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WLL Definations & Overview Introduction to CDMA
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Definition
Sometimes
called radio in the loop (RITL) or fixed-radio access (FRA), wireless
local loop (WLL) is a system that connects subscribers to the public
switched telephone network (PSTN) using radio signals as a substitute for
copper for all or part of the
connection between the subscriber and the switch. This includes cordless
access systems, proprietary fixed radio access, and fixed cellular systems. Overview Industry
analysts predict that the global WLL market will reach millions of
subscribers by the year 2000. Much of this growth will occur in emerging economies
where half the world's population lacks plain old telephone service (POTS).
Developing nations like China, India, Brazil, Russia, and Indonesia look to WLL technology as an efficient way to deploy POTS for millions
of subscribers—without
the expense of burying tons of copper wire.
In
developed economies, WLL will help unlock competition in the local loop,
enabling new operators to bypass existing wireline networks to deliver
POTS and data access. So the question isn't will the local loop go
wireless, but when and where. This tutorial discusses the basics of WLL
and examines the markets and future for this technology.
WLL Technology Shake-Out he
WLL revolution is underway. WLL suppliers and operators are flocking to
emerging markets, using whatever available wireless and line interface technologies
are at hand to achieve fast time to market. Because there are no
definitive WLL standards, vendors are faced with a bewildering choice of
fixed-access, mobile, and digital cordless technologies. Ultimately the
appropriate protocol technology will depend on an array of applications
considerations, such as size and population density of the geographic area
(rural versus urban) and the service needs of the subscriber base
(residential versus business; POTS versus data access). In fact, there are
many good reasons why different wireless technologies will serve some
applications better than others. The challenge for WLL vendors is to
identify the optimal wireless protocol for their unique application needs,
then reduce cost per subscriber through silicon and deliver integrated
solutions to the marketplace. WLL will be implemented across five
categories of wireless technology. They are digital cellular, analog
cellular, personal communications network (PCN)/personal
communications service (PCS), cordless telephones 2nd generation
(CT–2)/digital European cordless telecommunications (DECT), and
proprietary implementations. Each of these technologies has a mix of
strengths and weaknesses for WLL applications.
Analog
Cellular Given
its wide availability resulting from serving high-mobility markets, there
is significant momentum to use analog cellular for WLL. There are
currently three main
analog cellular system types operating in the world: advanced mobile phone
system (AMPS), Nordic mobile telephone (NMT), and total access
communications systems (TACS). AMPS and its cousin narrowband advanced
mobile phone system (NAMPS) dominate the analog cellular market with 69
percent of subscriber s, while TACS has 23 percent and NMT has only 8
percent. As
a WLL platform, analog cellular has some limitations in regards to
capacity and
functionality. Due to widespread deployment, analog cellular systems are
expected to be a major wireless platform for WLL, at least in the short
term. Given its characteristics, analog cellular is best suited to serve
low-density to medium - density markets that don't require landline-type
features. Analog cellular is forecasted to account for 19 percent of the
WLL subscribers in the year 2000. Digital
Cellular These
systems have seen rapid growth and are expected to outpace analog cellular
over the next few years. Major worldwide digital cellular standards include
global system for mobile communications (GSM), time-division multiple
access (TDMA), Hughes enhanced TDMA (E–TDMA), and code-division multiple
access (CDMA). GSM dominates the digital cellular market with 71 percent
of subscribers. Digital cellular is expected to play an important role in
providing WLL. Like analog cellular, digital cellular has the benefit of
wide availability. Digital cellular can support higher capacity
subscribers than analog cellular, and it offers
functionality
that is better suited to emulate capabilities of advanced wireline
networks. Its disadvantage is that it is not as scalable as analog
cellular. It is forecasted that approximately one-third of the installed
WLLs will use digital cellular technology in the year 2000. Although GSM
currently dominates mobile digital cellular, there has been little
activity in using GSM as a WLLplatform. Since GSM's architecture was
designed to handle international roaming, it carries a large amount of
overhead that makes it unwieldy and costly for WLL applications. In spite
of these limitations, it is likely that GSM WLL products will be developed
over the next few years. CDMA
appears
to be the standard best suited for WLL applications. CDMA employs a
spread-spectrum modulation technique in which a wide range of frequency is
used for transmission and the system's low-power signal is spread across
wide-frequency bands. It offers higher capacity than the other digital
standards (10 to15 times greater than analog cellular), relatively
high-quality voice, and a high level of privacy. The main disadvantage of
CDMA is that it is only now beginning
to
be deployed on a wide scale.
PCS/PCN
PCS/PCN incorporates elements of digital cellular and cordless standards as well as newly developed radio-frequency (RF) protocols. Its purpose is to offer low-mobility wireless service using low-power antennas and lightweight, inexpensive handsets. PCN is primarily seen as a city communications system with far less range than cellular. PCS is a broad range of individualized telecommunications services that let people or devices communicate regardless of where they are. Some of the services include personal numbers assigned to individuals rather than telephones, call
completion regardless of locations (find me), calls to the PCS customer
that can be paid by either the caller or the receiver, and call-management
services that give the called party greater control over incoming
calls.
It is not clear which standards, if any, will dominate the WLL portion of PCS/PCN. The candidate standards are CMDA, TDMA, GSM, personal access communication systems (PACS), omnipoint CDMA, TDMA, upbanded CDMA, personal handyphone system (PHS), and digital cordless telephone United States (DCT–U). These standards will probably be used in combination to provide both WLL and high-mobility wireless services. PCS/PCN has the advantage of being designed specifically to provide WLL by public wireless operators. The main weakness of PCS/PCN is that it is not yet commercially available.
CT–2/DECT
Cordless telephony was originally developed to provide wireless access within a residence or business between a base station and a handset. Since the base station is still hard-wired to the PSTN, this is not considered WLL. For the purposes of this study, DECT is considered WLL when a public network operator provides wireless service directly to the user via this technology. Although DECT does not appear to be ideally suited for rural or low-density applications, it has some significant advantages in medium-density to high-density areas. Cordless telephony has advantages in terms of scalability and functionality. As compared to cellular technology, DECT is capable of carrying higher levels of traffic, provides better voice quality, and can transmit data at higher rates. The microcell architecture of DECT allows it to be deployed in smaller increments that more closely match the subscriber demand, with reduced initial capital requirements. |