Produits de la Marque EnGenius

Remote System

Monitoring with

Communications

System

Analyzers

By Katrina Scally, software

engineer, General Dynamics

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

THE WIRELESS JOURNAL

Market Focus

*For your current pricing and our complete product offering, visit TESSCO.com or call TESSCO.

Baffled by Wi-Fi Connectors?

By Connie Jones, director of product management, RF Connectors, division of RF Industries

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Defeating NLOS and Cost to Win Last Mile Business

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