CCSS: Switched Capacitor Power Amplifiers for Digital MIMO Transmitters

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The desire for increases in data transmission rates to support hi-definition video content and increased availability of wireless has pushed data service providers to use more than one antenna for each communication link between a user's mobile device and the fixed point base station operated by the communications provider. This requires complex circuits and systems for beam-steering that can appropriately direct the signal from each antenna and form a communications beam that optimizes wireless data transmission. Current circuit architectures rely on highly linear transmitter and receiver circuitry that is inefficient in its energy usage. The goal of this research is to introduce new, highly energy efficient transmitter/amplifier architectures that are easily integrated with the beam-steering circuitry. Such circuits will enable longer battery lifetimes when used for mobile devices and will draw less energy from the electric grid due to increased efficiency. The theories and techniques that will be developed will enable engineers and researchers to design more efficient wireless transmission systems that operate with much higher data transmission rates. In addition to the scientific and engineering outcomes associated with the proposal, the educational objectives will seek to educate STEM students at the secondary education level and encourage them to enter into STEM fields and focus on topics such as using energy more efficiently. This will be accomplished by curriculum development, mentoring local secondary students and also incorporating undergraduate students into research at early stages in their post-secondary education.The objective of this proposal is to improve the resolution available in digital switched-capacitor power amplifiers (SCPAs). Increase in the resolution of these power amplifiers is necessary to enable their practical use in wireless systems, as the out-of-band noise, owing to signal quantization, limits their deployment in wireless systems. Presently SCPAs have been limited to amplitude quantization of less than eight bits. Not only does the SCPA need additional resolution to drive the out-of-band noise down to sufficient levels, additional amplitude control is needed to allow better focusing of the beam. Other digital power amplifiers have achieved higher resolution. However, they are not as linear; hence they must use digital pre-distortion for correction of the nonlinearity. The proposed research will address several of the current limitations in increasing the resolution of the SCPA using techniques that include: (1) Split-array techniques that allow higher resolution with practical capacitor sizing of capacitive elements in fine-line CMOS processes; (2) C-2C array techniques to improve the matching of the capacitors in the array that will improve the SCPA's linearity while reducing the need for on-chip hardware decoding; (3) Digital signal processing techniques such as dynamic range reduction and digital pre-distortion, that enhance the operation of the higher resolution arrays; and (4) Multiple-input, multiple-output (MIMO) system integration and validation. The designs and findings from the experiment can provide relevant guidelines for deployment of such digital power amplifiers in MIMO systems and further move the digital interface closer to the antenna, allowing promising solutions toward deployable software defined radios.