120 research outputs found
Vandermonde Frequency Division Multiplexing for Cognitive Radio
We consider a cognitive radio scenario where a primary and a secondary user
wish to communicate with their corresponding receivers simultaneously over
frequency selective channels. Under realistic assumptions that the secondary
transmitter has no side information about the primary's message and each
transmitter knows only its local channels, we propose a Vandermonde precoder
that cancels the interference from the secondary user by exploiting the
redundancy of a cyclic prefix. Our numerical examples show that VFDM, with an
appropriate design of the input covariance, enables the secondary user to
achieve a considerable rate while generating zero interference to the primary
user.Comment: Submitted to Signal Processing Advances in Wireless Communications,
200
Vandermonde-subspace Frequency Division Multiplexing for Two-Tiered Cognitive Radio Networks
Vandermonde-subspace frequency division multiplexing (VFDM) is an overlay
spectrum sharing technique for cognitive radio. VFDM makes use of a precoder
based on a Vandermonde structure to transmit information over a secondary
system, while keeping an orthogonal frequency division multiplexing
(OFDM)-based primary system interference-free. To do so, VFDM exploits
frequency selectivity and the use of cyclic prefixes by the primary system.
Herein, a global view of VFDM is presented, including also practical aspects
such as linear receivers and the impact of channel estimation. We show that
VFDM provides a spectral efficiency increase of up to 1 bps/Hz over cognitive
radio systems based on unused band detection. We also present some key design
parameters for its future implementation and a feasible channel estimation
protocol. Finally we show that, even when some of the theoretical assumptions
are relaxed, VFDM provides non-negligible rates while protecting the primary
system.Comment: 9 pages, accepted for publication in IEEE Transactions on
Communication
Cognitive Orthogonal Precoder for Two-tiered Networks Deployment
In this work, the problem of cross-tier interference in a two-tiered
(macro-cell and cognitive small-cells) network, under the complete spectrum
sharing paradigm, is studied. A new orthogonal precoder transmit scheme for the
small base stations, called multi-user Vandermonde-subspace frequency division
multiplexing (MU-VFDM), is proposed. MU-VFDM allows several cognitive small
base stations to coexist with legacy macro-cell receivers, by nulling the
small- to macro-cell cross-tier interference, without any cooperation between
the two tiers. This cleverly designed cascaded precoder structure, not only
cancels the cross-tier interference, but avoids the co-tier interference for
the small-cell network. The achievable sum-rate of the small-cell network,
satisfying the interference cancelation requirements, is evaluated for perfect
and imperfect channel state information at the transmitter. Simulation results
for the cascaded MU-VFDM precoder show a comparable performance to that of
state-of-the-art dirty paper coding technique, for the case of a dense cellular
layout. Finally, a comparison between MU-VFDM and a standard complete spectrum
separation strategy is proposed. Promising gains in terms of achievable
sum-rate are shown for the two-tiered network w.r.t. the traditional bandwidth
management approach.Comment: 11 pages, 9 figures, accepted and to appear in IEEE Journal on
Selected Areas in Communications: Cognitive Radio Series, 2013. Copyright
transferred to IEE
Opportunistic Interference Alignment in MIMO Interference Channels
We present two interference alignment techniques such that an opportunistic
point-to-point multiple input multiple output (MIMO) link can reuse, without
generating any additional interference, the same frequency band of a similar
pre-existing primary link. In this scenario, we exploit the fact that under
power constraints, although each radio maximizes independently its rate by
water-filling on their channel transfer matrix singular values, frequently, not
all of them are used. Therefore, by aligning the interference of the
opportunistic radio it is possible to transmit at a significant rate while
insuring zero-interference on the pre-existing link. We propose a linear
pre-coder for a perfect interference alignment and a power allocation scheme
which maximizes the individual data rate of the secondary link. Our numerical
results show that significant data rates are achieved even for a reduced number
of antennas.Comment: To appear in proc. IEEE PIMRC 2008 - Workshop in Emerging Network
Perspectives in Multiuser and Cooperative MIMO (NWMIMO). 5 pages and 4
figure
Network Coding with Multimedia Transmission and Cognitive Networking: An Implementation based on Software-Defined Radio
Network coding (NC) is considered a breakthrough to improve throughput, robustness, and security of wireless networks. Although the theoretical aspects of NC have been extensively investigated, there have been only few experiments with pure NC schematics. This paper presents an implementation of NC under a two-way relay model and extends it to two\ua0non-straightforward scenarios: (i) multimedia transmission with layered coding and multiple-description coding, and (ii) cognitive radio with Vandermonde frequency division multiplexing (VFDM). The implementation is in real time and based on software-defined radio (SDR). The experimental results show that, by combining NC and source coding, we can control the quality of the received multimedia content in an on-demand manner. Whereas in the VFDM-based cognitive radio, the quality of the received content in the primary receiver is low (due to imperfect channel estimation) yet retrievable. Our implementation results serve as a proof for the practicability of network coding in relevant applications
