RF Signal Strength Terminology
Because so many variables exist in a wireless environment, being able to quantify an RF signal as it is transmitted and received is handy. Other factors that affect the signal strength can be taken into account, too.
An RF signal can be measured as a function of its power or energy in units of Watts (W) or milliWatts (mW)—one milliWatt is one-thousandth of one Watt. To put signal power into perspective, Table 17-3 shows typical power output from a variety of sources.
|
Source |
Power Output |
|
Shortwave radio station |
500,000 W |
|
AM radio station |
50,000 W |
|
Microwave oven (2.4 GHz) |
600-1000W |
|
Cell phone |
200 mW |
|
Wireless LAN AP (2.4 GHz) |
1-100 mW |
Power values can vary over such a wide range that making comparisons or computations is difficult. The decibel (dB) is a more flexible way to deal with power quantities for two reasons:
■ dB measures a ratio of actual power to a known reference power
■ dB is logarithmic, representing a wide range of values on a linear scale To compute a power ratio in dB, use the following formula:
dB = 10logio pj
Here, Psig is the actual signal power and Pref is the reference power. Most often, a reference power of 1.0 mW or 1.0 W is used. In those cases, the decibel abbreviation also changes to reflect the reference power:
■ dBm—The signal power is compared to 1 mW (the "m" in dBm reflects the "m" in milliWatt)
■ dBw—The signal power is compared to 1 W
You will commonly see dBm used with wireless LANs because the power used is around 100 mW or less. As an example, suppose that a wireless AP transmits at 100 mW. In dB, the output power would be represented as 10log10(100 mW/1 mW) or 20 dBm.
If the output power was decreased to 1 mW, the result would be 10log10(1 mW/1 mW) or 0 dBm. Therefore, 0 dBm always represents an output power that is equal to the reference power.
So far in this discussion, dB values have all been positive numbers. Positive dB values result from power values that are greater than the reference power. Most often, transmitter power levels will have positive dBm values because they are stronger than the reference.
dB values can also be negative. This doesn't mean that a negative amount of power is produced— rather, it means a power level that is less than the reference. For example, a signal power of 0.5 mW would result in 10log10(0.5 mW/1 mW) or -3 dBm.
TIP Notice that the sample power of 0.5 mW is half of the previous example, 1.0 mW. The dB value changed from 0 to approximately -3 dB, respectively. This change is significant because it demonstrates an important rule of thumb:
■ Whenever a power value is halved, the dB change is approximately -3.
■ Whenever the power doubles, the dB change is approximately +3.
Negative dBm values are often given for receivers, rather than transmitters. This is because receivers must be very sensitive to low signal levels (much lower than the 1 mW reference power) so that poor signals can be received with clarity. Receiver power levels are referred to as receiver sensitivity.
Signal Loss
Whenever an RF signal leaves the transmitter, even before it reaches the antenna, it is subject to outside influences that will reduce its strength. This is known as signal loss.
Signal loss can come from any of the following:
■ Cable loss from the transmitter to the antenna
■ Free space loss as the signal travels through the air
■ External obstacles
■ External noise or interference
■ Cable loss from the receiver's antenna to the receiver
Notice that this list covers conditions as they are encountered along the signal path from transmitter to receiver. The losses are actually cumulative, working together to degrade the signal. The total loss end-to-end is known as the path loss.
In an indoor wireless LAN environment, the antenna cabling distances are so short they are negligible. Typically, the antennas are built right onto the wireless adapters or within a portable PC, or directly connected to the RF electronics in an AP. The majority of path loss is due to physical objects in a room or building and the distance between the AP and a wireless client.
Outdoor environments can experience losses with components from all the factors listed. In a line-of-sight wireless path, the cables between APs and their antennas can be quite long. External objects might not be an issue if the path has been carefully selected. External interference can be a problem due to other nearby wireless installations, conflicts over channel use, or antennas that aren't aligned carefully.
