IEEE SIGNAL PROCESSING LETTERS, VOL. 26, NO. 10, OCTOBER 2019 1531
Minimax Design of Constant Modulus MIMO
Waveforms for Active Sensing
Zhen Lin , Wenqiang Pu, and Zhi-Quan Luo , Fellow, IEEE
Abstract—Waveform optimization is a crucial step in the design
of a multiple-input multiple-output system. This letter considers
the joint optimization of constant modulus waveforms and mis-
matched (or matched) receive filters to suppress the auto- and cross
correlations using the minimax (
∞
-norm) design criterion. For
practical waveform length and system size, the waveform design
problem becomes quite challenging due to the large problem size
(more than 10
5
unimodular complex variables and 10
7
nonlinear
constraints). In addition to the large size, this problem is noncon-
vex, nonsmooth, and as such, cannot be handled effectively by the
existing waveform design algorithms or off-the-shelve optimization
tools. This letter develops an efficient primal–dual type algorithm
with low per-iteration complexity to solve this problem. Numerical
comparison shows that the waveforms based on the minimax de-
sign outperform those obtained from the existing
2
-norm design
by 4–5 dBs in terms of peak sidelobe levels.
Index Terms—MIMO system, mismatched (or matched) fil-
ters, unimodular waveforms, auto-/cross-correlation, primal-dual
method.
I. INTRODUCTION
M
ULTIPLE-INPUT multiple-output (MIMO) systems
have many important applications in radar active sensing
[1]–[3], such as target detection and localization [4], [5], tracking
[6], and radar imaging [7], [8]. A fundamental component of a
MIMO system is the set of signal waveforms used by its antenna
elements. The performance of a MIMO system, such as location
accuracy [4], detection performance [9] and parameter identifia-
bility [10], is known to be directly related to the good correlation
property of its signal waveforms. In particular, it is desirable to
have low auto-/cross-correlations among the signal waveforms.
Orthogonality between waveforms promises many advantages
in different applications, e.g., higher location accuracy [4],
better detection performance [9], and parameter identifiability
[10]. For practical implementation, the MIMO waveforms are
required to be constant modulus (i.e., unimodular).
In view of the importance of waveform design, various math-
ematical formulations and algorithms have been proposed in
Manuscript received March 30, 2019; revised June 10, 2019; accepted June
23, 2019. Date of publication July 1, 2019; date of current version September 11,
2019. This work was supported in part by the National Natural Science Foun-
dation of China under Grants 61571384 and 61731018, in part by the Leading
Talents of Guangdong Province Program (00201501), and in part by the De-
velopment and Reform Commission of Shenzhen Municipality. This letter was
presented in part at the Asilomar Conference on Signals, Systems, and Comput-
ers, Pacific Grove, CA, USA, October 2018. The associate editor coordinating
the review of this manuscript and approving it for publication was Prof. Luca
Venturino. (Corresponding author: Zhi-Quan Luo.)
The authors are with the Shenzhen Research Institute of Big Data, The Chi-
nese University of Hong Kong, Shenzhen 518172, China (e-mail: 114020243
@link.cuhk.edu.cn; wenqiangpu@outlook.com; luozq@cuhk.edu.cn).
Digital Object Identifier 10.1109/LSP.2019.2926020
the past decade for the MIMO waveform design problem. In
[11], a customized alternating direction method of multipliers
(ADMM) algorithm was proposed to approximate desired beam-
patterns using constant modulus waveforms with low spacial
auto- and cross-correlation sidelobes. In [12], [13], an
2
-norm
minimization based waveform design (or the so called integrated
sidelobe levels (ISL)) was proposed to minimize the squared
sum of the auto- and cross-correlations of the waveforms. A
cyclic iterative algorithm called CAN [12] and a majorization-
minimization (MM) algorithm [13] were proposed to solve the
2
-norm based waveform design problem by exploiting the effi-
cient FFT representation of the minimization problem. An alter-
native
∞
-norm based design has also been considered in [14] to
minimize the so called peak auto-/cross-correlation (or sidelobe)
levels, PSL for short, so as to mitigate the effect of unknown in-
terferences in the worst-case. In particular, the reference [14]
derived theoretical bounds on ISL and PSL of sequence correla-
tions, and proposed an alternating projection (AP) algorithm for
the MIMO waveform design problem. However, each iteration
of the AP algorithm requires an eigenvalue decomposition of a
MN × MN matrix, where M is the number of waveforms and
N is the length of each waveform; such a step is prohibitively
expensive for large values of M,N (say, MN =10
5
). The MM
method proposed in [15] approximates
∞
-norm by an
p
-norm
of the correlations, for a large p, to address the nonsmoothness
of the
∞
-norm, but is only applicable to the design of a single
waveform (M =1). Apart from the aforementioned work on
ISL and PSL minimization for waveform design, various other
design criteria were proposed in different application contexts,
see [16]–[21]. Another factor affecting the achievable PSL of
MIMO waveforms is the use of mismatched receive filter [22],
[23]. The latter has been proposed to improve robustness to the
Doppler shifting [17], mitigate interferences [24] and clutters
[25], and approximate a desired beampattern [26], among others.
This letter considers the joint optimization of constant mod-
ulus waveforms and mismatched (or matched) receive filters
to suppress the auto- and cross-correlations using the minimax
design criterion. For practical system size (say, M = 128 an-
tenna elements) and waveform length (N>10
3
), the waveform
design problem becomes quite challenging due to the large
problem size (more than 10
5
complex unimodular variables,
and approximately 10
7
nonlinear constraints). In addition to the
large size, this optimization problem is nonconvex, nonsmooth,
and as such, can not be handled effectively by the existing
2
/
∞
-norm waveform design algorithms or off-the-shelve
optimization tools. The main contribution of this letter is an
efficient primal-dual type algorithm with low per-iteration
complexity to solve this minimax waveform design problem.
Numerical comparison shows that the newly designed wave-
forms outperform those obtained from the existing algorithms
by 4–5 dBs in terms of peak sidelobe level suppression,
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