Integrated Circuits for Wireless Communication Systems
Project Member: Charles Wu
The goal of this research is to develop and implement an RF receiver architecture that are amenable to integration in standard digital CMOS process. Specifically, in this project, our focus will be in processing an RF signal using a discrete time ΣΔ modulator. The RF signal is first downcoverted using a current-commutating mixer with a single capacitor as the output load. This capacitor forms the first stage of a two stage passive switched capacitor filter that makes up the ΣΔ modulator loop filter. The switched-capacitor filter is run at radio frequencies which gives rise to a large oversampling ratio. Availability of very good switches is one of the advantages of scaling, as for a given on-resistance; the parasitic capacitance of MOS switch becomes increasingly smaller.
This receiver architecture also provides a very good platform to implement an interference cancellation scheme. First, the receiver actually captures the entire signal up to the sampling frequency, along with the desired RF signal. This means that large, potentially blocking signals are also available at the digital output, although with compromised signal-to-noise ratio. Furthermore, by default the ΣΔ modulator has a feedback path which can be placed very close to the antenna. A digital signal processor can then be used to synthesize a cancelling signal for large interferers based on the receiver digital output. This scheme would significantly reduce the dynamic-range requirement needed for the receiver.
Project Member: Sharon Xiao
As wireless usage grows exponentially in the coming decades, cognitive radios have been proposed as a promising solution to the problem of spectrum scarcity. The proposal allows for spectrum re-use, letting unlicensed users operate on licensed bands when their primary users are absent. In such a system, spectrum sensing is necessary to identify idle bands as well as to rapidly detect the return of a primary user. In order to assure non-interference with primary user activities, a cognitive radio system must be able to reliably detect extremely weak signals down to the low and negative SNR regimes. Furthermore, spectrum sensing must be implemented with minimal power and complexity overhead to be realizable in mobile applications.
This research explores techniques for robust and low power spectrum sensing, initially focusing on the UHF DTV bands which have been approved for cognitive radio operations by the FCC. For minimum power, we propose an analog system using sub-Nyquist equivalent-time sampling and simple energy detection. To meet sensitivity requirements for weak signals, we seek out the DTV pilot tone, a narrowband feature with an SNR improvement over overall channel SNR. We can further more robustly extract the pilot from noise by adding a second, time-delayed receive path and autocorrelating the pilot.