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28
Optimal resource allocation for MIMO ad hoc cognitive radio networks
 in Proc. 46th Annu. Allerton Conf. Commun., Control, Comput
, 2008
"... Abstract—Maximization of the weighted sumrate of secondary users (SUs) possibly equipped with multiantenna transmitters and receivers is considered in the context of cognitive radio (CR) networks with coexisting primary users (PUs). The total interference power received at the primary receiver is ..."
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Abstract—Maximization of the weighted sumrate of secondary users (SUs) possibly equipped with multiantenna transmitters and receivers is considered in the context of cognitive radio (CR) networks with coexisting primary users (PUs). The total interference power received at the primary receiver is constrained to maintain reliable communication for the PU. An interference channel configuration is considered for ad hoc networking, where the receivers treat the interference from undesired transmitters as noise. Without the CR constraint, a convergent distributed algorithm is developed to obtain (at least) a locally optimal solution. With the CR constraint, a semidistributed algorithm is introduced. An alternative centralized algorithm based on geometric programming and network duality is also developed. Numerical results show the efficacy of the proposed algorithms. The novel approach is flexible to accommodate modifications aiming at interference alignment. However, the standalone weighted sumrate optimal schemes proposed here have merits over interferencealignment alternatives especially for practical SNR values. Index Terms—Ad hoc network, cognitive radio, interference network, MIMO, optimization. I.
Linear precoding designs for amplifyandforward multiuser twoway relay systems
 in Proc. IEEE GLOBECOM
, 2011
"... AbstractWe investigate the linear precoding designs for multiuser twoway relay system (MUTWRS) where a multiantenna basestation (BS) communicates with multiple singleantenna mobile stations (MSs) via a multiantenna relay station (RS). The amplifyandforward (AF) relay protocol is employed. Th ..."
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AbstractWe investigate the linear precoding designs for multiuser twoway relay system (MUTWRS) where a multiantenna basestation (BS) communicates with multiple singleantenna mobile stations (MSs) via a multiantenna relay station (RS). The amplifyandforward (AF) relay protocol is employed. The design goal is to optimize the precodings at BS, RS or both so as to minimize the total meansquare error (MSE) of the uplink messages while maintaining the individual signaltointerferenceplusnoise ratio (SINR) requirement for each downlink signal. We show that the BS precoding design problem can be converted to a standard second order cone programming (SOCP), while the RS precoding is nonconvex for which a local optimal solution is obtained using an iterative algorithm. A joint BSRS precoding is also obtained by alternating optimization of BS precoding and RS precoding with guaranteed convergence. Numerical results show that RSprecoding is superior to BSprecoding. Furthermore, the joint BSRS precoding can significantly outperform the two individual precoding schemes. The implementation issues including complexity and feedback overhead are also discussed.
Geometry of the 3User MIMO interference channel
 in Proc. Allerton Conference on Communication, Control, and Computing (Allerton
, 2011
"... Abstract—This paper studies vector space interference alignment for the threeuser MIMO interference channel with no time or frequency diversity. The main result is a characterization of the feasibility of interference alignment in the symmetric case where all transmitters have M antennas and all ..."
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Abstract—This paper studies vector space interference alignment for the threeuser MIMO interference channel with no time or frequency diversity. The main result is a characterization of the feasibility of interference alignment in the symmetric case where all transmitters have M antennas and all receivers have N antennas. If N ≥M and all users desire d transmit dimensions, then alignment is feasible if and only if (2r+1)d ≤ max(rN, (r+1)M) for all nonnegative integers r. The analogous result holds with M and N switched if M ≥ N. It turns out that, just as for the 3user parallel interference channel [1], the length of alignment paths captures the essence of the problem. In fact, for each feasible value of M and N the maximum alignment path length dictates both the converse and achievability arguments. One of the implications of our feasibility criterion is that simply counting equations and comparing to the number of variables does not predict feasibility. Instead, a more careful investigation of the geometry of the alignment problem is required. The necessary condition obtained by counting equations is implied by our new feasibility criterion. I.
Giannakis, “Distributed optimal beamformers for cognitive radios robust to channel uncertainties
 IEEE Trans. Sig. Proc
, 2012
"... Abstract—Through spatial multiplexing and diversity, multiinput multioutput (MIMO) cognitive radio (CR) networks can markedly increase transmission rates and reliability, while controlling the interference inflicted to peer nodes and primary users (PUs) via beamforming. The present paper optimiz ..."
