Multipath is a propagation phenomenon that results in two or more paths of a signal arriv- ing at a receiving antenna at the same time or within nanoseconds of each other. Because of the natural broadening of the waves, the propagation behaviors of refl ection, scattering, diffraction, and refraction will occur differently in dissimilar environments. When a signal encounters an object, it may refl ect, scatter, refract, or diffract. These propagation behav- iors can all result in multiple paths of the same signal.
In an indoor environment, refl ected signals and echoes can be caused by long hallways, walls, desks, fl oors, fi le cabinets, and numerous other obstructions. Indoor environments with large amounts of metal surfaces such as airport hangars, warehouses, and factories are notoriously high-multipath environments because of all the refl ective surfaces. The propagation behavior of refl ection is typically the main cause of high-multipath environ- ments. In an outdoor environment, multipath can be caused by a fl at road, a large body of water, a building, or atmospheric conditions. Therefore, we have signals bouncing and bending in many different directions. The principal signal will still travel to the receiving antenna, but many of the bouncing and bent signals may also fi nd their way to the receiv- ing antenna via different paths. In other words, multiple paths of the RF signal arrive at the receiver, as seen in Figure 2.15.
It usually takes a bit longer for refl ected signals to arrive at the receiving antenna because they must travel a longer distance than the principal signal. The time differential between these signals can be measured in billionths of a second (nanoseconds). The time differential between these multiple paths is known as the delay spread. You will learn later in this book that certain spread spectrum technologies are more tolerant than others of delay spread.
So, what exactly happens when multipath presents itself? In television signal transmissions, multipath causes a ghost effect with a faded duplicate image to the right of the main image.
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c02.indd 08/22/2014 Page 54 F I G U R E 2 .1 5 Multipath
The four possible results of multipath are as follows:
Upfade This is increased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are in phase or partially out of phase with the primary wave, the result is an increase in signal strength (amplitude). Smaller phase differences of between 0 and 120 degrees will cause upfade. Please understand, however, that the fi nal received signal can never be stronger than the original transmitted signal because of free space path loss. Upfade is an example of constructive multipath.
Downfade This is decreased signal strength. When the multiple RF signal paths arrive at the receiver at the same time and are out of phase with the primary wave, the result is a decrease in signal strength (amplitude). Phase differences of between 121 and 179 degrees will cause downfade. Decreased amplitude as a result of multipath would be considered destructive multipath.
Nulling This is signal cancellation. When the multiple RF signal paths arrive at the receiver at the same time and are 180 degrees out of phase with the primary wave, the result will be nulling. Nulling is the complete cancellation of the RF signal. A complete can- cellation of the signal is obviously destructive.
Data Corruption Because of the difference in time between the primary signal and the refl ected signals known as the delay spread, along with the fact that there may be multiple refl ected signals, the receiver can have problems demodulating the RF signal’s information. The delay spread time differential can cause bits to overlap with each other, and the end result is corrupted data. This type of multipath interference is often known as intersymbol interference (ISI). Data corruption is the most common occurrence of destructive multipath.
The bad news is that high-multipath environments can result in data corruption because of intersymbol interference caused by the delay spread. The good news is that the receiving station will detect the errors through an 802.11-defi ned cyclic redundancy check (CRC) because the checksum will not calculate accurately. The 802.11 standard requires that most unicast frames be acknowledged by the receiving station with an acknowledgment (ACK) frame; otherwise, the transmitting station will have to retransmit the frame. The receiver will not acknowledge a frame that has failed the CRC. Therefore, unfortunately, the frame must be retransmitted, but this is better than it being misinterpreted.
Layer 2 retransmissions negatively affect the overall throughput of any 802.11 WLAN and can also affect the delivery of time-sensitive packets of applications, such as VoIP. In Chapter 12, “WLAN Troubleshooting,” we discuss the multiple causes of layer 2 retrans- missions and how to troubleshoot and minimize them. Multipath is one of the main causes of layer 2 retransmissions that negatively affect the throughput and latency of a legacy 802.11a/b/g WLAN.
So, how is a hapless WLAN engineer supposed to deal with destructive multipath issues?
Multipath can be a serious problem when working with legacy 802.11a/b/g equipment. The use of directional antennas will often reduce the number of refl ections, and antenna diver- sity can also be used to compensate for the negative effects of multipath. Sometimes, reduc- ing transmit power or using a lower-gain antenna can solve the problem as long as there is enough signal to provide connectivity to the remote end. In this chapter, we have mainly focused on the destructive effects that multipath has on legacy 802.11a/b/g radio transmis- sions. Multipath has a constructive effect with the now prevalent 802.11n and 802.11ac radio transmissions that utilize multiple-input, multiple-output (MIMO) antenna diversity and maximal ratio combining (MRC) signal processing techniques.
In the past, data corruption of 802.11a/b/g transmissions caused by multipath had to be dealt with, and using unidirectional antennas to cut down on refl ections was commonplace in high-multipath indoor environments. Now that MIMO technology used by 802.11n and 802.11ac radios is commonplace, multipath is now our friend and using unidirectional antennas is rarely needed indoors. However, unidirectional MIMO patch antennas are often used indoors to provide sectored coverage in high-density user environments.
E X E R C I S E 2 . 2
Visual Demonstration of Multipath and Phase
In this exercise, you will use a program called EMANIM to view the effect on amplitude due to various phases of two signals arriving at the same time.
1. From the book’s page at www.sybex.com/go/cwna4e, download and install the EMANIM program by double-clicking emanim_setup.exe.
2. From the main EMANIM menu, click Phenomenon.
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E X E R C I S E 2 . 2 ( c o n t i n u e d )
Two identical, vertically polarized waves are superposed (you might not see both of them because they cover each other). The result is a wave having double the ampli- tude of the component waves.
5. Click Exercise B.
Two identical, 70-degree out-of-phase waves are superposed. The result is a wave with an increased amplitude over the component waves.
6. Click Exercise C.
Two identical, 140-degree out-of-phase waves are superposed. The result is a wave with a decreased amplitude over the component waves.
7. Click on Exercise D.
Two identical, vertically polarized waves are superposed. The result is a cancellation of the two waves.