Network Coding with Multimedia Transmission and Cognitive Networking: An Implementation based on Software-Defined Radio
Network coding (NC) is considered a breakthrough to improve throughput, robustness, and security of wireless networks. Although the theoretical aspects of NC have been extensively investigated, there have been only few experiments with pure NC schematics. This paper presents an implementation of NC under a two-way relay model and extends it to two non-straightforward scenarios: (i) multimedia transmission with layered coding and multiple-description coding, and (ii) cognitive radio with Vandermonde frequency division multiplexing (VFDM). The implementation is in real time and based on software-defined radio (SDR). The experimental results show that, by combining NC and source coding, we can control the quality of the received multimedia content in an on-demand manner. Whereas in the VFDM-based cognitive radio, the quality of the received content in the primary receiver is low (due to imperfect channel estimation) yet retrievable. Our implementation results serve as a proof for the practicability of network coding in relevant applications
Cognitive Interference Alignment for OFDM Two-tiered Networks
In this contribution, we introduce an interference alignment scheme that
allows the coexistence of an orthogonal frequency division multiplexing (OFDM)
macro-cell and a cognitive small-cell, deployed in a two-tiered structure and
transmitting over the same bandwidth. We derive the optimal linear strategy for
the single antenna secondary base station, maximizing the spectral efficiency
of the opportunistic link, accounting for both signal sub-space structure and
power loading strategy. Our analytical and numerical findings prove that the
precoder structure proposed is optimal for the considered scenario in the face
of Rayleigh and exponential decaying channels.Comment: 5 pages, 4 figures. Accepted and presented at the IEEE 13th
International Workshop on Signal Processing Advances in Wireless
Communications (SPAWC), 2012. Authors' final version. Copyright transferred
to IEE
Orthogonal LTE two-tier Cellular Networks
International audienceIn previous works, Vandermonde-subspace fre- quency division multiplexing (VFDM) has been shown to promote overlay networks by enabling a secondary transmitter to cancel its interference to a primary receiver, while simultaneously transmitting useful information to its own receiver at non- negligible rates. Interference cancelation is achieved by exploiting the null-space of the channel from the secondary transmitter to the primary receiver. In the wake of a global deployment of the third generation partnership project's (3GPP) long term evolution (LTE), one of the open questions of VFDM concerns its applicability in a primary LTE-orthogonal frequency division multiple access (OFDMA) multi-user setting. In this work, we address this question by extending VFDM to the multi-user scenario where the primary system employs OFDMA, such as LTE. We show that by using at the secondary system a similar precoder structure to the ones previously introduced, we are able to cancel the interference towards multiple primary receivers while still achieving acceptable rates for the secondary system
ON THE SECRECY CAPACITY OF FREQUENCY-SELECTIVE FADING CHANNELS : A PRACTICAL VANDERMONDE PRECODING
International audienceWe study the secrecy capacity of the frequency-selective wiretap channel. Assuming that a guard interval of L symbols is inserted to avoid the inter-block interference and these symbols are discarded at the receiver, the single-carrier frequency-selective channel is modeled as a multiple-input multiple-output Toeplitz matrix. For this special case of the MIMO wiretap channel and under the assumption of perfect channel knowledge at transmitter (CSIT), we propose a practical Vandermonde precoding scheme that transmits the confidential messages on the zeros of the eavesdropper channel. It is proved that this Vandermonde precoding achieves the full multiplexing gain offered by the frequencyselective wiretap channel in the high SNR regime. For a more realistic case where the transmitter only knows the legitime channel we consider the âmask beamformingâ scheme where the artificial noise is sent on the zeros of the legitime channel via the Vandermonde precoding. This mask beamforming is shown to achieve the same multiplexing gain as the perfect CSIT
Open Issues on the Statistical Spectrum Characterization of Random Vandermonde Matrices
International audienceRecently, analytical methods for finding moments of random Vandermonde matrices with entries on the unit circle have been proposed in the literature. Vandermonde matrices play an important role in signal processing and wireless applications, among which the multiple-antenna channel modeling, precoding or sparse sampling theory. Recent investigations allowed to extend the combinatorial approach usually exploited to characterize the spectral behavior of large random matrices with independent and identically distributed (i.i.d.) entries to Vandermonde structured matrices, under fairly broad assumptions on the entries distributions. While in several cases explicit expressions of the moments of the associated Gram matrix, as well as more advanced models involving the Vandermonde matrix could be provided, several issues are still open in the spectral behavior characterization, with applications either in signal processing (deconvolution, compressed sensing) and/or wireless communications (capacity estimation, topology information retrieving, etc)
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