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241
A Tractable Approach to Coverage and Rate in Cellular Networks
 IEEE Trans. Commun
, 2011
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Modeling and analysis of Ktier downlink heterogeneous cellular networks
 IEEE J. Sel. Areas Commun
, 2012
"... Abstract—Cellular networks are in a major transition from a carefully planned set of large towermounted basestations (BSs) to an irregular deployment of heterogeneous infrastructure elements that often additionally includes micro, pico, and femtocells, as well as distributed antennas. In this pap ..."
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Cited by 154 (41 self)
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Abstract—Cellular networks are in a major transition from a carefully planned set of large towermounted basestations (BSs) to an irregular deployment of heterogeneous infrastructure elements that often additionally includes micro, pico, and femtocells, as well as distributed antennas. In this paper, we develop a tractable, flexible, and accurate model for a downlink heterogeneous cellular network (HCN) consisting of K tiers of randomly located BSs, where each tier may differ in terms of average transmit power, supported data rate and BS density. Assuming a mobile user connects to the strongest candidate BS, the resulting SignaltoInterferenceplusNoiseRatio (SINR) is greater than 1 when in coverage, Rayleigh fading, we derive an expression for the probability of coverage (equivalently outage) over the entire network under both open and closed access, which assumes a strikingly simple closedform in the high SINR regime and is accurate down to −4 dB even under weaker assumptions. For external validation, we compare against an actual LTE network (for tier 1) with the other K − 1 tiers being modeled as independent Poisson Point Processes. In this case as well, our model is accurate to within 12 dB. We also derive the average rate achieved by a randomly located mobile and the average load on each tier of BSs. One interesting observation for interferencelimited open access networks is that at a given SINR, adding more tiers and/or BSs neither increases nor decreases the probability of coverage or outage when all the tiers have the same targetSINR. Index Terms—Femtocells, heterogeneous cellular networks, stochastic geometry, point process theory, coverage probability. I.
Topological interference management through index coding
, 2013
"... While much recent progress on interference networks has come about under the assumption of abundant channel state information at the transmitters (CSIT), a complementary perspective is sought in this work through the study of interference networks with no CSIT except a coarse knowledge of the topolo ..."
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Cited by 30 (14 self)
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While much recent progress on interference networks has come about under the assumption of abundant channel state information at the transmitters (CSIT), a complementary perspective is sought in this work through the study of interference networks with no CSIT except a coarse knowledge of the topology of the network that only allows a distinction between weak and significant channels and no further knowledge of the channel coefficients ’ realizations. Modeled as a degreesoffreedom (DoF) study of a partially connected interference network with no CSIT, the problem is found to have a counterpart in the capacity analysis of wired networks with arbitrary linear network coding at intermediate nodes, under the assumption that the sources are aware only of the end to end topology of the network. The wireless (wired) network DoF (capacity) region, expressed in dimensionless units as a multiple of the DoF (capacity) of a single point to point channel (link), is found to be bounded above by the capacity of an index coding problem where the antidotes graph is the complement of the interference graph of the original network and the bottleneck link capacity is normalized to unity. The problems are shown to be equivalent under linear solutions over the same field. An interference alignment
Linear Transceiver Design for Interference Alignment: Complexity and Computation,” Available on arxiv:1009.3481
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Degrees of Freedom of the Network MIMO Channel With Distributed CSI
, 2013
"... Abstract—In this work, we discuss the joint precoding with finite rate feedback in the socalled network MIMO where the TXs share the knowledge of the data symbols to be transmitted. We introduce a distributed channel state information (DCSI) model where each TX has its own local estimate of the ove ..."
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Cited by 16 (6 self)
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Abstract—In this work, we discuss the joint precoding with finite rate feedback in the socalled network MIMO where the TXs share the knowledge of the data symbols to be transmitted. We introduce a distributed channel state information (DCSI) model where each TX has its own local estimate of the overall multiuser MIMO channel and must make a precoding decision solely based on the available local CSI. We refer to this channel as the DCSIMIMO channel and the precoding problem as distributed precoding. We extend to the DCSI setting the work from Jindal in [1] for the conventional MIMO Broadcast Channel (BC) in which the number of Degrees of Freedom (DoFs) achieved by Zero Forcing (ZF) was derived as a function of the scaling in the logarithm of the SignaltoNoise Ratio (SNR) of the number of quantizing bits. Particularly, we show the seemingly pessimistic result that the number of DoFs at each user is limited by the worst CSI across all users and across all TXs. This is in contrast to the conventional MIMO BC where the number of DoFs at one user is solely dependent on the quality of the estimation of his own feedback. Consequently, we provide precoding schemes improving on the achieved number of DoFs. For the twouser case, the derived novel precoder achieves a number of DoFs limited by the best CSI accuracy across the TXs instead of the worst with conventional ZF. We also advocate the use of hierarchical quantization of the CSI, for which we show that considerable gains are possible. Finally, we use the previous analysis to derive the DoFs optimal allocation of the feedback bits to the various TXs under a constraint on the size of the aggregate feedback in the network, in the case where conventional ZF is used.
