Subject: [NEXTEL-1] Center for Wireless Telecommunications

From: [email-masked]
Date: Mon, 19 May 2003 01:37:56 -0400

http://www.cwt.vt.edu/faq/default.htm

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Wireless Frequently Asked Questions

Wireless Terms

What
does the term “wireless” imply?

What
are some wireless applications?

What
is the component availability of wireless devices?

What
is RFID and how does it work?

What
types of modulation are used in some of the various wireless
applications?

What
is spread spectrum?

What are
the ISM bands?

What
is propagation, multipath, and fading?

What
is the difference between a cordless phone, a cellular phone,
and a car phone?

Wireless Capabilities

At
what frequencies do wireless applications operate?

How
wide is the range for wireless devices?

What
are some of the major wireless standards?

What
are the standards, by geographic location?

Wireless Regulations

Do
I need a license to operate a wireless link?

Is
RF hazardous to humans and/or the environment?

How
secure is wireless communication? Can others intercept my phone
calls?

Ultra Wideband

What is Ultra Wideband?

How are UWB signals generated?

What are some of the benefits of UWB over a narrowband signal?

What are the current applications of UWB?

What issues need to be resolved in UWB?

Questions and Comments

Send us your questions and
comments...

Wireless Links

Wireless
Glossary

What does the term “wireless” imply?

Wireless describes the products and standards that allow
communications without additional electrical connections.
Typically, the term wireless implies that electromagnetic waves
traveling through the air carry information from one place to
another. Depending on their frequency, these waves exhibit
different properties. For example, microwaves (on the order of
GHz) tend to bounce off walls and objects. At the opposite
extreme are low frequency waves (on the order of tens of MHz)
which tend to travel through walls and objects. The wireless
systems designer selects the operating frequency suitable for a
particular application, thereby taking advantage of some of
these properties.

In some areas, powerline
communication is also considered wireless because of the
lack of "additional" wiring.

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What are some current wireless applications?

Wireless is quickly emerging into many new applications:

networking computers

allowing remote monitoring and data acquisition

providing access control and security

Of course, wireless ideally accommodates environments where
wires are impossible, such as vehicles and hand-held devices.
Most wireless products can be categorized by application, some
of which include the following:

·
Voice and Messaging:

Cordless
phones

Cellular
phones

Beepers,
pagers, and messaging systems

Wireless
e-mail systems

CB
Radio

Commercial
two-way business radios

Intercom
systems

·
Computer Networking:

Wireless
Local Area Networks (WLANs)

Infrared
(IR) ports on computers, printers, and other devices

Radio
modems

·
Remote Data Acquisition:

Personal
Digital Assistants (PDA's)

Radio
Frequency (RF) modems

·
Commercial Home Products:

Security
and access control

900
MHz stereo distribution

Temperature
control systems

Remote
control

Keyless
entry systems

Garage
door openers

Remote
Controlled toys

TV
remotes

·
Global Positioning Systems (GPS)

Aviation
navigation

Nautical
navigation

Roadway
navigation

·
Radio Frequency Identification technology (RFID)

Tags
and readers - Inventory Control, Animal migration/tracking

Smart
cards - Access control, Identification, Debit cards

Merchant
RF security tags

Many other applications exist. In
fact, many companies take existing products, cluttered with
wires, and make them wireless.

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What is the component availability of these devices?

While wireless communications emerge as a technology, components
are readily available. In fact, integrated circuit (IC)
technology enabled the placement of entire subsystems (such as a
transceiver) onto a single chip. Most of the major semiconductor
manufacturers own several IC packages, which work at a wide
range of frequencies.

To find the components most
effectively:

·
contact some of the major IC manufacturers

·
exam wireless design magazines

·
search the World Wide Web

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What is RFID and how does it work?

RFID
systems are used to track and identify objects, animals,
and people. A typical RFID system consists of a tag, a
reader, and a data processing device (such as a computer). The
tag is attached to the object, animal, or person, and stores
relevant identification information. The reader communicates
with the tag via a wireless radio frequency link to obtain the
information stored in the tag. In many cases, the reader is
attached to a data processing device such as a computer via
either a wired or wireless link.

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What types of modulation are used in some of the various
wireless applications?

