Digital Wireless Technologies

LECTURE NO.: 07

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Overview
† As demand for viable, commercial telephone service
has increased dramatically since its inception in 1983,
service providers found that basic engineering
assumptions borrowed from wireline (landline)
networks did not hold true in mobile systems.

† Cellular providers had to find a way to derive more

capacity from the existing multiple-access methods
currently in use, namely frequency division multiple
access (FDMA).

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† A multiple-access method defines how
the radio spectrum is divided into
channels and how channels are
allocated to the many users of the
system.
† Multiple-access technology allows a
large number of users to share a
common pool of radio channels, and
any user can gain access to any
channel.
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† The spectacular growth in the number of wireless
customers has to be accommodated through a
continual increase in system capacity.

† The most extreme and costly method is to reduce

cell sizes (coverage areas) and introduce
additional base stations – in effect, implementing
cell splits.

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† However, in most large cities , it has
become increasingly difficult and costly to
obtain the necessary permits to erect base
stations and antennas.
† Therefore, cellular carriers wanted a
solution that made it possible to increase
system capacity significantly without
requiring more base stations.
† The solution was to introduce digital radio
technology, which allows for increased
spectral efficiency without an increase in
the total number of base stations.
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† The implementation and use of digital
wireless systems result in better use of
available radio spectrum and cleaner,
quieter signals than the AMPS analog
systems.
† Digital transmission also provides for
greater security against eavesdropping
and cloning fraud.
† There are three major digital radio
technologies developed and in use today:
TDMA, GSM and Code division multiple
access (CDMA).
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 Multiple-access methods for
wireless technology:
FDMA: Users are assigned
different frequencies within
a given band. This is an
access technique for
analogue systems.
TDMA: Users are
assigned different
frequencies and different
time slots.
CDMA: Users are
channelized by specified
codes with in a frequency
band that is 1.25 MHz
wide.

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Digital Wireless Systems Comparison
with Analog Wireless Systems
† The two key advantages of digital cellular
technologies over analog cellular networks include
increased capacity and security. There are some basic
differences between digital radio systems and analog
radio systems:
„ Given equal amounts of spectrum, all of the digital
wireless technologies allow for increased use of radio
spectrum, compared to analog systems.
„ The basic intention of deploying digital radio
technologies is to get maximum use of allotted radio
spectrum. To that end, all of the digital radio
technologies (TDMA, GSM, and CDMA) allow for use
of a substantially larger amount of radio channels
than analog AMPS systems.

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„ Digital radio base stations cost significantly less
than analog base stations. This is because digital
base stations employ advanced, smaller-scale
equipment that has a smaller footprint as well as
a smaller price tag.
„ All-digital systems can employ handoff
techniques in which the mobile handset has
more of a role in determining if and when a
handoff is required, and to what cell or cells.
„ This improved handoff capability increases the
efficiency of the wireless network because it
decreases the potential that calls will be dropped
during the handoff process.

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 Digital wireless technologies and
associated classifications

FDMA TDMA CDMA

(Frequency Division (Time Division (Code Division
Multiple access) Multiple access) Multiple access)

AMPS TACS IS-54(“D-AMPS”) GSM (Europe)

(Europe) IS-136(U.S. TDMA) DCS 1800 (Europe) IS-95
(U.S.)
DCS 1900
North American GSM

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Vocoders
† The deployment of digital cellular systems has been
enabled through the development and implementation of
equipment known as vocoders.
† Human speech is easy to reproduce. Vocoders are
equipment (chipsets) embedded in mobile handsets and in
base station transceivers that digitize human speech.
† Vocoders are what actually enable digital wireless systems
to exist; they are the foundation that allows for increased
use of radio spectrum.
† Vocoders sample transmissions of human speech, packetize
the samples, and send digital pulses containing the
packetized samples from the handset to the base station
instead of always having the mobile’s transmitter “key” on
the appropriate frequencies.
† Distant-end vocoders (at the base station) decode the
pulses and restore the speech to its original form.

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† In GSM systems, a full-rate vocoder allows
for eight users (conversations) on a single
30-kHz radio channel. Hence, there is an
8:1 increase in capacity compared to
analog AMPS systems.
† In TDMA systems (IS-136), a full-rate
vocoder allows for three users
(conversations) on a single 30-kHz radio
channel. Hence, there is a 3:1 increase in
capacity compared to analog AMPS
systems.

