## 2 Summary of the Paper
### Challenges of the Wideband Front End  
The main components of a cognitive radio transceiver are the radio front-end and the baseband processing unit. Each component can be reconfigured via a control bus to adapt to the time-varying RFenvironment. In the RF front-end, the received signal is amplified, mixed and A/D converted. In the baseband processing unit, the signal is modulated/demodulated and encoded/decoded. The baseband processing unit of a cognitive radio is essentially similar to existing transceivers. However, the novelty of the cognitive radio is the RF front-end. So we will first focus on the RF front-end of the cognitive radios [40].  
For wideband (multi-GHz) cognitive radio, there may be interfering signals that are much stronger than the CR signal of interest, resulting in signal to interference ratios as low as \(-50\) dB. This requires a large dynamic range for the front-end circuitry and in particular for the ADC which must accommodate the large interfering signals while still provides sufficient quantization performance for the weak CR signal [18]. In typical cognitive radio sensing scenario, the RF signal presented at the antenna includes signals from closely and widely separated transmitters, and from transmitters operating at widely different power levels and channel bandwidths. As a result, the large dynamic range becomes the main challenge as it sets the stringent requirements on circuit linearity and resolution of A/D converters [11; 41].  
Generally, a wideband front-end architecture of the cognitive radio has the following structure (Figure 3(b)). The components of a cognitive radio RF front-end are as follows: (1) RF filter: the RF filter selects the desired band by bandpass filtering the received RF signal. (2) Low noise amplifier (LNA): the LNA amplifies the desired signal while simultaneously minimizing noise component. The LNA should have minimal noise figure (e.g., 2-3 dB) in order to have good sensitivity at low power. (3) Mixer: in the mixer, the received signal is mixed with locally generated RF frequency and converted to the baseband or the intermediate frequency (IF). The mixers have to maintain the linearity across entire dynamic range and bandwidth. However, these are also conflicting requirement with respect to power consumption. (4) Voltage-controlled oscillator (VCO) and phase locked loop (PLL): the VCO and PLL need to quickly generate a signal at a specific frequency with fine resolution. This is a key challenge for wideband spectrum sensing. (5) Channel selection filter: The channel selection filter is used to select the desired channel and to reject the adjacent channels. (6) Automatic gain control (AGC): The AGC maintains the gain or output power level of an amplifier constant over a wide range of input signal levels.  
Furthermore, large dynamic range and sampling of wideband signals further require high precision and high speed A/D converters. Unfortunately, the design of high speed A/D converters has fundamental limits in terms of achievable resolution. The requirement of a multi-GHz speed A/D converter necessitates the dynamic range of the signal to be reduced before A/D conversion.  
In summary, the key challenge of the physical architecture of the cognitive radio is an accurate detection of weak signals of licensed users over a wide spectrum range. Hence, the implementation of RF wideband front-end and A/D converter are critical issues that will be addressed in this paper.