3G Wireless Systems:

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Nov 24, 2013 (3 years and 9 months ago)

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3G Wi
reless Systems:

WCDMA, CDMA2000, TD/S CDMA and Beyond



Abstract


The third
-
generation wireless system standard has been
enhanced to offer significantly increased performance for
data and broadcast services through the introduction of
high
-
speed do
wnlink packet access, enhanced uplink, and
multimedia broadcast multicast services. This article
provides an overview of the key elements and technologies
were used, the reasons behind their selection and their
integration.
There are
WCDMA, CDMA2000, TD/S
CDMA

and 3G beyond.

1.

Introduction


T
hird
-
generation
(3G) mobile

communication systems based
on the wideband code
-
division multiple access (WCDMA)
and CDMA2000 radio access technologies have seen
widespread deployment around the world. There are more than
16
0 3G systems in commercial operation in 75 countries with
a total of more than 230 million 3G subscribers as of
December 2005. The applications supported

by these
commercial systems range from circuits

witched services such
as voice and video telephony to
packet
-
switched services such
as video streaming, email, and file transfer. As more packet
-
based applications are invented and put into service, the need
for better support for different quality of service (QoS), higher
spectral efficiency, and higher data

rate for packet
-
switched
services increases in

order to further enhance user experience
while maintaining efficient use of the system resources. This
need has resulted in the creation

of CDMA2000 1x Evolution
Data
-
Optimized (1xEV
-
DO) Revision 0 and

Revisi
on A by
3G Partnership Project 2 (3GPP2), as well as the High Speed
Downlink Packet Access (HSDPA) and

EUL (Enhanced
Uplink) evolution of WCDMA in 3GPP. CDMA2000 1xEV
-
DO Revision 0, which was developed to

provide efficient
support for asymmetric best effor
t packet data, more than
doubled the forward link spectral efficiency of CDMA2000 1x
for packet data applications. However, since its first
commercial deployment in 2002, market feedback has
revealed that some popular packet data applications actually
resu
lt in symmetric traffic and some also pose stringent
latency requirements. This has led to the development of
CDMA2000 1xEV
-
DO Revision A, which includes significant
improvements on the 1xEV
-
DO reverse link, including
increased total sector throughput for
best effort applications
and shortened delay for applications with low
-
latency
requirements. Similarly, 3GPP developed HSDPA to address
the need for further improved WCDMA forward
-
link
(downlink) packet data access, followed by the development
of EUL to i
mprove the corresponding reverse link (uplink)
performance and capabilities. Both 3GPP and 3GPP2 have
also developed techniques to more efficiently support
broadcast and multicast services as an integrated part of the
3G networks.



Transmit diversity (TD)

is one of the key contributing
technologies to defining the ITU endorsed 3G systems W
-
CDMA and cdma2000.

Spatial diversity is introduced into the signal by transmitting
through multiple antennas. The antennas are spaced far
enough apart that the signals e
manating from them can be
assumed to undergo independent fading. In addition to
diversity gain, antenna gain can also be incorporated through
channel state feedback. This leads to the categorization of TD
methods into open loop and closed loop methods. Sev
eral
methods of transmit diversity in the forward link have been
either under consideration or adopted for the various 3G
standards.

2.

CDMA2000

A little bit history

The CDMA family of cellular networks grew out of work
undertaken by Qualcomm, a small company

in Californian.
Working on direct sequence spread spectrum techniques, by
using different spreading codes, a large number of users could
occupy the same channel and that could provide a multiple
access scheme for cellular telecommunications. The first
sta
ndard was the IS
-
95 and first network was launched in
Hong Kong in 1996 under the brand name CDMAOne.


CDMA system also has th
e following standard in its
developing stages: IS
-
95, IS
-
95A, IS
-
95B, cdma2000, 1x/EV
-
DO and 1xEV
-
DV. As we all know about it, the

very first
version of system standard IS
-
95 has never been launched for
commercial purpose due to it was pre
-
matured, then the IS
-
95A was applied for business since then and still been used
widely nowadays.
IS
-
95B was a short version since cdma2000
standa
rd was announced six months after it came out.
Technically, most of wireless business operation companies
decided that they would work around IS
-
95B and went
towards for CDMA2000.

