+g
       
MAPL * 147 dB i=d-h
       

MAPL = Maximum Alowed Path Loss

Some notes on the values in the tables:
  • The left coloumns show the 'downlink' from base station to hand held
  • The right coloumns show the 'uplink' from handheld to base station
  • The RX sensitivity is defined at 12 dB Sinad.
  • Antenna gain of the hand held is presumed a 'rubber duck'antenna. These perform badly so a negative gain is assumed.
  • Body loss is the loss that the body introduces on a hand held device when worn on the body (eg. Belt)

The coverage of the repeater

With the help of Radio Mobile we now can calculate the coverage of the repeater. When we use the linkbudget we know that the hand held radio can receive the repeater at a minimum field strenght of -112 dBm measured in a dipole antenna.

Now we set in Radio Mobile the target treshold at -112 dBm:


This gives the following result. The blue areas are the areas where the downlink signal signal is available >= -112 dBm.

 
Radio Mobile displays the downlink in 'Single Polar radio coverage' - 'Centre TX - Mobile RX'
The uplink is displayed in Radio Mobile in 'Single Polar radio coverage' - 'Centre RX - Mobile TX'

Linkbalance

The result of our calculation is that the MAPL for the downlink 2 dB more is than the uplink. this is caused by the difference in transmit power. In this case there is link-un-balance.

In the case that we increase the transmit power of the base station the link-un-balance increases:

Downlink     Formula
TX power 41.8 dBm a
Tx attenuation -3 dB b
Antenna gain 0 dBd c
Erp power 38.5 dBd d=a+b+c
       
RX sensitivity -122 dBm e
RX antennagain -4 dB f
RX Bodyloss -6 dB g
minimum RX level -112 dBm h=e-f-g
       
MAPL * 151 dB i=d-h
       
Uplink     Formula
TX power 36 dBm a
TX Bodyloss -4 dB b
Antenna gain -6 dBd c
Erp power 26 dBd d=a+b+c
       
RX sensitivity -124 dBm e
RX antennagain 0 dB f
TX Attenuation -3 dB g
minimum RX level -121 dBm h=e+f+g
       
MAPL * 147 dB i=d-h
       

MAPL = Maximum Alowed Path Loss

From this we can conclude that a simple increase of power at a base station will do no good because the uplink is not improved to compensate for the increase of power in the downlink Radio Mobile can easily display the link unbalance.

The blue color displays the coverage range of the downlink. The yellow color displays the coverage range of the upnlink.

 
Radio Mobile combines the uplink and downlink in 'Single Polar radio coverage' as 'Worst case'. When using 'Worst case' the result will display the boundary where the criteria for a linkbudget in balance is realised.

 

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