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Time Division Multiple Access
† Time division multiple access is the generic name for
an air interface technology that is used by a variety of
standardized digital radio systems, such as IS-136
and GSM.
† TDMA was the earliest incarnation of digital radio
technology. It assigns both different frequencies and
different time slots to each conversation on a wireless
system.
† GSM is a digital wireless standard in its own right,
even though it uses TDMA as its air interface
technology. This is because GSM has many special
features and attributes which make it a distinct radio
technology.

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† IS-54 was the earliest standard to use TDMA technology,
defining the migration path from analog to digital radio
systems.
† The IS-54 standard, also known as the D-AMPS (digital
AMPS) standard, referred to TDMA and digital radio in very
generic terms and contained few references to digital radio
feature sets. It focused on the migration from AMPS analog
systems to digital cellular systems.
† The revised, updated standard for TDMA systems, IS-136,
contains information on full-featured TDMA digital systems,
with references to features sets such as caller ID and short
message service (SMS), which provides text messaging.
† Currently, IS-136 provides three voice calls per 30 kHz of
bandwidth. Compared to the analog AMPS standard, this is
a 3:1 increase in system capacity per channel.

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Time Division Multiple Access
† TDMA systems can be more susceptible to
interference than other radio technologies when used
in combined AMPS/ TDMA systems by cellular carriers,
where calls could break up or mute. However, major
improvements in vocoders since the early 1990s have
allowed for vast improvements in dual-mode TDMA/
AMPS transmissions.
† For the most part, TDMA as a technology is being
supplanted today by CDMA because CDMA offers
many benefits over TDMA, and is ultimately more
cost-effective because more capacity can be gleaned
by an equivalent amount of network infrastructure.
† The one carrier today that continues to use and grow
TDMA in its network is AT&T Wireless.
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Code Division Multiple Access
† Overview:
„ Code division multiple access (CDMA) is
an American digital standard that was
developed by a company named
Qualcomm, based in California.
„ CDMA was originally deployed as a
battlefield communications system
because it is very hard if not completely
impossible to intercept CDMA
transmissions. The CDMA digital scheme
is titled Interim Standard 95 (IS-95).
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Code Division Multiple Access
† How CDMA Works
„ CDMA is a wideband, spread spectrum technology. A unique code is
assigned to all speech bits (conversations). Signals for all calls are
spread across a broad frequency spectrum, hence the term “spread
spectrum.” The dispersed signals are pulled out of what appears as
background noise by a receiver that knows the code for the call it must
handle this technique allowed numerous phone calls to be
simultaneously transmitted on one radio frequency. As a result, CDMA
systems can handle between 10 and 20 times the calling capacity of
conventional cellular systems.
„ There is a certain analogy that is frequently used to explain how CDMA
works. Imagine you are at a reception being held in a room at the
United Nations. There are four people around you, each speaking a
different native language: Spanish, Korean, Chinese, and English. Your
native language is English. You only understand the words of the
English speaker, and tune out the Spanish, Korean, and Chinese
speakers. You hear only what you know and recognize. The same is
true for CDMA. Multiple users share a frequency band at the same
time, yet users only hear their own conversations.

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Code Division Multiple Access
† When a mobile phone call is made using CDMA technology,
the sound of the user’s voice is converted into a digital
code. This digital signal is first “correlated” with a noise-like
code known as a pseudorandom noise (PN) code, also called
“Walsh” codes. The correlator yields an encrypted digital
representation of the original signal. This encrypted signal
is then spread over a very wide frequency spectrum (1.25
MHz). At the receiving terminal, the signal is “demodulated”
back to a narrow bandwidth, and then fed into a
“decorrelator”. This decorrelator uses its unique PN code to
extract only the information intended for it. A signal
correlated with a given PN code and decorrelated with the
same PN code returns the original signal. Decorrelating the
signal with the wrong PN code would result in pure noise,
containing no discernible information or sound.

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CDMA Architecture and Operations
† In CDMA systems, the standard is to use 1.25 MHz blocks of
radio spectrum to carry many conversations, using
pseudorandom noise codes. Because of the nature of spread
spectrum, CDMA systems employ an (N=1) frequency-reuse
format.
† Each CDMA base station can use the same 1.25-MHz carriers
at the same time. The only change between each block of
1.25-MHz spectrum at each base station is the
pseudorandom Walsh noise codes per 1.25-MHz carrier in
the CDMA modulation scheme.
† Prior to the widespread deployment of CDMA systems,
which were mainly spurred by the broadband PCS carriers,
there were concerns that CDMA systems could not handle
heavy traffic loads. This caused concerns in the industry
that CDMA systems couldn’t handle a huge acquisition of
customers in a short period of time. Nevertheless, CDMA
has many distinct attributes which make it attractive to
cellular and PCS providers

