LINK PLANNING

A basic communication system consists of two radios , each with it associated antenna,the two being seperated by the path to be covered.In order for the two to be communicating,the radios require a certain minimum signal to be collected and presented to their input socket.Link budget calculation is the process of determing the feasibility of the link.Weather or not signals can be passed betwween the radios depends on the quality of the equipment being used and on the diminishment of the signal due to distance ,called path loss.
Calculating the link budget.
The available power in an 802.11 system can be characterised by the following facters:

• Transmitt power:transmitt power ranges from 30mW to 200mW or more.This power is often dependent on the transmission rate and is either measured in mW or in dBm.Tx power of the given device should be specified in the literature provided by the manufacturer.
• Antenna Gain: Antennas are passive devices that create the effect of amplification by virtual of their physical shape.Thes devices have the same characteristics when transmitting ang when receiving.So a 12dBi antenna is simply a 12dBi antenna without specifying weathwe it is in transmission or reception mode.Parabolic antennas have a gain of 19-24dB,omnidirectional antennas have 5-12dBi while sectorial antennas have roughly a 12-15dBi gain.
• Minimum received signal level:This is simply the sensitivity of the receiver.The minimum RSL is always expressed as a negative dBm and is the lowest power of singal the radio can distingiush.The RSL specifications should be provided by the manufacturer of the equipment.
• Cable Losses:These are the signal energy losses that take place in cables ,connectors and other devices ,going from the radios to the antennas .These losses depend on the type of cables being used and also on their lengths.
• Path losses:Free space losses ,attenustion and scattering are effects that must be considered when dealing with path losses.Free space loss is the diminishing of signal power by geometric spreading of the wavefront.It depends only on the distance i.e the loss happens because the radiated signal energy expands as a function of the distance from the Tx.Attenuation takes place as some of the signal power is absorbed when the wave passes through solid objects such as trees,walls,windows and floors of buildings.Attenuation vary greatly depending on the structure of the object th signal is passing,and it is quite difficult to quantify.The processes that lead to attenuation include Absorption,Reflection,Diffraction and interferance. However, diffraction and interferance can be useful. Along the path , the RF energy leaves the TX and then it spreads out .Some of the RF energy reaches the antenna directly while some bounces off the ground.This is called scattering.The line of sight thet we need in order to have an optimal wireless connection from A to B is more than just a thin line.Its shape is more like that of a cigeror an an ellipse.Its width can be decribed by the concept of Fresnel zones.

To evaluate the link feasibility,one must knowthe characteristics of the equipmentbeing used and evaluate the path losses.For it to work the condition is that (Radio1 Tx power +Antenna gain-losses)>Radio2 sensitivity. This calculation is performed on both sides of the radios and the acceptable difference between the two sides should be atleast within(3-6dB).