LINK PLANNING AND LINK BUDGETTING

In a basic communication consisting of two radios,each with it's associated antenna and separated by a distance(x),the radio require a certain minimum signal to be collected by the antennas and passed on to their input socket.Link budget calculation refers to the determination of the feasibility of the link between two antennas.This feasibility depends on the quality of the equipment being used and on the diminishment of the signal due to distance,called path loss.
The power available for a particular wireless link can be characterised by the following factors:

• Transmit Power.It is usually expressed in dB and ranges from 30mw to 200mw or more,oftenly it is dependant on transmission rate.


• Antenna Gain. Antennas are passive devices that create the effect of amplification by virtue of their physical shape.They have the same characteristics when receiving and transmitting.Parabolic antenna have a gain of 19-24 dBi,omnidirectional antennas have 5-12dBi while sectorial antennas have roughly a 12-15 dBi gain.


• Minimum Received Signal Level.This is also referred to as the sensitivity of the received.The minimum RSL is always expressed as a negative dBm and is the lowest power of the signal the radio can distinguish. The minimum RSL is dependant on the rate , and as a general rule the lowest rate (1mbps) has the greatest sensitivity.


• Cable Losses.Some energy is inevitably lost in the cables,connectors and other devices,going from the radios to the antennas.The loss depends on the type of cable and used and on its length.

To go ahead and calculate the link budget for a particular one is required to calculate the antenna gain,Losses

Antenna gain is given by:Antenna gain of radio 1 plus Antenna gain of radio 2 if there are two radios in the system.

Losses are given by:cable losses in radio 1 plus cable losses in radio 2plus free space loss

Then finally the Link budget between radio 1 and 2 is given by:

Radio 1TX power plus Antenna gain Minus Losses.And for a feasible link this should be less than the sensitivity of radio 2 and viceversa.