352 research outputs found
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
Downlink SDMA with Limited Feedback in Interference-Limited Wireless Networks
The tremendous capacity gains promised by space division multiple access
(SDMA) depend critically on the accuracy of the transmit channel state
information. In the broadcast channel, even without any network interference,
it is known that such gains collapse due to interstream interference if the
feedback is delayed or low rate. In this paper, we investigate SDMA in the
presence of interference from many other simultaneously active transmitters
distributed randomly over the network. In particular we consider zero-forcing
beamforming in a decentralized (ad hoc) network where each receiver provides
feedback to its respective transmitter. We derive closed-form expressions for
the outage probability, network throughput, transmission capacity, and average
achievable rate and go on to quantify the degradation in network performance
due to residual self-interference as a function of key system parameters. One
particular finding is that as in the classical broadcast channel, the per-user
feedback rate must increase linearly with the number of transmit antennas and
SINR (in dB) for the full multiplexing gains to be preserved with limited
feedback. We derive the throughput-maximizing number of streams, establishing
that single-stream transmission is optimal in most practically relevant
settings. In short, SDMA does not appear to be a prudent design choice for
interference-limited wireless networks.Comment: Submitted to IEEE Transactions on Wireless Communication
Secrecy in the 2-User Symmetric Deterministic Interference Channel with Transmitter Cooperation
This work presents novel achievable schemes for the 2-user symmetric linear
deterministic interference channel with limited-rate transmitter cooperation
and perfect secrecy constraints at the receivers. The proposed achievable
scheme consists of a combination of interference cancelation, relaying of the
other user's data bits, time sharing, and transmission of random bits,
depending on the rate of the cooperative link and the relative strengths of the
signal and the interference. The results show, for example, that the proposed
scheme achieves the same rate as the capacity without the secrecy constraints,
in the initial part of the weak interference regime. Also, sharing random bits
through the cooperative link can achieve a higher secrecy rate compared to
sharing data bits, in the very high interference regime. The results highlight
the importance of limited transmitter cooperation in facilitating secure
communications over 2-user interference channels.Comment: 5 pages, submitted to SPAWC 201
On the Capacity Region of Multi-Antenna Gaussian Broadcast Channels with Estimation Error
In this paper we consider the effect of channel estimation error on the capacity region of MIMO Gaussian broadcast channels. It is assumed that the receivers and the transmitter have (the same) estimates of the channel coefficients (i.e., the feedback channel is noiseless). We obtain an achievable rate region based on the dirty paper coding scheme. We show that this region is given by the capacity region of a dual multi-access channel with a noise covariance that depends on the transmit power. We explore this duality to give the asymptotic behavior of the sum-rate for a system with a large number of user, i.e., n rarr infin. It is shown that as long as the estimation error is of fixed (w.r.t n) variance, the sum-capacity is of order M log log n, where M is the number of antennas deployed at the transmitter. We further obtain the sum-rate loss due to the estimation error. Finally, we consider a training-based scheme for block fading MISO Gaussian broadcast channels. We find the optimum length of the training interval as well as the optimum power used for training in order to maximize the achievable sum-rate
CHARACTERIZATION OF FUNDAMENTAL COMMUNICATION LIMITS OF STATE-DEPENDENT INTERFERENCE NETWORKS
Interference management is one of the key techniques that drive evolution of wireless networks from one generation to another. Techniques in current cellular networks to deal with interference follow the basic principle of orthogonalizing transmissions in time, frequency, code, and space. My PhD work investigate information theoretic models that represent a new perspective/technique for interference management. The idea is to explore the fact that an interferer knows the interference that it causes to other users noncausally and can/should exploit such information for canceling the interference. In this way, users can transmit simultaneously and the throughput of wireless networks can be substantially improved. We refer to the interference treated in such a way as ``dirty interference\u27\u27 or noncausal state .
Towards designing a dirty interference cancelation framework, my PhD thesis investigates two classes of information theoretic models and develops dirty interference cancelation schemes that achieve the fundamental communication limits. One class of models (referred to as state-dependent interference channels) capture the scenarios that users help each other to cancel dirty interference. The other class of models (referred to as state-dependent channels with helper) capture the scenarios that one dominate user interferes a number of other users and assists those users to cancel its dirty interference. For both classes of models, we develop dirty interference cancelation schemes and compared the corresponding achievable rate regions (i.e., inner bounds on the capacity region) with the outer bounds on the capacity region. We characterize the channel parameters under which the developed inner bounds meet the outer bounds either partially of fully, and thus establish the capacity regions or partial boundaries of the capacity regions
Mathematical optimization techniques for resource allocation and spatial multiplexing in spectrum sharing networks
Due to introduction of smart phones with data intensive multimedia and interactive applications and exponential growth of wireless devices, there is a shortage for useful radio spectrum. Even though the spectrum has become crowded, many spectrum occupancy measurements indicate that most of the allocated spectrum is underutilised. Hence radically new approaches in terms of allocation of wireless resources are required for better utilization of radio spectrum.
This has motivated the concept of opportunistic spectrum sharing or
the so-called cognitive radio technology that has great potential to improve spectrum utilization. The cognitive radio technology allows an opportunistic
user namely the secondary user to access the spectrum of the licensed user (known as primary user) provided that the secondary transmission does not harmfully affect the primary user. This is possible with the introduction
of advanced resource allocation techniques together with the use of wireless relays and spatial diversity techniques.
In this thesis, various mathematical optimization techniques have been developed for the efficient use of radio spectrum within the context of spectrum sharing networks. In particular, optimal power allocation techniques and centralised and distributed beamforming techniques have been developed. Initially, an optimization technique for subcarrier and power allocation
has been proposed for an Orthogonal Frequency Division Multiple Access (OFDMA) based secondary wireless network in the presence of multiple primary users. The solution is based on integer linear programming with
multiple interference leakage and transmission power constraints. In order to enhance the spectrum efficiency further, the work has been extended to allow multiple secondary users to occupy the same frequency band under a multiple-input and multiple-output (MIMO) framework. A sum rate maximization technique based on uplink-downlink duality and dirty paper coding has been developed for the MIMO based OFDMA network. The work has
also been extended to handle fading scenarios based on maximization of ergodic capacity. The optimization techniques for MIMO network has been extended to a spectrum sharing network with relays. This has the advantage
of extending the coverage of the secondary network and assisting the primary network in return for the use of the primary spectrum. Finally, instead of considering interference mitigation, the recently emerged concept of
interference alignment has been used for the resource allocation in spectrum sharing networks. The performances of all these new algorithms have been demonstrated using MATLAB based simulation studies
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