Recently, the Dynamic Spectrum Access (DSA) techniques are proposed to solve the problem of spectrum scarcity and help to use the limited spectrum resource as effectively as possible. The current ongoing spectrum reform opens up the possibilities to exploit the DSA techniques. This paper aims to provide a critical review on the various ongoing efforts towards the use of DSA concept for the frequency management of future wireless communications systems, especially from the Cognitive Radio (CR) perspective. The CR aims for an efficient and dynamic access to the spectrum, and provides a new method of spectrum management. This paper also highlights the various challenges associated with CR in order to realize the concept of DSA.

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A novel compact ultrawide band (UWB), bandpass filter (BPF) with reconfigurable and embedded notches at WiMAX (3.5 GHz) and WLAN (5.2 GHz) is proposed. UWB BPF is embedded with a In this article, we present a compact UWB BPF which has high selectivity filtering characteristics and relatively small size in which narrow rejection bands are obtained using SIR and a slit ring resonator using switches to make the filter more usable. FILTER DESIGNAnalysis is performed for rectangular resonator. The substrate dielectric constant (e r 5 10.2) and height (h 50.64 mm) of substrate from ground plane is used. Figure 1 show the proposed third order UWB filter for bandpass operation with stopbands in its passband. The microstrip line with length L 5 37.0 mm (kg/ 2) and width of 0.596 mm is folded inward and placed in between kg/4 shot circuited stubs and is connected through a via to finite ground plane to obtain the UWB bandpass nature from 3.1-10.6 GHz with a midband frequency of 6.85GHz. An open loop slit ring resonator of length kg/2 is placed inside the folded rectangular resonator such that both the resonators couple capacitive making no difference in the passband of the filter, when they are connected through a finite conducting patch which acts as a switch then the open loop slit ring resonator acts as an inductor and produces zeros at the desired frequencies and we obtain a stopband (Notch) at 5.2 GHz (WLAN). And a three section SIR is placed in symmetry plane to rectangular resonator to obtain sharp rejection at desired 3.5 GHz (WiMAX) band of frequency. The patches placed in between the three section resonator models a switching element which can be MEMS switch PIN diode, or an optical switch in our case it is a finite conducting patch which in ON state acts as a conductor and notch is inserted and in OFF state acts a insulator and removes the notch. The filter with Z 0 5 50 X is excited by ports 1 and 2.The coupling coefficient between rectangular resonator and the split ring resonator is given bySIR consists of two different characteristic impedances Z 1 and Z 2 and of electrical lengths h 1 and h 2 , and the fundamental resonance occurs at where R z is the ratio of characteristic impedance Z 1 to Z 2 and h 0 is the electrical length for the fundamental frequency. The first spurious resonance occurs at tan h s1 51where hs1is the electrical length for the first spurious frequency at f s1 , therefore, from the above equations we obtain thatThe spurious response of the notch can be controlled by the characteristic impedance ratio Rz. FILTER FABRICATION AND RESULTSThe third order reconfigurable notch band BPF is designed and fabricated on a Rogers substrate (RO3010) with a thickness of 0.635 mm, relative dielectric constant of 10.2, tan d 5 0.0023, and the metallization thickness of 0.017 mm and simulated using HFSS13. The size of the overall circuit is 9.3 3 16 mm 2 the fabricated model is as shown in Figure 3 filter was fabricated using standard photolithography. The simulation result for the filter with ...

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