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Wireless communication has emerged as the most promising and one of the leading sectors of the telecommunications industry in the 21st century. To appreciate the expansion of the wireless sector, in 1990, there were only ten million cellular phone subscribers worldwide that frequently use first-generation technology. Today, there are more than one billion subscribers, and it is expected to increase to more than three billion subscribers during 2007-2010. Attempting to more efficiently use the limited frequency bands available for mobile communications, the idea of overlaying a code-division multiple-access (CDMA) system on an existing narrowband system has been proposed and explored. The basic idea of a CDMA overlay is to allow wideband CDMA waveforms to share a common spectrum with narrowband conventional waveforms to take advantage of the unexploited bandwidth available in the narrowband systems. Theoretically, the separation of CDMA and narrowband waveforms can be accomplished through notch filters. However, the filtering techniques are not sufficient for eliminating the multicross interference problem. In this paper, as a means of reducing the interference that cannot be eliminated by the use of filters, we have introduced a new proposal of an overlaid system in which we reduce the coverage area of the time-division multiple-access (TDMA) interference slots (slots inside the CDMA bandwidth are henceforth cited as cochannel slots). With this new strategy, we significantly reduce the effect of TDMA interference over the CDMA system.
Date of Publication: March 2008