In terms of the technology used,
cdmaOne used a bandwidth of 1.25MHz channel
.
This was
chosen to enable it to fit in with existing band plans and
channel allocations. When using the system, different users
are allocated different Walsh codes, the orthogonal spreading
codes. QPSK was used as modulation form on the forward
channel a
nd OQPSK on the reverse channel.

Evolution

The original IS
-
95A standard only allowed for circuit
switched data at 14.4
kbit/s, and IS
-
95B provided up to
64
kbit/s
data rates
as well as a number of additional services.
A
major step improvement came later w
ith the development of
3G services. The first of 3G standard was known as cdma2000
1x which initially provided data rates up to 144kbit/s,
but with
further developments promise that to allow the maximum data
rate of 307kbits/s.


To achieve the performance
improvements a number of
changes were introduced. Firstly, the Walsh codes length was
changed from 64 bits to 128 and new channels introduced then.
Additionally, turbo codes were initially used for er
ror
correction. At this point, the development of cdma20
00
diverged. One development know as cdma2000 1xEV
-
DO
provided a data only or data optimized evolution, whereas
another development know as cdma2000 1xEV
-
DV provided
a data and voice evolution.


IS
-
95 to cdma2000

Existing IS
-
95
-
based CDMA systems support c
ircuit
-
mode
and packet
-
mode data services at a data rate limited to 9.6
-
14.4kb/s. Fueled by the explosive growth of the Internet,
applications and market opportunities are demanding that
higher capacity, higher data rates, and advanced multimedia
services
be supported in the near future. The evolution of the
IS
-
95 CDMA standard to higher data rates and more advanced
services occurs in t w o steps. The f i r s t step, IS
-
95
-
B, i s an
enhancement to the IS
-
95
-
A standard and offers the highest
possible perform
ance without breaking current IS
-
95
-
A air
interface design characteristics, thereby maintaining strict
compatibility with existing base station hardware. The
proposed standard will support a high data rate (64 kb/s) in
both directions, to and from the mobi
le device. A new burst
mode packet data service is defined to allow better
interference management and capacity utilization. The second
evolution step, cdma2000, provides next
-
generation capacity,
data rates, and services. The cdma2000 system includes a
gr
eatly enhanced air interface supporting CDMA over wider
bandwidths for improved capacity and higher data rates while
also maintaining backward compatibility with existing IS
-
95
CDMA end
-
user devices. The cdma2000 system also includes
a sophisticated MAC f
e a t u r e t o effectively support very
-
high
-
data
-
rate services (up to 2 Mb/s) and multiple concurrent
data and voice

services. This article describes the desiqn and
performance of the hiqh
-
data
-
rate service

provided by second
-

and third
-
generation CDMA s
ystems.



The IS
-
95l system has been deployed worldwide because of its
superior voice quality, robust performance, and large air
interface capacity. Therefore, it is only natural that the design
of the next
-
generation air interface would build on this prov
en
second
-
generation air interface and would be backward
compatible to current IS
-
95 networks to protect the large
capital investment in the already deployed base of networks.
The third generation of revolution as know as cdma2000 uses
wideband CDMA techno
logy to meet and exceed all the
requirements of IMT
-
2000. Moreover, cdma2000 provides a
graceful transition from second
-
generation IS
-
95 systems.
Service providers can gradually build up cdma2000 networks
in a seamless fashion in selected areas where addit
ional
capacity and advanced services are needed.


In order to be backward compatible with IS
-
95 networks, the

cdma2000

radio interface retains many of the attributes of

the
IS
-
95 air interface design. As mentioned above, in IS
-
95
-
B

higher data rates are pr
ovided through code aggregation. In

cdma2000, higher rates are achieved through either reduced

spreading, or multiple code channels. In addition, there are a

number of major enhancements in the cdma2000 physical

layer that facilitate advanced data services

with higher rates

and improved capacity. Table 1 contrasts the enhancements

with those in IS
-
95
.



Table 1. Physical
-
layer characteristics of IS
-
95 and cdma2000


CDMA2000 PACKET
DATA

The packet data MAC functions for IS
-
95 have only two states,
active
a
nd
dormant,
as described above. This simple approach
works well for fairly low
-
speed data services with relatively
low occupancy for any given user. However, this MAC model
is inadequate to meet the aggressive requirements for very
-
high
-
speed data services

with many competing users in third
-
generation systems. This is due to the excessive interference
caused by idle users in the active state, and the relatively long
time and high
system overhead required to transition from the
dormant to the active state. T
o address these requirements, the
cdma2000 system incorporates a sophisticated MAC
mechanism that includes two intermediate states between the
IS
-
95 active and dormant states (Fig.1)



Figure 1. Comparison between IS
-
95 and cdma2000 MACs.