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Abstract—Through spatial multiplexing and diversity, multiinput multioutput (MIMO) cognitive radio (CR) networks can markedly increase transmission rates and reliability, while controlling the interference inflicted to peer nodes and primary users (PUs) via beamforming. The present paper optimizes the design of transmit and receivebeamformers for ad hoc CR networks when CRtoCR channels are known, but CRtoPU channels cannot be estimated accurately. Capitalizing on a normbounded channel uncertainty model, the optimal beamforming design is formulated to minimize the overall meansquare error (MSE) from all data streams, while enforcing protection of the PU system when the CRtoPU channels are uncertain. Even though the resultant optimization problem is nonconvex, algorithms with provable convergence to stationary points are developed by resorting to block coordinate ascent iterations, along with suitable convex approximation techniques. Enticingly, the novel schemes also lend themselves naturally to distributed implementations. Numerical tests are reported to corroborate the analytical findings. Index Terms—Beamforming, channel uncertainty, cognitive radios, distributed algorithms, MIMO wireless networks, robust optimization. I.
Transmit Optimization with Improper Gaussian Signaling for Interference Channels
 IEEE TRANSACTIONS ON SIGNAL PROCESSING, ACCEPTED
, 2013
"... This paper studies the achievable rates of Gaussian interference channels with additive white Gaussian noise (AWGN), when improper or circularly asymmetric complex Gaussian signaling is applied. For the Gaussian multipleinput multipleoutput interference channel (MIMOIC) with the interference tre ..."
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This paper studies the achievable rates of Gaussian interference channels with additive white Gaussian noise (AWGN), when improper or circularly asymmetric complex Gaussian signaling is applied. For the Gaussian multipleinput multipleoutput interference channel (MIMOIC) with the interference treated as Gaussian noise, we show that the user’s achievable rate can be expressed as a summation of the rate achievable by the conventional proper or circularly symmetric complex Gaussian signaling in terms of the users ’ transmit covariance matrices, and an additional term, which is a function of both the users ’ transmit covariance and pseudocovariance matrices. The additional degrees of freedom in the pseudocovariance matrix, which is conventionally set to be zero for the case of proper Gaussian signaling, provide an opportunity to further improve the achievable rates of Gaussian MIMOICs by employing improper Gaussian signaling. To this end, this paper proposes widely linear precoding, which efficiently maps proper informationbearing signals to improper transmitted signals at each transmitter for any given pair of transmit covariance and pseudocovariance matrices. In particular, for the case of twouser Gaussian singleinput singleoutput interference channel (SISOIC), we propose a joint covariance and pseudocovariance optimization algorithm with improper Gaussian signaling to achieve the Paretooptimal rates. By utilizing the separable structure of the achievable rate expression, an alternative algorithm with separate covariance and pseudocovariance optimization is also proposed, which guarantees the rate improvement over conventional proper Gaussian signaling.
Distributed MIMO Interference Cancellation for Interfering Wireless Networks: Protocol and Initial Simulation
"... Abstract—In this report, the problem of interference in dense wireless network deployments is addressed. Two example scenarios are: 1) overlapping basic service sets (OBSSes) in wireless LAN deployments, and 2) interference among closeby femtocells. The proposed approach is to exploit the interfere ..."
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Abstract—In this report, the problem of interference in dense wireless network deployments is addressed. Two example scenarios are: 1) overlapping basic service sets (OBSSes) in wireless LAN deployments, and 2) interference among closeby femtocells. The proposed approach is to exploit the interference cancellation and spatial multiplexing capabilities of multipleinput multipleoutput (MIMO) links to mitigate interference and improve the performance of such networks. Both semidistributed and fully distributed protocols for 802.11based wireless networks standard are presented and evaluated. The philosophy of the approach is to minimize modifications to existing protocols, particularly within clientside devices. Thus, modifications are primarily made at the access points (APs). The semidistributed protocol was fully implemented within the 802.11 package of ns3 to evaluate the approach. Simulation results with two APs, and with either one client per AP or two clients per AP, show that within 5 seconds of network operation, our protocol increases the goodput on the downlink by about 50%, as compared against a standard 802.11n implementation. Index Terms—MIMO, multiple antennas, beamforming, combining, overlapping basic service set, dense wireless networks I.
INTERWEAVE/UNDERLAY COGNITIVE RADIO TECHNIQUES AND APPLICATIONS IN SATELLITE COMMUNICATION SYSTEMS
, 2014
"... The demand for precious radio spectrum is continuously increasing while the available radio frequency resource has become scarce due to spectrum segmentation and the dedicated frequency allocation of standardized wireless systems. This scarcity has led to the concept of cognitive radio communication ..."
