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Remote System
Monitoring with
Communications
System
Analyzers
By Katrina Scally, software
engineer, General Dynamics
I
n large distributed communica-
tions systems, it is often neces-
sary to locate base station
equipment in remote locations,
such as mountaintops, to take
advantage of their geographic fea-
tures. These systems also require
frequent monitoring and adjust-
ment for optimum performance.
Together, these two requirements
present a significant challenge to
those responsible for maintaining
and servicing these sites.
Traditionally, when a remote
site requires maintenance, the
service technician must bring all
equipment necessary to handle
any possible problem or risk
having to make multiple trips to
the remote site. A remote moni-
toring and control application
provides the technician with an
indication of scope of mainte-
nance prior to visiting the site,
permitting him to transport
only the necessary equipment.
Using readily available off-
the-shelf software and consumer
electronics, maintaining optimal
performance of remote commu-
nications sites is easy and cost
effective. By remote operation of
a communication analyzer from
a single Central Command site, a
technician can identify specific
maintenance needs, eliminate
unnecessary trips, and maintain
multiple remote base stations
with a reduction of personnel
and equipment.
Remote Monitoring Solution
Using a stand-alone personal
computer (PC) with a communi-
cations system analyzer for
remote monitoring, the service
technician can monitor all
aspects of the base station. All of
the analyzer’s functionality is pro-
vided remotely, including the
ability to monitor audio output
from the analyzer, as well as trans-
mitting from the analyzer using
audio sent via the network con-
nection from the user’s office PC.
First, the analyzer display is
distributed using a commercial
properly for a link to remain reli-
able. NLOS obstructions create
higher link attenuation than is
predicted by the free space path
loss equation. Transmission
models normally address this by
adopting a faster rate of RF level
attenuator as distance is increased
In addition, a fixed shadowing
term is added to address single
instance obstructions such as a
rooftop or walls.
The more drastic effect of
NLOS transmission is the
increased severity of multipath
and its effects. Multipath can be
likened to an acoustic echo—RF
reflections received by the radio
from multiple, indirect paths.
The echoes, although attenu-
ated from the main path (if
there is one), are delayed in
time. The distribution of echoes
over time, or delay spread, cre-
ate intersymbol interference
(ISI), a condition in which the
delayed energy from one trans-
mitted data symbol begins to
corrupt the symbol next arriv-
ing along a faster RF path.
Another consequence of
multipath is fading. As waves of
the same frequency, the radio is
sensitized to peaks and valleys
of power that are created by the
overlapping waves. The effects
can be severe: narrowband fades
on the order of 20 dB and more
are possible when the receiver is
repositioned by only inches.
Moreover, since the multipath is
often created by moving objects
—trees, cars and to a lesser
extent, buildings—the distribu-
tion varies as a function of time
as well as position.
A final consequence of multi-
path involves the motion of
reflecting objects. Any wave that
hits a moving object, for exam-
ple a car, will experience a slight
frequency shift. This Doppler
effect creates echoes that are not
only distributed in time but also
in frequency.
Implementation Example
In all, reliable NLOS links
require a radio system able to
tolerate increased path loss and
a number of distorted echoes of
the original signal. How can this
be accomplished?
For the past 15 years and
more, most BWA equipment
consisted of low-power, single-
carrier and DSSS radios. Suitable
for LOS applications, their util-
ity in NLOS links decreases dra-
matically. Their low power and
average receive sensitivity do
not allow them to operate in the
presence of increased path loss,
shadowing and multipath fades.
LOS link distances to 10 miles
and more quickly deteriorate to
under a mile due to power loss.
In addition, these older systems
are not designed to easily sur-
vive the ISI and other distor-
tions created by delay spread
and Doppler effects.
Orthogonal frequency divi-
sion multiplexing (OFDM) mod-
ulation has been growing in
usage due to its ability to over-
come the shortcomings of ear-
lier commercial technology.
Technology sectors such as
802.11a/g WLAN, DVB/DAB
broadcast and upcoming 802.16
WMAN products are all looking
to OFDM to address NLOS chal-
lenges. For outdoor use, OFDM
is a powerful asset.
OFDM creates a wideband
signal comprised of a number of
independent or orthogonal sub-
carriers, each carrying a low bit
rate data stream. The low data
rate bit stream allows for a size-
able guard band at the begin-
ning of each symbol, effectively
isolating the symbols from each
other and neutralizing the effect
of delay spread. In addition, the
subchannelized operation, in
conjunction with the proper
error correction system, proves
to be very tolerant of narrow-
band multipath fades. In most
cases, only a limited number of
subcarriers may be affected by a
fade, causing the loss of sym-
bols. With the remainder of the
wideband signal unaffected, the
error correction system takes
over and is able to reconstruct
the small percentage of missing
data bytes.