In any environment, free-space loss is significant. The power of any RF transmission is inversely proportional to the square of the distance from the source. This simply means that the level of the received signal falls off rapidly as the receiver moves away from the transmitter.
For example, Table 17-4 shows how the dB level decreases as the distance increases. The dBm values shown are relative—you should notice that as the distance increases by 10 times, the signal drops by 20 dBm. As the distance increases by 100 times, the signal drops by 40 dBm.
|
Distance |
Relative Power |
Relative Level (dBm) |
|
1 m |
100 mW |
+20 dBm |
|
5 m |
4.0 mW |
+6 dBm |
|
10 m |
1.0 mW |
0 dBm |
|
25 m |
0.16 mW |
-8 dBm |
|
50 m |
0.04 mW |
-14 dBm |
|
100 m |
0.01 mW |
-20 dBm |
A receiver can be located too far from the transmitter to receive an intelligible signal. It can also be located behind too many objects that absorb or distort the signal. For example, even ordinary building materials such as drywall, block or concrete walls, wooden or metal doors, door frames, and windows can contribute to signal loss. Some sources estimate that within 100 meters from an AP in an office environment, a wireless signal can have a loss of 100 dBm! Because of this, performing a site survey that is based on a live WLAN signal in the actual physical environment is always a good idea.
Signal Gain
An RF signal can also be influenced by factors that actually increase its strength over a path. Signal gain can be produced by any of the following:
■ Antenna gain at the transmitter
■ Antenna gain at the receiver
An antenna can't create or add power to a signal by itself. How can it have gain? First, consider a transmitter antenna. Its gain is just a measure of how well the antenna can take the RF signal and project it in a specific direction.
However, antennas can be designed to favor different directions or patterns. Is one antenna better than another simply because it focuses a signal in one direction over another? Not necessarily. Typically, one antenna has more gain than another if it can focus the RF energy into a narrower pattern. As a result, antennas with higher gains are usually able to operate over longer distances.
The antenna gain is just a way to gauge how well the signal energy is focused, in comparison to an antenna that can't focus energy at all. An isotropic antenna is such an antenna—a theoretical model that is shaped like a small dot. It propagates a signal in all directions equally, in a spherical pattern. Its inability to focus the RF energy makes it the standard for comparison.
Antenna gain is usually given in dBi, which is computed exactly like dBm. The only difference is that the reference power comes from an isotropic antenna (hence the "i").
Wireless Path Performance
Often you will see settings or ratings of transmit power on an AP. This rating is usually the power measured at the output of the transmitter, without an antenna or cabling. The actual power of the transmitted signal will depend on the type of antenna used and the length of the antenna cable.
A much more realistic rating is the Effective Isotropic Radiated Power (EIRP). To compute the EIRP, add the transmitter power (in dBm) to the transmit antenna gain (in dBi) and subtract any cabling loss (in dB).
For example, a transmitter power of 100 mW (20 dBm) connects to an antenna with a 16 dBi gain using a cable that has 3 dB loss. The EIRP of the transmitter system is 20 dBm + 16 dBi - 3 dB = 33 dBm.
To design a complete wireless system, you have to consider more than the power of the transmitter or AP. Instead, you need to account for every component that will add gain or subtract loss, from one end of the wireless link to the other.
The simplest way to determine the path performance or overall gain is to add all the gain or loss dB values. You can use the following formula as a rule of thumb:
System Gain = Transmit Power (dBm)
+ Transmit Antenna Gain (dBi) + Receive Antenna Gain (dBi)
- Transmit Cable Loss (dB)
- Receive Cable Loss (dB)
- Receiver Sensitivity (dB)
Notice here that the receiver sensitivity is considered to be a loss and is subtracted. The receiver sensitivity represents the minimum threshold that must be overcome in order to have a usable signal. Therefore, that threshold must be subtracted to see the remaining gain that is available.
The range of a wireless link is determined by the overall path performance. When the total path loss becomes equal or greater than the total path gain, the receiver will be out of range.
Continue reading here: WLAN Antennas
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