Cognitive radios aim at exploiting the scarcely available radio spectrum in a smart flexible way. Traditional TV bands between 50 and 900MHz are currently being freed for new applications. New licensed users are planned (e.g. DVB-H), but in addition new ideas for more flexible use of the spectrum are explored [1]. For higher frequencies similar ideas are developed. In general, regulatory organizations seem to move in the direction of providing more freedom to new standards, where only a minimum set of requirements are enforced. E.g. regulations might allow to exploit white spectrum, where “Detect And Avoid” rules are defined (e.g. response times, maximum interference levels to incumbent services). This will lead to new radio systems with different requirements on the radio software and hardware. In this chapter we will mainly focus on the impact of cognitive radio system requirements on the physical layer (PHY), and especially the radio frequency hardware. Flexible multi-phase clocking will turn out to play a crucial role, and will be discussed in detail.
To allow for flexible spectrum access, a flexible radio hardware platform is desired, allowing for flexible choice of the radio frequency depending on free available spectrum. Traditional radio hardware is primarily optimized for cost and low power, but not for flexibility. Low power is often achieved using inductors and capacitors in resonating circuits with a high quality factor, dissipating only a fraction of the maximum energy stored in the reactive components. However, such circuits only work effectively in a narrow band around their resonance frequency, and are hence application specific for a certain band. Micro-Electrical-Mechanical system (MEMs) technology may help to relax this problem; however for reasons of cost and form factor fully integrated solutions in mainstream CMOS technology are preferred if feasible. Thus we focus in this chapter on CMOS circuits and IC architectures. We will analyze the desired functionality of the radio interface for dynamic spectrum access, and look at some feasibility bottlenecks induced by CMOS circuit properties, like timing jitter, nonlinearity and time-variance. Some possible solution directions are reviewed, especially a recently proposed polyphase multipath technique. This technique allows for realizing a highly flexible radio transmitter for the DC-2.4GHz range on a CMOS chip without dedicated filters. It requires multiphase clocks for which the phase-accuracy is critical. Two competing techniques to realize such clocks, one based on a Shift Register (SR) and the other on a Delay Locked Loop (DLL), are discussed in the second half of this chapter, to show that SR-based clocking has fundamental advantages.