Code division duplexing

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Shashwat Shriparv
dwivedishashwat@gmail.com
InfinitySoft

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INTRODUCTION
 Reducing interference in a cellular
system is the most effective
approach to increasing radio capacity
and transmission data rate in the
wireless environment.
 Therefore, reducing interference is a
difficult and important challenge in
wireless communications.

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 Mobile radio networks are operated with finite,
limited resources (the spectrum of frequencies
available) and these resources have to be used
effectively to ensure that all users receive
service, that is quality of service is maintained.
 To utilize spectrum efficiently, two
transmission techniques need to be
considered:
 a multiple access scheme
 a duplexing system

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The industry has already established the best multiple access
scheme, code-division multiple access (CDMA), for 3G systems.

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DUPLEXING SYSTEM
 A duplex communication system is
a system composed of two connected
parties or devices which can
communicate with one another in
both directions
Currently there exist only two duplexing
technologies in wireless communications,
 Frequency division duplexing(FDD)
 Time division duplexing (TDD).

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FDD
 FDD has been the primary technology
used in the first three generations of
mobile wireless because of its ability to
isolate interference
 Uses different frequencies to handle
incoming and outgoing signals.
 Using an FDD system can eliminate
interference among the base stations of
adjacent cells to the base station of the
home cell.

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But…
Two types of interference remain in
FDD:
 adjacent base-to-home mobile
interference
 and adjacent mobile-to-home base
interference.

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TDD
 TDD is seemingly a more spectral
efficient technology but has found
limited use because of interference
and coverage problems.
 Using a TDD system causes four
kinds of interference:

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 Adjacent base-to-home mobile
 Adjacent mobile-to-home base
 Adjacent base-to-home base
 Adjacent mobile-to-home mobile
So it can only be used for small confined
area systems

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CDD
 Code-division duplexing (CDD) is an
innovative solution that can eliminate
all kinds of interference.
 CDMA is the best multiple access
scheme when compared to all others
for combating interference.
 A set of smart codes can make a high-
capacity CDMA system very effective
without adding other technologies.
 The smart code plus TDD is called CDD.

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SMART CODES
 The Large Area Synchronous (LAS)
Codes are a set of smart codes that
can reduce interference very
effectively.
 The effectiveness of smart codes
applied to TDD makes it the right
choice in cellular systems.
 The application of LAS Codes in a
TDD system (called TD-LAS system)
creates a CDD system.

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cont…
 The CDMA system based on this is known
as the LAS-CDMA.
 LAS-CDMA is a new coding technology
that will increase the capacity and spectral
efficiency of mobile networks.
 The advanced technology uses a set of
smart codes to restrict interference, a
property that adversely affects the
efficiency of CDMA networks.

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Cont..
 The code used in today’s CDMA scheme
is the Walsh code, which is not too
smart.
 Walsh codes have the orthogonality
property among codes while the time
shift T= 0 (i.e., no time shift t )
 However, in the mobile radio
environment the signal arrival can have
a long time shift.
 The property of Walsh codes cannot
properly be applied to this environment

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Cont…
 Now there is a set of smart codes
that have orthogonality among the
codes for time shift T≠0.
 The codes arrive at the terminals at
different time shifts; because of the
orthogonal nature, all undesired
codes are blocked.
 Thus, smart codes are the proper
codes to handle this situation.

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The properties of smart codes have to
meet the following requirements:
 Auto-correlation
Rxx(T )= 0 for T =0
=d for T!=0,within window T0
 Cross-correlation
Rxy (T) = d for all T, within window
T0,Where T0 is a correlation window,
d can be zero or low correlation
value.

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 Outside the correlation window is
beyond the time delay spread range
of the received signal.
 Although the correlation value
outside the range is high, there is no
impact on our desired signal

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With this property, we can illustrate the
merit of using this smart code.

