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.
Additional Advantages 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. 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 differences in TDD and CDD for cellular systems can be shown in Fig. 8. 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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