Abstract
Opposite to the conventional wisdom, practical considerations for out-of-band emission (OOBE) suppression and adjacent channel interference (ACI) rejection in orthogonal frequency division multiplexing (OFDM) -based cognitive radio systems are not completely alike. For example, ACI will increase after discarding the cyclic prefix (CP) because the interference signal is also truncated during the CP removal process. Furthermore, effective OOBE is much larger than the estimated OOBE if there is a timing offset (TO) between the primary and secondary users. To the best of our knowledge, these and some other practical issues have not been well addressed. Various important practical issues (including multipath delay spread, high peak-to-average power ratio (PAPR), spectral efficiency loss, high complexity, sensitivities to frequency and timing offsets, non-contiguous and dynamically changing spectrum, CP removal, spectral containment, etc.) concerning OOBE suppression and ACI rejection are studied in this paper. Numerical results show that none of the existing approaches can deal with all practical issues satisfactorily. Hybrid combinations of frequency-domain approaches (spectral precoding and PAPR precoding) and time-domain approaches (windowing and filtering) are developed to maintain the merits and mitigate the drawbacks of each constituent approach in order to deal with these practical challenges. It is shown that a combination of filtering and precoding provides the best OOBE suppression and a combination of windowing and precoding provides the best ACI rejection characteristics in our numerical examples.
Highlights
Cognitive radio (CR), as a promising solution to the spectrum congestion problem brought by the rapid advance of wireless communication techniques and the drastic increase of wireless devices, has drawn a lot of attentions recently
Since filtering is less sensitive to timing offset (TO), a hybrid combination of precoding and filtering is more appropriate for out-of-band emission (OOBE) suppression for orthogonal frequency division multiplexing (OFDM) systems
For F-OFDM, to suppress the OOBE at the transmitter, the cyclic prefix (CP)-OFDM signal is convolved with a transmit filter, ftx(t), with a length of αtxTs, where αtx is the normalized transmit filter length, and to reject the adjacent channel interference (ACI) at the receiver, the received signal is convolved with a receive filter, ftx(t), with a length of αrxTs, where αrx is the normalized receive filter length
Summary
Cognitive radio (CR), as a promising solution to the spectrum congestion problem brought by the rapid advance of wireless communication techniques and the drastic increase of wireless devices, has drawn a lot of attentions recently (e.g., see [1,2,3]). For the frequency-domain example, spectral precoding approach is very efficient for reducing OOBE and can be extended to reduce PAPR simultaneously by combining with PAPR precoding [16] It suffers spectral efficiency loss and has high computational complexity when the number of subcarriers is large or when the spectrum is non-contiguous or dynamically changing. Hybrid approaches which combine the frequency-domain spectral coding and either one of the two time-domain approaches (windowing and filtering) are proposed to deal with various abovementioned. Detailed analyses are made on the complexity, spectral efficiency, spectral containments, PAPR and BER of the windowing, filtering, and spectral precoding approaches, and their possible combination schemes under practical conditions (broadband channels with radio impairments such as FO and TO). Ir represents an r by r identity matrix. a ⊗ b denotes the convolution of a and b
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