Link planning

A basic commjunication system consists of two radios , each with its asosociated antenna , the two being separated by the path to be covered . In order to have a communication between the two , the radios require a certain minimum singnal to be collected by the antennas and presented tot heir input socket . Determining if the link is feasible is a process called link budget calculation . Whether or not signals can be passed between the radios depends on the qualit of the equipment being used and on the diminishment of the signal due to distance , called path loss .

Calculateing the link budget

The power abailable in an 802.11 system can be characterized by the following factors:
- Transmit power . It is expressed in milliwatts or in dBm . Transmit Power ranges from 30mW to 200mW  or more . TX power is often dependent on the transmission rate . The TX power of a given device should be specified in the literature providd by the manufacture , but can sometimes be difficult to find . Online database such as the one provided by Seatle Wireless

- Antena Gain . Antenans are passive devices that create the effect of amplification by virtue of their physical shape . Antennas have the same characteristics when receiving and transmitting . So a 12 dBi antenna is simply a 12 dBi antenna , without specifying if it is in transmission or reception mode . Parabolic antennas have a gain of 19-24 dBi , omnidirectional antennas have 5-12 dBi , sectorial antennas have roughly a 12-15 dBi gain .

-Minimum Received Signal Level , or simply , the sensitivity of the receiver . The minimum RSL is always expressed s a negative dBm ( -dBm ) and is the lowest power of singnal the radio can distinguish . The  minimum RSL is dependent upon rate , and as a general rule the lowest rate ( 1Mbps ) has the greatest sensitivity . The minimum will be typically in the range of -75 to -95 dBm . Like TX power , the RSL specifications should be provided by the manufacturer of the equipment .

-Cable losses . ome o the signal's energy is lost in the cables, the connectors and other devices , going from  the radios to the antennas . The loss depends on the type of cabl;e used and on its length . Signal loss for short coaxial cables including connectors is quite low , in the range of 2-3 dB /.It is better to have cables as short as possible .

When calculating the path loss , several effects must be considered . one has to take into account the free space loss , attenuation and scattering . Signal power is diminished by geometric spreading of the wavefront , commonly known as free space loss . Ignoring evertthing else , the further away the two penent rom the environment , depending only on the distance . This loss happends because the radiated signal eenergy expands as a function of the distance from the transmitter .

Using decibels to express the loss and using 2.45 Ghz as the signal frequency , the equation for the free space loss is

Lfsi = 40 + 20*log

Where Lisi is expressed in dB and r is the distance between the transmitter and receiver in meter .

The second contribution to the path loss is given by attenuation . This takes place as some of the signal power is absorbed when the wave passes through solid objects such as trees , walls , windws and floors of buildings . Attenuation can vary greatly depending upon the structure of the object the signal is passing through , and it is very difficult to quantity . The most conventient way to express its contribution to the total loss is by adding an "allowed loss" to the free space . For example , experience shows that trees add 10 to 20 dB of loss per tree in the direct path , while walls contribute 10 to 15 dB depending upon the contruction .

Along the link path , the RF energy leaves the transmitting antenna and energy spreads out . Some of the RF energy reaches the receiving antenna directly , while some bounces off the ground . Part of the RF energy which bounces off the ground reches the receiving antenna . Since the reflected signal has a longer way to travel , it arrives at the receiving antenna later than the direct signal . The effect is called mutyipath , or singla dispersion . In some cases reflected signal add together and cause no problem . When they add together out of phase , the received signal is almost worthless . In some cases the signal at the receiving antenan can be zeroed by the reflected signals . This is known as extreme fading , or nulling . There is a simple technique  that is used to  deal with multipath , called antenna diversity . it consists of adding a second antenna to the radio . Multipath is in fact a very location  -specific phenomenon . If two singnal add out of phase at one lacation , they will not add desctructively at a second , nearby location . If there are two antennas , at least one of them should be able to receive a usable signal , even if the other is receiving a distorted one . In commercialdevices , antenna switching diversity is used : there are multiple antennas on multiple inputs , with a single receiver . The signal is thus received through only one antenna at a time  . When transmitting , the radio uses the antenna last used for reception . The distortion iven by multipath degrades the abi,lity of the receoiver to recover the signal in a manner much like signal loss . A simple way of applying the effects of scattering in the clculation of the path loss is to change the exponent of the distance factor of the free space loss formula . The exponent tends to increase witht the range in an environment with a lot of scattering . An exponent of 3 can be used in an outdoor environment with trees , while one of 4 can be used for an indoor environment .

When free space loss , attenuation , and scattering are combined , the path loss is :

L(dB = 40 + 10 *n*log (r) + L ( allowed )

For a rough estimate of the link feasiblility , one can evaluate just the free space loss . The environment can bring further signal loss , and should be considered for an exact evaluation of the link . The environment is in fact a very imortant factor , and should never be neglected .

To evaluate if a link is feasible , one must know the characteristic of the equipment being used and evaluate the path loss . Note that when performing this calculation , you should only add the TX power  of one side of the link . If you are using different radios on either side of the link , you should calculate the path loss twice , once for each direction  ( using the appropriate TX power for each calculation ) . Adding up all the gains and subtracting all the losses gives

TX power Radio 1
+ Antenna Gain Radio 1
- cable losses radio 1
+ Antenna Gain Radio 22
- Cable Losses Radio 2
= Total Gain

Subtractin the Path Loss from the Total Gain  :
Total Gaoin - Path Loss = signal level at one side of the link

If the resulting signal level is greater than the minimum rteceived signal level , then the link is feasible ! . The received signal is powerful enough for the radios to use it . Remember that the minimum RSL is alwasys espressed as a negative dBm  , so -56 is greater than -70 dBm . On a given path , the variation in path loss over a period of time can be large , so a certain margin ( difference between the signal level and the minimum received signal level ) should be considered . This margin is the amount of signal aboive the sensitivity of radio that should be received in order to ensure a stable , high quality radio link during bad weather adn other atmospheric disturbances . A margin of 10 to 15 dB is fine . To give some space for attenuation and multipath in the received radio signal , a margin of 20dB should be safe enough .

Once you have calculated the link budget in one direction , repeat the calculation for the other direction . Substitute the transmit power for that of the second radio , and compare the result against the minimum received signal level of the first radio .