Energy Efficient and Intrinsically Linear Digital Polar Transmitters

One of the biggest challenges in modern transmitter (TX) design, when going from the fourth generation (4G) to fifth generation (5G) communications network, is to handle the increased linearity requirements without introducing any compromise in the energy efficiency of the TX line-up. In analog systems, high quality for the TX signal can be only achieved when using very linear operation of the (analog) power amplifier (PA). This severely limits the achievable efficiency in practical TX line-ups. Alternatively, a nonlinear PA can be used, which is linearized by digital pre-distortion (DPD) circuitry. This later approach is commonly used in (4G) macro-cell base stations, but it comes at the cost of increased system complexity and high supply power for the advanced DPD unit. When going towards 5G handset, or massive -multiple - input -multiple - output (mMIMO) 5G base station units, that facilitate beamforming and higher data rates to their end users. The required RF output power per individual transmitter is rather low (at most only a few watts). However, since many more transmitters are used in 5G applications (e.g. a factor 64 x to 256 x more than in 4G base stations) the use of an advanced DPD units in each individual TX-lineup, with their related high-power consumption becomes simply impractical. Consequently, to address these changing needs, it is highly desirable to find new circuit-level TX solutions, that overcome the traditional linearity-efficiency trade-off. To achieve this goal, this PhD work is focused on the utilization and tailoring of digital device operation, as facilitated by advanced CMOS technologies, towards the needs of modern wireless applications with their wideband complex modulated TX signals. The circuit techniques developed within this thesis, target an inherently linear amplitude-code-word (ACW) to TX output signal transfer, as such omitting completely the need for a power hungry advanced DPD unit, or alternatively, rely on a much more simple and consequently less power hungry DPD unit for the most demanding applications (e.g. when handling large modulation bandwidths). The circuit techniques developed in thesis, allow excellent drain and TX line-up efficiency, while being compatible with wideband efficiency enhancement techniques like Doherty. The proposed circuit techniques are also able to correct for process, voltage, load and temperature variations of the application.