Calculating the Link Budget

The power available 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 provided by the manufacturer, but can sometimes be difficult to find. Online databases such as the one provided by SeattleWireless may help.

• Antenna Gain. Antennas 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 dBm, 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 as a negative dBm (-dBm) and is the lowest power of signal the radio can distinguish. The minimum RSL is dependent upon rate, and as a general rule the lowest rate (1 Mbps) 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. Some of 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 cable 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 everything else, the further away the two radios, the smaller the received signal is due to free space loss. This is independent from the environment, depending only on the distance. This loss happens because the radiated signal energy 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

Lfsl = 40 + 20*log(r)

where Lfsl is expressed in dB and r is the distance between the transmitter and receiver, in meters.

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, windows 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 quantify. The most convenient 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 construction.

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 reaches the receiving antenna. Since the reflected signal has a longer way to travel, it arrives at the receiving antenna later than the direct signal. This effect is called multipath, fading or signal dispersion. In some cases reflected signals add together and cause no problem. When they add together out of phase, the received signal is almost worthless. In same cases, the signal at the receiving antenna can be zeroed by the reflected signals. This is known as nulling. There is a simple technique that is used to deal with multipath, called antenna diversity. It consists in adding a second antenna to the radio. Multipath is in fact a very location-specific phenomenon. If two signals add out of phase at one location, they will not add destructively 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 commercial devices, 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 given by multipath degrades the ability of the receiver to recover the signal in a manner much like signal loss. A simple way of applying the effects of scattering in the calculation of the path loss is to change the exponent of the distance factor of the free space loss formula. The exponent tends to increase with 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 feasibility, 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 important factor, and should never be neglected.

To evaluate if a link is feasible, one must know the characteristics 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 2
      - Cable Losses Radio 2
      -----------------------
      = Total Gain

Subtracting the Path Loss from the Total Gain:

        Total Gain
      - Path Loss
      ---------------
      = Signal Level at one side of the link

If the resulting signal level is greater than the minimum received 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 always expressed as a negative dBm, so -56dBm is greater than -70dBm. 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 above the sensitivity of radio that should be received in order to ensure a stable, high quality radio link during bad weather and other atmospheric disturbances. A margin of error of 10-15 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.