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Monday, March 11, 2013

What is meant by Code Division Duplexing?

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:
  1. Frequency division duplexing or FDD and
  2. 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:
  1. Adjacent base – to – home mobile interference and
  2. 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:
  1. A multiple access scheme and
  2. 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. 

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