Modulation is a technique in which a carrier wave is used
to carry information from one place to another. The sine wave is
modified in either amplitude, phase, or frequency, so that the
information is present on the wave, and can be decoded at the
receiving end. The three major types of digital modulation are:

·
Amplitude Shift Keying (ASK) - ASK is rarely used
in wireless applications because multipath
effects can dramatically influence the amplitude of the
information-bearing signal. This causes errors when the signal
is received and decoded.

·
Phase Shift Keying (PSK)

·
Frequency Shift Keying (FSK)

PSK and FSK are more common in
wireless systems because of their robustness in the presence of
multipath.

Many hybrid forms of ASK, PSK, and FSK exist. For example,
Binary Phase Shift Keying (BPSK) is a form of PSK where only two
phase states are allowed. BPSK is highly clear of multipath
propagation, but is unused in high data rate applications
because of bandwidth inefficiency. Other forms of PSK, such as
Quadrature PSK (QPSK), allow for more phase states, which
provide greater spectral efficiency at the cost of more
susceptibility to multipath.

One special form of modulation that has actually become a
buzzword in the wireless industry is spread
spectrum. Spread spectrum and many other hybrid techniques
are used in some of the major
cellular and data networking standards.

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What is spread spectrum?

Spread
spectrum is presently one of the biggest buzzwords in
wireless. This special modulation technique spreads the
transmitted signal over a frequency range much wider than the
minimum bandwidth required to send the signal. Widening the
signal bandwidth in this fashion increases the probability that
received information will closely match the transmitted
information.

There are two basic types of
spread spectrum modulation:

·
Direct
Sequence (DS)

·
Frequency
Hopped (FH)

Some hybrid spread spectrum
systems exist that combine both types.

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What are the ISM bands?

The Industrial,
Scientific, and Medicine (ISM) frequency bands were
allocated by the Federal Communications Commission (FCC) in the
mid 1980's. The bands encompass
the frequency ranges 902 - 928 Mhz, 2400 - 2483.5 Mhz, and 5725
- 5850 Mhz. The advantage of the ISM bands is that they are
unlicensed as long as a device operating in the ISM bands meets
special FCC regulations. While many devices operating in the ISM
bands use spread
spectrum modulation, the FCC does permit narrowband
modulation techniques to be used (provided that the respective
device meets stringent power requirements).

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What is propagation, multipath, and fading?

Propagation

Electromagnetic wave propagation describes how
a wave gets from one point to another depending upon the
environmental conditions it encounters along the way. For
example, a low frequency wave that encounters a plaster wall
will likely travel through it. On the other hand, a much higher
frequency wave will bounce off the wall. And a wave of an
intermediate frequency might bounce off the wall but lose a
substantial amount of energy in the process. It is apparent that
propagation can have severe effects on the received signal.

Multipath

In a typical wireless environment, no direct line-of-sight (LOS)
path between the transmitter and the receiver exists.  In
fact, the electromagnetic waves emitted by the transmitter
usually take different paths to get to the receiver, depending
upon the obstacles in the environment. As a result, the
received signal comprises a sum of the various contributions,
each of which differs in both amplitude and phase. In many
cases, the signals combine in a destructive manner, thereby
severely degrading the signal's strength. The receiver faces the
difficult task of properly demodulating and decoding the signal
into something that resembles the original. Despite the emerging
receiver technologies multipath problems can still seriously
inhibit a wireless system's performance.

Fading

Fading describes rapid
changes in a radio signal's amplitude over a short period of
time or travel distance. Fading results from multipath. In
fact, fading results from interference between multiple replicas
of the transmitted signal.

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What is the difference between a cordless phone, a cellular
phone, and a car phone?

Cordless Phones

A cordless phone refers to a device
whose range of use operates in a relatively small area,
(such as within a house). In fact, when you purchase a cordless
phone, you receive both the handset or mobile, and a base that
connects directly to the phone line in your house. The base
communicates with the mobile to ensure that phone calls are
handled properly. As a result, the user can only stay within a
relatively short distance of the base.

Cellular Phones/Car Phones

Cellular phones and car phones refer to the same device, and permit
wireless operation over large distances. Cellular phones,
like cordless phones, require that the mobile be able to
communicate with a base. However, a cellular phone user does not
need to be aware of the base location, which may be located
miles away. The user can travel over large distances. In fact,
modern cellular systems permit the user to travel from an area
covered by one base to an area covered by another base. The
user's call is automatically and seamlessly handed over
to the new base.

