Wi-Fi as a Metro-Access Deployment Option

The Wi-Fi certification addresses interoperability across
IEEE 802.11 standards-based products. The IEEE 802.11
standard, with specific revisions, was designed to address
wireless local area coverage.

External modifications to the standard through hardware and
software allow Wi-Fi products to become a metro-access
deployment option. These two major modifications address
two different usage models:
• Fixed-access or last-mile usage—802.11 with highgain
antennas
• Portable-access or hot-zone usage—802.11
mesh networks

Wi-Fi products associated with the metro-access deployment
option use these different radio frequencies:
• The 802.11a standard uses 5 GHz in an AP-to-AP interlink.
• The 802.11b and 802.11g standards use 2.4 GHz.

The 802.11a, 802.11b and 802.11g standards use different
frequency bands; devices based on these standards do not
interfere with one another. On the other hand, devices on
different bands cannot communicate; for example, an
802.11a radio cannot talk to an 802.11b radio.
The most common deployments by WISPs for wireless metro
access to date are the 802.11b and 802.11g standards
because of interoperability and the greater range they achieve
in the 2.4-GHz band.

Each standard also differs in the type of radio-modulation
technology used, as follows:
• The 802.11b standard uses direct-sequence spread spectrum
(DSSS) and supports bandwidth speeds up to 11 Mbps.
• The 802.11a and 802.11g standards use orthogonal
frequency division multiplexing (OFDM) and support speeds
up to 54 Mbps. Because OFDM is more adaptable to
outdoor environments and interference, it is most commonly
used for metro-access solutions.

OFDM technology uses sub-carrier optimization, which assigns
small sub-carriers to users based on radio frequency conditions.
Orthogonal means that the frequencies into which the carrier
is divided are chosen such that the peak of one frequency
coincides with the nulls of the adjacent frequency. The data
stream is converted from serial to parallel, and each parallel
data stream is mapped by a modulation block. The
modulated data is fed to an inverse fast Fourier transform
(IFFT) block for processing. The IFFT block converts the
discrete modulated frequencies into a time-domain signal,
which is used to drive the radio frequency (RF) amplifier.
This enhanced spectral efficiency is a great benefit to OFDM
networks, making them well suited for high-speed data
connections in both fixed and mobile solutions.

The 802.11 standard provides for 64 subcarriers. These
individual carriers are sent from the base station (BS) or AP to
the subscriber station (SS) or client and are then reconstituted
at the client side. In non-line-of-sight (NLOS) situations, these
carriers will hit walls, buildings, trees and other objects, which
then reflect the signal, creating multi-path interference.
By the time the carrier signals reach the client for
reconstitution, the individual carrier signals are time delayed.
For example, one carrier may have been reflected once and
arrived 1 μs later than another, and a second carrier may have
been reflected twice and arrive 2 μs later. The larger number of
subcarriers over the same band results in narrower
subcarriers, which is the equivalent to larger OFDM symbol
periods. Consequently, the same percentage of guard time or
cyclic prefix (CP) will provide larger absolute values in time for
larger delays, improving resistance to multi-path interference.
Because the 802.11a and 802.11g standards use OFDM, they
are more resilient than the 802.11b standard in outdoor multipath-
prone environments. These factors were taken into
account when developing the 802.16-2004 standard. The
802.11a and 802.11g standards have one-fourth of the OFDM
symbol options for CP than in the 802.16-2004 standard.

Wi-Fi standards at a glance.

Wi-Fi Standard Frequency Modulation
802.11a 5 GHz OFDM
802.11b 2.4 GHz DSSS
802.11g 2.4 GHz OFDM

The 802.11g standard is often selected for a last-mile solution
for three reasons.
• Speed
• The ability to handle interference
• Interoperability with 802.11b-based devices