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25
Downlink MIMO HetNets: Modeling, Ordering Results and Performance Analysis
 IEEE TRANS. ON WIRELESS COMMUN
, 2013
"... We develop a general downlink model for multiantenna heterogeneous cellular networks (HetNets), where base stations (BSs) across tiers may differ in terms of transmit power, target signaltointerferenceratio (SIR), deployment density, number of transmit antennas and the type of multiantenna tr ..."
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Cited by 14 (6 self)
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We develop a general downlink model for multiantenna heterogeneous cellular networks (HetNets), where base stations (BSs) across tiers may differ in terms of transmit power, target signaltointerferenceratio (SIR), deployment density, number of transmit antennas and the type of multiantenna transmission. In particular, we consider and compare space division multiple access (SDMA), single user beamforming (SUBF), and baseline singleinput singleoutput (SISO) transmission. For this general model, the main contributions are: (i) ordering results for both coverage probability and per user rate in closed form for any BS distribution for the three considered techniques, using novel tools from stochastic orders, (ii) upper bounds on the coverage probability assuming a Poisson BS distribution, and (iii) a comparison of the area spectral efficiency (ASE). The analysis concretely demonstrates, for example, that for a given total number of transmit antennas in the network, it is preferable to spread them across many singleantenna BSs vs. fewer multiantenna BSs. Another observation is that SUBF provides higher coverage and per user data rate than SDMA, but SDMA is in some cases better in terms of ASE.
MISO Broadcast Channels with Delayed FiniteRate Feedback: Predict or Observe?
"... Abstract—Most multiuser precoding techniques require accurate channel state information at the transmitter (CSIT) to maintain orthogonality between the users. Such techniques have proven quite fragile in timevarying channels because the CSIT is inherently imperfect due to quantization error and fe ..."
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Cited by 10 (2 self)
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Abstract—Most multiuser precoding techniques require accurate channel state information at the transmitter (CSIT) to maintain orthogonality between the users. Such techniques have proven quite fragile in timevarying channels because the CSIT is inherently imperfect due to quantization error and feedback delay. An alternative approach recently proposed by MaddahAli and Tse (MAT) allows for significant multiplexing gain in the multiinput singleoutput (MISO) broadcast channel (BC) even with CSIT that is “completely stale”, i.e., uncorrelated with the current channel state. With K users, their scheme claims to lose only a log(K) factor relative to the full K degrees of freedom (DoF) attainable in the MISO BC with perfect CSIT for large K. However, their result does not consider the cost of the feedback, which is potentially very large in high mobility (short channel coherence time). In this paper, we more closely examine the MAT scheme and compare its maximum net DoF gain to single user transmission (which always achieves 1 DoF) and partial CSIT linear precoding (which achieves up to K). In particular, assuming the channel coherence time isN symbol periods and the feedback delay is Nfd, we show that when N < (1+o(1))K logK (short coherence time), single user transmission performs best, whereas for N> (1+o(1))(Nfd+K / logK)(1−log−1K)−1 (long coherence time), zeroforcing precoding outperforms the other two. The MAT scheme is optimal for intermediate coherence times, which for practical parameter choices is indeed quite a large and significant range, even accounting for the feedback cost. Index Terms—MIMO, channel state information, quantization. I.
TwoStage Channel Feedback for Beamforming and Scheduling in Network MIMO Systems
"... Abstract—This paper proposes an efficient twostage beamforming and scheduling algorithm for the limitedfeedback cooperative multipoint (CoMP) systems. The system includes multiple basestations cooperatively transmitting data to a pool of users, which share a ratelimited feedback channel for sen ..."
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Cited by 6 (2 self)
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Abstract—This paper proposes an efficient twostage beamforming and scheduling algorithm for the limitedfeedback cooperative multipoint (CoMP) systems. The system includes multiple basestations cooperatively transmitting data to a pool of users, which share a ratelimited feedback channel for sending back the channel state information (CSI). The feedback mechanism is divided into two stages that are used separately for scheduling and beamforming. In the first stage, the users report their best channel gain from all the basestation antennas and the basestations schedule the best user for each of their antennas. The scheduled users are then polled in the second stage to feedback their quantized channel vectors. The paper proposes an analytical framework to derive the bit allocation between the two feedback stages and the bit allocation for quantizing each user’s CSI. For a total number of feedback bits B, it is shown that the number of bits assigned to the second feedback stage should scale as log B. Furthermore, in quantizing channel vectors from different basestations, each user should allocate its feedback budget in proportion to the logarithm of the corresponding channel gains. These bit allocation are then used to show that the overall system performance scales doublelogarithmically with B and logarithmically with the transmit SNR. The paper further presents several numerical results to show that, in comparison with other beamformingscheduling algorithms in the literature, the proposed scheme provides a consistent improvement in downlink sum rate and network utility. Such improvements, in particular, are achieved in spite of a significant reduction in the beamformingscheduling computational complexity, which makes the proposed scheme an attractive solution for practical system implementations. I.