EV
-
DO

cdma2000 1
xEV
-
DO was originally not on the development
roadmap for cdma2000. So as a result, EV
-
DO was defined
under IS
-
856 rather than IS
-
2000 and carries only data

at
broadband speed. The first commercial CDMA2000 1xEV
-
DO network was deployed by Korea company SK t
elecom in
January 2002. The system is becoming more widespread with
30 networks live across Asiam Americas and Europe and 37
more networks scheduled to be deployed in 2006.



The evolution of cdma2000 1xEV
-
DO systems to multi
-
carrier
EV
-
DO (supported by 1x
EV
-
DO Revision B) is discussed in
this article. Multi
-
carrier EV
-
DO offers a backward
-
compatible upgrade to leverage existing 1xEV
-
DO networks
and terminals. It allows a software upgrade to multi
-
carrier
EV
-
DO using 1xEV
-
DO Revision A base station hardware
.
Multi
-
carrier operation achieves higher efficiencies relative to
single
-
carrier by exploiting channel frequency selectivity,
improved transmit efficiencies on the reverse link, and
adaptive load balancing across carriers. Multi
-
carrier EV
-
DO
enables very

high

speed download, high
-
resolution video
telephony, and improved user experience with concurrent
applications. The sources of higher efficiency are discussed in
detail in this article. It also enables hybrid frequency reuse
deployment scenarios that ena
ble spectrally efficient operation
and significant improvement in edge coverage
performance
with
hardware

efficient implementations. The evolved wider
bandwidth
systems up to 20 MHz

based on multi
-
carrier EV
-
DO offer operators a cost
-
effective solution tha
t competes
favorably with other technologies.


Multi
-
carrier EV
-
DO offers both operators and end
-
users
significant benefits over that of single

carrier systems. Some
of the benefits of multi
-
carrier EV
-
DO are:

• Backward compatibility

• Reuse of existing i
nfrastructure hardware, lower
development cost, and rapid time to market

• Higher peak data rates, reduced latency and improved
support for quality of service (QoS)
-
sensitive applications
• Improved transmit efficiency (reverse link)

• Higher spectral effi
ciency via exploiting frequency
-

selective fading across carriers

• Adaptive load balancing across carriers

• Alternate deployment scenarios due to use of flexible
duplex assignment

EV
-
DV

The original roadmap for development of cdma2000 1x was to
adopt th
e data and voice system, but it unlikely to be deployed,
but could be capable of providing data at speeds up to 3.09
Mbits/s in the forward direction. In order to meet the
requirements of 1xEV
-
DV, there are number of new features
to be implemented. These i
ncluded the addition of new
channels, an adaptive modulation and coding scheme, the
addition of ARQ to the Physical layer and Cell Switching.


The main driver is the growing demand for wireless data
services such as mobile Internet access, music and video

streaming, online interactive gaming, and multimedia
messaging. This has led to the development of the so
-
called
third
-
generation (3G) evolutionary systems. One such system,
commonly known as 1xEV
-
DV (cdma2000 1x Evolution for
high
-
speed integrated data a
nd voice), has just recently been
standardized. It represents the latest evolution of the
cdma2000 family of standards.


The cdma2000 system is the extension of the first commercial
wireless communication system using code
-
division multiple
access (CDMA)
technology: IS
-
95. Commercial IS
-
95
systems were mainly voice
-
oriented systems, while the first
commercial cdma2000 systems, commonly referred to as
cdma2000 1x systems, offered support for packet

data service
as well as increased voice capacity. This was
accomplished
using dedicated code division multiplexed (CDM) channels to
support both voice and packet data service. Although the
approach employing dedicated channels is well suited to real
-
time traffic, such as voice and live video streaming, it cannot
e
xploit either the burst

nature and delay tolerance of packet
data or multi
-
user diversity via opportunistic scheduling of
mobile stations (MSs) experiencing independent or relatively
uncorrelated fading.


