There are many issues involved to get the best signal range and quality in a WLAN deployment. One concern comes from multipath distortion. Consider the wireless connection shown in Figure 2-2.
Figure 2-2. Multipath Distortion Occurs When Obstacles and Other Environmental Obstructions Cause Radio Frequency Signals to Arrive Out of Synch
When a radio frequency broadcast is transmitted from the antenna, part of the signal can travel directly to its destination, although other parts bounce off of obstructions.
As 802.11 signals are broadcast, the signal bounces around the room, creating multiple copies of the signal. Because the signals that bounce around travel a longer path than a signal that is received directly from the antenna, the multiple signals are no longer in phase with each other. Instead, they are in a random phase. Multiple signals that are in-phase are ideal; they create better reception. However, out-of-phase signals can cancel each other out or cause packet retries to occur.
Multipath distortion occurs most often in places with highly RF-reflective surfaces, such as metal, coated glass, or furniture. Unfortunately, places where multipath distortion is prevalent are also those places where wireless networking is so useful, such as:
The solution to the problem of multipath distortion is diversity.
Configuring Multiple Antennas for Diversity
To mitigate multipath distortion, two antennas are used in tandem. Diversity uses these antennas, placed a short distance apart, for each radio. Figure 2-3 shows this.
Figure 2-3. Diversity Antennas Use Two Antennas Located a Short Distance Apart
The idea behind this solution is that with two antennas deployed, the odds of the radio that receives an acceptable signal from one of the two antennas is increased. Multipath distortion is localized to a specific location (either antenna A or antenna B). Because of this, if the distortion vexes antenna A, then antenna B does not suffer the same distortion problems. The radio simply switches to whichever antenna receives the strongest signal. Either antenna can be repositioned to a better location to ensure good reception and reduced multipath distortion.
To get a better understanding of how diversity works, it's important to realize that the receiver does not simultaneously listen to both antennas. Instead, it listens to only one antenna at a time. When it encounters multipath distortion, it checks the other antenna to see if it gets a better reception. If so, it switches to that antenna.
As you might surmise, if the receiver listened to both antennas, then a multipath environment would occur because the device would receive the radio signals at slightly different times.
Optimal Antenna Placement for Diversity
Because each antenna is used independently from the other, to select and locate antennas for decreasing multipath distortion, you must follow two important guidelines:
Because it is necessary for the antennas to be physically separated from each other, it's easy to assume that they should be located on opposite ends of the building. Not so. In fact, most often the antennas should be located between .3 and 2.5 meters (1 to 8 feet) apart. On the other hand, antennas placed closer than .15 meters (6 inches) do not experience the diversity needed to quash multipath distortion.
The farther apart you locate the antennas, the more disparate the areas the antennas cover. If they are too far apart, the antennas cover different clients. Whenever the AP decides to switch to the other antenna, the group of clients covered is cut off, and vice versa. Diversity relies on the antennas to cover the same radio cell.