98 research outputs found
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
A Distributed Approach to Interference Alignment in OFDM-based Two-tiered Networks
In this contribution, we consider a two-tiered network and focus on the
coexistence between the two tiers at physical layer. We target our efforts on a
long term evolution advanced (LTE-A) orthogonal frequency division multiple
access (OFDMA) macro-cell sharing the spectrum with a randomly deployed second
tier of small-cells. In such networks, high levels of co-channel interference
between the macro and small base stations (MBS/SBS) may largely limit the
potential spectral efficiency gains provided by the frequency reuse 1. To
address this issue, we propose a novel cognitive interference alignment based
scheme to protect the macro-cell from the cross-tier interference, while
mitigating the co-tier interference in the second tier. Remarkably, only local
channel state information (CSI) and autonomous operations are required in the
second tier, resulting in a completely self-organizing approach for the SBSs.
The optimal precoder that maximizes the spectral efficiency of the link between
each SBS and its served user equipment is found by means of a distributed
one-shot strategy. Numerical findings reveal non-negligible spectral efficiency
enhancements with respect to traditional time division multiple access
approaches at any signal to noise (SNR) regime. Additionally, the proposed
technique exhibits significant robustness to channel estimation errors,
achieving remarkable results for the imperfect CSI case and yielding consistent
performance enhancements to the network.Comment: 15 pages, 10 figures, accepted and to appear in IEEE Transactions on
Vehicular Technology Special Section: Self-Organizing Radio Networks, 2013.
Authors' final version. Copyright transferred to IEE
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
Space Alignment Based on Regularized Inversion Precoding in Cognitive Transmission
For a two-tier Multiple-Input Multiple-Output (MIMO) cognitive network with common receiver, the precoding matrix has a compact relationship with the capacity performance in the unlicensed secondary system. To increase the capacity of secondary system, an improved precoder based on the idea of regularized inversion for secondary transmitter is proposed. An iterative space alignment algorithm is also presented to ensure the Quality of Service (QoS) for primary system. The simulations reveal that, on the premise of achieving QoS for primary system, our proposed algorithm can get larger capacity in secondary system at low Signal-to-Noise Ratio (SNR), which proves the effectiveness of the algorithm
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A composite approach to self-sustainable transmissions : rethinking OFDM
This paper proposes two novel strategies to extend the battery life of an orthogonal frequency division multiplexing (OFDM) receiver, by exploiting the concept of wireless power transfer (WPT). First a new receiver architecture is devised that does not discard the cyclic prefix (CP), but instead, exploits it to extract power from the received signal, realizing a WPT between the transmitter and the receiver. Subsequently, a flexible composite transmit strategy is designed, in which the OFDM transmitter transmits to the receiver two independent signals coexisting in the same band. It is shown that, by means of this approach, the transmitter can arbitrarily increase the power concentrated within the CP at the OFDM receiver, without increasing the redundancy of the transmission. The feasibility conditions for the self-sustainability of the transmission are derived, in terms of power consumption at the receiver, for both legacy and composite transmission. Numerical findings show that, under reasonable conditions, the amount of power carried in the CP could be made sufficient to decode the information symbols, making the transmission fully self-sustainable. The potential of the proposed approach is confirmed by the encouraging results obtained when the full self-sustainability constraint is relaxed, and partially self-sustainable OFDM transmissions are analyzed
Transmission Cooperative Strategies for MIMO-OFDM Heterogeneous Networks
Mobile traffic in cellular networks is increasing
exponentially, mainly due to the use of data intensive services
like video. One way to cope with these demands is to
reduce the cell-size by deploying small-cells along the
coverage area of the current macro-cell system. The deployment
of small-cells significantly improves indoor coverage.
Nevertheless, as additional spectrum licenses are
difficult and expensive to acquire it is expected that the
macro and small-cells will coexist under the same spectrum.
The coexistence of the two systems results in crosstier/
inter-system interference. In this context, we design
several interference alignment based techniques for the
downlink of heterogeneous networks, in order to cancel the
interference generated from macro-cell at small-cell user
terminals. More specifically, in this contribution we design
interference alignment methods under different levels of
inter-system coordination and the constraint that the performance
of macro-cell system is kept close to the case
where small-cell system is switched-off. Numerical results
demonstrate that the proposed methods achieve close to
the optimal performance with low overhea
Physical-Layer Transmission Cooperative Strategies for Heterogeneous Networks
The deployment of small cells within the boundaries of a macro-cell is considered to be an effective solution to cope with the current trend of higher data rates and improved system capacity. In the current heterogeneous configuration with the mass deployment of small cells, it is preferred that these two cell types coexist over the same spectrum, because acquiring additional spectrum licenses for small cells is difficult and expensive. However, the coexistence leads to cross-tier/inter-system interference. In this context, this contribution investigates interference alignment (IA) methods in order to mitigate the interference of macro-cell base station towards the small cell user terminals. More specifically, we design a diversity-oriented interference alignment scheme with space-frequency block codes (SFBC). The main motivation for joint interference alignment with SFBC is to allow the coexistence of two systems under minor inter-system information exchange. The small cells just need to know what space-frequency block code is used by the macro-cell system and no inter-system channels need to be exchanged, contrarily to other schemes recently proposed. Numerical results show that the proposed method achieves a performance close to the case where full-cooperation between the tiers is allowed
Joint time-frequency alignment for PAPR and OOBE suppression of OFDM-based waveforms
WOS: 000418138700009Orthogonal frequency division multiplexing (OFDM) waveform has been adopted by many wireless standards due to its numerous advantages. However, OFDM symbols have two critical drawbacks: high out-of-band emission (OOBE) and high peak-to-average power ratio (PAPR). Cyclic prefix (CP) alignment is one of the promising methods that jointly suppresses the OOBE and PAPR of OFDM symbols without introducing extra receiver complexity. However, its suppression performance remains limited in practical scenarios as it only exploits the degrees of freedom (DoFs) provided by the CP part of the OFDM symbols. In this letter, we propose a novel approach which jointly exploits the time domain resources, i.e., CP, and the frequency domain resources that are not effectively used by the receiver, i.e., guard tones and the subcarriers faded by the multipath channel. Thus, the available DoFs are substantially increased and more efficiently utilized for alignment purpose. It is shown that the OOBE and PAPR suppression performance is significantly enhanced with the proposed method as compared with the original approach
Joint IA and SFBC Macrocells and Small-Cells Coexistence under Minor Information Exchange
Thedeployment of small-cells within the boundaries of a macrocell is considered to be an effective solution to cope with the current
trend of higher data rates and improved system capacity. In the current heterogeneous configuration with the mass deployment of
small-cells, it is preferred that these two cell types will coexist over the same spectrum, because acquiring additional spectrum
licenses for small-cells is difficult and expensive. However, the coexistence leads to cross-tier/intersystem interference. In this
context, this contribution investigates interference alignment (IA) methods in order to mitigate the interference of macrocell base
station towards the small-cell user terminals.More specifically, we design a diversity-oriented interference alignment scheme with
space-frequency block codes (SFBCs).The main motivation for joint interference alignment with SFBC is to allow the coexistence
of two systems under minor intersysteminformation exchange.The small-cells just need to know what space-frequency block code
is used by the macrocell system and no intersystem channels need to be exchanged, contrarily to other schemes recently proposed.
Numerical results show that the proposed method achieves a performance close to the case where full cooperation between the
tiers is allowed
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