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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.
In
every two-way communication system it is necessary to use separate channels to
transmit information in each direction. This is called duplexing. Currently there
exist only two duplexing technologies in wireless communications, Frequency division
duplexing (FDD) and time division duplexing (TDD). FDD has been the primary technology
used in the first three generations of mobile wireless because of its ability
to isolate interference. TDD is seemingly a more spectral efficient technology
but has found limited use because of interference and coverage problems.
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. 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. The smart code plus TDD is called CDD. This paper will
elaborate on a set of smart codes that will make an efficient CDD system a reality.
The CDMA system based on this is known as the LAS-CDMA, where LAS is a set of
smart codes. 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.
To utilize spectrum efficiently, two transmission
techniques need to be considered: one is a multiple access scheme and the other
a duplexing system. There are three multiple access schemes namely TDMA, FDMA
and CDMA. The industry has already established the best multiple access scheme,
code-division multiple access (CDMA), for 3G systems. The next step is to select
the best duplexing system. Duplexing systems are used for two-way communications.
Presently, there are only two duplexing systems used: frequency-division duplexing
(FDD), and time-division duplexing (TDD). The former uses different frequencies
to handle incoming and outgoing signals. The latter uses a single frequency but
different time slots to handle incoming and outgoing signals.
In the current cellular duplexing systems, FDD has been the appropriate choice,
not TDD. Currently, all cellular systems use frequency-division duplexing in an
attempt to eliminate interference from adjacent cells. The use of many technologies
has limited the effects of interference but still certain types of interference
remain. Time-division duplexing has not been used for mobile cellular systems
because it is even more susceptible to different forms of interference. TDD can
only be used for small confined area systems. Code-division duplexing is an innovative
solution that can eliminate all kinds of interference. Eliminating all types of
interference makes CDD the most spectrum efficient duplexing system.
CDMA
overview
Interference
and Capacity
One of the key criteria in evaluating a communication system
is its spectral efficiency, or the system capacity, for a given system bandwidth,
or sometimes, the total data rate supported by the system. For a given bandwidth,
the system capacity for narrow band radio systems is dimension limited, while
the system capacity of a traditional CDMA system is interference limited.
Traditional
CDMA systems are all self-interference system. Three types of interference are
usually considered. By ISI we mean Inter Symbol Interference, which is created
by the multi-path replica of the useful signal itself; MAI, or Mutual Access Interference,
which is the interference created by the signals and their multi-path replica
from the other users onto the useful signal; and ACI, or Adjacent Cell Interference,
which is all the interfering signals from the adjacent cells onto the useful signal.
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