Research on propulsion algorithm based on digital pre-distortion platform

To meet the linearity and spectrum requirements of the air interface, the input signal level to the power amplifier is reduced so that it operates in the linear portion of the transfer curve, but this results in poor power efficiency. Although this approach is simple, it increases system cost; to achieve the required power output, a larger and more expensive PA must be used. In a typical 3G mobile base transceiver station (BTS), the transmission efficiency is less than 10%, which means that more than 90% of the DC power is converted into heat and is not used.

1 Digital Pre-Distortion Technology

A more cost-effective solution to this dilemma is to use a cleverly designed DSP. Digital predistortion (DPD) is a technique that predistorts the transmit signal to meet spectral requirements while effectively linearizing the PA transistors operating in the high-efficiency saturation region. DPD requires an observation receiver where a coupled version of the PA output is downconverted by a high-bandwidth ADC. The digital version of the transmit waveform is compared to the receive waveform, and an adaptive algorithm calculates or updates a series of parameters to preload the next transmit waveform. When the adaptive algorithm converges, the transmitter output is linearized even when the PA is operating in a highly nonlinear portion of the transfer function. DPD can increase transmitter efficiency from less than 10% to more than 35%, depending on the algorithm and power amplifier topology used.

Radio system design that includes complex closed-loop algorithms such as DPD cannot be done in isolation. Modeling the analog behavior of the signal chain and the electrical and thermal memory effects of the PA is also not a trivial task. The number of distortion mechanisms increases rapidly with the order of nonlinearity, which means that the input drive level of the PA can significantly change the distortion behavior. A complete closed-loop estimation platform is invaluable in optimizing the DPD algorithm for a given PA.

Analog Devices has developed a 3G/4G compatible transmit radio platform that enables designers of wireless infrastructure equipment to estimate closed-loop performance results using power amplifiers and digital predistortion techniques. This mixed-signal digital predistortion platform (MSDPD), shown in Figure 1, combines high-performance linear and mixed-signal components into an advanced transmitter and DPD observation receiver.

Figure 1: Mixed Signal Digital Predistortion (MSDPD) Development Board

2 FPGA Advantages of DPD Platform

Many DPD users today use solutions based on either fixed-function ASICs or FPGAs. FPGAs are programmable, giving users the flexibility to optimize their solutions and adapt to future developments in data converter and power transistor technology. Fixed-function ASICs do not allow designers to easily change algorithms or support different versions of standards. The benefits of programmable devices are faster time to market, flexible and cost-effective adaptation to new and evolving standards without the need to redesign like ASICs.

With the advancement of FPGA technology, the entire radio modem can now be implemented using a single FPGA device, supporting multiple standards and multiple antennas, thus eliminating many signal processing and connection ICs, reducing circuit board space and BOM costs. In addition, this level of integration brings the industry one step closer to software-defined radio (SDR), helping equipment manufacturers to quickly respond to the needs of network providers.

The MSDPD development platform is the only solution on the market that provides FPGA functionality to designers of wireless infrastructure equipment. The MSDPD board seamlessly interfaces with a variety of FPGA development kits: Altera Stratix IV through the HSMC interface, and Xilinx Virtex 6 through the FMC interface. Directly interfacing with the FPGA provides designers with an instant and convenient framework to quickly evaluate third-party DPD algorithms, or to design and optimize their own algorithms in a closed-loop environment by simply reprogramming the FPGA.