RF Fundamentals for WLANs

Though WLANs are easy to deploy, the network administrator or IT professional will benefit from some basic knowledge about radio wave propagation.Although it is possible to utilize infrared technology (which always requires line of sight between elements of the network), this paper deals only with Radio Frequency (RF)wireless networks, which have become the industry accepted standard for WLANs.

The reason for using RF is simple.It can pass through solid objects such as office walls. However,radio waves do not go on forever in all directions without weakening or being affected by physical barriers.The user needs to have some understanding of their propagation characteristics, as well as the relationship between power levels and data rates, before a wireless network can be designed.

Propagation Characteristics Must Be Considered

Reflection -Radio waves can be reflected by some materials.This phenomenon is often used to steer microwave signals between stations that are not line-of-sight, but in an office environment it can create multipath (see below).

Absorption -Radio waves can be absorbed by many materials such as water,plastic, sheetrock,and carpet. Geometric Spreading loss -Radio waves,like light waves, get weaker as they expand outward away from their source.This loss grows as the square of the distance.This means that if a device is moved twice as far away,the signal power drops by one fourth.

Path loss -The above phenomena lead to path loss,or an unavoidable weakening of the signal ’s power as it propagates outward.In an office environment,the placement of furniture and walls,and even the movement and location of people,will contribute to the amount of path loss.

Multipath -If a received signal is made up of radio waves from the same signal that has dispersed and arrived from different paths,i.e.some of the original energy was reflected off an object before arriving at the receiver,then the effects of multipath are seen.Television sets connected to antennas often exhibit this as ghosting.Network users may likewise experience its digital counterpart -referred to as intersymbol interference .This is caused when the difference in time between radio waves arriving from the same signal,referred to as delay spread , is enough to cause symbol overlap in the digital data.As the data transmission speed gets faster, the time between received data bits get smaller and more susceptible to intersymbol interference,so multipath places an upper limit on data transmission speed.

Propagation characteristics are frequency dependent:
At lower frequencies (longer wavelengths),less RF energy is absorbed by obstructions. Signals can pass through solid objects (walls)more readily. At higher frequencies (shorter wavelengths),smaller antennas can be used.However,if antennas are scaled down proportionately with wavelength,the received signal power will decrease as a function of frequency squared,due to less signal energy being intercepted by the smaller antenna.This shortcoming can be overcome by using higher gain antennas.

Data Rate and Range are Related
The signal strength at the receiver is often stated in reference to the noise level as the signal-to-noise ratio (S/N ratio or SNR).Information theory defines a relationship between the SNR of a received signal,and how much information can be reliably transmitted over that channel.The result for the user is that for a given SNR (and corresponding transmitted power level),the bit error rate (BER)increases as the data rate increases.Thus there exists a tradeoff between data rate and range.All things (transmitter power,encoding method, antenna gain,etc.)being equal,if a wireless network is operated at a higher data rate, the effective range will decrease.Conversely,if it is operated at a greater range,the data rate may need to be dropped to obtain an acceptable BER. The only way to increase range without decreasing data rate is to increase power – and the FCC puts a ceiling on power levels.

It is not difficult to get a wireless network up and running,but it is often advantageous, especially for large corporate LANs,to have a specialist do an RF site survey first.The site survey will assist in the network design process by providing details about how particular conditions in a building contribute to path loss and multipath,and thus to data rate and range.