RF Frontend for Spectrum Analysis in Cognitive Radio

Abstract

Advances in wireless technology have sparked a plethora of mobile communication standards to support a variety of applications. FCC predicts a looming crisis due to the exponentially growing demand for spectrum and it recommends to increase the efficiency of spectrum utilization. Cognitive Radio (CR) is envisioned as a radio technology which detects and exploits empty spectrum to improve the quality of communication. Spectrum analyzer for detecting spectrum holes is a key component required for implementing cognitive radio. Mitola's vision of using an RF Analog-to-Digital (ADC) to digitize the entire spectrum is not yet a reality. The traditional spectrum analysis technique based on a RF Front end using an LO Sweep is too slow, making it unsuitable to track fast hopping signals. In this work, we demonstrate an RF Frontend that can simplify the ADC's requirement by splitting the input spectrum into multiple channels. It avoids the problem of PLL settling by incorporating LO synthesis within the signal path using a concept called Iterative Down Converter. An example 0.75GHz-11.25GHz RF Channelizer is designed in 65nm Standard CMOS Process. The channelizer splits the input spectrum (10.5GHz bandwidth) into seven channels (each of bandwidth 1.5GHz). The channelizer shows the ability to rapidly switch from one channel to another (within a few ns) as well as down-converting multiple channels simultaneously (concurrency). The channelizer achieves a dynamic range of 54dB for a bandwidth of 10.5GHz, while consuming 540mW of power. Harmonic rejection mixer plays a key role in a broadband receiver. A novel order scalable harmonic rejection mixer architecture is described in this research. A proof-of-principle prototype has been designed and fabricated in a 45nm SOI technology. Experimental results demonstrate an operation range of 0.5GHz to 1.5GHz for the LO frequency while offering harmonic rejection better than 55dB for the 3rd harmonic and 58dB for the 5th harmonic across LO frequencies. While cognitive radio solves the spectrum efficiency problem in frequency domain, the electronic beam steering provides a spatial domain solution. Electronic beam forming using phased arrays have been claimed to improve spectrum efficiency by serving more number of users for a given bandwidth. A LO path phase-shifter with frequency-doubling is demonstrated for WiMAX applications