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Wireless
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Research interests include wireless networks, MC-CDMA, OFDM, communication over multipath
and fading channels, and Random Access.
- At Philips Research, we developed effective methos for canceling
ICI in mobile OFDM systems, in particular DVB-H
- At PIMRC 1993, I was the first to use the term (Orthogonal) Multi-Carrier
CDMA (MC-CDMA) for a spread spectrum transmission method which
combines Direct-Sequence CDMA with Orthogonal Frequency Division Multiplexing
(OFDM). Recently, I studied OFDM over Doppler
channels. See a tutorial.
- During my M.Sc. and Ph.D. project I worked on mathematical methods
to estimate the outage probability in cellular networks. See a spreadsheet
outage calculator, developed for Wireless
Communication, The Interactive Multimedia CD-ROM.
- The use of Laplace Transforms of probability densities of received
signal powers also allowed me to develop a mathematical framework
for the analysis of the effect of multipath fading in random access
systems such as ALOHA and CSMA. After a lengthy derivation, it appears
that a simple "weighing" of the traffic intensity exactly
models wireless propagation effects. See: Animated
audio tutorial
- My packet queueing analysis shows that the optimum frequency reuse
pattern in wireless data or multimedia networks is "1",
i.e., every cell uses the same channel. DS-CDMA does not improve
delay performance in such case, but protocols to resolve packet collisions
within cells and between cells are essential.
- To jointly address spatial frequency reuse and random access, I
developed the Virtual Cell Network concept and Space-Time Reservation
Multiple Access (STRMA).
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Watermarks, Conditional access
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At Philips Research, my research group mainly works on Electronic
Watermarks, Conditional Access and Information Security.
- I developed a method biometric identification in which the prover
does a priori not have any information about the biometric data of
the prover. In the existing methods, a dishonest verifier knows (or
can learn) the biometrics of the prover even without actually
observing the prover.
- Electronic Watermarking for DVD video
- Watermark detection can be enhanced substantially by exploiting
redundancy in the image (whitening)
- Many of the previously published calculation on the error rate of
watermark detectors are overly optimistic: they ignore the correlation
in image pixels.
- I developed the ticket concept for copy generation control (ESORICS
'98)
- My sensitivity attack on public watermarking (IHW,
Portland, '98) refutes the common misbelief that it requires O(2^N)
attempts to guess a watermark in an image of N pixels. In stead,
it can be estimated in only O(N) attempts.
- At the 104 Audio Engineering Society Convention in May 1998, I chaired
the Workshop on Copy Control.
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