Consequently, 1xEV
-
DV has been developed to exploi
t the
delay tolerance and diversity of multi
-
user packet data traffic
via numerous air interface innovations. It has been developed
in a cooperative fashion with the support and effort of the
companies involved in the Third Generation Partnership
Project 2

(3GPP2).


Near future for CDMA2000

With cdma2000 1x and EV
-
DO release 0 well established,
further development is looking to providing greater data
speeds, better spectral efficiency and improved network
manageability. The evolution continues through EV
-
D
O
. It
aggregates multiple channels for higher peak rates and
supports up to 20 MHz bandwidth with peak rates increase
linearly. In this way, cdma2000 1xEV
-
DO would be able to
compete with wireless LANs while maintaining the mobility
expected much more form

a wireless technology.

3.

WCDMA

Introduction and Evolution

Wideband Code
-
Division Multiple Access (WCDMA) was
initially proposed and engineered with a vision that already
has shown its future proof. WCDMA was designed to be a
high performance system able to
support future applications
requiring simultaneous transmission of several bit streams that
require individual quality of service (QoS).


Before going into technical solutions in 3G
-
revolution, it is
most essential to understand the need for such evolutio
n and
essential differences from what the original 3G WCDMA
system can offer. It is important to realize that most often
when different generations are discussed (e.g., 2G and 3G),
people are referring to major changes in air interface standards.
This actu
ally is a good approach, because both core network
and applications are developing at their own speeds, and we
cannot really see clear differences between “generations”
anymore. Looking back to 2G Global System for Mobile
Communications (GSM) evolution and

first 3G WCDMA
systems, some essential differences can be seen in the air
interface implementation. As an example, one key WCDMA
feature has been support of multiple simultaneous services
with different QoS parameters. Another key development has
been in
data rates, where original GSM could not efficiently
support many user needs (e.g., email downloading). However,
GSM capabilities have evolved to being close to original
WCDMA targets when WCDMA is launched. These kinds of
new system characteristics, such
as support of simultaneous
services or increased data rate, first need to be understood
before the technical solutions are decided.


DEVELOPMENT OBJECTIVES

The development of 3G will follow a few key trends, and the
evolution following these trends will c
ontinue as long as the
physical limitations or backward compatibility requirements
do not force the development to move from evolution to
revolution. The key trends include:



Voice services will also stay important in foreseeable
future, which means that c
apacity optimization for voice
services will continue.



Together with increasing use of IP
-
based applications, the
importance of data as well as simultaneous voice and data
will increase.



Increased need for data means that efficiency of data
services needs
to be improved as well as it delay, and
average and peak user data rates.



When more and more attractive multimedia terminals
emerge in the markets, the usage of such terminals will
spread from office, homes, and airports to roads, and
finally everywhere. T
his means that high
-
quality high
-
data
-
rate applications will be needed everywhere.



When the volume of data increases, the cost per
transmitted bit needs to decrease in order to make new
services and applications affordable for everybody. The
data rate tren
ds are summarized in Fig2. The other current
trend in Fig. 1 indicates that in the 3G evolution path very
high data rates are achieved in hot spots with WLAN
rather than cellular
-
based standards.

Broadband/Broadcast Services

The rapid widespread deployment

of WCDMA and an
increasing uptake of third
-
generation services are raising
expectations with regard to new services. Packet data services
such as Web surfing and file transfer are already provided in
the first release of WCDMA networks, release 99. Althou
gh
this is a significant improvement compared to 2G networks,
where such services have no or limited support, WCDMA is
continuously evolving to provide even better performance.
Release 5 of WCDMA, finalized in early 2002 and with
products starting to appea
r, introduced improved support for
downlink packet data, often referred to as high
-
speed
downlink packet access (HSDPA). In release 6, finalized early
2005, the packet data capabilities in the uplink (enhanced
uplink) were improved. Release 6 also brought
support for
broadcast services through multimedia broadcast multicast
services (MBMS), enabling applications such as mobile TV.
WCDMA has been evolving to meet the increasing demands
for high
-
speed data access and broadcast services. These two
types of ser
vices have different characteristics, which
influence the design of the enhancements. For high
-
speed data
access, data typically arrives in bursts, posing rapidly varying
requirements on the amount of radio resources required. The
transmission is typically

bidirectional and low delays are
required for a good end
-
user experience. As the data is
intended for a single user, feedback can be used to optimize
the transmission parameters.




