Networking

How to make the best of 802.11 multipath environments

In a post titled Wi-Fi 101: Multipath environments and how they affect Wi-Fi propagation, I examined what happens to 802.11 RF signals when they encounter physical obstacles. I would like to take the next logical step and develop quantifiable relationships between three different Line of Sight (LoS) environments and the affect each one has on an 802.11 RF signal.

RF LoS environments

LoS: When RF signals are traveling in a Fresnel Clearance Zone, Free Space Path Loss is the only measurable propagation factor affecting the expanding RF wave front which makes it simple to model. But, you must be careful as Free Space Path Loss can be wrongly interpreted. It is easy to believe that free space attenuates an electromagnetic wave, but that's not the case. Free Space Path Loss is the resultant of two variables. One variable is the diffusing of an electromagnetic wave in free space based on the inverse square law. The second variable is how well the receiving antenna can pick up power from an incoming electromagnetic wave. Free Space Loss calculations are fairly accurate indicators and used by RF engineers to calculate System Operating Margins. Near-LoS: This is an environment, where received RF signal components start to vary in space, time, and power levels due to multipath events. These variations are called Rician fading and measured by the Rician K-factor, which is the ratio between the signal power of the dominant (LoS) component and the signal power of the scattered/reflected (Near-LoS) components. Because the Rician K-factor is such a good representation of Near-LoS conditions, many equipment developers incorporate it in their algorithms for determining bit error rates. Non-LoS: This refers to a RF environment involving two distinct components, path fade, or blockage of the Fresnel Clearance Zone and Rayleigh fading. What makes an environment displaying Rayleigh fading unique is the lack of a dominant LoS RF signal component and knowing that the RF signal components arriving at the receiving antenna only do so because of multipath interference. The Rayleigh Distribution model is used to gain insight as to whether multiple RF signals will combine in a positive or negative manner at the receiving antenna.

Using these analytical tools, RF engineers have been able to develop specific technologies that mitigate the negative effects of multipath interference. Even more interesting is the discovery-due to copious amounts of research into Rayleigh fading-of methods using multipath environments to actually enhance RF propagation. Let's now move on to the resultant technology or what RF engineers refer to as diversity schemes.

Reactive diversity schemes

The following diversity schemes are hardware/firmware solutions attempting to minimize the negative effects of multipath interference (with special emphasis on "minimize.")

Frequency diversity: This is where the signal is sent using several frequency channels or spread over a wide spectrum. This effectively alters the multipath environment when the carrier frequency changes. Although not well known for lessening the negative effects of multipath interference, OFMD modulation and spread spectrum are just that. Using multiple sub carriers over a particular bandwidth that's intentionally spread in the frequency domain dramatically increases protection from interference and jamming. Polarization diversity: In a normal linearly polarized receive/transmit antenna pair, a polarity misalignment of 45 degrees between the transmitting antenna and the receiving antenna could degrade the signal by up to 3 dB. If the misalignment is 90 degrees the attenuation can be more than 20 dB. In real-world situations, even with the receive/transmit antennas aligned perfectly, some RF signal components will have different polarities, due in part to the signal being altered-by multipath interference-on its way to the receiving antenna.

Antenna arrays with multiple elements, each having a different polarity, is one way to reduce the effects of environmentally induced polarity misalignment. One example is a yagi style antenna with two sets of elements, set 90 degrees apart. The whole antenna is then rotated along the main axis so the elements are at a 45 degree angle to the horizon. The idea behind this approach is to try and present a surface that will capture as many of the misaligned RF signals as possible. Circular polarized antennas use the same science and are gaining some traction as being a solution for dealing with polarity misalignment. Awareness of polarization diversity and the need to experimentally determine the optimal antenna attitude can lead to significant signal strength gains.

Time diversity: this is a transmitting scheme that sends multiple versions of the identical signal but at different time slots. Usually Forward Error Correction (FEC) code is added and bit-interleaving is used to make sure the size of the transmitted frame is such that even with error bursts the frame is recoverable. Antenna diversity: Antenna diversity is a popular technology where two or more receiving antennas are used to eliminate multipath signal distortion. The premise behind antenna diversity is that if one receiving antenna is experiencing low signal levels due to fading, the other receiving antenna is probably not experiencing the same conditions, since the antenna is in a different location. The receiver examines the RSSI level of each antenna at the beginning of the receive burst and selects the one with the best ratio. Most people are skeptical about how two antennas used on 802.11 devices could make that much difference since they're so close together. Why it works has to do with the fact that the wavelength of 802.11b/g frequencies is approximately five inches long and research has shown that antennas even a quarter wavelength apart can realize appreciable antenna gain.

Proactive diversity schemes

Proactive diversity schemes are the exact opposite of reactive diversity schemes in that multipath interference is leveraged to improve RF signal quality. Simply put, multipath interference exists in virtually every 802.11 network, so why not use it to your advantage? You need very sophisticated technology to accomplish this and the following diversity schemes describe some recent engineering breakthroughs.

Transmit diversity: This is used to diminish the effects of fading by transmitting the same information from two or more antennas. The data being transmitted from each antenna is encoded differently which allows the receiver to recognize that the information is coming from multiple locations and properly decode the data. Diversity reception: I first wanted to mention that I have no clue as to why the naming order is reversed for this scheme. Simply stated, diversity reception is where the transmitted information is received multiple times, thereby enabling the receiver to have more opportunities to determine what was transmitted. If you remember, reactive diversity schemes were set up to isolate and focus only on one RF signal, which hopefully is the dominant (LoS) component. Diversity reception receivers have significantly more intelligence and can actively process multiple RF signal components at the same time. Diversity reception receivers also use what is called diversity combining, an additive process to create a resultant RF signal that is actually stronger than the single dominant (LoS) component.

Technology has progressed a great deal in recent years and hopefully these posts have been insightful enough to point out the major contributions. I only mention this as the discussion about multipath environments now turns to today's 802.11n technology.

In my next blog, I'll talk about MIMO and how it will revolutionize 802.11 wireless networking.

About Michael Kassner

Information is my field...Writing is my passion...Coupling the two is my mission.

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