Concurrent Multi-Band Envelope Tracking Power Amplifiers for New Emerging Wireless Communications
Emerging wireless communication are shifting toward data-centric broadband services which has resulted in employment of sophisticated and spectrum efficient modulation and access techniques, yielding communication signals with large peak-to-average power ratio (PAPR) and stringent linearity requirements. For example, future wireless communication standards, such as long term evolution advanced (LTE-A) require adoption of carrier aggregation technique to improve the effective modulation bandwidth. The carrier aggregation technique in LTE-A incorporates multiple carrier over a wide frequency range to create a wider bandwidth of up to 100MHz. This will require future power amplifiers (PAs) and transmitters to efficiently amplify concurrent multi-band signals with large PAPR, while maintaining good linearity.
Different back-off efficiency enhancement techniques are available in literature, such as envelope tracking (ET) and Doherty. ET has gained a lot of attention recently as it can be applied to both base station and mobile transmitters. Unfortunately, few publications have investigated concurrent dual-band amplification using ET PAs, mainly due to the limited bandwidth of the envelope amplifier.
In this thesis, a novel approach to enable concurrent amplification of multiband signals using a single ET PA will be presented. This thesis begins by studying the sources of nonlinearities in single-band and dual-band PAs. Based on the analysis, a design methodology is proposed to reduce the sources of memory effects in single-band and dual-band PAs from the circuit design stage and improve their linearizability. Using the proposed design methodology a 45W GaN PA was designed. The PA was linearized using simple memoryless digital pre-distortion (DPD) with 8 and 28 coefficients when driven with single-band and dual-band signals, respectively. Hence, in practice, the PA can be linearized using simple power efficient linearization techniques, such as lookup table (LUT) or memoryless polynomial DPD. Note that the power dissipation of the linearization engine becomes crucial as we move toward small base station cells, such as femto- and pico-cells, where complicated DPD models cannot be implemented due to their large power overhead. This analysis is also very important when implementing a multi-band ET PA system, where the sources of memory effects in the PA itself are minimized through the proposed design methodology.
Next, the principle of concurrent dual-band ET operation using the low frequency component (LFC) of the envelope of the dual-band signal is presented. The proposed dual-band ET PA modulates the drain voltage of the PA using the LFC of the envelope of the dual-band signal. This will enable concurrent dual-band operation of the ET PA without posing extra bandwidth requirement on the envelope amplifier. A detailed efficiency and linearity analysis of the dual-band ET PA is also presented. Furthermore, a new dual-band DPD model with supply dependency is proposed in this thesis, capable of capturing and compensating for the sources of distortions in the dual-band ET PA. To the best of our knowledge, concurrent dual-band operation of ET PAs using the LFC of the envelope of the dual-band signal is presented for the first time in literature. The proposed dual-band ET operation is validated using the measurement results of several GaN ET PAs.
Lastly, the principle of concurrent dual-band ET operation is extended to multi-band signals using the LFC of the envelope of the multi-band signal. The proposed multi-band ET operation is validated using the measurement results of a tri-band ET PA. To the best of our knowledge, this is the first reported tri-band ET PA in literature. The tri-band ET PA is linearized using a new tri-band DPD model with supply dependency, presented in this thesis.