Signal Processing for Energy-Efficient Burst-Mode RF Transmitters

Modern wireless communication systems utilize complex modulated signals such as OFDM signals to achieve increased data rates and spectral efficiency. These signals are characterized by a high peak-to-average-power ratio (PAPR). Thus, highly linear transmitters are required to provide sufficient transmission signal linearity. Conventional linear PAs, such as Class A or Class AB, produce high efficiency only near or at the peak output power region. As a result, the average efficiency is quite low for high PAPR signals. For non-portable devices such as base stations or mobile devices like mobile phones, low PA efficiency means higher heat dissipation which is often a design criterion. In addition, in mobile devices, a direct consequence of the low PA efficiency is the reduced battery lifetime, especially when the mobile device is required to operate at quite different output power levels. This thesis addresses the challenge of the efficiency enhancement while maintaining the linearity of wireless transmitters for signals with high PAPRs and output powers over a large range of power levels from the signal processing viewpoint. In the first part of the thesis, the burst-mode RF transmitter that employs pulse-width modulation (PWM) is introduced as an efficiency enhancement transmitter architecture. The PWM is analyzed in detail, where mathematical descriptions of PWM signals of different kinds are provided to explore the possibility of signal processing in burst-mode RF transmitters. In the second part of the thesis, the analysis of continuous-time PWM based burst-mode RF transmitters is carried out. The image problem is addressed when the baseband PWM signal is upconverted to the passband, and image distortion mitigation methods are provided. Furthermore, according to the presented power efficiency analysis, an efficiency optimization procedure is proposed for multilevel burst-mode RF transmitters. By using the efficiency optimization method, the maximum average transmitter efficiency can be achieved by proper choice of the threshold values of the input magnitude according to its probability density function (PDF). In the third and final part of the thesis, the investigation of discrete-time PWM based burst-mode RF transmitters is performed. Discrete-time PWM signals inherently suffer from aliasing distortion due to the sampling of non-bandlimited baseband PWM signals, which degrades the signal quality. To deal with the distortion, the aliasing-free PWM (AFPWM) method is described which allows avoiding the destructive aliasing distortion by limiting the number of harmonics in the generated PWM signals. The power efficiency analysis shows that although the AFPWM method imposes amplitude variation onto the time-domain signal which reduces the PA efficiency, the higher coding efficiency compensates for the RF PA efficiency degradation. An important result obtained is that a good efficiency-linearity trade-off can be achieved for AFPWM based burst-mode RF transmitters by selecting a proper number of harmonics in the generated PWM signals.