Multipath a Potential WLAN Problem

Unforeseen RF transmission impairments, such as cordless phones that use the
2.4GHz band, can cause frustration when deploying wireless LANs. In order to
maintain happy users, it’s a good idea to perform an RF
site survey
before installing access points. The survey can uncover problems
such as RF interference and multipath propagation, two troubles that you should
tackle before moving to far forward. We’ve already discussed RF interference
in a previous
tutorial
, so let’s discuss multipath propagation and its impacts on wireless
LANs.

Multipath propagation defined

Multipath propagation occurs when an RF signal takes different paths when propagating
from a source (e.g., a radio NIC) to a destination node (e.g., access point).
While the signal is en route, walls, chairs, desks, and other items get in the
way and cause the signal to bounce in different directions. A portion of the
signal may go directly to the destination, and another part may bounce from
a chair to the ceiling, and then to the destination. As a result, some of the
signal will encounter delay and travel longer paths to the receiver.

Multipath delay causes the information symbols represented in an 802.11 signal
to overlap, which confuses the receiver. This is often referred to as intersymbol interference
(ISI). Because the shape of the signal conveys the information being transmitted,
the receiver will make mistakes when demodulating the signal’s information.
If the delays are great enough, bit errors in the packet will occur. The receiver
won’t be able to distinguish the symbols and interpret the corresponding bits
correctly.

When multipath strikes in this way, the receiving station will detect the errors
through 802.11’s error checking process. The CRC (cyclic redundancy check) checksum
will not compute correctly, indicating that there are errors in the packet.
In response to bit errors, the receiving station will not send an 802.11 acknowledgement
to the source. The source will then eventually retransmit the signal after regaining
access to the medium.

Because of retransmissions, users will encounter lower throughput when multipath
is significant. The reduction in throughput depends on the environment. As examples,
802.11 signals in homes and offices may encounter 50 nanoseconds multipath delay
while a manufacturing plant could be as high as 300 nanoseconds. Based on these
values, multipath isn’t too much of a problem in homes and offices. Metal machinery
and racks in a plant, however, provide a lot of reflective surfaces for RF signals
to bounce from and take erratic paths. As a result, be wary of multipath problems
in warehouses, processing plants, and other areas full of irregular, metal obstacles.

802.11b suffers the most

When comparing FHSS (frequency
hopping spread spectrum), DSSS (direct sequence spread spectrum) and OFDM (orthogonal
frequency division multiplexing), DSSS used by 802.11b
networks is the most susceptible to multipath propagation. The frequency elements
of a wider band signal will vary greatly in terms of reflectivity as they encounter
obstacles in the facility. FHSS uses relatively narrow channels (1 MHz) and
changes transmit frequency often, making it difficult for multipath to occur.
OFDM (used by 802.11a and 802.11g) transmits information on many narrow subchannels,
which also reduces the impacts of multipath.

DSSS, however, transmits information continuously over a relatively wide channel,
nearly 30MHz. This leaves enough room for lower frequency elements of the DSSS
signal to reflect off obstacles much differently than the higher frequency elements
of the signal. The differences in reflectivity will cause a wider range of signal
paths. Thus, 802.11b systems are more susceptible to multipath delays.

If radio NIC logs or site survey software provided by wireless LAN vendors
indicates that the wireless LAN is retransmitting packets often, then the network
may be experiencing severe multipath propagation. Keep in mind, however, that
RF interference from an external radio source could be causing these same indications.
As a result, you should verify that RF interference is not also causing the
problem. The best method to differentiate the problem is to use a handheld tester
capable of measuring multipath delays (e.g., Berkeley Varitronics’ Scorpion).

Diversity offers some comfort

What can you do if multipath is causing problems? Aside from clearing desks
and chairs from your building, diversity seems to be the best solution to combat
the perils of multipath. Diversity is the use of two antennas for each radio
in order to increase the odds of receiving a better signal on either of the
antennas.

Diversity antennas have physical separation from the radio to ensure that one
of the antennas will encounter less multipath propagation affects than the other.
In other words, the composite signal that one antenna receives may be closer
to the original than what’s found at the other antenna. The receiver uses signal-filtering
and decision-making software to choose the best signal for demodulation. In
fact the reverse is also true: the transmitter will then choose the best antenna
for transmitting in the opposite direction.

If diversity doesn’t solve a severe multipath problem, then consider deploying
an 802.11a network. You’ll also gain the benefits of higher performance and
freedom from RF interference, but of course you’ll need to consider interoperability
issues and possibly higher costs.

Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book,
Wireless LANs (2nd Edition), and regularly instructs
workshops on wireless
LANs.

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