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 Assume that eight smart codes are
transmitted
 They arrive at the receiver of Code C1 at
different times due to the multipaths
caused by different reflectors.
 Because of the cross-correlation property,
the desired code to be received is C1
 However, many C1 codes can be received
due to the effects of the multipaths.
 This does not occur though; due to the
auto-correlation property, only Code C1 at
time t1 is received

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 We do not need the strength of more
than one path signal to be added for
increasing carrier-to-interference
ratio (C/I) since we are only
receiving carrier-to-noise ratio (C/N),
not C/I.
 The rest of the signals from different
paths do not cause any interference,
and there is no need to use any
means to collect them for the
purpose of reducing interference

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USING SMART CODE SEQUENCES TO
REDUCE INTERFERENCE
Smart code sequences have two
properties:
 Auto-correlation equals zero, which
causes no multipath interference.
 Cross-correlation equals zero, which
causes no multi-user interference
 Smart code sequences will effectively
work in TDD scenarios and become a
CDD system

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Adv: of smart codes
 The two different smart codes can isolate
interference at the same frequency and in
the same time frame.
 The capabilities of smart codes eliminate
the known near-far effect in mobile
communication systems.
 The inherent properties of smart codes
eliminate the need for the following
technologies: take receiver, power control,
joint detection etc..

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Adv: of using CDD
 Any single (unpaired) spectrum band with
a bandwidth equal to 1.6 MHz can be used
for the CDD application.
 The system is simple.
 The handset is also simple.
 Since no duplexer is needed, the handset
cost can be lower and the size of the
handset smaller.
 Power consumption is lower due to only
one single spectrum band being operable.

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 CDD is like TDD, the ideal system for
asymmetrical traffic.
 CDD has the highest spectrum
efficiency and can be used in the new
third-generation (3G) systems.
 The CDD system changes the
environment from an interference-
limited one to a noise-limited one

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 In other TDD systems the codes are
not smart, so the receiver not only
receives its desired code, but also
receives interference (I) from other
cells.
 Sometimes, the interference is so
high it can mask the desired signal.
 In the LAS CDD system, because of
the smart codes, the receiver only
receives the desired code, no
interference.

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THE TRIAL OF THE CDD SYSTEM
 A working trial of the CDD system has
been in Shanghai, China, since the
beginning of September 2001.
 Voice and video was transmitted at
384 kb/s through a 1.6 MHz unpaired
band while the mobile was traveling at
speeds in excess of 30 mph.
 The trial successfully proved the
technology and concept of a CDD system

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A WIRELESS IP NETWORK SOLUTION
 A CDD system is ideal for Internet
communications, which makes a total
wireless IP network solution highly
desirable.
 On December 15, 1999, four companies —
Vodafone Airtouch, Cisco Systems,
Hyundai, and Telos Technology— made a
demo for a switch less IP core network in
Reno, Nevada [6].
 The iBTS connects to the IP core network,
as shown in Fig.

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 The function of the switch is broken down
into two:
mobility management and call
processing.
 The handoff function is carried by the
selective distribution unit (SDU).
 The separated functions are placed on the
gateways to the core network.
 The demo of carrying voice over IP from
mobile to mobile and mobile to landline
was successful.
 The CDD system’s future IP core network
will follow this approach.
 The cost will be lower, and add-on
features can be very flexible and easy.

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CDD TECHNOLOGY IS A
NEW MILESTONE
 The LAS smart codes are the next breakthrough
technology.
 Applying LAS technology to a TDD system
becomes a CDD system.
 The benefit of using a TDD system for
communications is that the sending and receiving
are alternating in time at a single frequency.
 Without the sending and receiving occurring at
the same time we can use the same smart code
for both sending and receiving;
 thus, LAS + TDD = CDD.
 CDD is no more like TDD and is a better system
for cellular.

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REDUCE INTERFERENCE-LIMITED
ENVIRONMENT
TO NOISE-LIMITED ENVIRONMENT
 Before 1989, cellular systems were using multiple
cells with multiple frequencies assigned in each
cell.
 These systems generate an interference-
limited environment.
 After 1989, multiple cells used the same
frequency. Although the capacity increased using
CDMA, the system is still in an interference-
limited environment.
 After 2000, the CDD system is developed.
 This system changes the environment from an
interference- limited one to a noise-limited one.

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CONCLUSION
 We are convinced that CDMA is the best
multiple access scheme when compared to
all others for combating interference.
 However, the codes in CDMA can be more
than one type of code.
 A set of smart codes can make a high-
capacity CDMA system very effective
without adding other technologies.

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 The smart code plus TDD is called CDD,
which is a new 3G technology.
 A CDD system is simpler in design and
lower cost.
 CDD is an ideal system for a total IP
network solution: new 3G technology + IP
core network.
 Hopefully, a CDD system will be the
system of choice for future 3G networks.

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Shashwat Shriparv
dwivedishashwat@gmail.com
InfinitySoft

Code division duplexing

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