Multiuser Spectral Precoding for OFDM-Based Cognitive Radio Systems

Abstract

Orthogonal frequency-division multiplexing (OFDM) is an ideal transmission technique for cognitive radio (CR) systems because of its flexible nature to support dynamic spectrum access. However, the out-of-band (OOB) radiation of OFDM signals from different CR users must be strictly controlled to protect licensed users operating in the adjacent frequency bands. In this paper, we propose a spectral precoding approach for multiple OFDM-based CR users to reduce OOB leakage and enhance spectrum compactness. By constructing individual precoders to render selected spectrum nulls, our approach suppresses the overall OOB radiation without sacrificing biterror rate performance of CR users. The proposed approach also ensures user independence thus with low encoding and decoding complexities. Furthermore, our approach can improve bandwidth efficiency by carefully selecting notched frequencies. As a comprehensive application of the proposed approach, two simplified multiuser spectral precoding schemes are provided to reduce the computational complexity. Simulation results demonstrate that our spectral precoding schemes effectively limit OOB radiation and enable efficient spectrum sharing.Index Terms-Cognitive radio, dynamic spectrum access, orthogonal frequency-division multiplexing, out-of-band radiation, spectral precoding.

OFDM-based overlay cognitive radios with improved spectral leakage suppression for future generation communications

“…Note that the sidelobe rolls off asymptotically as of f −2 for the conventional OFDM and of f −4 for the schemes in [21]- [23]. As shown in (4), in the Bohman window-based scheme the sidelobe rolls off asymptotically as of f −8 .…”

“…The pulse shape used in the OOB-reduction schemes [21]- [23] as g(t) is a rectangular window function which has a high highest sidelobe power of −13 dB and a low sidelobe fall rate of −6 dB/Octave among the well-known windows studied in the signal processing literature [24]. Such windows as the Hamming window, the minimum 4-sample Blackman-Harris window, and the Kaiser-Bessel window with α = 3.5 (where α is a factor discussed in [24]) have very low highest sidelobe power; however, the low sidelobe fall rate (−6 dB/Octave) makes these windows unsuitable for SUs in overlay CR.…”

“…Recently, a lowcomplexity spectrum shaping scheme was proposed based on antipodal symbol pair mapping [21], [22] on adjacent edge subcarriers to suppress spectral leakage of the SU in OFDM-based overlay CRs. This spectral precoding approach was extended for multiuser CRs [23] with enhanced spectrum compactness and efficient spectrum sharing in OFDM-based overlay CRs.…”

“…We propose a transmitter architecture for OFDM-based overlay CR to reduce the spectral leakage of SUs. By applying the Bohman window [24] based pulse shape with very high sidelobe fall rate and very low highest sidelobe power at the edge subcarriers of the utilized spectrum, the power spectral density (PSD) rolls off much faster than recently proposed OOB-reduction schemes [21]- [23]. Antipodal symbol pairs are used on the lower-and upperedge Bohman-windowed subcarriers to overcome the problem of a relatively large 3-dB bandwidth of the Bohman window.…”

Two Stage Precoding in Ofdm Based Cognitive Radio

“…In [11], we discuss spectral p recoding scheme. In these approaches, the informat ion symbols are mapped to a new set of precoded symbols under some constraints.…”