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

Transceiver design and interference alignment in wireless networks: complexity and solvability

University of Minnesota M.S. thesis. November 2013. Major: Mathematics. Advisor: Gennady Lyubeznik. 1 computer file (PDF); vi, 58 pages.This thesis aims to theoretically study a modern linear transceiver design strategy, namely interference alignment, in wireless networks. We consider an interference channel whereby each transmitter and receiver are equipped with multiple antennas. The basic problem is to design optimal linear transceivers (or beamformers) that can maximize the system throughput. The recent work [1] suggests that optimal beamformers should maximize the total degrees of freedom through the interference alignment equations. In this thesis, we first state the interference alignment equations and study the computational complexity of solving these equations. In particular, we prove that the problem of maximizing the total degrees of freedom for a given interference channel is NP-hard. Moreover, it is shown that even checking the achievability of a given tuple of degrees of freedom is NP-hard when each receiver is equipped with at least three antennas. Interestingly, the same problem becomes polynomial time solvable when each transmit/receive node is equipped with no more than two antennas.The second part of this thesis answers an open theoretical question about interference alignment on generic channels: What degrees of freedom tuples (d1, d2, ..., dK) are achievable through linear interference alignment for generic channels? We partially answer this question by establishing a general condition that must be satisfied by any degrees of freedom tuple (d1, d2, ..., dK) achievable through linear interference alignment. For a symmetric system with dk = d for all k, this condition implies that the total achievable DoF cannot grow linearly with K, and is in fact no more than K(M + N)/(K + 1), where M and N are the number of transmit and receive antennas, respectively. We also show that this bound is tight when the number of antennas at each transceiver is divisible by the number of data streams

On the Performance of SSK Modulation over Multiple-Access Rayleigh Fading Channels”, IEEE Global Communications Conference

International audienceSpatial Modulation (SM) is a recently proposed joint coding and modulation scheme for Multiple–Input-Multiple–Output (MIMO) wireless systems, which is receiving a growing interest. SM offers a low-complexity alternative to the design of MIMO wireless systems, which avoids multiple Radio Frequency (RF) chains at the transmitter and high–complexity interference cancelation algorithms at the receiver, but still guarantees a multiplexing gain that only depends on the number of antennas at the transmitter. This makes this technology especially suitable for the downlink with low–complexity mobile units. So far, the feasibility and performance of SM have been assessed and studied only for point–to–point communication systems, i.e., the single–user scenario. However, the performance achievable by the vast majority of wireless communication networks is interference limited, due to the simultaneous transmission of various users over the same physical wireless channel. Therefore, the adoption of SM in the next generation of wireless communication systems requires a deep understanding of its performance over interference channels. Motivated by this consideration, in this paper we study the performance of SM over the reference multiple–access fading channel composed by two transmitters and one receiver. Two detectors at the receiver are studied, i.e., the single– and the multi–user detector. In particular, analysis and Monte Carlo simulations show that the single–user detector does not offer, in general, good error performance for arbitrary channel conditions, while the multi–user detector achieves error performance very close to the single–user lower–bound. These results clearly highlight that SM can be adopted for enabling data transmission over multiple–access fading channels as well

Degrees of Freedom of Interference Channels with CoMP Transmission and Reception

We study the Degrees of Freedom (DoF) of the K-user interference channel with coordinated multi-point (CoMP) transmission and reception. Each message is jointly transmitted by M_t successive transmitters, and is jointly received by M_r successive receivers. We refer to this channel as the CoMP channel with a transmit cooperation order of M_t and receive cooperation order of M_r. Since the channel has a total of K transmit antennas and K receive antennas, the maximum possible DoF is equal to K. We show that the CoMP channel has K DoF if and only if M_t + M_r is greater than or equal to K+1. For the general case, we derive an outer bound that states that the DoF is bounded above by the ceiling of (K+M_t+M_r-2)/2. For the special case with only CoMP transmission, i.e, M_r = 1, we propose a scheme that can achieve (K+M_t-1)/2 DoF for all K < 10, and conjecture that the result holds true for all K . The achievability proofs are based on the notion of algebraic independence from algebraic geometry.Comment: Submitted to IEEE Transactions on Information Theor

Robust adaptive filtering algorithms for system identification and array signal processing in non-Gaussian environment

This dissertation proposes four new algorithms based on fractionally lower order statistics for adaptive filtering in a non-Gaussian interference environment. One is the affine projection sign algorithm (APSA) based on L₁ norm minimization, which combines the ability of decorrelating colored input and suppressing divergence when an outlier occurs. The second one is the variable-step-size normalized sign algorithm (VSS-NSA), which adjusts its step size automatically by matching the L₁ norm of the a posteriori error to that of noise. The third one adopts the same variable-step-size scheme but extends L₁ minimization to Lp minimization and the variable step-size normalized fractionally lower-order moment (VSS-NFLOM) algorithms are generalized. Instead of variable step size, the variable order is another trial to facilitate adaptive algorithms where no a priori statistics are available, which leads to the variable-order least mean pth norm (VO-LMP) algorithm, as the fourth one. These algorithms are applied to system identification for impulsive interference suppression, echo cancelation, and noise reduction. They are also applied to a phased array radar system with space-time adaptive processing (beamforming) to combat heavy-tailed non-Gaussian clutters. The proposed algorithms are tested by extensive computer simulations. The results demonstrate significant performance improvements in terms of convergence rate, steady-state error, computational simplicity, and robustness against impulsive noise and interference --Abstract, page iv

Achievable sum DoF of the K-user MIMO interference channel with delayed CSIT

This paper considers a K-user multiple-inputmultiple-output (MIMO) interference channel (IC) where 1) the channel state information obtained by the transmitters (CSIT) is completely outdated, and 2) the number of transmit antennas at each transmitter, i.e., M, is greater than the number of receive antennas at each user, i.e., N. The usefulness of the delayed CSIT was firstly identified in a K-phase Retrospective Interference Alignment (RIA) scheme proposed by Maddah-Ali et al for the Multiple-Input-Single-Output Broadcast Channel, but the extension to the MIMO IC is a non-trivial step as each transmitter only has the message intended for the corresponding user. Recently, Abdoli et al focused on a Single-Input-SingleOutput IC and solved such bottleneck by inventing a K-phase RIA with distributed overheard interference retransmission. In this paper, we propose two K-phase RIA schemes suitable for the MIMO IC by generalizing and integrating some key features of both Abdoli’s and Maddah-Ali’s works. The two schemes jointly yield the best known sum Degrees-of-Freedom (DoF) performance so far. For the case M N ≥K, the achieved sum DoF is asymptotically given by 64 15N when K→∞