IEEE TRANSACTIONS ON BROADCASTING, VOL. 53, NO. 1, MARCH 2007 127
Analysis on LUT Based Predistortion Method for HPA with Memory
Bo Ai, Member, IEEE, Zhi-xing Yang, Chang-yong Pan, Shi-gang Tang, and Tao-tao Zhang
Abstract—Most existing methods for HPA predistortion with
memory effects are based on polynomial or indirect learning
structures. Few papers concern about the memory predistortion
with LUT based techniques. Based on the previously presented
method, we propose an improved two-dimensional LUT indexing
method for HPA predistortion with memory. Computer simula-
tions and corresponding analysis demonstrate that the proposed
method has more effective predistortion capabilities but lower
computational complexity.
Index Terms—HPA, LUT, memory effects, predistortion.
I. INTRODUCTION
I
N WIRELESS communication systems, the operation of
high power amplifiers (HPAs) in a more effective way will
introduce serious in-band and out-of-band (spectral re-growth)
distortions, resulting in the BER and adjacent channel interfer-
ence (ACI) performance degradation [1]. In most of previous
related research works, HPAs are assumed to be memoryless.
With the development of digital TV and 3G mobile systems, the
influence of the HPAs memory effects on system performance
can no longer be ignored for the wide-band transmitted signals.
Detailed analysis about the inherent electrical or electro-thermal
causes of memory effects has been carried out in [2]. As will
be shown in later sections, memoryless predistortion for HPAs
with memory yields poor linearization performance.
As for the memory predistorter, most literatures have dealt
with the memory predistortion by using memory polynomials
[3], [4], the indirect learning architecture [5], envelope filtering
and envelope injection [6] etc. Few papers, however, employ
the look-up table (LUT) based method to compensate for HPA
nonlinearities with memory. Based on the method presented in
[7], an improved two-dimensional indexing method for LUT
is proposed in this paper. Computer simulations and analysis
demonstrate its excellent performances in terms of storage units
and computational complexity.
The HPA models are introduced in Section II and in
Section III, the simple description of the method in [7] and
our improved indexing method for LUT are presented. Com-
puter simulations and the corresponding analysis are given in
Section IV with some conclusions drawn in Section V.
Manuscript received August 17, 2006; revised September 18, 2006. This
work was supported by the Postdoctoral Foundation of China under Grant
2005038094 and in part by the National Natural Science Funds in China under
Grant 60332030.
B. Ai is with the Department of E&E, Tsinghua University, State Key Labo-
ratory on Microwave and Digital Communications, Beijing 100084 China. He is
also with the Engineering College of Armed Police Force, Xi’an 710086, China
(e-mail: abeffort_apple@yahoo.com.cn).
Z. Yang, C. Pan, S. Tang, and T. Zhang are with the Department of E&E,
Tsinghua University, State Key Laboratory on Microwave and Digital Commu-
nications, Beijing 100084, China.
Digital Object Identifier 10.1109/TBC.2006.885872
II. HPA M
ODELS
Various HPA models have been developed for theoretical
analysis in the past decades. They can be generally classified
into two categories: memoryless and memory HPA models.
Saleh [8] and Rapp model [9] are the typical memoryless
HPA model for Traveling Wave Tube Amplifier (TWTA) and
Solid State Power Amplifier (SSPA), respectively. Some HPA
models considering memory effects have been established: the
Wiener model [10], the parallel Wiener model [11], the Volterra
polynomial model [4], and other behavioral models for HPAs
are mentioned in [12]. As is claimed in [3]: in applications that
we are interested in, predistortion linearization is the ultimate
objective, whereas accurate PA modeling is only a secondary
concern. We, therefore, take the following memory polynomial
model for simulations and analysis in this paper [3]:
(1)
where
and represent the input and output signal of
PA, respectively. The odd polynomial order is denoted by
and denotes the HPA memory depth. The coefficients are
extracted from an actual Class AB power amplifier.
(2)
The AM/AM and AM/PM transfer characteristic simulation
curves derived from the above HPA model are presented in
Fig. 1.
Fig. 1 shows that both the AM/AM and AM/PM transfer
characteristics are no longer a single point-to-point curve but
a bundle of stochastic discrete points due to HPA memory ef-
fects.
III. T
HE IMPROVED TWO-DIMENSIONAL
LUT INDEXING
METHOD
Some typical predistortion methods for HPA with memory
have been developed with both advantages and disadvantages.
For example, the Volterra polynomial predistortion technique
yields high computational complexity due to a high-order
kernel. The truncated Volterra serials, though, is considered
a good means to resolve the computational complexity, it
suffers from the difficulty of real-time implementation. In
[14], imperfections of some other HPA predistortion methods
have been analysed in detail. The LUT based method has high
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