Cellular systems are able to achieve wide coverage and high
capacity because they employ frequency re-use. This means
that a channel frequency being employed in one area can also be
used in another area provided that the distance between the two
areas is large enough to prevent interference.

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At what frequencies do wireless applications operate?

The frequency varies tremendously depending on the
specific device and the application being used. Below is a
general run-down of several major frequency bands, along with
some common devices operating in them:

Applications

Frequency Bands

VLF band radios

Beacons

170 - 190 kHz

AM broadcast communications

Low power voice and data

550 - 1600kHz

CB radios

Low power voice and data

27 MHz

Remote control

Cordless telephones

Low power voice and data

49 MHz

FM radios

Low power voice and data

88 - 108 MHz

Commercial two-way voice

Pager services

150 - 170 MHz

303 MHz garage door openers

Keyless entry systems

Security alarms

260 - 470 MHz

Cellular Phones

824 - 849 MHz and

869 - 894 MHz

ISM band Wireless LAN’s

Part 15 devices (spread
spectrum cordless phones, etc.)

Military radiolocation systems

Federal mobile communications

902 - 928 MHz

902 - 928 MHz

Pager services with a high transmitter power

930 MHz

Amateur satellite

Part 15 devices

Microwave ovens and systems

Army packet radio development

2.4 – 2.4385 GHz

Amateur satellite

Part 15 devices

Naval radar systems

Test range instrumentation radars

5.727 – 5.875 GHz

Radio navigation

24 – 24.25 GHz

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How wide is the range for wireless devices?

Depending on the design and application of the device, range
varies tremendously. While some devices (such as RF tags)
operate over a few inches, other devices (such as those used in
wireless computer networking) operate over tens of miles.

Range depends on many factors,
which include:

·
The environment – A highly dependent factor in
the range of a device. Certain environments may cause excessive multipath,
which can reduce both the range and data rate of the device.

·
The operating frequency of the device

·
The output power

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What are some of the major wireless standards?

AMPS

Advanced
Mobile Phone System. AMPS is the first analog cellular standard
in the

U.S.

Although AMPS is still in use, it is anticipated that it will be
replaced by the United States Digital Cellular(USDC) standard.

CDPD

Cellular
Digital Packet Data is a digital standard for packet data
services. CDPD was designed to overlay with existing cellular
infrastructure, thereby permitting simple and inexpensive
installation.

CEBus

The
Consumer Electronics Bus (CEBus) standard was created by the
Electronic Industries Association (EIA). CEBus is an engineering
standard for home automation products. It supports carrier
current, RF, IR, coaxial cable, twisted pair, and eventually
fiber optic cable.

DECT

Digital
European Cordless Telephone is a universal cordless telephone
standard developed by the European Telecommunications Standard
Institute (ETSI). DECT offers services for both voice and data
communications.

GSM
–

Global
System for

Mobile

. The GSM standard was developed in
Europe
to standardize cellular communications among European countries.
GSM has demonstrated substantial success and continues as one of
the world's most popular standards for new cellular radio and
personal communications equipment.

IEEE
802.11 –

This
is the IEEE standard for wireless LAN's. The goal of the IEEE
802.11 committee is to standardize wireless LAN development in
the ISM
band. The standard focuses on the media access control (MAC)
and the physical (PHY) protocol levels. The IEEE 802.11 standard
is still under development, but is anticipated to become the
wireless LAN standard.

IrDA

The
Infrared Data Association (IrDA) was formed to develop a
standard for wireless communication using infrared (IR)
technology. Some of the chief goals of the committee are to
develop a standard that permits low cost, low power,
point-to-point user communications using IR as the transmission
medium.

IS-54

Interim
Standard 54. See USDC.

IS-95

IS-95
is a

U.S.

digital cellular standard that uses a Code Division Multiple
Access (CDMA) scheme. In a CDMA system, users share time and
frequency resources simultaneously. This occurs through
assigning each user a distinct digital code. This code is added
to the information data and modulated onto the carrier, using spread
spectrum techniques. Although it anticipates providing
significant capacity improvement and increased interference
rejection over other digital cellular standards, IS-95 remains
somewhat controversial because of its wide bandwidth
requirements.

PHS

The
Personal Handphone System standard was developed in

Japan

specifically for indoor and microcell usage.