Figure 2. Data Rate Trends


Broadcast/multicast services carry data int
ended for multiple
users. Consequently, user specific adaptation of the
transmission parameters is cumbersome and diversity not
requiring feedback is crucial. Due to the unidirectional nature
of broadcasted data, the low delay for transmission is not as
im
portant as for high
-
speed data access.


DATA ACCESS AND ENHANCED
UPLINK


In WCDMA, the shared downlink resource consists of
transmission power and channelization codes in
node B
(the
base station), while in the uplink the shared radio resource is
the inter
ference at the base station.
Fast scheduling
is used to
control allocation

of the shared resource among users on a rapid basis.
Additionally,
fast hybrid ARQ with soft combining
enables fast
retransmission of erroneous data packets.

To meet the requirement

on low delays and

rapid
resource (re)allocation, the corresponding

functionality must be located close to the air

interface. In WCDMA this has been solved by

locating the enhancements in the base station as part of
additions to the MAC layer.




Figure
3. The Path ti WCDMA evolved


An illustration of this is found in Fig. 3, where the overall
UTRAN architecture with high
-
speed downlink packet access
(HSDPA) and enhanced uplink is illustrated. A number of
radio network controllers (RNCs) are connected to
the core
network. Each RNC controls one or several node Bs, which in
turn communicate with the user equipment.



D
OWNLINK
P
ACKET
A
CCESS

A key characteristic of HSDPA is the use of
shared
-
channel
transmission
. This implies that a certain fraction of the tot
al
downlink radio resources available within a cell,
channelization codes and transmission power, is seen as a
common resource that is dynamically shared between users,
primarily in the time domain. The use of shared
-
channel
transmission, in WCDMA implemen
ted through the high
-
speed downlink shared channel (HS
-
DSCH), enables the
possibility to rapidly allocate a large amount of the downlink
resources to a user when needed.


Link Adaptation


Link adaptation is implemented by
adjusting the channel
-
coding rate
, and selecting between
QPSK and 16
-
QAM. Higher
-
order modulation such as 16
-
QAM makes more efficient use of bandwidth than QPSK, but
requires greater received
Eb
/
N
0. Consequently, 16
-
QAM is
mainly useful in advantageous channel conditions. In addition,
the

data rate also depends on the number of channelization
codes assigned for HSDSCH transmission in a TTI. The data
rate is selected independently for each 2 ms TTI by node B,
and the link
-
adaptation mechanism can therefore track rapid
channel variations.


S
cheduling


The scheduler is a key element and to a large
extent determines the overall downlink performance,
especially in a highly loaded network. A practical scheduler
strategy exploits the short
-
term variations (e.g., due to
multipath fading and fast
nterference variations) while
maintaining some degree of long
-
term fairness between the
users.


Hybrid ARQ


The third key feature of HSDPA is hybrid
ARQ with soft combining, which allows the terminal to
rapidly request retransmission of erroneously recei
ved
transport blocks, essentially fine
-
tuning the effective code rate
and compensating for errors made by the link
-
adaptation
mechanism. The terminal attempts to decode each transport
block it receives and reports to node B its success or failure 5
ms afte
r the reception of the transport block.

CONCLUSION for WCDMA

With the recent evolution to the WCDMA standard, support
for packet data and broadcast services has been considerably
improved to meet future demands. Fast adaptation to rapidly
varying traffic a
nd channel conditions has been applied to
WCDMA through HSDPA and enhanced uplink, thereby
providing high data rates to cellular users. Similarly, by
combining the transmissions from multiple sites, true
broadcast services are possible in WCDMA with the
in
troduction of MBMS. The article has described how these
techniques have been introduced into WCDMA while
maintaining compatibility with existing WCDMA
deployments. HSDPA, enhanced uplink, and MBMS offer an
attractive way for 3G operators to enhance the net
work in
order to offer new services.

4.

TD
-
SCDMA

What is TD
-
SCDMA

Transmit diversity (TD) is one of the key

contributing technologies to defining the ITU

endorsed 3G systems W
-
CDMA and cdma2000.

