## 1 Introduction  
Being capable of gathering information from the environment with simple processing, low complexity, and low power consumption, wireless sensor networks (WSN) have attracted an increasing amount of attention and have seen rapid development, especially in micro-electro-mechanical systems (MEMS) technology [1]. Generally, since the Industrial Scientific Medical (ISM) band, where WSNs operate, is shared with various kinds of successful communication technologies [2], the detection performance in a complex electro-magnetic environment is affected by false alarms or missed detection, and inevitably degrades. Instead of the traditional fixed-spectrum arrangement method, cognitive radio (CR) has the ability to sense underutilized or unutilized bands [3] to increase the spectrum efficiency to some extent. Hence, CR-WSN is supposed to be a promising technique, taking into account both spectrum sensing accuracy and efficiency [4; 5; 6]. At present, most studies regarding CR-WSN in published papers focus on the development of sensing techniques [7; 8; 9; 10; 11; 12; 13]. The literature [7] provides a spectrum-sensing technique based on the autocorrelation of received samples and decides whether signals exist by means of a derived Euclidean distance. In Reference [8], a spectrum sensor scheduling algorithm and a data sensor resource allocation algorithm are proposed for a heterogeneous cognitive radio sensor network (HCRSN) to achieve the sustainability of spectrum sensors and the conservation of data sensors. Except for the development on improving spectrum sensing performance, the sensing performance evaluation of CR networks is studied in Reference [9]. By contrast, the limitation of radios, as another essential reason causing vacant spectrum inaccessibility and deteriorated sensing performance, should be discussed further. Normally, the broadband RF front-end of the sensor in CRs offers the secondary users (SUs) [14] unoccupied spectrum opportunistically and, in the meantime, protects licensed primary users (PUs) from interference. If there is vacant spectrum available for SUs during a certain period, the sub-band of interest can be used for signal reception by a flexible change of the filter in the digital domain. However, strong signals from PUs in adjacent channels may generate unwanted intermodulation to interfere with SUs' signals or occupy vacant spectrum. In this case, detection mechanisms in a CR-WSN are very likely to make the wrong judgment on the service condition of the spectrum, resulting in spectrum efficiency degradation. Therefore, for the sake of facilitating the development of CR-WSN, a broadband RF front-end with sufficient spurious-free dynamic range (SFDR) is an important guarantee. Distortions, namely the RF front-end impairments, contain nonlinear distortion, in-phase/quadrature (I/Q) imbalance, in-band group delay distortion, carrier frequency offset, and phase noise [15]. I/Q imbalance usually takes place at the direct-conversion receiver (DCR) whose I/Q mixer fails to provide an exactly 90-degree phase shift as desired. This can be avoided through digital down-conversion (DDC) after the analog to digital converter (ADC). Additionally, de-noise methods [16; 17] have been widely studied and have achieved a certain degree of noise reduction effect. Loosely speaking, compared with other kinds of distortions in the RF front-end, nonlinear distortion is more difficult to mitigate [18], especially the spectral hyperplasia generated by strong signals inside or outside the sub-band of interest which occupies free bandwidth or hides weaker target signals away. In Reference [19], circuits were designed to receive the I/Q signal and calculate its different kinds of nonlinear distortions to compensate for the distorted baseband signals. In Reference [20], intermodulation products were generated by a tunable nonlinear circuit and depressed nonlinearities in the original signals after adaptive filtering.