System-level Design Considerations for Digital Predistortion of Wireless Base Station Transmitters
The use of digital predistortion (DPD) techniques for the linearization base station transmitter (BTS) power amplifiers is now commonplace in cellular wireless infrastructure. Digital predistortion is a classic example an adaptive control system, in which we are controlling the output of the plant – the power amplifier – using algorithms implemented in the digital domain, often using an FPGA or custom digital IC. It is a mixed-domain control system: the signal we wish to control is in an RF signal, and the controller is in digital hardware. Conversion of the RF signal to a digital signal, and back again, must be achieved without introducing further distortions and limitations. This places strict requirements on the data converters, frequency translation components, filters, and amplifiers that comprise the hardware aspect of the DPD system. Such considerations include the analog nonlinearity, IQ imbalance, and frequency response, and digital impairments such as jitter, thermal noise, and dynamic range.