Spatial diversity is introduced into the signal by

transmitting
through multiple antennas. The

antennas are spaced far enough apart that the

signals
emanating from them can be assumed to

undergo
independent fading. In addition to

diversity gain,
antenna gain can also be incorporated

through
channel state feedback. This

leads

to the
categorization of TD methods into open

loop and
closed loop methods. Several methods

of transmit
diversity in the forward link have

been either under
consideration or adopted for

the various 3G
standards.

T
RANSMIT
D
IVERSITY
B
ASICS

Most mobile

communication channels must combat

the effects of fading caused by multipath
propagation.

An important way of quantifying fading is

in terms
of a measure called the
coherence bandwidth

which
indicates the amount of bandwidth

that will fade in
a correlated

fashion at any instant

in time.


Transmit diversity can improve the receiver

performance in the presence of flat fading. It

reduces the impact of fading by offering multiple

independent copies of the digitally modulated

waveform at the receiver, where the

chance that

all
copies are simultaneously in a fade is very

small.
Common methods of TD employing spatially

separated antennas utilize either temporal

or
frequency techniques, or combinations of

these
techniques.


T
RANSMIT
D
IVERSITY IN
3G

The earliest ope
n loop diversity techniques

were
simple in their configuration, for example,

phase
-
switched TD
(PSTD) and
time
-
switched TD

(TSTD).
PSTD introduces a known periodically

varying
phase difference between the symbols

transmitted
through different antennas to s
imulate

fast fading.

O
RTHOGONAL
T
RANSMIT
D
IVERSITY

Orthogonal TD [2] is an open loop method in

which
coded interleaved symbols are split into

even and
odd symbol streams and transmitted

using two
different Walsh codes. The length of

the Walsh
code is doubl
ed so that the total number

of Walsh
codes available is not reduced as a

result of splitting
the data, and the data rate will

remain more constant
than is the case with no

data splitting.


T
RANSMIT
D
IVERSITY VIA
S
PACE
-
T
IME
C
ODING

In general, coding with sp
ace and time redundancy

is accomplished by finding an efficient way

to
allocate different symbols to different antennas

while adding, jointly across antennas, some

type of
time redundancy for implementing forward

error
correction.

The noteworthy fact about

this

approach
is that it isolates TD from those forms

of diversity
associated with the radio channel. Nevertheless,
spread

spectrum systems in frequency selective
channels

can benefit equally from coding with
space and

time redundancy, as outlined below.

A COMPARISON OF TRANSMIT
DIVERSITY METHODS

the performance of different

OL and CL methods.
The results were

generated in a symbol
-
level
simulation environment

for the CDMA2000
standard.
T
he average power per chip required to

transmit at a given frame error

rate with power

control. It can be seen that the open loop methods

are robust at higher velocities, while TXAA

provides the biggest benefit at the lower velocities.

Rece
nt Advances on TD
-
SCDMA

China has fully embraced the remarkable

growth
and unprecedent
ed penetration of mobile

services,
and has become the world’s largest

mobile cellular
market. TD
-
SCDMA was proposed

by the China
Wireless Technology Standard

(CWTS) Group in
1998, approved as one

of the 3G standards by ITU
in May 2000, and

joined 3GPP in M
arch 2001. This
has been a

major effort by China to advance its
mobile

communication systems and facilitate its
own

technological development in this critical area.

TD
-
SCDMA, a combination of TDD and
synchronous

CDMA, offers several unique
advantages

over
its alternatives, WCDMA and

cdma2000, such as flexible spectrum allocation,

low
-
cost implementation, and easier migration

from
GSM systems. This article reviews the

development,
key technical features, and deployment

of TD
-
SCDMA in China.

3G STANDARD IN CH
INA

The initial development of mobile communications

satisfied people’s basic needs. 2G and the

2.5G
systems also offer data services using General

Pack
Radio Service (GPRS) and Enhanced

Data Rate for
GSM Evolution (EDGE) technologies

that enable
transmiss
ion rates up to 144

kb/s and 384 kb/s.
While basic voice, text messaging,

and simple
image services have proved to

be a massive success
for 2G and 2.5G mobile

communications systems,
they cannot meet the

growing demands, especially
with the recent

integrat
ion of mobile
telecommunications and

the Internet. In addition, it
is estimated that the

capacity of 2G system will be
exhausted within

the next 10 years in densely
populated areas.

