4 Scott C. Douglas and Malay Gupta
the received signal shape is completely different from that of the originally-
transmitted signal. It also has connections to human perception, in that
human hearing system is able to focus attention on particular sound sources
in the presence of significant noise and interference. It is not surprising
that early contributions in the development of BSS techniques were made
in two different areas of study: blind equalization/deconvolution in sig-
nal processing, and the analysis of neurological structures in the brain in
medicine.
1.1.1 Blind Equalization and Deconvolution
Blind equalization and deconvolution originally arose in the context of com-
munication systems involving symbols that are transmitted using modulated
analog waveforms. Since the goal of communications is signal recovery, the
channel effects must be removed from the observed signal, leading to the con-
cept of channel identification to guide the design of a second system applied
to the received signals that reverses the channel’s effects. Training signals
are often transmitted periodically to aid in this recovery, and these trans-
mitted symbols lower the effective data rate of the communications link.
In the blind counterpart to the above communications task, termed blind
equalization or deconvolution, no training data is used, and the goal is to
recover the original transmitted symbols based only on the received signal
measurements.
Blind signal recovery is possible due to certain structural or statistical
constraints that are often placed on the transmitted sources in communica-
tions tasks. This structural constraint can be on the amplitude of the signals
being transmitted, as in the case of the constant modulus algorithm [2] and
the Sato algorithm [1]. Alternatively, a constraint on the probability density
function (p.d.f.) of the sources, such as their non-Gaussianity [3], can also be
used, in which case the goal of signal recovery is to match the density of the
equalized signal to that of the transmitted symbols. Further simplifications
to this approach were suggested in [4], in which higher-order statistics are
exploited for the equalization task. A similar method was originally proposed
by Wiggins [5]. This work illustrates that non-Gaussian statistics have played
an important role in many blind signal recovery algorithms.
In BSS, multiple unknown sources are transmitted through an unknown
multiple-input, multiple-output channel. Recovery of the sources is hampered
by an additional difficulty not present in the blind equalization or deconvo-
lution task: the source distributions may differ from one another.
1.1.2 Neurological Structures and Information Representation
The study of brain function naturally leads one to consider the coding strategies
employed by neurological structures such as the brains of higher animals in