Linearity Challenges of LTE-Advanced Mobile Transmitters: Requirements and Potential Solutions

In order to provide higher data rates and to improve radio spectrum utilization, 3GPP has introduced the concept of CA in its Release 10 and onward commonly known as LTE-Advanced. The CA technology, particularly when applied in a noncontiguous manner, poses serious design and implementation challenges for radio transceivers, mainly due to the allowed flexibility in the transmitted signal characteristics and the nonlinear RF components in the TX and RX chains. As a consequence, substantial nonlinear distortion may occur that not only degrades the transmitted signal quality but can also affect the concurrent operation of the coexisting receiver when operating in the FDD mode. In this article, the key technical design challenges in terms of linearity requirements for LTE-Advanced mobile terminals are reviewed, and the corresponding self-interference problem related to the potential desensitization of the device's own receiver is highlighted. Then technical solutions to mitigate the self-interference at the RX band due to a nonlinear PA in the transmitter chain are reviewed, with specific emphasis on digital self-interference cancellation methods. As demonstrated through simulation and actual RF measurement examples, the cancellation solutions can substantially mitigate the RX desensitization problem, thus relaxing the RF isolation requirements between the TX and RX chains. Such cancellation methods are one potential enabling technique toward the full exploitation of the fragmented RF spectrum and the CA technology in future LTE-Advanced and beyond mobile networks.