Broadband DSP based feedforward amplifier lineariser

Abstract

The use of linear modulation and complex access schemes has increased the linearity requirements of the power amplifiers used in wireless communication systems. To maintain system capacity and avoid interference, external linearization circuitry must be added to correct the distortion created by the amplifier. Feedforward linearization is a technique commonly used in wireless base station applications, as it offers a wide bandwidth and excellent linearity correction. However, this linearization technique is expensive, complex, and very difficult to implement. This thesis presents a hybrid linearization technique that integrates DSP into the structure of the basic analog feedforward linearizer. This allows the implementation of equalizers to correct the frequency response of the components in the system. The novel linearizer maintains the excellent performance of feedforward, while overcoming many of its drawbacks. An experimental circuit is designed and tested to demonstrate the capabilities of the hybrid linearizer.

Steady-State Performance Analysis and Step-Size Selection for LMS-Adaptive Wideband Feedforward Power Amplifier Linearizer

“…The delay elements are removed from the model, implying that the upper and lower branches are assumed to be perfectly synchronized at subtraction points. Even though the effects of timing synchronization are not discussed in this paper, it is not less of an importance and some related discussion can be found in [8] and [24]. …”

“…Based on these, the optimum values of SCL and ECL coefficients and the corresponding linearizer output are restated below as (6) Based on the last term in expression (6), an IMD-free, linearized signal can be obtained by feedforward linearizer with well-adjusted coefficients. However, there are certain practical considerations in real world implementations causing imperfect linearization [8], [24], and [26]. These can mainly be categorized under error amplifier linearity demands as well as delay and coefficient mismatches of the loops.…”

“…The PA output in that case is given by (13) and the cross-correlation between the filter input and reference signal is (27) where the approximation is stemming from the second property in (24). The MMSE solution is then given by (28) The expression in (28) points out that in general there is a certain offset between the MSE-optimum filter coefficient and the optimum linearization coefficient in (18).…”

“…1, the idea in feedforward linearization is to regenerate the interfering IMD products using a reference signal branch, and subtract them out from the final radio frequency (RF) waveform. One of the practical problems, however, is that the above feedforward method is actually fairly sensitive to the misadjustment of the underlying component parameters as shown, e.g., in [8], [24], and [26]. Therefore, adaptive feedforward techniques have been developed [5], [7]- [9], [19] with online parameter monitoring, to have better IMD suppression performance under implementation inaccuracies or varying operating conditions.…”

DSP oriented implementation of a feedforward power amplifier linearizer

“…There have been some attempts reported to implement the feedforward linearizer, at least partially, in the digital domain. Randall et al [2] have implemented the generation of the baseband input signal v m , needed to generate the error signal v e , in the digital baseband. This eliminates the need of a splitter in the RF part and provides flexibility to manipulate the baseband input signal v m and the amplifier input separately.…”