154 research outputs found
Cognitive Radio from Hell: Flipping Attack on Direct-Sequence Spread Spectrum
In this paper, we introduce a strong adversarial attack, referred to as the
flipping attack, on Direct-Sequence Spread Spectrum (DSSS) systems. In this
attack, the attacker, which is appropriately positioned between the transmitter
and the receiver, instantaneously flips the transmitted symbols in the air at
50% rate, thereby driving the channel capacity to zero. Unlike the traditional
jamming attack, this attack, when perfectly executed, cannot be detected at the
receiver using signal-to-noise-ratio measurements. However, this attack
necessitates the attacker to perfectly know the realizations of all the
channels in the model. We first introduce the consequences of the flipping
attack on narrowband frequency-flat channels, and subsequently discuss its
feasibility in wideband frequency-selective channels. From the legitimate
users' perspective, we present a method to detect this attack and also propose
heuristics to improve the error-performance under the attack. We emphasize that
future cyber-physical systems that employ DSSS should design transceivers to
detect the proposed flipping attack, and then apply appropriate
countermeasures
High Rate Single-Symbol Decodable Precoded DSTBCs for Cooperative Networks
Distributed Orthogonal Space-Time Block Codes (DOSTBCs) achieving full
diversity order and single-symbol ML decodability have been introduced recently
for cooperative networks and an upper-bound on the maximal rate of such codes
along with code constructions has been presented. In this report, we introduce
a new class of Distributed STBCs called Semi-orthogonal Precoded Distributed
Single-Symbol Decodable STBCs (S-PDSSDC) wherein, the source performs
co-ordinate interleaving of information symbols appropriately before
transmitting it to all the relays. It is shown that DOSTBCs are a special case
of S-PDSSDCs. A special class of S-PDSSDCs having diagonal covariance matrix at
the destination is studied and an upper bound on the maximal rate of such codes
is derived. The bounds obtained are approximately twice larger than that of the
DOSTBCs. A systematic construction of S-PDSSDCs is presented when the number of
relays . The constructed codes are shown to achieve the upper-bound
on the rate when is of the form 0 modulo 4 or 3 modulo 4. For the rest of
the values of , the constructed codes are shown to have rates higher than
that of DOSTBCs. It is also shown that S-PDSSDCs cannot be constructed with any
form of linear processing at the relays when the source doesn't perform
co-ordinate interleaving of the information symbols.Comment: A technical report of DRDO-IISc Programme on Advanced Research in
Mathematical Engineerin
A Novel Power Allocation Scheme for Two-User GMAC with Finite Input Constellations
Constellation Constrained (CC) capacity regions of two-user Gaussian Multiple
Access Channels (GMAC) have been recently reported, wherein an appropriate
angle of rotation between the constellations of the two users is shown to
enlarge the CC capacity region. We refer to such a scheme as the Constellation
Rotation (CR) scheme. In this paper, we propose a novel scheme called the
Constellation Power Allocation (CPA) scheme, wherein the instantaneous transmit
power of the two users are varied by maintaining their average power
constraints. We show that the CPA scheme offers CC sum capacities equal (at low
SNR values) or close (at high SNR values) to those offered by the CR scheme
with reduced decoding complexity for QAM constellations. We study the
robustness of the CPA scheme for random phase offsets in the channel and
unequal average power constraints for the two users. With random phase offsets
in the channel, we show that the CC sum capacity offered by the CPA scheme is
more than the CR scheme at high SNR values. With unequal average power
constraints, we show that the CPA scheme provides maximum gain when the power
levels are close, and the advantage diminishes with the increase in the power
difference.Comment: To appear in IEEE Transactions on Wireless Communications, 10 pages
and 7 figure
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