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Power Control
† CDMA base stations control the power of all mobiles for
interference reduction purposes. All mobile signals must arrive at
the base station at the same power level so that the signals can be
properly coded. Power control is a required operational parameter
of CDMA digital systems. For example, if a mobile station that is
right next to the base station is transmitting at very high power,
and a mobile station 10 mi away from the base station is
transmitting at very low power, the power of the mobile next to
the base station is throttled down to a given level while the power
of the mobile 10 mi away from the base station is raised to a given
level. Power control is necessary to maintain system capacity. A
beneficial by-product of power control is reduced power costs at
the base station, as well as increased battery life in the mobile
phone.
† Power control exists in AMPS, TDMA, and GSM systems, but it is
simply a benefit that can be utilized to make the systems perform
better. Power control in CDMA systems is a critical item; it is
absolutely required in order for the system to operate effectively.

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† CDMA systems thrive on multi-path radio signals.
Multi-path fading can actually be of benefit in CDMA
systems.

† The multi-path signals are additives to the direct

signal to obtain the cleanest, strongest signal
possible.

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Soft Handoff
† “Soft” call handoffs are different from “hard” call handoffs in that a soft
handoff allows both the original cell and one to two new cells to temporarily
service a call during the handoff transition. The handoff transition is from
the original cell carrying the call to one or more new cells and then to the
final new cell.
† With soft handoff, the wireless call is actually carried by two or more cells
simultaneously. In this regard, the analog system (and TDMA and GSM
digital systems as well ) provide a “break-before-make” switching function
in relation to call handoff. In contrast, the CDMA-based soft handoff system
provides a “make-before-break” switching function with relation to call
handoff.
† CDMA systems require a Global Positioning System (GPS) antenna at every
cell base station. The GPS antennas synchronize all the cell sites to one
timing source – the GPS. This is an absolute necessity for soft handoffs
because timing is critical among the multiple sites that may simultaneously
handle a call during the soft handoff process.
† Not only does soft handoff greatly minimize the probability of a dropped
call, but it also makes the handoff virtually undetectable to the user. Soft
handoffs are directed by the mobile telephone. As such, soft handoff is also
known as mobile-directed handoff.

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Soft Handoff Operations
† The sequence of events in a soft handoff is as follows:
„ After a mobile call is initiated, the mobile station continues
to scan the neighboring cells to determine if the signal from
another cell becomes stronger than that of the original cell.
„ When this happens, the mobile station knows that the call
has entered a new cell’s coverage area and that a handoff
can be initiated.
„ The mobile station transmits a control message to the MSC
which states that the mobile is receiving a stronger signal
from the new cell site, and the mobile identifies that new
cell site.
„ The MSC initiates the handoff by establishing a link to the
mobile station through the new cell while maintaining the
old link.
„ While the mobile station is located in the transition region
between the two cell sites, the call is supported by
communication through both cells. This eliminates the ping-
pong effect of repeated requests to hand the call back and
forth between two cell sites.
„ The original cell site will discontinue handling the call only
when the mobile station is firmly established in the new
cell. 23
† In CDMA systems that coexist with analog/ AMPS
technologies (i.e., cellular carriers), hard handoffs do
exist.
† A hard handoff in a CDMA system describes a call
handoff from a CDMA carrier to an analog/ AMPS RF
channel.
† It should be noted that in CDMA systems like this, a
wireless call can “step down” from a CDMA carrier to
an analog channel. Yet the reverse can never happen:
† a wireless call can never be handed off from an
analog RF channel to a CDMA-based channel.

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Wideband CDMA
† There is currently a newer version of CDMA under
development, known as wideband CDMA (W-CDMA),
sometimes known as “CDMA 2000.”

† Instead of utilizing a 1.25- MHz carrier, W-CDMA will

utilize a 5-MHz (or greater) carrier. This new
technology is supposed to significantly step up the
time frame by which CDMA systems will be able to
offer voice, data, and at least half-rate motion video
from CDMA handsets.

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† This technology falls under the heading of
third-generation (3G) wireless technology.
† The first generation is cellular, the second
generation is all-digital (PCS) systems, and
the third generation of wireless technologies
are those technologies that the industry is
trying to have standardized through national
and international standards bodies to offer
multimedia capabilities

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Benefits of CDMA Technology
† CDMA radio technology offers the following benefits to wireless
systems that implement this technology:

„ Increased capacity over other technologies( i.e., FDMA,

TDMA) by allowing for reuse of the same (carrier)
frequency in all sectors and cells.

