Digital Compensation Techniques for Transmitters in Wireless Communications Networks Terrestrial and Satellite Systems

Since they appeared, wireless technologies have deeply transformed our society. Today, wireless internet access and other wireless applications demand increasingly more traffic. However, the continuous traffic increase can be unbearable and requires rethinking and redesigning the wireless technologies in many different aspects. Aiming to respond to the increasing needs of wireless traffic, we are witnessing a rapidly evolving wireless technology scenario. This thesis addresses various aspects of the transmitters used in wireless communications. Transmitters present several hardware (HW) impairments that create distortions, polluting the radio spectrum and decreasing the achievable traffic in the network. Digital platforms are now flexible, robust and cheap enough to enable compensation of HW impairments at the digital base-band signal. This has been coined as ’dirty radio’. Dirty radio is expected in future transmitters where HW impairments may arise to reduce transmitter cost or to enhance power efficiency. This thesis covers the software (SW) compensation schemes of dirty radio developed for wireless transmitters. As described in the thesis, these schemes can be further enhanced with knowledge of the specific signal transmission or scenarios, e.g., developing cognitive digital compensation schemes. This can be valuable in today’s rapidly evolving scenarios where multiple signals may co-exist, sharing the resources at the same radio frequency (RF) front-end. In the first part, this thesis focuses on the instrumentation challenges and HW impairments encountered at the transmitter. A synthetic instrument (SI) that performs network analysis is designed to suit the instrumentation needs. Furthermore, how to perform nonlinear network analysis using the developed instrument is discussed. Two transmitter HW impairments are studied: the measurement noise and the load impedance mismatch at the transmitter, as is their coupling with the state-of-the-art digital compensation techniques. These two studied impairments are inherent to measurement systems and are expected in future wireless transmitters. In the second part, the thesis surveys the area of behavioral modeling and digital compensation techniques for wireless transmitters. Emphasis is placed on low computational complexity techniques. The low complexity is motivated by a predicted increase in the number of transmitters deployed in the network, from base stations (BS), access points and hand-held devices. A modeling methodology is developed that allows modeling transmitters to achieve both reduced computational complexity and low modeling error. Finally, the thesis discusses the emerging architectures of multi-channel transmitters and describes their digital compensation techniques. It revises the MIMO Volterra series formulation to address the general modeling problem and drafts possible solutions to tackle its dimensionality. In the framework of multi-channel transmitters, a technique to compensate nonlinear multi-carrier satellite transponders is presented. This technique is cognitive because it uses the frequency link planning and the pulse-shaping filters of the individual carriers. This technique shows enhanced compensation ability at reduced computational complexity compared to the state-of-the-art techniques and enables the efficient operation of satellite transponders.