Optimal channel training in uplink network MIMO systems
 IEEE Trans. Signal Processing, to be published. THE RAPIDLY INCREASING DEMAND FOR WIRELESS DATA TRAFFIC POSES THE CHALLENGE OF HOW TO INCREASE THE CAPACITY OF CELLULAR NETWORKS IN AN ECONOMICAL AND ECOLOGICAL WAY. MARCH2011  IEEEVEHICULARTECHNOLOGYMAGAZI
"... We study a multicell frequencyselective fading uplink channel from K user terminals (UTs) to B base stations (BSs). The BSs, assumed to be oblivious of the applied encoding scheme, compress and forward their observations to a central station (CS) via capacity limited backhaul links. The CS joint ..."
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Cited by 15 (6 self)
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We study a multicell frequencyselective fading uplink channel from K user terminals (UTs) to B base stations (BSs). The BSs, assumed to be oblivious of the applied encoding scheme, compress and forward their observations to a central station (CS) via capacity limited backhaul links. The CS jointly decodes the messages from all UTs. Since we assume no prior channel state information, the channel needs to be estimated during its coherence time. Based on a lower bound of the ergodic mutual information, we determine the optimal fraction of the coherence time used for channel training. We then study how the optimal training length is impacted by the backhaul capacity. Our analysis is based on large random matrix theory but shown by simulations to be tight for even small system dimensions. Index Terms — Coordinated MultiPoint (CoMP), network MIMO, channel estimation, random matrix theory
Uplink Multicell Processing with Limited Backhaul via PerBaseStation Successive Interference Cancellation
"... Abstract—This paper studies an uplink multicell joint processing model in which the basestations are connected to a centralized processing server via ratelimited digital backhaul links. We propose a simple scheme that performs WynerZiv compressandforward relaying on a perbasestation basis f ..."
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Cited by 12 (1 self)
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Abstract—This paper studies an uplink multicell joint processing model in which the basestations are connected to a centralized processing server via ratelimited digital backhaul links. We propose a simple scheme that performs WynerZiv compressandforward relaying on a perbasestation basis followed by successive interference cancellation (SIC) at the central processor. The proposed scheme has a significantly reduced complexity as compared to joint decoding, resulting in an easily computable achievable rate region. Although suboptimal in general, this paper shows that the proposed perbasestation SIC scheme can achieve the sum capacity of a special Wyner cellular model to within a constant gap. This paper also establishes that in order to achieve to within a constant gap to the maximum SIC rate with infinite backhaul, the limitedbackhaul system must have backhaul capacities that scale logarithmically with the signaltointerferenceandnoise ratios (SINRs) at the basestations. Further, this paper studies the optimal backhaul rate allocation problem for the perbasestation SIC model with a total backhaul capacity constraint, and shows that the sumrate maximizing allocation should also have individual backhaul rates that scale logarithmically with the SINR at each basestation. Finally, the proposed perbasestation SIC scheme is evaluated in a practical multicell network to quantify the performance gain brought by multicell processing. Index Terms—Coordinated multipoint (CoMP), interference channel, limited backhaul network multipleinput multipleoutput (MIMO), relay channel, successive interference cancellation, WynerZiv coding I.
Energy cooperation in cellular networks with renewable powered base stations
 IEEE Trans. Wireless Commun.. [Online]. Available
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Asymptotic moments for interference mitigation in correlated fading channels
 in Proc. IEEE ISIT
, 2011
"... Abstract—We consider a certain class of large random matrices, composed of independent column vectors with zero mean and different covariance matrices, and derive asymptotically tight deterministic approximations of their moments. This random matrix model arises in several wireless communication sy ..."
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Cited by 12 (3 self)
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Abstract—We consider a certain class of large random matrices, composed of independent column vectors with zero mean and different covariance matrices, and derive asymptotically tight deterministic approximations of their moments. This random matrix model arises in several wireless communication systems of recent interest, such as distributed antenna systems or large antenna arrays. Computing the linear minimum mean square error (LMMSE) detector in such systems requires the inversion of a large covariance matrix which becomes prohibitively complex as the number of antennas and users grows. We apply the derived moment results to the design of a lowcomplexity polynomial expansion detector which approximates the matrix inverse by a matrix polynomial and study its asymptotic performance. Simulation results corroborate the analysis and evaluate the performance for finite system dimensions. I.
Optimal coordinated beamforming in the multicell downlink with transceiver impairments
 in Proc. IEEE GLOBECOM
, 2012
"... Abstract—Physical wireless transceivers suffer from a variety of impairments that distort the transmitted and received signals. Their degrading impact is particularly evident in modern systems with multiuser transmission, high transmit power, and lowcost devices, but their existence is routinely ig ..."
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Cited by 11 (5 self)
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Abstract—Physical wireless transceivers suffer from a variety of impairments that distort the transmitted and received signals. Their degrading impact is particularly evident in modern systems with multiuser transmission, high transmit power, and lowcost devices, but their existence is routinely ignored in the optimization literature for multicell transmission. This paper provides a detailed analysis of coordinated beamforming in the multicell downlink. We solve two optimization problems under a transceiver impairment model and derive the structure of the optimal solutions. We show numerically that these solutions greatly reduce the impact of impairments, compared with beamforming developed for ideal transceivers. Although the socalled multiplexing gain is zero under transceiver impairments, we show that the gain of multiplexing can be large at practical SNRs. I.