3G wireless networks are expected
to transmit

data rates up to 384 kb/s an
d more. With
the

rapid development of rich media applications

and deployment of wireless Internet services, 3G

systems have to provide both symmetric circuit

switched

services for real
-
time applications such as
voice and video, and growing asymmetric

packe
t
-
switched services for non
-
real
-
time applications

such as mobile Internet access.



There are now three main de facto 3G standards

b
eing adopted in China: wideband CDMA

(WCDMA) proposed by Europe, cdma2000 by

the
United States, and TD
-
SCDMA by China.

They

are
all CDMA
-
based wireless radio technologies,

as
CDMA technology demonstrates

several key
advantages such as greater coverage

with fewer cell
sites, better frequency reuse, and

higher capacity.
Figure 2 illustrates the different

migration paths to
3G sy
stems [1], which have

been abstracted as
follows:



GSM to GPRS and/or EDGE to WCDMA

or
TD
-
SCDMA



GSM to TD
-
SCDMA directly



CDMA IS95 to cdma2000 1x to cdma2000



1x EV
-
DO/DV or cdma2000 3x

TECHNOLOGY ASPECTS OF TD
-
SCDMA

TD
-
SCDMA, proposed by the China Academy

f
or
Telecommunications Technology (CATT),

was
formally approved by the International

Telecommunication Union (ITU) as one of

three 3G
transmission standards in May 2000. In

March 2001,
TD
-
SCDMA was accepted by the

3G Partnership
Project (3GPP), which is a c
lear

indication that TD
-
SCDMA will be supported by

global industry
alliances, including service operators

and system
suppliers.

Figure
4

shows the typical multiplexing
structure

of a TD
-
SCDMA radio channel. The
channel

includes three carriers using a low c
hip rate
mode

of 1.28 Mchips/s that corresponds to a carrier

bandwidth of 1.6 MHz. This helps provide high

flexibility in spectrum usage and network design,

especially in densely populated areas.

In addition,

each TDMA frame of 5 ms duration is divided

int
o
7 time slots, which can be flexibly assigned to

either
multiple users or a single user that might

require
multiple time slots.




In addition to the

TDMA/TDD principle, TD
-
SCDMA uses a different

CDMA mode from that
of cdma2000 and

WCDMA systems, in w
hich TD
-
SCDMA limits

the number of codes for each time
slot to a maximum

of 16 as shown in Fig. 3. This
helps to

reduce multiple access interference (MAI)
and

increase system capacity
.

O
PERATOR

S
C
HOICE

There are four major operators in China, including
t
wo fixed
-
line operators (China Telecom and
China Netcom), as well as two mobile
Communication operators (i.e., China Mobile and
China Unicom). Although the number of mobile
subscribers in China exceeded 269 million at the
end of 2003, the actual market pen
etration still
remains very low. All these four major operators
desire future 3G licenses, but which 3G system

will be adopted is still unclear. Here we only give

a
path to 3G from the current network situation.

T
he
major competitor to China Mobile,

China
Unicom
has begun to focus on deploying

its cdma2000 1x
network.
Therefore, the path to

3G of China
Unicom is from IS
-
95 to cdma2000

1x and further
to cdma2000 EV
-
DO/DV, as

mentioned earlier.
Ranked as the largest mobile

operator in the world,
China Mobile
has not yet

officially decided which
3G system to deploy. It

needs to make use of the
existing GSM resources

and is expected to upgrade
its current networks

to WCDMA.


R
EALITY
C
HECK FOR
TD
-
CDMA

Time
-
division synchronous code
-
division multiple

access (TD
-
SC
DMA) technology for 3G mobile
communication,

proposed by the China Wireless
Telecommunications

Standard Group (CWTS),
emerged as one of the

radio transmission
technologies (RTTs) considered by

the
International Telecommunication Union (ITU) and

was adopted

as the low chip rate (LCR) version of
the

3G Partnership Project (3GPP) time
-
division
duplex

(TDD) standard. Relying on a combination
of TDD and

synchronous CDMA, TD
-
SCDMA
offers a number of

attractive features, including
unpaired frequencies, suitability

for IP services,
and capability to support asymmetric

services in
up/down link. In addition, TD
-
SCDMA

systems
also incorporate some new or unique technologies

such as joint detection, adaptive antennas, dynamic

channel allocation, and baton handover.


