Aerospace and Electronic Systems Magazine December 2017 - 54

Feature Article:

DOI. No. 10.1109/MAES.2017.170057

Experimental Results for MIMO Methods Applied in
Over-the-Horizon Radar
Gordon J. Frazer, Australian Department of Defence, Edinburgh, SA, Australia

INTRODUCTION
Multiple-Input Multiple-Output (MIMO) radar has been an active topic in the radar research community for more than a decade
[1]-[3]. MIMO based approaches have been investigated for improving target detectability, improving target localization, achieving higher spatial resolution, and clutter reduction using transmit
or joint transmit-receive beamforming. This paper describes two
series of experiments undertaken by the author and coworkers in
learning how we might apply MIMO radar techniques to challenges in skywave Over-the-Horizon Radar (OTHR).1
There are four broad areas constituting MIMO radar. The
first is statistical MIMO [4]-[8], wherein multiple transmit and
multiple receive "views" of a target are generated concurrently to
achieve diversity in the measurement aspect under the assumption
that the radar-cross section of the target is sufficiently varying,
with a target aspect that is best characterized statistically. Under certain assumptions for this statistical characterization it can
be shown that there is a detectability advantage compared with
the single transmitter single receiver situation. This work builds
upon the established bi/multistatic radar literature [9]-[12]. The
second class of MIMO methods are concerned with improving
target localization using multilateration [13]. This also has similarities with the bi/multistatic radar literature and with precision
navigation methods, such as used in GPS [14]. The third category
of MIMO methods build upon the sum coarray ideas summarized in [15] and achieve improved spatial resolution in the joint
transmitter-receiver [2]. This approach also admits the prospect
of adaptive beamforming for the transmit system and applications
in airborne space-time adaptive processing have been reported
[16]-[18]. Finally, the fourth area of MIMO radar is concerned
with transmitter beamforming, or, more generally, spatial coding
of transmitter signals in radar systems. The pioneering work in
1

We use the term skywave and over-the-horizon interchangeably, however acknowledge that other propagation modalities
including surface-wave and meteor scatter can achieve beyondhorizon coverage.

Author's current address: DST, NSID, 2 Commercial Rd, Edinburgh, South Australia 5111, Australia, E-mail: (frazer@ieee.org).
Manuscript received March 3, 2017, revised and ready for
publication April 28, 2017.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 Crown
52

this area is encapsulated in the ground-breaking Synthetic Antenna and Impulse Radar (RIAS) radar [19]-[22]. It is this class of
MIMO radar methods that have been extended and employed in
OTHR applications and are the basis of the experiments reported
herein [16], [23]-[29].

EXPERIMENTAL SYSTEMS
Of particular relevance to the work reported here are examples of
experimental MIMO systems. In addition to our own work, there
have been a number of demonstrations that explore the applicability of MIMO in radar. The original concept of spatially coding
the transmitted signal was reported by researchers at the Office
National d'Etudes et de Recherches Aérospatiales (ONERA) in
France in 1984, with the RIAS system [19], [20] and with experimental results given in [21]. The remarkable RIAS is the first
known MIMO radar. A similar system to RIAS is described by Wu
and coauthors from the East China Research Institute of Electrical
Engineering and Xidian University in [22] in 2001.
The earliest reported experimental use of MIMO since the
RIAS class systems is the work on distributed apertures by Robey
and Coutts and their coauthors at MIT Lincoln Laboratory in
2004 [16], [30]. They demonstrated coherent radar action across
multiple distributed apertures using MIMO radar concepts and
several line-of-sight radars operating at X-band. More recently
the same group has been applying MIMO to OTHR, although
they have not published in the open literature [31], [32]. Experimental results showing the use of MIMO techniques to enhance
performance in airborne radar applied to ground moving target
indication (GMTI) is reported by Kantor, Davis, and Bliss, also of
MIT Lincoln Laboratory, in [17], [18]. While unrelated to OTHR,
the results described in their work make a compelling case for
MIMO use in airborne GMTI radar. A group at Bauman Moscow
State Technical University in Russia has reported MIMO radar
experimental results in [33] and refer to earlier published material from 1994 and 2001 covering MIMO radar (albeit in Russian). Krolik, Mecca, and Yu at Duke University have reported
MIMO experimentation using both radar and acoustic systems
at low-power in the laboratory [28], [34]. The same group have
also published theoretical and algorithm investigations concerning OTHR applications of MIMO techniques [35] (and the references therein).
In an application similar to our own, interest concerning clutter
mitigation in high frequency (HF) radar, Riddolls and coauthors at

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