Distributive power control algorithm for multicarrier interference network over timevarying fading channels – tracking performance analysis and optimization,” submitted to
 IEEE Trans. Signal Process
, 2009
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Mode Selection and Power Optimization for Energy Efficiency in Uplink Virtual MIMO Systems
"... Abstract—Driven by green communications, energyefficient transmission is becoming an important design criterion for wireless systems, aiming to extend the life cycle of batteries in mobile devices. In this paper, we tackle the energy efficiency (EE) issue in uplink virtual multipleinput multipleo ..."
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Cited by 3 (1 self)
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Abstract—Driven by green communications, energyefficient transmission is becoming an important design criterion for wireless systems, aiming to extend the life cycle of batteries in mobile devices. In this paper, we tackle the energy efficiency (EE) issue in uplink virtual multipleinput multipleoutput (MIMO) systems, which requires the optimization of two interlaced parameters: the number of constituent mobile users in the virtual MIMO and their corresponding power allocation. The former parameter is a structural parameter defining the size of the virtual MIMO (usually known as the transmission mode) and its optimization relies on the method of enumeration. The difficulty is further aggravated by the fact that the EE is a nonconvex function of power, even for a given transmission mode. By exploiting the fact that increasing the number of active users can increase the number of contributors to the total EE on one hand but reducing the diversity order for each single user on the other, we can show the existence of an optimal transmission mode and find a simple way for its search. Through indepth analysis, we show the existence of a unique globally optimal power allocator for the case without power constraints under the assumption of zeroforcing receivers, and further reveal the impact of power constraints upon power allocation, as compared to its global counterpart, aiming to provide a powerful means for powerconstrained EE optimization. Finally, we establish theories, for isometric networks, to narrow down the search range for possible transmission modes, leading to a significant reduction of computational complexity in optimization. Simulation results are presented to substantiate the proposed schemes and the corresponding theories.
Performance Analysis of SDMA in Multicell Wireless Networks
"... Abstract—Multiantenna transmission, or MIMO, is a major enabling technique for broadband cellular networks. The current implementation, however, is mainly for the pointtopoint link, and its potential for SpaceDivision Multiple Access (SDMA) has not been fully exploited. In this paper, we will an ..."
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Cited by 3 (2 self)
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Abstract—Multiantenna transmission, or MIMO, is a major enabling technique for broadband cellular networks. The current implementation, however, is mainly for the pointtopoint link, and its potential for SpaceDivision Multiple Access (SDMA) has not been fully exploited. In this paper, we will analytically evaluate the performance of SDMA in multicell networks based on a spatial random network model, where both the base stations (BSs) and users are modeled as two independent Poisson point processes. The main difficulty is the evaluation of the interference distribution, for which we propose a novel BS grouping approach that leads to a closedform expression for the network area spectral efficiency. We find that the number of active users (U) served with SDMA is critical, as it affects the spatial multiplexing gain, the aggregated interference, and the diversity gain for each user. The optimal value of U can be selected based on our analytical result, with which SDMA is shown to outperform both the singleuser beamforming and fullSDMA for which U is the same as the number of BS antennas. In particular, it is shown that the performance gain of SDMA is higher when the BS density is relatively small compared to the user density, but the optimal value of U is almost the same for different scenarios, which is close to half of the BS antenna number.
Performance analysis for physical layer security in multiantenna downlink networks with limited CSI feedback
 IEEE Wireless Commun. Lett
, 2013
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Fundamentals of Intercell Overhead Signaling in Heterogeneous Cellular Networks
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Modeling and Mitigation of Interference in Wireless Receivers with Multiple Antennae
, 2011
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Optimal power allocation and user loading for multiuser MISO channels with regularized channel inversion
 IEEE Trans. Commun
, 2013
"... Abstract—We consider a multiuser system where a single transmitter equipped with multiple antennas (the base station) communicates with multiple users each with a single antenna. Regularized channel inversion is employed as the precoding strategy at the base station. Within this scenario we are inte ..."
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Cited by 2 (0 self)
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Abstract—We consider a multiuser system where a single transmitter equipped with multiple antennas (the base station) communicates with multiple users each with a single antenna. Regularized channel inversion is employed as the precoding strategy at the base station. Within this scenario we are interested in the problems of power allocation and user admission control so as to maximize the system throughput, i.e., which users should we communicate with and what power should we use for each of the admitted users so as to get the highest sum rate. This is in general a very difficult problem but we do two things to allow some progress to be made. Firstly we consider the large system regime where the number of antennas at the base station is large along with the number of users. Secondly we cluster the downlink path gains of users into a finite number of groups. By doing this we are able to show that the optimal power allocation under an average transmit power constraint follows the wellknown water filling scheme. We also investigate the user admission problem which reduces in the large system regime to optimization of the user loading in the system. Index Terms—Multiuser precoding, regularized channel inversion, power allocation, large system analysis. I.