Tec
hnology and standard development have been
followed

by strong commercialization efforts. The
TDSCDMA

industry alliance is focused on
accelerating all

aspects of industry development
including system equipment,

access network,
handsets, core chipsets, softw
are

modules, and test
equipment [2]. Lessons learned from

the earlier
days of other 3G technologies (e.g., lack of reliable

terminals) are well understood and a primary target

Maturing the technology

has been conducted

through the Master Trial Net(MTnet) d
eployed in
several

urban areas in China, including

Beijing and
Shanghai, with the

major goal of removing
uncertainties

due to TD
-
SCDMA technology

not
being deployed

previously.


Today for TD
-
SCDMA

2006 will be challenging year for TD
-
SCDMA
technology


a c
ombination of technical maturity
and the economics

of deployment must strike a
balance to result in

success. As we have often been
reminded in the past

through the history of the
cellular industry, this balance is

delicate with
consumers being exposed to a

number of

choices.
Having a market of over 350 million mobile
subscribers

that is adding 4 to 5 million new users
every

month,

the world’s attention will be focused
on China

eagerly awaiting not only the
Olympics but also 3G deployments.

What is at
stake

is a reality check for TD
-
SCDMA
,
establishing a strong position in China may be a
stepping

stone to the global telecom technology
supplier position in

the future.


5.

Conclusion

3G
AND
B
EYOND

The
services, applications, and

even the core
network are evolving

at high speeds,

and
distinguishing different generations is not

really
possible anymore. The evolution, and sometimes

revolution, is a very significant trend, but in

this
article 4G is seen as a revolution of the air

interface rather than a new phase of e
volution.

The
other major trend is that access methods will

be
less tightly coupled to the network.


After a certain point, evolution is not no longer

an
answer to air
interface development, and
revo
lutionary concepts must also be considered.
Figure

5

illu
strates the evolution of 2G/3G cellular

and WLAN standards and the revolutionary step

toward future wireless systems. GSM evolution

will continue in parallel with WCDMA. In the

United States, cdma2000
-
1X will be followed by

1XeV
-
DO (high
-
bit
-
rate data only
) and 1X
EV
-
D
V

(high
-
bit
-
rate data and voice)

standards.


Looking at development in the Internet and

applications, it is clear that the complexity of the

transferred content is rapidly increasing and will

increase further in the future. Generally it can

be
said that the more bandwidth is available, the

more
bandwidth applications will consume.

In order to
justify the need for a new air

interface, targets need
to be set high enough to

ensure that the system will
be able to serve us

long into the future. A
rea
sonable approach

would be to aim at 100 Mb/s
full
-
mobility wide

area coverage and 1 Gb/s low
-
mobility local area

coverage with a next
-
generation
cellular system

in about 2010 in standards fora.
Also, the future

application and service
requirements will bri
ng

new requirements to the air
interface and new

emphasis on air interface design.
One such issue,

which already strongly impacts 3G




Figure 5. the path toward 4G from a radio perspective


R
evolution

is

the need to support IP and IP
-
based
multimedia.

If both technology and spectrum to
meet such

requirements cannot be found, the whole
discussion

of 4G may become obsolete.


Mobile wireless technologies beyond 3G are being
investigated and discussed

in industry and
universities, in government

and interna
tional
organizations, and in

professional

conferences,
journals
.

These reflect a view of wireless that
continues to

r
evolve, but which is also at the
beginning of a

significant change in wireless
systems and services.

N
ext
-
generation wireless
involves the

concept that the next generation of
wireless communications

will be a major move
toward ubiquitous

wireless communications
systems and

seamless high
-
quality wireless
services.
4G

mobile communications involves a
mix of concepts and technologies in the maki
ng.

Some can be recognized as derived from 3G and
are

called
evolutionary
(e.g., evolutions of
WCDMA and

cdma2000), while others involve
new approaches to wireless

mobile and are
sometimes
labeled
revolutionary


like OFDM/
WCDMA
. What is

important, though
, is the
common understanding that technologies

beyond
3G are of fundamental relevance in the

movement
toward a new wireless world that is a total
convergence

of wireless mobile and wireless
access communications.

Any of these terms are
meant to signify fu
ndamentally better wireless
mobile communications in the future.