USDC

United
States Digital Cellular, also known as IS-54 (Interim Standard
54), was developed to replace the AMPS standard, particularly in
urban areas where AMPS did not provide adequate channel
capacity. USDC allows the co-existence of AMPS so that providers
can gradually phase out AMPS as needed.

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What are the standards, by geographic location?

Standards by geographic region can be found in detail on
the following page.

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Do I need a license to operate a wireless link?

Licensing and other legal requirements for operating any radio
transmitter are established by the Federal
Communications Commission (FCC). Because these alter with
time, the reader should consult the FCC Regulations for the
latest information.

The FCC permits unlicensed operation in portions of the spectrum
called ISM
(Industrial, Scientific, and Medical) bands, provided that
certain technical restrictions on transmitter power and
modulation are met.

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Is RF hazardous to humans and/or the environment?

Thermal waves are the
only electromagnetic waves confirmed as a biological hazard.
This hazard stems from the fact that radio waves can heat
biological material (this is how microwave ovens work). Although
observed, non-thermal effects have not proven harmful. The
FCC establishes legal power limits for cellular telephones,
hand-held radios, and any other transmitters the public might
come in contact. These limits are well
below the power levels that prove to have any harmful effects,
and the legal presumption is that any transmitter operating in
compliance with the FCC regulations is safe.

Readers concerned about the safety of wireless devices should
consult the following references:

EMF-Link
-- A biomedical science and engineering clearinghouse of
information on the biological and health effects of EMF from
commonly used appliances.

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How secure is wireless communication? Can others intercept
my phone calls?

The
cordless (49 MHz) and cellular FM phones are not secure. If
you don't want anyone to hear you, don't use cordless or
cellular. The new
cordless (900 MHz) spread
spectrum and
cellular non-FM (ie IS-95, etc.) are secure because of the
spreading code used
in the modulation technique. In general, spread spectrum
systems provide a much higher level of security than the more
traditional narrowband systems.

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What is Ultra Wideband?

Ultra Wideband is the transmission of a signal where the fractional bandwidth is greater than .25.

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How are UWB signals generated?

There are several methods of generating a UWB signal, but all involve the formation of a very short pulse in the time-domain. This pulse, typically sub-nanosecond in length, can then be transmitted directly or filtered depending on whether a DC component in the UWB signal is desired or not.

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What are some of the benefits of UWB over a narrowband signal?

UWB signals are not as affected by multi-path as narrowband signals, due to shorter pulse widths.

It has high data rates.

It has increased security (low probability of detection) due to low spectral power density.

Signals transmitted below the noise floor can be recovered due to inherent processing gain. These signals will be invisible to narrow band users allowing for more efficient use of the spectrum.

It has multi-path resolution leading to <50ft object resolution.

It allows Multiple User Access for a given portion of the spectrum.

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What are the current applications of UWB?

UWB RADAR has been researched for some time as a method of defeating stealth aircraft and as electronic counter measures. Because of the bandwidth occupied by UWB RADAR, it is difficult to develop a system that can jam the RADAR or can effectively appear invisible to the RADAR. Several civilian systems take advantage of these properties, including:

Secure Communication Systems (hard to jam or detect),

Ground Penetrating RADAR, and

Obstacle Avoidance RADAR.

Both civilian RADAR systems take advantage of the fact that while there may be no one specific frequency that is ideal (or adequate) for the application, the UWB RADAR examines many different frequency components at the same time.

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What issues need to be resolved in UWB?

Although UWB signals can be transmitted below the noise floor, it may be desirable to transmit with more power. If a UWB system is operated under current FCC Part 15 regulations, narrow band signals operating in the same band will be adversely affected. The narrow band users will see the UWB signal as an increase in the noise floor by about 10 dB. The FCC is currently proposing new regulations to resolve this issue. On suggested solution would be to only allow UWB operation above a certain frequency.

Currently UWB Transmitter and Receiver must be time synchronized. GPS makes this task feasible for production systems, however a system where synchronization is not required is desirable.

High sampling rate receivers are required.

Efficient Pulse Shaping is required.

Current Transmitter designs consume large amounts of power in the generation and processing of the pulses. Although the period of high power consumption is very short, and probably will not affect battery life compared to a narrow band system, it does lead to problems in designing a transmitter on a chip.

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[email-masked]

Last Updated October 31, 2002 SRE