OFDM is a key element of the
NLOS solution, although addi-
tional tricks are required to
begin approaching the holy
grail of indoor deployment. The
sheer magnitude of the link loss
between a base station and a
NLOS client, due to obstructions
and multipath fading, mandates
that additional measures be
taken to increase link gain.
The most obvious is to
increase the link gain of the sys-
tem by increasing the output
power of the transmitting radio
or receive sensitivity of the
receiver. Due to strict linearity
requirements for an OFDM sys-
tem, a high-power OFDM radio
requires more care design and
construction than previous gen-
erations of design.
Recent developments to
combat link attenuation involve
using sophisticated methods of
spatial and time diversity. Space-
time coding and multi-input/
Today’s wireless market utilizes
many new, and some familiar,
coaxial connectors. The wide
range of wireless broadband
equipment now available to
meet Wi-Fi IEEE 802.11a/b/g
requirements can sometimes
seem bewildering.
What are the input and out-
put connectors used with Wi-Fi-
certified products, which include
access points, gateways, routers,
cellular devices, point-to-point
networks, Bluetooth, antennas,
residential gateways, PDAs, PCI
cards, PCMCIA cards, USB
devices, wireless print servers,
WLAN-enabled computers, PC
peripherals, antennas, LANs and
Internet access devices? Many of
these connectors are not easily
recognizable so read on for help
in identifying them.
Variations on old standards
In addition to specialized inter-
faces that are relatively new to
the coaxial market, such as
DMX, MC Card, MHF, there are
variations on standard RF con-
nectors’ styles that satisfy FCC
Part 15 and 802.11 require-
ments. The most popular com-
pliance method creates reverse
polarity, or gender, versions of
BNC, MCX, MMCX, N, SMA,
SMB, SSMB and TNC connec-
tors. You will also find reverse,
or left-handed thread, versions
of N, SMA and TNC connectors.
Another compliance method
replaces unified threads with
metric threads to prevent mat-
ing with standard connectors.
(If you try to mate threaded
connectors, stop at once if you
meet any resistance! You may
be working with a device
connector having reverse or
metric threads. To force mating
can result in damage to both
connectors.)
When choosing adapters or
pigtails for your Wi-Fi devices,
you will gain better perform-
ance by using straight rather
than right-angle connectors. If
you have a choice, consider the
trade-off between compactness
and performance.
Specialized Wi-Fi connectors
MC Card connectors were
designed for wireless and tele-
com devices operating at fre-
quencies up to 8 GHz where
board and chassis space is lim-
ited. They are used on many
Apple, Buffalo, Dell, Enterasys,
Filotex, Lucent, and Proxim/
Orinoco products.
DMC connectors are special-
ized snap-on connectors
designed for DC to 6 GHz and
are larger than the MC Card
connector. They are compatible
with Orinoco AP600 and
AP4000 devices.
MCX connectors share the
same inner contact and dielec-
tric dimensions as SMBs, but the
outer diameter is approximately
30 percent smaller. MCXs offer
broadband performance from
DC to 6 GHz and coupled con-
nectors can be rotated 360
degrees for precision alignment
without performance loss.
MCX plugs have a center
pin, insulation that extends
beyond the pin, and six slotted
spring fingers surrounding that
insulation. The MCX plug inserts
into the jack body. MCX jacks
have a larger diameter body
which houses the locking mech-
anism for the plug’s springs,
recessed insulation and a slotted
socket contact. MCX connectors
are found on Apple Airport
Extreme and SMC devices.
MMCX, miniature MCX
connectors, are about half the
size of SMBs. Their lock-snap
coupling mechanism allows 360
degree rotations without loss of
broadband performance with
low reflection from DC to 6
GHz. MMCX connectors are
used on devices manufactured
by 3Com, Cisco/Aironet,
EnGenius, Proxim/Orinoco,
Samsung, Senao and Symbol.
MMCX plugs have no shell, a
center pin and a visible external
snap-ring. MMCX jacks have a
larger body, center socket con-
tact and internal spring slots to
receive the external snap-ring of
the plug. To differentiate MCX
and MMCX plugs, remember
that the MCX has slotted exter-
nal spring fingers while the
MMCX has a solid external con-
tact with snap-ring.
Reverse MMCX plugs have
the dielectric and center socket
of the MMCX jack inside the
MMCX body. If you find a
socket contact in a body with
external snap-ring, you have an
RP MMCX plug. They will not
mate with standard MMCX
jacks. To attempt to do so will
damage contacts in both con-
nectors. All performance and
function characteristics of the
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Market Focus
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3
Baffled by Wi-Fi Connectors?
By Connie Jones, director of product management, RF Connectors, division of RF Industries
(continued on page 17)
Defeating NLOS and Cost to Win Last Mile Business
(continued from page 1)
(continued on page 8)
(continued on page 19)