Digital predistortion for wideband wireless transmitters with carrier aggregation

Abstract

When the use of mobile devices is soaring, the demand for higher data rates over wireless networks is continuously increasing. To meet the demand, one of the obvious solutions is to increase the bandwidth of the transmission frequency band. However, no matter how much you are willing to pay for the spectrum, you can no longer easily obtain a large contiguous bandwidth because most spectrum bands have been assigned for different wireless applications. To resolve this issue, carrier aggregation (CA) technique has been proposed to combine multiple bands at different carriers to form a wide bandwidth in order to conduct high data-rate transmissions. This requires concurrent multi-band transmitters to be deployed in wireless systems. Similar to single band transmitters, distortion introduced by radio frequency (RF) power amplifiers (PAs) in multi-band transmitters not only reduces the quality of the signal but also affects users in the adjacent channels. Because of low cost and high accuracy, digital predistortion (DPD) is expected to be employed in the new systems to resolve the linearity issue. Because the application scenarios are very different, conventional DPDs that were mainly designed for single band transmitters cannot been directly employed and new approaches must be proposed to deal with concurrent multi-band transmitters with carrier aggregation. In this talk, DPD techniques will be discussed in three typical CA scenarios firstly, mainly based on the research work of University College Dublin (UCD), Ireland and Southeast University (SEU), China. In the first CA scenario (the intra-band contiguous CA), a novel technique, called band-limited DPD, has been proposed [1] to effectively realize the linearization for five-carrier 100 MHz long term evolution-advance (LTE-A) signal. This method is achieved by inserting a band-limiting function into the conventional Volterra series to accommodate for the scenarios with limited bandwidth. With the assistance of this technique, the sampling rate of analog-to-digital converter (ADC) and digital-to-analog converter (DAC) can be significantly reduced and the bandwidth for the transmitter and receiver chain can be also decreased to much lower level, while keeping similar linearization performance. Also, the same 100 MHz LTE-A signal can be linearized with the conventional piecewise dynamic deviation reduction-based Volterra series (DDR) by capturing all the distortion (400 MHz) with an advanced wideband DPD platform developed by SEU group [2]. For the second scenario (intra-band non-contiguous CA), since all the carriers are still located within the 100 MHz frequency range, the band-limited DPD is applicable to achieve great performance, as demonstrated in [1]. In the third scenario (inter-band non-contiguous CA), a novel multi-band DPD method, called single-model single-feedback DPD [3], will be introduced, based on the concept of employing only one model and one feedback loop in DPD operation. This technique can significantly reduce the DPD complexity for signal processing and also the number of feedback loop for data acquisition while still being capable of dealing with multiple bands. It is worth mentioning that this method provides more flexibility for system re-configurability, which has been validated to concurrently support up to four bands that is reported for the first time to date. Besides these three scenarios, some recent research work on DPD technique are also carried out in the context of millimeter-wave operation to accommodate with the forth-coming 5G wireless system. In summary, this talk has provided a set of solutions to successfully resolve the linearization issues for all three typical scenarios deployed in CA system, which will be further developed towards future 5G system architectures.