Reduction of out of band radiation in ofdm system, Computer Networking

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THE high spectral efficiency makes OFDM a suitable technology to meet the demands of wireless data traffic. A currently discussed new application are OFDM based overlay systems. In this concept, an overlay system shares the same frequency band with a licensed system by filling existing frequency gaps with OFDM subcarriers. However, OFDM suffers from high out-of-band radiation caused by sidelobes of the modulated subcarriers. Well-known techniques for sidelobe suppression are windowing the transmission (Tx) signal in time domain and the insertion of guard bands. With the former method, the symbol duration is prolonged, whereas with the latter the scarce spectral resources are wasted. Both methods reduce the system throughput and, in general, do not achieve sufficient sidelobe suppression for OFDM based overlay systems. To overcome the drawbacks of the existing techniques for sidelobe suppression we propose the concept of cancellation carriers. The basic idea is to insert a few well-defined subcarriers, referred to as cancellation carriers (CCs), at both sides of the spectrum which are weighted such that their sidelobes cancel the sidelobes of the Tx signal. In, a similar concept based on the insertion of dummy tones has been proposed for amplitude shift-keying modulation in wire line applications. However, these dummy tones are restricted to carry weighted sums of the data on other subcarriers.

 


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