The rapid development in the area of cognitive radio technology leads the society to higher standards of spectrum sensing performance, particularly in low signal-to-noise ratio (SNR) environment. This article proposes an adaptive double-threshold energy sensing method based on Markov model (ADEMM). When using the double-threshold energy sensing method, the modified Markov model that accounts for the time varying characteristic of the channel occupancy was presented to resolve the ‘confused’ channel state. Furthermore, in order to overcome the effect of noise uncertainty, the findings of this article introduce an adaptive double-threshold spectrum sensing method that adjusts its thresholds according to the achievable maximal detection probability. Numerical simulations show that the proposed ADEMM achieves better detection performance than the conventional double-threshold energy sensing schemes, especially in very low SNR region.

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To generate linearly chirped microwave signals with a large frequency tunable range, a photonic approach is proposed. Firstly, A dual-output dual-parallel Mach-Zehnder modulator (DPMZM) followed by the polarization beam combiner and an optical filter is utilized to generate orthogonally polarized ± second-order optical sidebands. Then a polarization modulator is employed to achieve the phase modulation of the two wavelengths. Finally, the balanced detection is applied to suppress the distortion and background noise. The key advantages of the proposed scheme are the central frequency multiplying operation and large frequency tunable range. Simulation results show that a linearly chirped pulse product with time-bandwidth as well as a compression ratio for the pulse of 11 and 9.3 respectively, and a peak-to-sidelobe ratio (PSR) of 7.4 dB is generated. The system has both good reconfigurability and tunability, its frequency can be continuously adjusted from about 10 GHz to as much as 50 GHz in principle.

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