„ Studies show an estimated increase of about 6 to 18 times

the capacity of AMPS systems. This number may rise
significantly over time with the honing of current
technology and the development of newer technologies
such as W-CDMA.

„ Simplified RF engineering, due to the N=1 reuse pattern in

CDMA systems. This reduces the time and effort required to
expand or modify CDMA systems.

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„ Increased performance over the weakest link in the
wireless system, the radio system. This is mainly due
to the use of rake receivers to resolve multi-path
fading (Rayleigh fading).
„ Lower transmitted power levels. This equates to
lower power bills at the base station level, and longer
battery life for CDMA handsets. Power adjustments
are constantly being made in the handset to reduce
the amount of interference introduced to other
conversations. Since a CDMA systems is noise-
limited, the less interference introduced by one
conversation the greater the system capacity left.
„ Greater security due to the encoding of CDMA
signals.
„ Enhanced performance and voice quality due to the
ability to accomplish soft handoffs.

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CDMA radiated power
† Because signal paths change continuously with
moving units, mobile units perform power
adjustments as many as 800 times per second
(once every 1.25 ms) under control of the base
station. Base stations instruct mobile units to
increase or decrease their transmitted power in 1-
dB increments (±0.5dB).
† When a mobile unit is first turned on, it measures
the power of the signal received from the base
station. The mobile unit assumes that the signal
loss is the same in each direction (forward and
reverse)and adjusts its transmit power on the
basis of the power level of the signal it receives
from the base station. This process is called open
loop power setting.

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Cont’d
† A typical formula used by mobile
units for determining their transmit
power is:
† Pt (dBm) = -76dB – pr
† Where Pt = transmit power in dBm
† Where Pr = received power in dBm
† The maximum radiated power of base
stations is limited to 100W per
1.23-MHz CDMA channel.
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Question
† Determine the transmit power for a
CDMA mobile unit that is receiving a
signal from the base station at -
100dBm.

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Solution:

† Pt (dBm) = -76dB – pr
† Pt (dBm) = -76 – (-100)
Pt = 24 dBm, or 250mW

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Global System for Mobile
Communications (GSM)

† At the time GSM was developed, there were six

incompatible cellular systems in operation throughout
Europe. A mobile designed for one system could not be
used with another system. This situation served as the
catalyst for the development of an all-European System.
† Over time, GSM has become a worldwide digital wireless
standard, along with becoming the standard pan-European
digital wireless system. This is evidenced by the fact that
many American PCS carriers chose GSM as their digital
radio technology standard.
† Since the performance of wireless systems is restricted
primarily by cochannel interference, a digital standard was
sought in order to obtain improvements in spectral
efficiency and increase capacity.

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† GSM systems use TDMA technology as their transport
format (the air interface). However the standardized
approach, distinctive features [e.g., subscriber
identity module (SIM) cards], and subsystem
architecture of GSM make it a completely different
and separate digital radio technology.
† GSM operates under a strictly controlled series of
standards known as the Memorandum of
Understanding (MoU). These standards dictate how
the system will operate in intricate detail. By building
the system to these standards, planners could be
assured that GSM systems around the world would be
completely compatible.

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Adoption of the GSM Standard
† When the broadband PCS auctions concluded in the United
States in March 1995, winners of the broadband PCS
licenses began their decision-making processes to choose
their respective digital wireless technology standards. Many
of these license winners have chosen the GSM standard as
their technology, such as Voicestream Wireless, PowerTel,
and BellSouth Mobility.
† As of May 1998, GSM Systems in the United States are
serving almost 2 million customers in almost 1500 cities
throughout 40states and the District of Columbia, covering
nearly 60% of U.S. POPs. The GSM carriers in the United
States have spent billions to build out and launch their
networks. They have created 8000 new jobs directly, and
an estimated 20,000 related jobs.
† Since 1992, GSM systems have grown tremendously, to the
point where there are 280 GSM networks in over 100
countries worldwide.

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The GSM Architecture
† The GSM offers an open architecture
according to the Open System Interconnect
(OSI) model, for layers 1,2, and 3. This
approach represents a significant departure
from the previous analog systems. Also, a
key benefit is afforded to GSM carriers
because the architecture is a fully open
system: they can go to any supplier of GSM
equipment to build out their systems; they
are not beholden to any one equipment
supplier due to proprietary schemes.

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GSM Subsystems
† The GSM network (standard) is
divided into four main subsystems:
„ The base station subsystem
„ The network subsystem
„ The operations and support subsystem
„ The mobile station subsystem

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† TO BE CONT’D

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