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The demand for precious radio spectrum is continuously increasing while the available radio frequency resource has become scarce due to spectrum segmentation and the dedicated frequency allocation of standardized wireless systems. This scarcity has led to the concept of cognitive radio communication which comprises a variety of techniques capable of allowing the coexistence of licensed and unlicensed systems over the same spectrum. In this context, this thesis focuses on interweave and underlay cognitive radio paradigms which are widely considered as important enablers for realising cognitive radio technology. In the interweave paradigm, an unlicensed user explores the spectral holes by means of some spectrum awareness methods and utilizes the available spectral availabilities opportunistically while in the underlay paradigm, an unlicensed user is allowed to coexist with the licensed user only if sufficient protection to the licensed user can be guaranteed. Starting with a detailed overview of the existing techniques, this thesis provides contributions in both theoretical and the practical aspects of these paradigms. This thesis is structured into two main parts as described below. One of the important capabilities required for a cognitive radio transceiver is to be able to acquire the knowledge of its surrounding radio environment in order to utilize the available spectral opportunities
1A Feasibility Test for Linear Interference Alignment in MIMO Channels with Constant Coefficients
"... Abstract—In this paper, we consider the feasibility of linear interference alignment (IA) for multipleinput multipleoutput (MIMO) channels with constant coefficients for any number of users, antennas and streams per user; and propose a polynomialtime test for this problem. Combining algebraic geo ..."
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Abstract—In this paper, we consider the feasibility of linear interference alignment (IA) for multipleinput multipleoutput (MIMO) channels with constant coefficients for any number of users, antennas and streams per user; and propose a polynomialtime test for this problem. Combining algebraic geometry techniques with differential topology ones, we first prove a result that generalizes those previously published on this topic. Specifically, we consider the input set (complex projective space of MIMO interference channels), the output set (precoder and decoder Grassmannians) and the solution set (channels, decoders and precoders satisfying the IA polynomial equations), not only as algebraic sets but also as smooth compact manifolds. Using this mathematical framework, we prove that the linear alignment problem is feasible when the algebraic dimension of the solution variety is larger than or equal to the dimension of the input space and the linear mapping between the tangent spaces of both smooth manifolds given by the first projection is generically surjective. If that mapping is not surjective, then the solution variety projects into the input space in a singular way and the projection is a zeromeasure set. This result naturally yields a simple feasibility test, which amounts to checking the rank of a matrix. We also provide an exact arithmetic version of the test, which proves that testing the feasibility of IA for generic MIMO channels belongs to the boundederror probabilistic polynomial (BPP) complexity class. Index Terms—Interference alignment, MIMO interference channel, polynomial equations, algebraic geometry, differential topology. I.
On Precoding for Constant KUser MIMO Gaussian Interference Channel with Finite Constellation Inputs”, Available at: arXiv:1210.3819v1
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Signal Alignment: Enabling Physical Layer Network Coding for MIMO Networking
"... Abstract—We apply signal alignment (SA), a wireless communication technique that enables physical layer network coding (PNC) in multiinput multioutput (MIMO) wireless networks. Through calculated precoding, SA contracts the perceived signal space at a node to match its receive capability, and hen ..."
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Abstract—We apply signal alignment (SA), a wireless communication technique that enables physical layer network coding (PNC) in multiinput multioutput (MIMO) wireless networks. Through calculated precoding, SA contracts the perceived signal space at a node to match its receive capability, and hence facilitates the demodulation of linearly combined data packets. PNC coupled with SA (PNCSA) has the potential of fully exploiting the precoding space at the senders, and can better utilize the spatial diversity of a MIMO network for higher system degreesoffreedom (DoF). PNCSA adopts the idea of ‘demodulating a linear combination ’ from PNC. The design of PNCSA is also inspired by recent advances in IA, though SA aligns signals not interferences. We study the optimal precoding and power allocation problem of PNCSA, for SNR (singaltonoiseratio) maximization at the receiver. The mapping from SNR to BER is then analyzed, revealing that the DoF gain of PNCSA does not come with a sacrifice in BER. We then design a general PNCSA algorithm in larger systems, and demonstrate general applications of PNCSA, and show via network level simulations that it can substantially increase the throughput of unicast and multicast sessions, by opening previously unexplored solution spaces in multihop MIMO routing. Index Terms—Network coding; physical layer letwork coding; interference alignment; signal alignment; MIMO networks. I.