CDD or the Code Division Duplexing is a duplexing system that is perhaps the most spectrum – efficient
one. In this article we discuss about this system.
But first let us look at the
scenario of the spectrum efficiency.
- Elimination of interference is very
complicated as well as an important issue when it comes to the implementation
of the wireless communications.
- With the increasing spectrum efficiency,
interference becomes compulsory to be eliminated.
- Reducing the interference is
very much important in order to increase the transmission data rate and the
radio capacity of the wireless communications.
- By duplexing, we mean that using two separate channels for the
transmission of information in a two way communication in two different
directions.
- Currently we have only two working technologies in this regard
namely:
- Frequency
division duplexing or FDD and
- Time
– division technology or TDD
- The
former is the primary technology used in wireless because of its characteristic
ability of interference isolation.
- The latter is kind of a spectral efficient
technology.
- At
present, frequency–division duplexing or FDD is mostly used by all the
cellular systems for eliminating the interference that occurs in the adjacent
cells.
- A number of technologies are used today that have been successful in
limiting the effects of the interference.
- But they have not been successful in
eliminating two particular types of interference namely:
- Adjacent base – to – home mobile interference
and
- Adjacent mobile – to – home base interference
- Time Division Duplexing or TDD has not been put to use for the cellular systems
since this one is much susceptible to different types of interference
- TDD is
not just subjected to the above mentioned two forms of interference rather it
faces additional adjacent mobile to home and base to home interference.
- This limits the use of time division duplexing to the systems confined to a small
area.
- In Code–Division Duplexing, all these types of interference are
eliminated making it the most spectrum efficient one.
- However, proper CDD
systems are not a perfect reality yet.
- CDD
represents an innovative solution.
- Another best multiple access scheme is the
CDMA (code – division multiple access) since it is more effective in
eliminating the interference when compared to the other technologies.
- Code –
division duplexing is a combination of TDD and smart codes.
- The CDD can be
turned in to reality only with these smart codes.
- CDMA based on these smart
codes is termed as LAS – CDMA where LAS represents the smart codes set.
- This
technology holds the promise for increasing the spectral efficiency as well as
capacity of the mobile networks.
- For the efficient utilization of the spectrum,
two technologies are required:
- A multiple access scheme and
- A duplexing system
- The
industry has already established CDMA as the best out of three available (the
other two are FDMA and TDMA).
- Next, a duplexing system remains to be selected.
- FDD
has been the most common choice till now.
- It makes use of different frequencies
for handling the outgoing and incoming signals.
- On the other hand, TDD operates
on a single frequency but uses different time slots.
- Using a TDD system
provides a benefit of alternating sending and receiving in a single frequency.
- Smart
codes ensure that the code–division duplexing system is not like TDD but much
better than that.
- An interference limited environment was generated by the
cellular systems using multiple frequencies in multiple cells.
- Later, cells
started using same frequency but still the environment was interference
limited.
- But the CDD system promises to change this environment in to one that
is noise limited.
